Chapter 14: Human Nutrition
In 1822, at a fur-trading post on Mackinac Island in North America, a young trader named Alexis St. Martin was hit by an accidental gunshot at close range. The army surgeon who treated him, William Beaumont, did not expect him to survive. He did survive — but the wound in his side healed in an unusual way. Instead of sealing shut, it left a small permanent opening leading directly into his stomach.
Beaumont realised he was looking at something no one had ever been able to watch before: human digestion, happening live. Over the next several years he ran a long series of observations. He tied small pieces of food — meat, bread, vegetables — to a silk thread, lowered them through the opening into the stomach, and pulled them out at fixed intervals to see how much had dissolved and how fast. He also drew out samples of the stomach’s own fluid, placed food in it in a warm glass vial, and found that the food broke down there too, outside the body altogether.
That last result mattered enormously. It showed that the stomach does not simply mash food mechanically; it pours out a chemical fluid that takes food apart molecule by molecule. Every idea in this chapter — enzymes, juices, glands, absorption — grows from that insight.
Diagram
Fig. 14.0 – Opening illustration: a cutaway view of a human torso showing the alimentary canal from mouth to anus, with the stomach highlighted and a small side panel showing a piece of food suspended on a thread inside the stomach cavity.
Each of these questions is answered somewhere in this chapter. Watch for them as you read — the answers will appear at the exact point where the chapter builds the idea behind them.
14.1 Human Digestive System
What Nutrition Means
Every activity you perform — walking to school, thinking through a problem, even breathing while asleep — runs on energy. That energy, and the raw material your body uses to build and repair itself, comes from food. But food in the form we eat it is not directly usable by our cells, and this is where nutrition as a whole process begins.
Nutrition is the sum of all the processes by which an organism takes in food substances and puts them to use. The World Health Organisation (WHO) defines nutrition as the intake of food considered in relation to the body’s dietary needs — in other words, it is not just about eating, but about eating what the body actually requires.
The dietary needs of a healthy human being include carbohydrates, proteins, fats, vitamins, minerals, water and fibres, all in adequate amounts. Carbohydrates and fats are the body’s main energy sources, proteins supply material for growth and tissue repair, and water and fibres keep the digestive system itself working smoothly.
Vitamins and minerals are needed only in small quantities, but “small” does not mean “unimportant”. They act as helpers for enzymes and as parts of key body structures, and a shortage of even one of them produces a clear deficiency disorder.
The word nutrition covers a whole chain of processes: ingestion (taking food in), digestion (breaking it down), absorption (taking the products into blood and lymph), assimilation (building them into the body’s own material) and egestion (throwing out what is left over). The sections of this chapter follow this chain in exactly that order.
Why Food Must Be Digested
Food as we eat it is made of complex molecules. These molecules are non-diffusible (too large to pass through a living membrane) and non-absorbable (they cannot be taken up by the cells lining the gut). Digestion solves this problem by chemically cutting them into simple, small units.
Notice how carefully that definition is worded. It does not say food is merely “broken into small pieces” — chewing does that. It says the food is changed into substances that can diffuse and can be assimilated, that is, actually built into the body. That is a chemical change, not just a physical one.
The human digestive system that carries out this work has two parts: a long tube called the alimentary canal, and the associated digestive glands that pour their secretions into it. This section deals with the canal itself; the glands come later in the chapter.
The Alimentary Canal
The alimentary canal is a long tubular structure that begins at the mouth and ends at the anus. In an adult it is about 8–10 metres long, coiled and packed neatly within the body cavity.
It is helpful to picture the canal as a single continuous passage with specialised stations along its length, each doing one particular job before handing the food on. The overall path is shown in Fig. 14.1.1 and summarised as a sequence in FC 14.1.
Diagram
Fig. 14.1.1 – Human alimentary canal in situ, labelling mouth, pharynx, oesophagus, stomach, duodenum, jejunum, ileum, caecum with appendix, ascending/transverse/descending colon, rectum and anus.
Flowchart
FC 14.1 – Path of food through the alimentary canal: mouth (buccal cavity) → pharynx → oesophagus → stomach (cardia → fundus → body → pylorus) → duodenum → jejunum → ileum → caecum → ascending colon → transverse colon → descending colon → rectum → anus
The organs of the alimentary canal, in order, are the mouth, pharynx, oesophagus, stomach, small intestine and large intestine, ending at the anus. We will now follow food along this route.
Mouth
The mouth, also called the oral or buccal cavity, is where food enters the body. It is bounded in front by fleshy lips, its side walls are formed by the cheeks, its roof by the palate, and its floor by the tongue.
Internally the buccal cavity is lined by a mucous membrane — a moist lining that protects the underlying tissue and keeps the surface slippery. The ducts of the salivary glands open into this cavity, so food is moistened almost as soon as it enters.
Teeth
An adult human being has 32 teeth in the buccal cavity. Human dentition is described by three terms together: thecodont, diphyodont and heterodont.
It is called thecodont because each tooth is fixed in a separate socket in the jaw bone, held there by a fibrous immovable joint called gomphosis. This socket arrangement is what allows a tooth to withstand the strong forces of biting and grinding without being pushed out.
It is called diphyodont because in our lifetime we get only two sets of teeth — the milk (temporary) teeth of childhood and the permanent teeth that replace them. Once a permanent tooth is lost, no third set grows in its place, which is exactly why dental care matters so much.
It is called heterodont because we have four different types of teeth, each shaped for a different job: incisors (I) for cutting, canines (C) for tearing, premolars (PM) and molars (M) for crushing and grinding.
Each half of each jaw carries two incisors, one canine, two premolars and three molars, as shown in Fig. 14.1.2.
Diagram
Fig. 14.1.2 – One half of the upper jaw seen from below, showing the arrangement 2 incisors, 1 canine, 2 premolars, 3 molars, with each type labelled.
Thus the dental formula of an adult human can be represented as:
\[I\,\frac{2}{2},\; C\,\frac{1}{1},\; PM\,\frac{2}{2},\; M\,\frac{3}{3} \;=\; \frac{2,1,2,3}{2,1,2,3}\]
The upper number in each fraction is the count in one half of the upper jaw, and the lower number is the count in one half of the lower jaw. Adding across one half-jaw gives \(2+1+2+3 = 8\) teeth, so
\[8 \times 2 = 16 \text{ teeth in each jaw} \;\Rightarrow\; 32 \text{ teeth in all}\]
The study of teeth with respect to their number, arrangement, development and so on is known as dentition.
Structure of a Tooth
A tooth has two visible divisions, shown clearly in Fig. 14.1. The portion that projects above the gum is the crown, and the portion made up of two or three projections embedded in the gum is the root. A short neck connects the crown with the root.
Diagram
Fig. 14.1 – Longitudinal section of a tooth labelling enamel, dentin, pulp, gums, bone, root canal, nerves and blood vessels, with crown and root marked.
The crown is covered by enamel, the hardest substance in the human body. Enamel is made up of calcium phosphate, \(Ca_3(PO_4)_2\), and calcium carbonate, \(CaCO_3\) — mineral salts that give it its extreme hardness and its resistance to the wear of a lifetime of chewing.
The basic shape of a tooth is derived from dentin, a calcified connective tissue lying beneath the enamel. Dentin is hard but slightly softer than enamel, which lets it absorb the shock of biting without cracking.
The dentin encloses a cavity called the pulp cavity, filled with connective tissue pulp. This pulp cavity contains blood vessels and nerves — which is why a deep cavity in a tooth is so painful, while a shallow chip in the enamel is not felt at all.
The pulp cavity has an extension running down into the root, called the root canal. The dentin of the root is covered by cementum, a bone-like substance that attaches the root firmly to the surrounding socket in the gum.
Tongue
The tongue is a muscular, fleshy organ, roughly triangular in shape, lying along the floor of the buccal cavity. Being muscular, it can move food around the mouth, position it between the teeth, and finally push it backwards for swallowing.
The upper surface of the tongue bears numerous small projections called papillae. Some papillae carry sensory receptors called taste buds, which is how the tongue serves as the organ of taste as well as an aid to chewing and swallowing.
Pharynx
The buccal cavity leads into a short pharynx. The pharynx is important because it is a common passage for both food and air — the routes of the two cross here.
The pharynx opens into the trachea (the windpipe) through an opening called the glottis. The glottis is guarded by a cartilaginous flap called the epiglottis.
During the swallowing action, known as deglutition, the epiglottis closes over the glottis and prevents food from entering the trachea. If this flap fails to close in time — for instance when we talk or laugh while eating — food enters the airway and we cough violently to expel it.
The branching at the pharynx is shown in FC 14.2.
Flowchart
FC 14.2 – Branching at the pharynx: material reaches pharynx → if it is air → glottis open → trachea → lungs; if it is food (deglutition) → epiglottis closes glottis → oropharynx → gullet → oesophagus
The lower region of the pharynx is called the oropharynx. The oropharynx opens into the oesophagus through the gullet.
Oesophagus
The oesophagus is a thin, muscular tube lying behind the trachea. This tube, about \(\approx 25\) cm long, passes through the neck and the central aspect of the rib cage, pierces the diaphragm, and joins the stomach.
It is lined by mucus cells. The mucus they secrete lubricates the passageway, so that a fairly dry bolus can slide down without injuring the lining.
Histologically, the oesophagus is made up of longitudinal and circular muscles. A rhythmic wave of contraction and relaxation of these muscles, called peristalsis, pushes the food along.
Stomach
The stomach is located in the upper left portion of the abdominal cavity. It is a muscular, sac-like, ‘J’ shaped organ, around 25 to 30 cm in length, and is divided into an upper cardiac region and a lower pyloric region. Its parts are shown in Fig. 14.2.
Diagram
Fig. 14.2 – External view of the stomach labelling oesophagus, gastroesophageal junction, cardia, fundus, body, lesser curvature, greater curvature, pylorus and duodenum.
Cardia or cardiac region is the first part, into which the oesophagus opens. The cardia surrounds a band of circular muscles present at the junction of the oesophagus and stomach, called the cardiac sphincter. A sphincter is simply a ring of muscle that can tighten to close an opening.
The cardiac sphincter prevents back flow, or regurgitation, of food from the stomach into the oesophagus. This matters because stomach contents are strongly acidic, while the oesophagus has no protection against acid.
Fundus is the dome-shaped region above and to the left of the cardia. Body forms the large central portion of the stomach that stores the food. Pylorus is a narrow posterior region of the stomach; it opens into the duodenum, the initial part of the small intestine.
This opening is guarded by a set of sphincter muscles called the pyloric sphincter, which regulates the flow of food from the stomach into the small intestine. By releasing food a little at a time, it makes sure the small intestine is never flooded faster than it can handle.
The stomach temporarily stores food and gives the feeling of satiety, that is, the sense of being full. It also churns the food and helps in mixing the food with gastric juice.
Small Intestine
In human beings, the small intestine is about 6 metres long and 2.5 cm broad, a tube coiled within the abdominal cavity. Its coils are held together by mesenteries — sheets of tissue that support the blood vessels, lymph vessels and nerves running to the gut.
The mesenteries also answer a question students often ask: with six metres of tubing packed into the abdomen, why does it not tangle when we jump or run? The mesenteric attachment keeps each loop anchored in place while still allowing it to move.
The small intestine is divided into three parts. Duodenum is about 26 cm long and is a ‘U’ shaped structure; it turns towards the left side of the abdominal cavity below the stomach. Jejunum is about 2.5 metres long and forms the coiled middle portion; it is narrower than the duodenum. Ileum is about 3.5 metres long, highly coiled and a little broader than the jejunum.
The ileum opens into the caecum of the large intestine at the ileocaecal junction, as seen in Fig. 14.1.3.
Diagram
Fig. 14.1.3 – Small and large intestine in position, labelling duodenum, jejunum, ileum, ileocaecal junction, caecum, vermiform appendix, ascending colon, transverse colon, descending colon, rectum and anus.
Large Intestine
The ileum opens into the large intestine. The large intestine is 1.5 metres in length. It is wider in diameter but shorter than the small intestine — which is exactly why it carries the name “large” despite being the shorter of the two.
The large intestine consists of the caecum, colon and rectum.
Caecum is a small, blind sac present at the junction of the ileum and colon. It is 6 cm in length and hosts some symbiotic micro-organisms — bacteria that live there permanently and benefit both themselves and the host.
An elongated, worm-like vermiform appendix arises from the caecum. In human beings the appendix is a vestigial organ, that is, a reduced structure with no useful function left. In herbivorous animals the corresponding structure is functional and helps in the digestion of cellulose, which their gut enzymes cannot break down on their own.
Colon — the caecum opens into the colon. The colon is a tube-like organ consisting of three parts: the ascending colon, the transverse colon and the descending colon. It is internally lined by mucosal cells.
Rectum is the posterior region of the large intestine. It temporarily stores the undigested waste material called faeces until it is egested out through the anus.
Anus is the terminal opening of the alimentary canal. It is guarded by a sphincter, and expels faecal matter by a process called egestion or defaecation.
14.2 Histological Structure of Alimentary Canal
The Four Basic Layers
So far we have looked at the alimentary canal organ by organ. If we now cut across the wall of that canal and examine it under a microscope, a surprising pattern appears: from the oesophagus right down to the rectum, the wall is built to the same basic plan.
The entire gastrointestinal tract is lined by four basic layers. Named from inside to outside, they are mucosa, submucosa, muscularis and serosa. The arrangement is seen in Fig. 14.3, and the order is summarised in FC 14.3.
Diagram
Fig. 14.3 – Transverse section through the wall of the alimentary canal, labelling serosa, longitudinal muscle layer, circular muscle layer, submucosa, glandular epithelium of mucosa and blood vessel.
Flowchart
FC 14.3 – Layers of the gut wall from the lumen outwards: lumen → mucosa → submucosa → muscularis (inner oblique → middle circular → outermost longitudinal) → serosa
These four layers show modification depending on the location and the function of the organ concerned. That single sentence is the key to this whole section. The plan stays the same, but each organ adjusts the layers to suit its own job — a thicker muscle coat where food must be churned, a folded lining where food must be absorbed.
Serosa
Serosa is the outermost layer. It is made up of a layer of squamous epithelium called mesothelium, together with an inner layer of connective tissue.
Being smooth and moist, this outer covering lets neighbouring loops of the gut slide past one another as the canal moves, instead of rubbing and sticking together.
Muscularis
Muscularis is formed of smooth muscles — the involuntary muscle type, which is why the movements of the gut continue without our conscious control. These muscles are usually arranged in three concentric layers: the outermost layer shows longitudinal muscles, the middle shows circular muscles and the inner shows oblique muscles.
The thickness of this layer is not the same everywhere. It is wider in the stomach and comparatively thin in the intestinal region, and the layer of oblique muscles is absent in the intestine.
Submucosa
Submucosa is formed of loose connective tissue containing blood vessels, lymph vessels and nerves. It is the service layer of the gut wall: the vessels here collect absorbed nutrients and carry them away, while the nerves coordinate the muscle contractions above and the secretions below.
The duodenal submucosa shows the presence of glands, a modification not found along the rest of the canal.
Mucosa
The lumen of the alimentary canal — that is, its inner cavity — is lined by the mucosa. Throughout the length of the alimentary canal, the mucosa layer shows the presence of goblet cells, which secrete mucus. This mucus lubricates the lumen of the alimentary canal and protects the lining from the food and juices passing over it.
This layer shows modification in different regions of the alimentary canal. In the stomach, it is thrown into irregular folds called rugae. These folds allow the stomach wall to stretch out as a meal arrives and to fold back again when it empties.
In the stomach, the mucosa layer also forms gastric glands that secrete gastric juice. So the same layer that protects the lining is also the layer that produces the digestive secretion — a division of labour between different cells of one tissue.
Modifications in the Small Intestine
The mucosa of the small intestine forms finger-like foldings called villi. Each villus is a tiny projection into the lumen, and there are enormous numbers of them, as seen in Fig. 14.2.1.
Diagram
Fig. 14.2.1 – A single intestinal villus in longitudinal section, labelling brush border epithelial cells with microvilli, capillary network, lacteal, and a crypt of Lieberkuhn at the base between two villi.
The intestinal villi are lined by brush border or epithelial cells having microvilli at the free surface. Microvilli are far smaller projections on the surface of each cell, so the folding happens at two levels — the villus folds the tissue, and the microvilli fold the cell surface.
Villi are supplied with a network of capillaries and with lymph vessels called lacteals. This double supply matters: the capillaries pick up water-soluble products of digestion, while the lacteals pick up the fatty products, which cannot enter the blood capillaries directly.
Mucosa also forms crypts in between the bases of villi in the intestine, called crypts of Lieberkuhn. These are intestinal glands, and they supply the intestinal secretion that we will meet again when we study the physiology of digestion.
14.3 Digestive Glands
The alimentary canal is only half of the digestive system. On its own, a tube can move food and absorb the products of digestion, but it cannot supply all the chemicals needed to break that food down. That work belongs to the digestive glands.
The digestive glands associated with the alimentary canal include the salivary glands, the liver and the pancreas. Each of them lies outside the canal but empties its secretion into it through a duct, as set out in FC 14.4.
Flowchart
FC 14.4 – Digestive glands and where they pour their secretions: digestive glands → salivary glands (parotid, submandibular, sublingual) → buccal cavity; liver → bile → duodenum; pancreas → exocrine part (acini, pancreatic juice) → duodenum, and endocrine part (islets of Langerhans, hormones) → blood
Salivary Glands
There are three pairs of salivary glands, all of which open into the buccal cavity. The parotid glands are present in front of the ear. The submandibular glands are present below the lower jaw. The glands present below the tongue are called sublingual.
Salivary glands are made up of two types of cells, and this is the reason saliva is able to do two very different jobs at once. Serous cells secrete a fluid containing the digestive enzyme salivary amylase. Mucous cells produce mucus that lubricates food and helps swallowing.
An enzyme, as you know, is a biological catalyst — a protein that speeds up a particular chemical reaction without being used up itself. Salivary amylase is the enzyme that begins the chemical breakdown of starch, and we will follow its action closely in the next section.
Liver
The liver is a dark reddish-brown coloured gland present just below the diaphragm. It occupies the right upper portion of the abdominal cavity. It is the largest gland of the body and weighs about 1.2 to 1.5 kg in an adult human being.
The liver is bilobed, that is, it has two lobes. Each lobe of this gland is covered by a thin covering called Glisson’s capsule, which is made up of connective tissue.
Each lobe is divided into several structural and functional units of the liver called hepatic lobules. Calling them “structural and functional” units means that the lobule is both the building block of the organ and the smallest piece that can still perform the organ’s work — much as the nephron is for the kidney.
Each hepatic lobule is polygonal in shape. At the junction of adjacent lobules, a triangular portal area is present. In this portal area, a branch of each of the following is present: the hepatic artery, the hepatic portal vein and the bile duct.
Diagram
Fig. 14.3.1 – Liver and pancreas in position, labelling liver, gall bladder, bile ducts, hepatic duct, cystic duct, common hepatic duct, common bile duct, pancreas, pancreatic duct, hepatopancreatic sphincter and duodenum.
Structure of a Hepatic Lobule
A lobule consists of cords of hepatic cells which are arranged around a central vein. In between the cords of hepatic cells, spaces called sinusoids are present, through which the blood flows.
This arrangement is worth pausing on. Because the liver cells are stacked in cords with blood channels running between them, almost every hepatic cell is in direct contact with flowing blood. Nothing dissolved in that blood can slip past unexamined.
In the sinusoids, phagocytic cells called Kupffer cells are present. Phagocytic means these cells engulf and destroy particles. Kupffer cells destroy toxic substances, dead and worn-out blood cells, and microorganisms.
Bile and Its Path
Hepatic cells produce bile juice. It is collected and carried through the bile duct and stored in a sac-like structure called the gall bladder.
The duct of the gall bladder and the hepatic duct together form the common bile duct. Bile juice secreted by the liver emulsifies fats and makes food alkaline — two roles we will return to when we study digestion in the small intestine.
Flowchart
FC 14.5 – Bile pathway: hepatic cells → bile duct → gall bladder (storage) → duct of gall bladder + hepatic duct → common bile duct → joins pancreatic duct → hepato-pancreatic duct → sphincter of Oddi → duodenum
Other Functions of the Liver
The liver is a vital organ, meaning life is not possible without it. Its work extends far beyond producing bile.
It stores excess glucose in the form of glycogen. Glycogen is a large storage carbohydrate, and converting glucose into it keeps the blood sugar level from rising too high after a meal, while providing a reserve to draw on later.
Deamination of excess amino acids to ammonia, and the further conversion of that ammonia to urea, takes place in the liver. Deamination means the removal of the amino group from an amino acid. This matters because the body cannot store surplus protein, and the ammonia released is highly toxic — converting it to the far less harmful urea makes it safe to carry in the blood until the kidneys excrete it.
The liver is also involved in the synthesis of vitamins A, D, K and \(B_{12}\). It produces blood proteins like prothrombin and fibrinogen, both of which are essential for the clotting of blood.
During early development, the liver acts as a haemopoietic organ, that is, an organ that produces blood cells — a role later taken over largely by the bone marrow. Kupffer cells help in the detoxification process and in the destruction of old RBCs.
Pancreas
The pancreas is a leaf-shaped heterocrine gland present in the gap formed by the bend of the duodenum, under the stomach.
The exocrine part of the pancreas is made up of acini. Acinar cells secrete alkaline pancreatic juice that contains various digestive enzymes. The alkaline nature is deliberate: the material arriving from the stomach is strongly acidic, and the enzymes of the small intestine can work only in an alkaline medium.
Pancreatic juice is collected and carried to the duodenum by the pancreatic duct. The common bile duct joins the pancreatic duct to form the hepato-pancreatic duct, which opens into the duodenum. The opening of the hepato-pancreatic duct is guarded by the sphincter of Oddi.
The Endocrine Part of the Pancreas
The endocrine part of the pancreas is made up of groups of cells called islets of Langerhans, present between the acini. Their arrangement among the acinar tissue is seen in Fig. 14.4.
Diagram
Fig. 14.4 – Histological structure of the pancreas, labelling acinar cells, interlobular ducts, central duct, capillary network, islet of Langerhans with alpha cell, beta cell and delta cell.
Islets contain three types of cells. \(\alpha\)-cells secrete glucagon, \(\beta\)-cells secrete insulin, and somatostatin hormone is secreted by \(\delta\)-cells.
Glucagon and insulin together control the blood-sugar level. They work in opposite directions — a pattern seen again and again in the body, where two opposing signals hold a value steady rather than one signal pushing it in a single direction. Somatostatin hormone inhibits glucagon and insulin secretion.
14.4 Physiology of Digestion
We are already aware that the food we consume needs to be processed in order to utilise it completely. Physiology of digestion deals with the various processes involved in the simplification of food — what happens to a mouthful of food, step by step, from the moment it enters the mouth.
Mechanical and Chemical Digestion
The digestion process is carried out by both mechanical as well as biochemical methods, and the two work together rather than separately.
Mechanical digestion includes the various movements of the alimentary canal that help chemical digestion. Mastication or chewing of food by the teeth, churning in the stomach, and peristaltic movements of the gastrointestinal tract bring about mechanical digestion in the human body.
Chemical digestion is a series of catabolic (breaking down) reactions that hydrolyse the food. Hydrolysis means splitting a large molecule by adding water across the bond that holds its units together — which is precisely how a starch chain or a protein chain is cut into its building blocks.
Let us now study the process of digestion from the point where food enters the body, that is, the mouth.
Digestion in the Buccal Cavity
Both mechanical and chemical digestion processes take place in the mouth. Mastication or chewing of food takes place with the help of the teeth and the tongue. The teeth crush and grind the food, while the tongue manipulates it, keeping it positioned between the grinding surfaces.
Crushing of food becomes easier when it gets moistened by saliva. Mucus in the saliva lubricates the food, and it also helps in binding the food particles into a mass of food called bolus.
The bolus is swallowed by deglutition. The tongue presses against the palate and pushes the bolus into the pharynx, and from there the bolus passes on to the oesophagus.
Composition and Role of Saliva
Saliva contains 98% water and 2% other constituents. These other constituents include electrolytes — sodium, potassium, calcium, chloride and bicarbonates — and the digestive enzyme salivary amylase.
The only chemical digestion that takes place in the mouth is by the action of salivary amylase. It helps in the conversion of starch into maltose. About 30% of the starch gets converted to maltose in the mouth.
\[\text{Starch (Polysaccharide)} \xrightarrow[\text{pH } 6.8]{\text{Salivary amylase}} \text{Maltose (Disaccharide)}\]
Read that reaction carefully. A polysaccharide is a long chain of sugar units; a disaccharide is just two units joined together. So the enzyme has not finished the job — it has only chopped a long chain into short pieces. The final cut into single sugars happens much later, in the small intestine.
The pH value written below the arrow is not decoration. Every enzyme works best within a narrow pH range, and salivary amylase needs a nearly neutral medium of about \(pH\ 6.8\). Move it far from that value and the enzyme stops working — a fact that becomes important the moment food reaches the stomach.
Saliva also contains lysozyme. It acts as an antibacterial agent that prevents infections, giving the mouth a chemical defence against the many microorganisms that enter along with food.
From Mouth to Stomach
The bolus further passes down through the oesophagus by peristalsis. Sometimes regurgitation or vomiting takes place due to reverse spasmodic peristalsis, in which the wave travels in the wrong direction and pushes contents upward instead of downward.
Food from the oesophagus enters the stomach. The gastro-oesophageal sphincter controls the passage of food into the stomach.
Digestion in the Stomach
Both mechanical and chemical digestion take place in the stomach. The stomach stores the food for 4–5 hours, which gives the gastric juice ample time to act.
Physical digestion here happens by churning of the food. The thick muscular wall of the stomach helps in the churning process. Churning further breaks down the food particles and also helps in thorough mixing of gastric juice with the food.
The Cells of the Gastric Gland
The mucosa layer of the stomach has gastric glands. Each gastric gland has three major types of cells, namely mucus cells, peptic or chief cells, and parietal or oxyntic cells.
Mucus cells secrete mucus. Peptic cells secrete the proenzyme pepsinogen — a proenzyme, also called a zymogen, being an inactive form of an enzyme that must be activated before it can work.
Parietal cells secrete HCl and intrinsic factor, which is essential for the absorption of vitamin \(B_{12}\). Thus, gastric juice contains mucus, the inactive enzyme pepsinogen, HCl and intrinsic factor. In infants, the stomach also secretes rennin.
Mucus protects the inner lining of the stomach from the HCl present in gastric juice.
Action of Gastric Juice
HCl in gastric juice makes the food acidic and stops the action of salivary amylase. This is the answer to the question raised earlier: amylase needs a nearly neutral pH, and the acid destroys that condition. It also kills the germs that might be present in the food.
Pepsinogen gets converted into the active enzyme pepsin in the acidic medium provided by HCl.
\[\text{Pepsinogen (Inactive enzyme)} \xrightarrow[\text{pH }(1.8)]{\text{HCl}} \text{Pepsin (Active enzyme)}\]
In the presence of pepsin, proteins in the food get converted into simpler forms like peptones and proteoses.
\[\text{Proteins} \xrightarrow[\text{Acidic medium}]{\text{Pepsin}} \text{Peptones} + \text{Proteoses}\]
Rennin found in the gastric juice of infants acts on casein, a protein present in milk. It brings about curdling of milk proteins with the help of calcium. The coagulated milk protein is further digested with the help of pepsin. Rennin is absent in adults.
Curdling is a sensible first step for an infant. Milk is a liquid and would otherwise pass quickly out of the stomach; turning its protein into a soft solid holds it back long enough for pepsin to work on it.
At the end of gastric digestion, food is converted to a semifluid acidic mass of partially digested food, which is called chyme. The chyme from the stomach is pushed into the small intestine through the pyloric sphincter for further digestion.
Digestion in the Small Intestine
In the small intestine, intestinal juice, bile juice and pancreatic juice are mixed with the food. Peristaltic movements of the muscularis layer help in the proper mixing of these digestive juices with the chyme.
Bile juice and pancreatic juice are poured into the duodenum through the hepato-pancreatic duct, the shared passage we traced in the previous section.
Bile Juice
Bile juice is a dark green coloured fluid that contains bile pigments (bilirubin and biliverdin), bile salts (Na-glycocholate and Na-taurocholate), cholesterol and phospholipid.
Bile does not contain any digestive enzyme. This is worth emphasising, because bile is nevertheless essential to fat digestion — it prepares the fat rather than breaking it down.
Bile salts neutralise the acidity of chyme and make it alkaline. This shift is necessary because the enzymes arriving from the pancreas and the intestinal mucosa can work only in an alkaline medium, just as pepsin needed an acidic one.
Bile salts also bring about emulsification of fats. Fats do not dissolve in water, so a large fat droplet offers very little surface for a water-soluble enzyme to attack. Emulsification breaks big droplets into a very large number of tiny ones, multiplying the surface available for lipase.
Bile also activates the lipid-digesting enzymes, or lipases. Bile pigments impart colour to faecal matter, and we shall see in the last section of this chapter what happens when those pigments cannot reach the intestine.
Pancreatic Juice and Intestinal Juice
Pancreatic juice secreted by the pancreas contains pancreatic amylases, lipases, and the inactive enzymes trypsinogen and chymotrypsinogen. Pancreatic juice also contains nucleases — the enzymes that digest nucleic acids.
The intestinal mucosa secretes digestive enzymes, and the goblet cells of the mucosa produce mucus. Mucus plus intestinal enzymes together constitute the intestinal juice, or succus entericus. The intestinal juice contains various enzymes like dipeptidases, lipases, disaccharidases and others.
Action of Pancreatic Juice
Pancreatic amylase acts on glycogen and starch and converts those to disaccharides. This completes the job that salivary amylase had only begun in the mouth.
Lipases hydrolyse fat molecules into fatty acids and monoglycerides.
Inactive trypsinogen present in pancreatic juice is converted to its active form, trypsin. This conversion is brought about by enterokinase present in intestinal juice.
Notice the safety logic here. The pancreas sends out its protein-digesting enzymes in inactive form, and the activating agent waits in the intestine — so the enzymes cannot begin digesting the pancreas itself on the way.
Trypsin converts proteins as well as proteoses and peptones to polypeptides. It also converts chymotrypsinogen to active chymotrypsin. Chymotrypsin converts polypeptides to dipeptides.
Both pancreatic and intestinal lipases initially convert fats into fatty acid and diglycerides. Diglycerides are further converted to monoglycerides by the removal of a fatty acid from glycerol.
The mucus and bicarbonates present in pancreatic juice protect the intestinal mucosa and provide the alkaline medium for enzymatic action. Sub-mucosal Brunner’s glands help in the action of goblet cells. Most of the digestion gets over in the small intestine.
Digestion of Nucleic Acids
Nucleases present in pancreatic juice help in the digestion of nucleic acids to pentose sugar and nitrogenous base. This happens in three successive steps, each carried out by a different enzyme.
\[\text{Nucleic acids} \xrightarrow{\text{Nucleases}} \text{Nucleotides}\]
\[\text{Nucleotides} \xrightarrow{\text{Nucleotidases}} \text{Nucleosides} + \text{phosphate}\]
\[\text{Nucleosides} \xrightarrow{\text{Nucleosidases}} \text{Sugar} + \text{base}\]
The sequence is shown together in FC 14.9. Note that each enzyme removes one part at a time: first the chain is cut into nucleotides, then the phosphate is taken off, and finally the sugar and the nitrogenous base are separated.
Flowchart
FC 14.9 – Digestion of nucleic acids: nucleic acids → (nucleases) → nucleotides → (nucleotidases) → nucleosides + phosphate → (nucleosidases) → pentose sugar + nitrogenous base
Action of Intestinal Juice
The intestinal juice carries out the final cuts. The disaccharides left over from carbohydrate digestion are split into single sugars, and dipeptides are split into amino acids.
\[\text{Maltose} \xrightarrow{\text{Maltase}} \text{glucose}\]
\[\text{Sucrose} \xrightarrow{\text{Sucrase}} \text{glucose} + \text{fructose}\]
\[\text{Lactose} \xrightarrow{\text{Lactase}} \text{glucose} + \text{galactose}\]
\[\text{Dipeptides} \xrightarrow{\text{Dipeptidase}} \text{Amino acids}\]
\[\text{Emulsified fats} \xrightarrow{\text{Lipase}} \text{fatty acids} + \text{monoglycerides}\]
The complete route taken by each class of food is easier to follow as a chain than as a paragraph, so the three main pathways are set out in FC 14.6, FC 14.7 and FC 14.8.
Flowchart
FC 14.6 – Digestion of carbohydrates: starch (mouth) → (salivary amylase, pH 6.8) → maltose; starch and glycogen (duodenum) → (pancreatic amylase) → disaccharides → (maltase) → glucose; (sucrase) → glucose + fructose; (lactase) → glucose + galactose
Flowchart
FC 14.7 – Digestion of proteins: proteins (stomach) → (pepsin, acidic medium) → peptones + proteoses → (trypsin, small intestine) → polypeptides → (chymotrypsin) → dipeptides → (dipeptidase) → amino acids
Flowchart
FC 14.8 – Digestion of fats: fats → (bile salts) → emulsified fats → (pancreatic and intestinal lipase) → fatty acids + diglycerides → (removal of a fatty acid from glycerol) → monoglycerides + fatty acids
The End of Digestion
The conversion of proteins into amino acids, fats to fatty acids and monoglycerides, nucleic acids to sugar and nitrogenous base, and carbohydrates to monosaccharides marks the end of digestion of food.
Food is now called chyle. Chyle is an alkaline slurry which contains various nutrients ready for absorption. The nutrients are absorbed, and the undigested remains are transported to the large intestine.
What Happens in the Large Intestine
Remember, the mucosa of the large intestine produces mucus but no enzymes. So whatever chemical breakdown occurs here is not the body’s own work.
Some carbohydrates and proteins do enter the large intestine. These are digested by the action of bacteria that live in the large intestine.
Carbohydrates are fermented by bacterial action, and hydrogen, carbon dioxide and methane gas are produced in the colon. Protein digestion in the large intestine ends up in the production of substances like indole, skatole and \(H_2S\). These are the reason for the odour of faeces.
These bacteria also synthesise several vitamins like the B vitamins and vitamin K. This is a genuine partnership: the bacteria get shelter and leftover food, and we get vitamins we could not otherwise make.
Neurohormonal Control of Digestion
It is essential that the digestive enzymes and juices are produced in a sequential manner and at a proper time. Pouring out gastric juice when the stomach is empty would damage the lining; releasing bile when there is no fat to emulsify would simply waste it. These secretions are therefore under neurohormonal control — that is, controlled by nerves and hormones together.
Sight, smell and even the thought of food trigger saliva secretion. The tenth cranial nerve stimulates the secretion of gastric juice in the stomach, and the hormone gastrin brings about the same effect.
You must have experienced hunger pangs at your regular meal times. Can you now reason out why it happens? The body has learned to anticipate the meal, and the nervous signals begin preparing the stomach before the food actually arrives.
The intestinal mucosa produces hormones like secretin, cholecystokinin (CCK) and gastric inhibiting peptide (GIP). Secretin inhibits secretion of gastric juice. It stimulates the secretion of bile juice from the liver, pancreatic juice and intestinal juice.
CCK brings about a similar action and induces satiety, that is, the feeling of fullness or satisfaction. GIP also inhibits gastric secretion. The overall pattern of nervous and hormonal control is summarised in Fig. 14.5.
Diagram
Fig. 14.5 – Regulation of gastric function, showing sensory and mental input acting through the vagus nerve, the long vagovagal reflex with sensory and motor fibres, gastrin release from the stomach, and secretin plus CCK from the intestine inhibiting gastric activity.
14.5 Absorption, Assimilation and Egestion
Digestion has now converted food into simple end products, but those products are still lying inside the lumen of the alimentary canal. Strictly speaking, they are still outside the body’s tissues. Getting them across the lining is the next step.
The passage of end products of digestion through the mucosal lining of the alimentary canal into blood and lymph is called absorption.
How Substances Are Absorbed
Absorption takes place by various ways like simple diffusion, osmosis, facilitated transport and active transport. Which route a particular substance takes depends on its size, its solubility and whether it has to move with or against a concentration gradient.
About 90% of absorption takes place in the small intestine, and the rest in the mouth, stomach and large intestine. Given the villi and microvilli we studied earlier, this distribution is exactly what we should expect.
Absorption of part of the glucose, amino acids and some electrolytes like chloride ions takes place by simple diffusion, depending on the concentration gradient. In simple diffusion a substance simply moves from where it is more concentrated to where it is less concentrated, so no energy is spent by the cell.
Some amino acids, as well as substances like fructose, are absorbed by facilitated transport. In this method, carrier ions like \(Na^+\) bring about absorption. The substance still moves down its gradient, but it needs a carrier in the membrane to help it across.
Some ions are absorbed against the concentration gradient. This requires energy. This type of absorption of a mineral like sodium is called active transport. Water is absorbed along the concentration gradient, that is, by osmosis.
Where Absorption Takes Place
Different regions of the alimentary canal absorb different substances, as summarised in Fig. 14.5.1.
Diagram
Fig. 14.5.1 – Chart of the alimentary canal showing absorption at four sites: mouth, stomach, small intestine and large intestine, with the substances absorbed at each site listed beside it.
In the mouth, absorption takes place through the mucosa of the mouth and the lower side of the tongue into the blood capillaries — for example, some drugs like certain painkillers.
In the stomach, the gastric mucosa is impermeable to most substances, hence nutrients reach the small intestine unabsorbed. Little water, electrolytes, alcohol and drugs like aspirin get absorbed in the stomach.
In the small intestine, glucose, fructose, galactose, amino acids, minerals and water-soluble vitamins are absorbed into the blood capillaries in the villi. Lipids and fat-soluble vitamins (A, D, E, K) are absorbed into the lacteals.
In the large intestine, absorption of water, electrolytes like sodium and chloride, drugs and some vitamins takes place.
Absorption of Fats
Monoglycerides and fatty acids cannot be absorbed into the blood. Being insoluble in water, they cannot simply dissolve in the watery contents of the intestine and cross into a capillary.
These dissolve in the centre of spherical aggregates formed by bile salts, called micelles. A micelle is arranged so that its water-loving surface faces the surrounding fluid while the fat sits protected in the middle.
Micelles enter into the intestinal villi. Here, they are reformed into chylomicrons. Chylomicrons are small, protein-coated fat globules.
Chylomicrons are transported into the lymph vessels called lacteals. From here, they are transported to the blood stream. The whole route is shown in Fig. 14.5.2 and set out step by step in FC 14.10.
Diagram
Fig. 14.5.2 – Absorption of fat in a villus: fatty acids and monoglycerides forming micelles with bile salts in the lumen, entry into the epithelial cell, re-formation as chylomicrons, and passage into the central lacteal.
Flowchart
FC 14.10 – Absorption of fats: fats digested to fatty acids + monoglycerides → dissolved in the centre of micelles formed by bile salts → micelles enter the intestinal villi → reformed as protein-coated chylomicrons → pass into lacteals (lymph vessels) → carried to the blood stream
Assimilation
The absorbed food material finally reaches the tissue and becomes a part of the protoplasm. This is called assimilation.
Absorption and assimilation are often confused, but they are two different stages. Absorption gets the nutrient into the transport system; assimilation is what the cell finally does with it — building an amino acid into a muscle protein, or storing glucose as glycogen.
Egestion
Undigested waste is converted to faeces in the colon and reaches the rectum. Faeces contain water, inorganic salts, sloughed off mucosal cells, bacteria and undigested food.
It is worth noticing that faeces are not simply leftover food. A large share of their bulk is made of bacteria and of cells shed from the gut lining — a reminder that the alimentary canal is constantly renewing its own surface.
Distension of the rectum stimulates pressure-sensitive receptors that initiate a neural reflex for defecation or egestion. That is, the stretching of the rectal wall as faeces collect is itself the signal that sets the reflex going.
It is a voluntary process that takes place through the anal opening guarded by sphincter muscles. Being voluntary means the final decision rests with us — the reflex creates the urge, but the sphincter can be held closed until a suitable time.
Flowchart
FC 14.11 – Egestion: undigested residue → colon (water absorbed, faeces formed) → rectum → distension of rectal wall → pressure-sensitive receptors stimulated → neural reflex for defaecation → voluntary relaxation of anal sphincters → egestion
14.6 Nutritional Disorders and Disorders of Digestive System
| Sr. No. | Food Component | Gross calorific value (Kcal/g) | Physiological value (Kcal/g) |
|---|---|---|---|
| 1. | Fats | 9.45 | 9.0 |
| 2. | Proteins | 5.65 | 4.0 |
| 3. | Carbohydrates | 4.1 | 4.0 |
Nutrition-related disorders can be categorised based on the food that an individual consumes, and on conditions that develop due to the malfunctioning of the organ or glands associated with the digestive system. This gives us two broad groups, as set out in FC 14.12.
Flowchart
FC 14.12 – Classification of nutrition and digestive disorders: nutrition-related disorders → (a) disorders due to the diet consumed → Protein Energy Malnutrition (PEM) → Kwashiorkar, Marasmus; (b) disorders due to malfunctioning of digestive organs or glands → indigestion, constipation, diarrhoea, jaundice, vomiting
Protein Energy Malnutrition
You are already aware that a little extra or a little less of nutrition can lead to a dietary disorder. Inadequate intake of proteins causes Protein Energy Malnutrition (PEM). It can be associated with an inadequacy of vitamins and minerals in the diet as well.
Proteins matter here more than any other nutrient because they are the body’s building material. Carbohydrates and fats can be burnt for energy, but only proteins supply the amino acids needed to build muscle, enzymes, blood proteins and the growing brain — which is why the effects of PEM show up most severely in young children.
PEM can cause diseases like Kwashiorkar and Marasmus.
Kwashiorkar
Kwashiorkar is a protein deficiency disorder found generally in children between one to three years of age.
Children suffering from Kwashiorkar are underweight and show stunted growth, poor brain development, loss of appetite, anaemia, protruding belly, slender legs, bulging eyes, oedema of the lower legs and face, and a change in skin and hair colour.
The oedema — swelling caused by fluid collecting in the tissues — is the feature that gives this condition its characteristic appearance. It also makes the disorder deceptive, because a swollen child may not look thin even while being severely undernourished.
Marasmus
Marasmus is a prolonged protein energy malnutrition (PEM) found in infants under one year of age. In this disease, protein deficiency is coupled with a lower total food calorific value.
In other words, the child in Kwashiorkar is short of protein, while the child in Marasmus is short of protein and of total energy. The body therefore begins to consume its own reserves.
Inadequate diet impairs physical growth and retards mental development. Subcutaneous fat disappears, the ribs become prominent, the limbs become thin, and the skin becomes dry, thin and wrinkled. There is loss of weight, and digestion and absorption of food stops due to atrophy of the digestive glands. There is no oedema.
That last point is easy to overlook but is exam-critical: oedema is present in Kwashiorkar and absent in Marasmus.
Diagram
Fig. 14.6.1 – Comparative illustration of a child with Kwashiorkar (oedema of legs and face, protruding belly, slender legs, altered hair colour) and a child with Marasmus (loss of subcutaneous fat, prominent ribs, thin limbs, wrinkled skin), with the distinguishing features labelled.
Why These Disorders Occur
The major cause of these disorders is the unavailability of nutritious food. Poverty, large family size, ill spacing of children, early termination of breast feeding and overdiluted milk are a few causes.
Notice that most of these are social and economic causes rather than medical ones. That is an important idea in public health: a disease can have a purely biological mechanism and still have its real origin in a family’s circumstances.
Because of malnutrition, infectious diseases become opportunistic, and this worsens the condition. A malnourished body cannot mount a proper immune response, so infections that a healthy child would shake off become serious. Proper diet can help in the reversal of symptoms.
Indigestion
We now turn to the second group of disorders — those arising from the malfunctioning of the digestive organs themselves.
Overeating, inadequate enzyme secretion, spicy food and anxiety can cause discomfort and various symptoms. This is called indigestion. Improperly digested food or food poisoning can also cause indigestion.
Anxiety appearing on that list is not surprising once you recall that digestive secretions are under neurohormonal control. The nervous system that prepares the stomach for a meal is the same system that responds to stress, so emotional states genuinely affect digestion.
Indigestion leads to loss of appetite, acidity (acid reflux), heartburn, regurgitation, dyspepsia (upper abdominal pain) and stomach pain.
Avoiding eating large meals, lying down after a meal, spicy and oily junk food, smoking and alcohol are the preventive measures for indigestion.
Constipation
When the frequency of defaecation is reduced to less than once per week, the condition is called constipation. Difficulty in defaecation may result in abdominal pain and distortion, and rarely perforation.
The causes are affected colonic mobility due to neurological dysfunction like spinal cord injury, a low fibre diet, inadequate fluid intake and inactivity.
Each of these makes sense in terms of what the colon does. The colon absorbs water; if too little fluid is taken, the residue becomes hard and difficult to move. Fibre adds bulk that stimulates the muscular wall, and physical activity encourages the movements of the canal.
Roughage, sufficient fluids in the diet, and exercise can help improve the condition.
Diarrhoea
Passing loose watery stools more than three times a day is called diarrhoea. Diarrhoea can lead to dehydration, which is the most serious immediate danger, particularly in small children.
The other symptoms are blood in the stool, nausea, bloating and fever, depending on the cause and the severity of the disorder.
The causes of diarrhoea are infection through food and water, or disorders like ulcer, colitis, inflammation of the intestine, or irritable bowel syndrome.
Jaundice
We all associate jaundice with yellowness of the conjunctiva of the eyes and skin, and whitish stool. These are the symptoms of the condition called jaundice.
It is a sign of abnormal bilirubin metabolism and excretion. Jaundice develops if excessive breakdown of red blood cells takes place, raising the bilirubin level beyond what the liver can handle, or if there is an obstruction in the flow of bile from the liver to the duodenum.
Both routes are shown in FC 14.13. In either case the result is the same: bilirubin builds up in the blood instead of leaving through the bile.
Flowchart
FC 14.13 – Origin of jaundice: haemoglobin of old RBCs broken down → bilirubin formed → normally handled by liver → excreted in bile → duodenum → colours the faeces; branch 1: excessive RBC breakdown → bilirubin exceeds liver’s capacity → accumulates in blood; branch 2: obstruction to bile flow from liver to duodenum → bilirubin accumulates in blood and no pigment reaches the gut → yellow eyes and skin, whitish stool
Bilirubin produced from the breakdown of haemoglobin is either water soluble or fat soluble. Fat soluble bilirubin is toxic to brain cells. Hence serum bilirubin values have great diagnostic importance — the test tells the doctor not only that bilirubin is raised, but how dangerous the particular form of it may be.
There is no specific treatment for jaundice. Supportive care and proper rest are the treatments given to the patient.
Vomiting
In this condition, the stomach contents are thrown out of the mouth due to reverse peristaltic movements of the gastric wall. Instead of travelling towards the intestine, the wave of contraction travels in the opposite direction.
It is controlled by the non-vital vomiting centre of the medulla, and it is typically associated with a nauseatic feeling.
Although unpleasant, vomiting is often protective: it removes irritant or contaminated material from the stomach before it can be absorbed. The danger lies in repeated vomiting, which, like diarrhoea, causes loss of water and electrolytes.
14.7 Summary
Nutrition is the sum of the processes by which an organism consumes and utilises food substances. The WHO defines it as the intake of food considered in relation to the body’s dietary needs. Those needs include carbohydrates, proteins, fats, vitamins, minerals, water and fibres in adequate amounts.
Nutrition covers five processes: ingestion, digestion, absorption, assimilation and egestion. Food provides energy and organic material for growth and tissue repair.
Digestion is the process by which complex, non-diffusible and non-absorbable food substances are converted into simple, diffusible and assimilable substances.
The human digestive system consists of the alimentary canal and the associated digestive glands. The alimentary canal is about 8–10 metres long and runs from the mouth to the anus.
The buccal cavity is bounded by lips, cheeks, palate and tongue. An adult has 32 teeth. Human dentition is thecodont (each tooth in a separate socket, by gomphosis), diphyodont (two sets — milk and permanent) and heterodont (four types — incisors, canines, premolars, molars). The dental formula is \(I\frac{2}{2}, C\frac{1}{1}, PM\frac{2}{2}, M\frac{3}{3} = \frac{2,1,2,3}{2,1,2,3}\), giving 16 teeth in each jaw and 32 in all.
A tooth has a crown, neck and root. The crown is covered by enamel, the hardest substance of the body; the bulk of the tooth is dentin, which encloses the pulp cavity with blood vessels and nerves, extending into the root canal. Cementum covers the root and attaches it to the socket.
The tongue is muscular and bears papillae, some of which carry taste buds. The pharynx is the common passage for food and air; the glottis is guarded by the epiglottis, which closes during deglutition. The oropharynx opens into the oesophagus through the gullet.
The oesophagus is a \(\approx 25\) cm muscular tube with longitudinal and circular muscles; peristalsis moves food along it.
The stomach is a ‘J’ shaped muscular organ, 25 to 30 cm long, with cardia (guarded by the cardiac sphincter), fundus, body and pylorus (guarded by the pyloric sphincter). It stores food, gives satiety and churns food with gastric juice.
The small intestine is about 6 metres long and 2.5 cm broad, held by mesenteries, and has three parts — duodenum (about 26 cm, ‘U’ shaped), jejunum (about 2.5 m) and ileum (about 3.5 m). The ileum opens into the caecum at the ileocaecal junction.
The large intestine is 1.5 metres long, wider and shorter than the small intestine, and consists of caecum (6 cm, with the vestigial vermiform appendix and symbiotic micro-organisms), colon (ascending, transverse, descending) and rectum. The anus is guarded by a sphincter and expels faeces by egestion.
The gut wall has four layers, from inside outwards: mucosa, submucosa, muscularis and serosa. Serosa is mesothelium plus connective tissue. Muscularis has longitudinal, circular and oblique smooth muscle layers; it is thicker in the stomach, and the oblique layer is absent in the intestine. Submucosa has blood vessels, lymph vessels and nerves, with glands in the duodenum.
Mucosa carries goblet cells that secrete mucus. In the stomach it forms rugae and gastric glands; in the small intestine it forms villi with microvilli (brush border), each villus containing a capillary network and a lacteal, with crypts of Lieberkuhn between the villi.
The digestive glands are the salivary glands, liver and pancreas. The three pairs of salivary glands are parotid, submandibular and sublingual; serous cells secrete salivary amylase and mucous cells secrete mucus.
The liver is the largest gland, weighing about 1.2 to 1.5 kg, bilobed and covered by Glisson’s capsule. Its units are hepatic lobules, made of cords of hepatic cells around a central vein, with sinusoids containing phagocytic Kupffer cells. A portal area between lobules carries branches of the hepatic artery, hepatic portal vein and bile duct.
The liver produces bile, which is stored in the gall bladder; the duct of the gall bladder and the hepatic duct form the common bile duct. Bile emulsifies fats and makes food alkaline. The liver also stores glycogen, carries out deamination and urea formation, synthesises vitamins A, D, K and \(B_{12}\), produces prothrombin and fibrinogen, is haemopoietic in early development, and destroys toxins and old RBCs through Kupffer cells.
The pancreas is a heterocrine gland. Its exocrine acini secrete alkaline pancreatic juice into the pancreatic duct, which joins the common bile duct to form the hepato-pancreatic duct, guarded by the sphincter of Oddi. Its endocrine islets of Langerhans contain \(\alpha\)-cells (glucagon), \(\beta\)-cells (insulin) and \(\delta\)-cells (somatostatin); glucagon and insulin control blood sugar, and somatostatin inhibits both.
Digestion is mechanical (mastication, churning, peristalsis) and chemical (catabolic hydrolysis by enzymes).
In the buccal cavity, food is chewed and bound into a bolus. Saliva is 98% water and 2% other constituents including electrolytes and salivary amylase, which converts about 30% of starch to maltose at \(pH\ 6.8\). Lysozyme in saliva is antibacterial. The bolus travels down the oesophagus by peristalsis.
In the stomach, food is stored for 4–5 hours and churned. Gastric glands have mucus cells (mucus), peptic or chief cells (pepsinogen) and parietal or oxyntic cells (HCl and intrinsic factor, needed for vitamin \(B_{12}\) absorption). HCl makes the food acidic, stops salivary amylase, kills germs and converts pepsinogen to pepsin at \(pH\ 1.8\); pepsin converts proteins to peptones and proteoses. Rennin, present in infants, curdles milk casein with calcium. The product is acidic chyme, released through the pyloric sphincter.
In the small intestine, bile, pancreatic juice and intestinal juice act on chyme. Bile contains bile pigments (bilirubin, biliverdin), bile salts (Na-glycocholate, Na-taurocholate), cholesterol and phospholipid, but no enzyme; it neutralises acidity, emulsifies fats, activates lipases, and its pigments colour the faeces.
Pancreatic juice contains amylases, lipases, trypsinogen, chymotrypsinogen and nucleases. Enterokinase of intestinal juice activates trypsinogen to trypsin; trypsin converts proteins, proteoses and peptones to polypeptides and activates chymotrypsin, which converts polypeptides to dipeptides. Lipases give fatty acids and monoglycerides. Nucleases, nucleotidases and nucleosidases break nucleic acids down to sugar and nitrogenous base.
Intestinal juice (succus entericus) contains dipeptidases, lipases and disaccharidases. Maltase, sucrase and lactase give monosaccharides, and dipeptidases give amino acids. The end product of complete digestion is chyle, an alkaline slurry ready for absorption.
In the large intestine the mucosa secretes mucus but no enzymes. Bacteria ferment carbohydrates, releasing hydrogen, carbon dioxide and methane, and act on proteins to give indole, skatole and \(H_2S\), which cause the odour of faeces. These bacteria also synthesise B vitamins and vitamin K.
Digestive secretions are under neurohormonal control. Sight, smell and thought of food trigger saliva; the tenth cranial nerve and the hormone gastrin stimulate gastric juice. Secretin inhibits gastric secretion and stimulates bile, pancreatic and intestinal juice; CCK acts similarly and induces satiety; GIP also inhibits gastric secretion.
Absorption is the passage of the end products of digestion through the mucosal lining into blood and lymph, by simple diffusion, osmosis, facilitated transport and active transport. About 90% of absorption occurs in the small intestine.
Small amounts are absorbed elsewhere: some drugs in the mouth; water, electrolytes, alcohol and aspirin in the stomach; water, electrolytes, drugs and some vitamins in the large intestine. In the small intestine, glucose, fructose, galactose, amino acids, minerals and water-soluble vitamins enter the blood capillaries of the villi, while lipids and fat-soluble vitamins (A, D, E, K) enter the lacteals.
Fatty acids and monoglycerides cannot be absorbed into blood directly. They dissolve in micelles formed by bile salts, enter the villi, are reformed into protein-coated chylomicrons, and pass through the lacteals into the blood stream.
Assimilation is the process by which absorbed food reaches the tissues and becomes part of the protoplasm. Egestion is the removal of faeces — containing water, inorganic salts, sloughed mucosal cells, bacteria and undigested food — through the anus. Distension of the rectum triggers a neural reflex, but the act is voluntary.
Gross calorific value is the heat liberated by complete combustion of 1 g of food in a bomb calorimeter, while physiological value is the energy actually produced by 1 g of food in the body.
Inadequate protein intake causes Protein Energy Malnutrition (PEM), leading to Kwashiorkar and Marasmus. Kwashiorkar affects children of one to three years, with oedema, protruding belly, stunted growth, anaemia and changes in skin and hair. Marasmus affects infants under one year, with protein deficiency plus low calorific intake, loss of subcutaneous fat, prominent ribs, wrinkled skin, atrophy of digestive glands, and no oedema. Their major causes are social and economic, and proper diet can reverse the symptoms.
Disorders of the digestive system include indigestion (overeating, poor enzyme secretion, spicy food, anxiety), constipation (defaecation less than once per week, from low fibre, low fluid intake or inactivity), diarrhoea (loose watery stools more than three times a day, leading to dehydration), jaundice (abnormal bilirubin metabolism or obstructed bile flow, causing yellow eyes and skin and whitish stool) and vomiting (reverse peristalsis of the gastric wall, controlled by the vomiting centre of the medulla).
The whole sequence of human nutrition is condensed in FC 14.14.
Flowchart
FC 14.14 – Human nutrition at a glance: ingestion (mouth: mastication, salivary amylase, starch → maltose, bolus) → deglutition → oesophagus (peristalsis) → stomach (churning, HCl, pepsin, proteins → peptones + proteoses, chyme) → duodenum (bile emulsifies fat; pancreatic juice: amylase, lipase, trypsin, chymotrypsin, nucleases) → jejunum and ileum (intestinal juice: disaccharidases, dipeptidases, lipases → monosaccharides, amino acids, fatty acids + monoglycerides = chyle) → absorption (villi: capillaries for water-soluble products, lacteals for fats via micelles and chylomicrons) → assimilation (tissues, protoplasm) → large intestine (bacterial action, water and electrolyte absorption) → rectum → egestion
14.8 Points to Ponder
The alimentary canal is a continuous tube open at both ends. In what sense, then, is the food inside it still “outside” the body? At what exact moment does a nutrient truly enter you?
Digestion begins in the mouth with salivary amylase, yet the mouth holds food for only a few seconds. If so little starch is digested there, why has the body kept this step at all?
The stomach secretes an acid strong enough to kill most microorganisms, and an enzyme that digests protein — while its own wall is made of protein. Think about the different ways a living organ can protect itself from the very chemicals it produces.
Trypsinogen, chymotrypsinogen and pepsinogen are all released in inactive form, and each is activated only after it has left the gland that made it. What general principle of biological safety does this repeated pattern suggest to you?
Bile contains no enzyme, yet without it the digestion of fats is severely impaired. Can a substance be essential to a chemical process without taking part in the chemistry itself?
The small intestine is folded into villi, and each villus cell is folded again into microvilli. Where else in the human body do you find surface area increased in this repeated, layered way, and what does that tell you about the problems living systems must solve?
Bacteria of the large intestine feed on our undigested food and, in return, supply us with vitamins. Should such bacteria be thought of as part of our body, as guests within it, or as an entirely separate organism living alongside us?
The appendix is described as vestigial in humans and functional in herbivores. What does the existence of such an organ suggest about how bodies change over long spans of time?
Digestive secretions can be triggered by the sight, smell or even the mere thought of food. Consider what it means that a purely mental event can set off a chemical process in an organ far from the brain.
Secretin slows the stomach while stimulating the intestinal juices, and insulin and glucagon pull blood sugar in opposite directions. Why might a body regulate itself using pairs of opposing signals instead of one strong controlling signal?
The gross calorific value of a protein is much higher than its physiological value, while for carbohydrates the two are nearly equal. What does this difference tell you about the limits of judging a diet by calorie figures alone?
Kwashiorkar and Marasmus both arise from a shortage of food, yet a child with Kwashiorkar may appear swollen rather than thin. How might such an appearance delay recognition of the disorder, and what would you look for instead?
Malnutrition is described as having causes such as poverty, family size and early stopping of breast feeding. If the immediate cause of a disease is biological but its root cause is social, where should the effort to prevent it be directed?
A person can live without a gall bladder, and even without a stomach after surgery, but not without a liver. What makes an organ genuinely irreplaceable, and how would you decide this for any organ you study?
14.9 Check Your Concepts
Digestion is defined as the conversion of complex, non-diffusible and non-absorbable substances into simple, diffusible and assimilable ones. Explain why the words “non-diffusible” and “non-absorbable” are essential to this definition, and why chewing alone does not count as digestion.
Arrange the following in the correct sequence in which a bolus encounters them: pyloric sphincter, epiglottis, cardiac sphincter, oropharynx, gullet, duodenum.
A student writes that “the large intestine is called large because it is longer than the small intestine.” Identify the error and give the correct basis for the two names, with figures.
Draw a labelled diagram of the longitudinal section of a tooth and mark the enamel, dentin, pulp cavity, root canal and cementum. Beside each label, write one line on what that part does.
A small chip in the enamel causes no pain, but a deep cavity causes severe toothache. Explain this difference using the internal structure of a tooth.
Write the dental formula of an adult human being and show, step by step, how it gives a total of 32 teeth. Why is this formula written as a fraction rather than as a single row of numbers?
What would happen to the process of deglutition if the epiglottis failed to close over the glottis? Name the passage the food would enter and the reflex that would follow.
Name the four layers of the gut wall in order from the lumen outwards, and state one way in which each layer is modified in a particular organ of the alimentary canal.
Distinguish between villi and microvilli. Why does the small intestine need folding at both these levels rather than at just one?
A patient’s intestinal villi are severely flattened, though all the digestive enzymes are being secreted normally. Predict the effect on the patient’s nutrition and explain your reasoning.
Why does each intestinal villus need a lacteal in addition to a capillary network? Which products of digestion would be left stranded if the lacteals were absent?
Explain, with reference to the arrangement of hepatic cells and sinusoids, why the liver is so well suited to detoxifying substances absorbed from the intestine.
Trace the path of bile from the hepatic cell that makes it to the point where it acts on food, naming every duct and sphincter it passes through.
What is a proenzyme? Name three proenzymes involved in digestion, state where each is secreted, and name the agent that activates each one. What advantage does the body gain by secreting these enzymes in an inactive form?
The action of salivary amylase continues for 15 to 30 minutes after swallowing and then stops. Explain what stops it, using the idea of an optimum pH.
Rennin is present in the gastric juice of infants but absent in adults. Explain why this enzyme is useful specifically to an infant, and state what happens to the curdled protein afterwards.
Complete the sequence for protein digestion, naming the enzyme and the medium at each step: proteins → …… → …… → dipeptides → ……
Differentiate between chyme and chyle on the basis of the site of formation, nature, pH and contents.
Emulsification and fat digestion are often confused. Distinguish between them, stating clearly which is a physical change and which is a chemical change, and name the agent responsible for each.
What would be the effect on digestion if the sphincter of Oddi remained permanently closed? Name two juices that would fail to reach the duodenum and state which class of food would be worst affected.
The mucosa of the large intestine secretes no digestive enzymes, yet some digestion still occurs there. Explain how, and name the useful substances the body gains from this arrangement.
Secretin inhibits gastric secretion but stimulates the secretion of bile, pancreatic juice and intestinal juice. Explain how these two seemingly opposite effects serve a single purpose.
Arrange the four modes of absorption — simple diffusion, osmosis, facilitated transport and active transport — into those that require energy and those that do not, and give one example of a substance absorbed by each.
Why can monoglycerides and fatty acids not be absorbed directly into the blood? Describe the route they take instead, naming the two structures formed on the way.
Distinguish between absorption and assimilation. At which of these two stages does food actually become a part of the body?
Faeces are not made up only of undigested food. List the other components present in them, and explain what the presence of sloughed-off mucosal cells tells us about the lining of the alimentary canal.
Defaecation is described as a voluntary process, yet it begins with a reflex. Explain how both statements can be true, naming the stimulus that starts the reflex.
Compare Kwashiorkar and Marasmus with respect to the age group affected, the nature of the deficiency, and the presence or absence of oedema. Why might a child with Kwashiorkar not appear thin?
In Marasmus, digestion and absorption of food stop due to atrophy of the digestive glands. Explain why this makes treatment more difficult than simply providing more food.
The gross calorific value of protein is 5.65 Kcal/g while its physiological value is only 4.0 Kcal/g, whereas for carbohydrates the two values are 4.1 and 4.0 Kcal/g. Account for the larger difference in the case of protein.
A patient has yellow conjunctiva and skin along with whitish stools. Name the condition, name the pigment responsible, and explain how one substance produces both of these signs.
Explain why a low-fibre diet, inadequate fluid intake and physical inactivity all lead to the same disorder. Name the disorder and relate each cause to the normal working of the colon.
Vomiting is described as being caused by reverse peristaltic movements. Explain what this means, name the centre that controls it, and give one way in which vomiting can be protective as well as harmful.
Draw a flow chart showing the complete digestion of a carbohydrate, beginning with starch in the mouth and ending with monosaccharides in the small intestine. Name the enzyme and the site at each step.
For the school athletic meet, Shriya was advised to consume either Glucon-D or fruit juice but no sugarcane juice. Why must it be so?
Alcoholic people may suffer from liver disorder. Do you agree? Explain your answer.
The digestive action of pepsin comes to a stop when food reaches the small intestine. Justify.
The small intestine is very long and coiled. Even if we jump and run, why does it not get twisted? What can happen if it gets twisted?
Write down the explanation: Digestive enzymes are secreted at the appropriate time in our body. How does it happen?
Write down the explanation: Explain the structure of a tooth. Explain why human dentition is considered as thecodont, diphydont and heterodont.
Write down the explanation: Explain the heterocrine nature of the pancreas with the help of its histological structure.
Write a short note on: Position and function of salivary glands.
Write a short note on: Jaundice.
Observe the diagram. This is the histological structure of the stomach. Identify and comment on the significance of the layer marked by the arrow.
Diagram
Fig. Ex.14.1 – Transverse section of the wall of the stomach as in Fig. 14.3, with an unlabelled arrow pointing to the innermost (mucosal) layer for the student to identify.
- Find out the pH maxima for salivary amylase, trypsin, nucleotidase and pepsin, and place them on the given pH scale.
Diagram
Fig. Ex.14.2 – A horizontal pH scale marked from 1 to 14 with arrowheads at both ends, left blank for the student to mark the four enzymes.
Write the name of a protein deficiency disorder and write the symptoms of it.
Observe the diagram given below, label A, B, C, D and E, and write the function of A and C in detail.
Diagram
Fig. Ex.14.3 – Liver, gall bladder, pancreas and duodenum in position as in Fig. 14.3.1, with five unlabelled leader lines marked A, B, C, D and E.
14.10 Objective Questions
- Acinar cells are present in ………..
- liver
- pancreas
- gastric glands
- intestinal glands
- Which type of teeth are maximum in number in the human buccal cavity?
- Incisors
- Canines
- Premolars
- Molars
- Select the odd one out on the basis of the digestive functions of the tongue.
- Taste
- Swallowing
- Talking
- Mixing of saliva in food
- Complete the analogy: Ptyalin : Amylase : : Pepsin : ……………..
- Lipase
- Galactose
- Proenzyme
- Protease
- Assertion (A): The oblique muscle layer of the muscularis is absent in the intestine. Reason (R): The intestine has to churn food as vigorously as the stomach does. Choose
- Both A and R are true and R is the correct explanation of A
- Both A and R are true but R is not the correct explanation of A
- A is true, R is false
- A is false, R is true
- Assertion (A): Bile is essential for the digestion of fats. Reason (R): Bile contains the enzyme lipase, which hydrolyses fats. Choose
- Both A and R are true and R is the correct explanation of A
- Both A and R are true but R is not the correct explanation of A
- A is true, R is false
- A is false, R is true
- Assertion (A): A person suffering from diarrhoea should be given Oral Rehydration Solution rather than plain water. Reason (R): Sodium is absorbed more effectively from the intestine when glucose is absorbed along with it, and water then follows by osmosis. Choose
- Both A and R are true and R is the correct explanation of A
- Both A and R are true but R is not the correct explanation of A
- A is true, R is false
- A is false, R is true
- Match the following:
| Column A | Column B |
|---|---|
| a. Parietal cells | i. Pepsinogen |
| b. Peptic cells | ii. Glucagon |
| c. Acinar cells | iii. HCl and intrinsic factor |
| d. \(\alpha\)-cells | iv. Insulin |
| e. \(\beta\)-cells | v. Alkaline pancreatic juice |