Chapter 12: The Endocrine System

In 1902, in a laboratory at University College London, William Bayliss and Ernest Starling set out to settle an argument about the pancreas.

Everyone knew that when acidic food left the stomach and entered the duodenum, the pancreas answered by pouring in its digestive juice. The accepted explanation was that a nerve carried the message. Ivan Pavlov, then the leading authority on digestion, had shown how closely nerves controlled the gut, and few doubted him.

Bayliss and Starling took a short length of small intestine and cut every nerve running to it, leaving only its blood supply intact. If the accepted explanation was right, acid placed in that loop could tell the pancreas nothing. They put acid in. Soon afterwards, pancreatic juice began to flow.

Something had travelled — but not along a nerve. They scraped the lining of that piece of intestine, made an extract from it, and injected the liquid straight into a vein. The pancreas responded again, and more strongly. The message was a chemical, carried in the blood.

They named the substance secretin. Three years later Starling gave the whole class of such substances a name borrowed from a Greek word meaning to set in motion: hormone.

This was a new idea about the body. Distant organs could speak to one another without wires, using the bloodstream to carry the message. The two explanations their experiment had to choose between are set out in Fig. 12.0-1.

Diagram

Fig. 12.0-1 — Two Possible Ways A Message Could Reach The Pancreas Simple two-panel schematic line drawing, no animal shown. Panel A, the nerve explanation: a loop of duodenum with acid inside, a nerve drawn from the duodenum to the pancreas, an arrow from the pancreas back to the duodenum marked pancreatic juice; label duodenum, acid, nerve, pancreas, pancreatic juice. Panel B, the chemical explanation: the same loop with its nerves drawn cut and marked with a cross, a blood vessel leaving the duodenal wall carrying small dots labelled chemical messenger in blood, the vessel reaching the pancreas, and the same arrow of pancreatic juice returning to the duodenum; label duodenum, acid, cut nerves, blood vessel, chemical messenger in blood, pancreas, pancreatic juice. Labels on straight, non-crossing lead lines.

Curiosity Corner

Q1. The body already has nerves running everywhere. Why does it need chemical messengers as well? Q2. A hormone travels in blood, so it reaches every organ you have. Why, then, does it change only some of them? Q3. Just before you stand up to speak in front of a crowd, your mouth goes dry and your heart thumps. What is giving those orders? Q4. Once a gland has made enough of its hormone, what tells it to stop? Q5. Why is the packet of salt in your kitchen sold with iodine added to it?

12.1 Need For The Regulation Of Body Activities

At this moment your body is doing a great many things at once. It is digesting a meal, replacing worn-out cells, holding your temperature steady and keeping your blood sugar within a narrow range. These activities cannot be allowed to run at random. Each has to happen at the proper time and in the correct sequence.

Digestion shows why the timing matters. Gastric juice, bile and pancreatic juice are useful only when there is food in the food canal. If they were poured in at other times they would be wasted, and the food would meet no juice when it did arrive. Something must decide the moment of release.

Two systems share this work of regulation. One is the nervous system, which you studied in Chapter 10. The other works by chemical means, using regulators called hormones — chemical messengers made in one part of the body that control activity in another.

12.1.1 Hormones, The Chemical Regulators

A hormone is made by a particular group of cells, or by a whole gland built for the purpose. It is not carried along a tube to where it is needed. Instead it passes straight into the blood, and the blood carries it everywhere the blood goes.

This raises an obvious difficulty. If a hormone reaches every organ in the body, why does only one organ change? The answer is that the hormone acts only on the cells that are built to recognise it. These are called its target organ or target cells; every other cell simply lets the hormone pass by. A single messenger can therefore travel through the whole body and still deliver its instruction to one address.

Notice also that a hormone works inside the individual that made it. It coordinates two parts of one body that may be far apart, such as a gland in the neck and a bone in the leg. The complete route is set out in FC 12.1.

Flowchart

FC 12.1 — The Route Taken By A Hormone Endocrine gland makes the hormone → hormone passes directly into the blood → blood carries it to every part of the body → only the target cells recognise it → activity of the target organ changes

Definition

Hormone A chemical secretion from a glandular part of the body, poured directly into the blood, which acts on the target organs or cells of the same individual and brings about coordination between distant parts of the body.

Do Not Mix Up

Hormone And Enzyme Both are made by cells and both are needed in tiny amounts. An enzyme speeds up a chemical reaction at the place where it is made or released. A hormone carries an instruction through the blood to a distant target.

Curiosity Corner

Q2. A hormone travels in blood, so it reaches every organ you have. Why, then, does it change only some of them?

A. Because only the cells of its target organ are built to recognise that particular hormone. The rest of the body’s cells are bathed in it and ignore it, so the message is delivered to one address even though it is carried everywhere.

12.1.2 Hormonal Control Compared With Nervous Control

Both systems regulate the body, but they do it in quite different ways, and the differences are worth setting out carefully. A nerve impulse travels as an electrochemical signal along a nerve fibre, and crosses to the next nerve cell chemically at a synapse — the tiny junction between one neuron and the next. A hormone has no such fibre. It is simply released into the blood and drifts to its target.

That single difference explains most of the others. A signal on a wire arrives at once and stops at once; a chemical in the bloodstream takes time to arrive and takes time to disappear. The seven main differences are listed in Table 12.1.

Compare Table

Curiosity Corner

Q1. The body already has nerves running everywhere. Why does it need chemical messengers as well?

A. Because nerves are good at exactly what hormones are bad at, and the reverse. Nerves give speed and precision for a moment, reaching one muscle at once. Hormones give reach and duration, changing the chemistry of many organs together and holding that change for hours, months or years. Growth and metabolism need the second kind of control.

12.1.3 Endocrine Glands And Exocrine Glands

Most hormones are made by special glands called endocrine glands (endo: inside, crine: secrete), meaning glands that secrete internally. They are also called ductless glands, because their secretion is poured directly into the blood and not through any tube.

Compare these with the glands you already know from digestion and from the skin. A salivary gland, a sweat gland, a tear gland or a gastric gland has a duct — a tube that carries the secretion to the surface or cavity where it is used. Such a gland is an exocrine gland (exo: outside). The two designs are drawn in Fig. 12.1-1.

Diagram

Fig. 12.1-1 — An Exocrine Gland Compared With An Endocrine Gland Two simple schematic line drawings side by side. Left, exocrine gland: a rounded cluster of secretory cells emptying into a duct, the duct opening onto a body surface, with a few blood capillaries drawn nearby but not receiving the secretion; label secretory cells, duct, body surface, capillary. Right, endocrine gland: a similar cluster of secretory cells with no duct, closely surrounded by a dense network of blood capillaries, with small dots labelled hormone passing from the cells into the capillaries; label secretory cells, no duct present, capillary network, hormone entering blood. Labels on straight, non-crossing lead lines.

The differences follow from the presence or absence of that one tube, and are collected in Table 12.2.

Compare Table

Not every hormone comes from a gland built only for that purpose. Some body parts whose main business is something else also release hormones of their own; the lining of the stomach and of the duodenum are two such parts, although digestion is their chief work.

12.1.4 The Endocrine System

A system, in biology, is a group of organs that together perform one overall function. The endocrine glands qualify, because they are not a set of separate workers. They act in a coordinated manner, and one gland commonly switches another on.

Together they are called the endocrine system, and their overall function is chemical coordination — keeping the body’s many activities in step with one another by chemical means rather than by nerve signals. Each later section of this chapter looks at one of its glands, but it is worth holding on to the fact that they work as one.

Zoom In / Out

Where You Are Zoom in: a hormone is a molecule, made by a gland cell. Zoom out: gland cells form a gland, an organ; the glands together form the endocrine system; that system helps run the whole organism.

12.2 General Properties Of Hormones

The glands you will meet in this chapter sit in different places and do very different jobs. Their secretions, however, share a set of properties, and knowing these makes every gland that follows easier to understand.

How A Hormone Reaches Its Work

A hormone is released from its source, an endocrine gland, straight into the blood. It then acts on target organs or cells that usually lie some distance away from that source. The gland and the organ it controls need not be anywhere near each other.

What A Hormone Does To A Cell

Hormones regulate the physiological processes of the body by chemical means. What they chiefly do is alter the enzyme systems of their target cells, speeding some reactions up and slowing others down.

This is why hormones can control metabolism — the whole set of chemical reactions going on inside the body, including those that release energy and those that build new material. A messenger that reaches the enzymes reaches the chemistry itself.

Active In Very Small Amounts

Hormones are produced in very small quantities, yet they are biologically very active. The total amount of a hormone circulating in all your blood at a given moment can be far less than a single grain of sugar, and it is still enough to change the behaviour of a whole organ.

Such power in such small amounts has a consequence worth noticing. A slight change in the quantity a gland releases is not a slight matter for the body, which is why both excess and shortage show up as illness.

Side Note

Hyper- And Hypo- Two Greek prefixes run through this whole chapter. Hyper- means above or too much; hypo- means below or too little. Meet them once and you can read hypersecretion, hyperglycaemia, hypothyroidism and their opposites without being told.

The Chemical Nature Of Hormones

Hormones are not all built the same way. Chemically they fall into three groups, set out in Table 12.3, and the group a hormone belongs to decides whether it dissolves in water or in fat.

Compare Table

The Same Hormone In Other Species

A hormone produced in one species usually shows a similar influence in another. The chemistry of chemical coordination is old and is largely shared across the animal kingdom, so a messenger that means one thing in a sheep tends to mean much the same thing in a human being.

Real-World Application

Two Practical Consequences Of A Hormone Being A Protein Because a hormone from one species works in another, people with diabetes were treated for decades with insulin extracted from the pancreas of cattle and pigs; human insulin made by bacteria came much later.

Because insulin is a protein, it cannot be swallowed. Taken by mouth it would be digested into amino acids in the food canal, exactly like the protein in a meal, and would never reach the blood as insulin. It is therefore injected under the skin.

Too Much And Too Little

A hormone released in excess is described as hypersecretion or oversecretion. A hormone released in short supply is hyposecretion or undersecretion. Both may lead to serious consequences, and much of the rest of this chapter is an account of what those consequences are.

It is worth being clear that hyposecretion is not simply the harmless absence of an effect. A body that has grown used to a hormone is disturbed when it disappears, just as it is disturbed when the hormone floods in.

Not Stored In The Body

Hormones are not stored in the body. Once a hormone has done its work it is broken down or excreted from the system, so a gland must keep producing more if the effect is to continue.

That sounds wasteful, but it is what makes fine control possible. Because old hormone is constantly being cleared away, the level in the blood follows what the gland is doing now, and the body can raise or lower an effect quickly. Section 12.8 shows how it decides when to do so.

Try Yourself

Think It Through Suppose a gland could store a large stock of its hormone and release it whenever needed. Write down two ways this could make the body harder, not easier, to control.

12.3 The Endocrine Glands Of The Human Body

The organs of most systems are joined to one another. The parts of the food canal run end to end, and the blood vessels of the circulatory system form one continuous network. The endocrine glands are different. They lie scattered from the head to the lower abdomen, with nothing joining them.

What holds them together is the blood. Every one of these glands empties its secretion into the same circulating fluid, so a gland in the neck can act on a gland above the kidney without any physical connection between them. Their positions are shown in Fig. 12.3-1.

Diagram

Fig. 12.3-1 — The Principal Endocrine Glands In The Human Body Front-view outline of the human body, drawn as a simple line figure with the internal organs faintly shown in position. Mark and label: hypothalamus and pituitary gland in the head; thyroid gland in the front of the neck; thymus behind the upper part of the breastbone; parathyroid glands behind the thyroid; adrenal gland on the top of each kidney; pancreas below and behind the stomach; lining of the stomach and lining of the small intestine; ovaries in the lower abdomen marked in females and testes marked in males, both shown on the same outline with a bracket noting that the two sexes have the same endocrine glands except these. Labels on straight, non-crossing lead lines.

The Principal Glands

The main hormone-producing glands of the body are these.

Adrenal — one gland capping each kidney.
Pancreas — lying below and behind the stomach.
Thyroid — in the front of the neck.
Pituitary — hanging from the base of the brain.
Parathyroid — four small glands on the back of the thyroid.
Thymus — behind the upper part of the breastbone.
Gonads — the sex glands, which are the ovaries in females and the testes in males.

Four of these are examined closely in this chapter: the adrenal glands, the pancreas, the thyroid and the pituitary. For each one you will meet its position and shape, the hormones it releases, and what goes wrong when it releases too much or too little.

One structure in Fig. 12.3-1 is not a gland of this list at all. The hypothalamus is a small region at the base of the brain, and it is the point at which the nervous system and the endocrine system meet. It sits directly above the pituitary and gives it orders, as Sections 12.7 and 12.8 will show.

Do Not Mix Up

The Gland And Its Hormone The thyroid is a gland; thyroxine is the hormone it makes. The pancreas is a gland; insulin is one of its hormones. A gland is a structure you could point to in the body. A hormone is a chemical it releases into the blood.

Try Yourself

Read The Figure Cover the labels of Fig. 12.3-1. Name each gland from the head downwards, and say for each whether it lies above or below the diaphragm.

12.4 The Adrenal Glands

12.4.1 Position And Parts Of The Adrenal Gland

There are two adrenal glands, and each sits like a cap on the top of one kidney. The name records the position: ad means near and renal means kidney.

Cut one open and you find it is really two glands in one. The outer layer is the adrenal cortex, and the small central region it encloses is the adrenal medulla. The two parts are shown in Fig. 12.4-1.

The distinction matters more than it looks. Cortex and medulla make completely different hormones, are switched on in completely different ways, and produce completely different disorders when they go wrong. Everything that follows in this section depends on keeping them apart.

Diagram

Fig. 12.4-1 — The Adrenal Gland And Its Two Parts Two-part line drawing. Left, whole view: a kidney with the adrenal gland sitting as a cap on its upper end, with the renal blood vessels shown; label adrenal gland, kidney, renal artery, renal vein. Right, cut section of the adrenal gland alone: an outer band clearly distinguished from a central region; label adrenal cortex (outer region), adrenal medulla (central region), capsule. Labels on straight, non-crossing lead lines.

Structure and Function

Adrenal Medulla Supplied directly by nerve endings of the autonomic nervous system → the hormone is released the instant the nerve fires → the body’s quickest hormonal response, fast enough to be useful in an emergency.

12.4.2 The Adrenal Medulla And Adrenaline

The medulla secretes adrenaline, also called epinephrine, together with a closely related hormone called noradrenaline. Adrenaline is the one to concentrate on, and it has a single clear task: it prepares the body for an emergency.

That emergency may call for either of two responses. The body may have to face the danger, which is described as fight, or get away from it, which is described as flight. Both need the same thing — more fuel and more oxygen delivered to the muscles, quickly. Adrenaline arranges exactly that, which is why it is known as the emergency hormone.

The gland does not wait for a slow chemical order. It is stimulated by the nerve endings of the autonomic nervous system, the part of the nervous system that runs your internal organs without your having to think about it. Extra adrenaline pours into the blood at times of emotional stress, and when you are frightened, excited or angry your adrenals release a great deal of it.

The route from the danger to the response is set out in FC 12.2.

Flowchart

FC 12.2 — From Alarm To Response Danger is seen or heard → brain registers alarm → autonomic nerves carry impulses to the adrenal medulla → adrenaline released into the blood → blood carries it to heart, lungs, muscles, liver and arterioles → each target organ responds → body is ready for fight or flight

What Adrenaline Does, Organ By Organ

Adrenaline reaches every organ, but its effects fit one plan: send blood, oxygen and fuel to the muscles, and take them away from everything that can wait. Table 12.4 sets out those effects, why each one helps, and what it feels like from the inside.

Compare Table

Adrenaline does a few other things at the same time. It increases the clotting capacity of the blood, which limits loss from a wound. It widens the pupil of the eye, letting in more light. It acts on the small muscles of the hair follicles, and it stimulates contraction of the uterus during labour. Taken together, these effects are much the same as those produced by the sympathetic part of the autonomic nervous system.

Trace It

The First Few Seconds Of A Fright 1. A dog runs at you. Your eyes send the news to the brain. 2. Autonomic nerve endings in the adrenal medulla are stimulated at once. 3. The medulla releases adrenaline straight into the blood. 4. Your heart beats faster and harder, so blood moves round the body more quickly. 5. Arterioles in the skin and in the gut wall narrow, while the muscles receive more blood. 6. Breathing deepens, so the blood arriving at the muscles carries more oxygen. 7. The liver breaks glycogen down to glucose and releases it into the blood. 8. Within seconds your leg muscles have the fuel, the oxygen and the blood supply for a sprint you had not planned.

Curiosity Corner

Q3. Just before you stand up to speak in front of a crowd, your mouth goes dry and your heart thumps. What is giving those orders?

A. Adrenaline from your adrenal medulla. The body cannot tell the difference between a physical danger and an audience, so it prepares for both in the same way. The arterioles supplying your digestive system narrow, which is why the mouth goes dry, and the heart beats faster and harder, which is why you can feel it.

Try Yourself

Name The Sensation Before a race or a stage performance, run through the last column of Table 12.4. Which of those sensations can you recognise, and which body part was each one coming from?

12.4.3 The Adrenal Cortex And The Cortical Hormones

The cortex works quite differently. It is not switched on by nerves in a moment of alarm; it releases its hormones steadily, and they govern the slow, everyday chemistry of the body.

It secretes many hormones, together called the cortical hormones. The best known of them is cortisol, which among other things suppresses inflammation. In older accounts this hormone is often referred to as cortisone.

The cortical hormones fall into three groups.

Mineralocorticoids regulate mineral metabolism, and in particular the balance of sodium (\(Na^+\)) and potassium (\(K^+\)) ions in the body.

Glucocorticoids regulate the metabolism of carbohydrate, protein and fat. Cortisol belongs to this group.

Sex corticoids are cortical hormones that behave like sex hormones. Both male and female types are produced in both sexes. An overgrowth of the cortex in a young child can therefore bring on sexual maturity earlier than usual.

Taken as a whole, the cortical hormones do four things. They increase the concentration of glucose in the blood; they influence fat and protein metabolism; they regulate the balance of salt and water in the body; and they adapt the body to stresses such as extreme heat or cold, burns and infections.

Do Not Mix Up

Mineralocorticoids And Glucocorticoids Read the first half of each name. Mineralo- points to minerals, that is, salts such as sodium and potassium. Gluco- points to glucose, and so to the metabolism of carbohydrate, protein and fat.

12.4.4 Disorders Of The Adrenal Cortex

Because the cortical hormones touch blood sugar, salt balance and the body’s response to stress, both too little and too much of them show up plainly.

A shortage of cortisol causes Addison’s disease. A person with this condition loses energy and loses weight, feels nauseated, develops a darkening of the skin, and becomes unusually sensitive to cold and to pain. Blood sugar falls below the normal level and infections take hold more easily.

An excess of cortisol causes Cushing’s syndrome. Here the person gains weight, blood sugar rises above the normal level, salt and water are retained in the body, the muscles weaken, and the bones lose material and become fragile, a condition known as osteoporosis. The two are set against each other in Table 12.5.

Compare Table

Read the two columns against each other and a pattern appears. One hormone, released in too small an amount and in too large an amount, produces two different illnesses whose signs are in several places the exact reverse of one another.

An excess of the sex corticoids produces a different picture again, called adrenal virilism. A woman with this condition develops some male secondary characters, such as facial hair and a deeper voice; a man may develop some female secondary characters. It is not a change of sex, but a change in the balance of hormones that both sexes already produce.

12.5 The Pancreas

12.5.1 A Gland With Two Kinds Of Secretion

Every gland you have met so far has been of one kind or the other. The pancreas is both at once, and it is worth pausing on how unusual that is.

As a duct gland it makes pancreatic juice, a digestive fluid, and delivers it through a duct into the duodenum. That is exocrine work. As a ductless gland it makes hormones and passes them straight into the blood. That is endocrine work. The same organ therefore has one set of cells emptying into a tube and another set emptying into capillaries, as Fig. 12.5-1 shows.

Diagram

Fig. 12.5-1 — The Pancreas And Its Two Kinds Of Secretion Line drawing of the pancreas in position, lying below and behind the stomach, with the duodenum curving round its head. Show the pancreatic duct running the length of the gland and opening into the duodenum, with an arrow labelled pancreatic juice to digestion. Separately show a blood capillary within the gland with an arrow labelled hormones into blood. Label stomach, duodenum, pancreas, pancreatic duct, blood capillary. Labels on straight, non-crossing lead lines.

12.5.2 The Islets Of Langerhans

The hormone-making cells are not spread evenly through the pancreas. They are gathered into small, separate groups scattered throughout the gland, and these groups are called the Islets of Langerhans. The name is apt, since islets means little islands: each group sits like an island in a sea of enzyme-secreting tissue. One is drawn in Fig. 12.5-2.

Each islet contains more than one kind of cell, and the different kinds make different hormones. Beta cells make insulin, alpha cells make glucagon, and delta cells make a third hormone, somatostatin, which slows the release of the other two.

Diagram

Fig. 12.5-2 — An Islet Of Langerhans Line drawing of a section of pancreas tissue as seen under the microscope. Show a rounded, paler group of cells set among the darker, densely packed enzyme-secreting cells around it, with blood capillaries running through the group. Within the group, mark two kinds of cell distinctly. Label islet of Langerhans, alpha cell, beta cell, blood capillary, enzyme-secreting cells. Labels on straight, non-crossing lead lines.

Structure and Function

Islets Of Langerhans Small islands of cells scattered through the whole gland and threaded with capillaries → every islet has blood running past it → the hormone enters the circulation the moment it is made, wherever in the pancreas that happens.

12.5.3 Insulin

Insulin is secreted by the beta cells, and its task is to check a rise in the sugar level of the blood. It does this in two principal ways.

First, it promotes the utilisation of glucose by the body cells. Glucose that is being taken in and used is glucose that is no longer in the blood, so the blood sugar level comes down.

Second, it stimulates the deposition of the extra glucose as glycogen — the storage carbohydrate of animals — in the liver and in the muscles. Sugar that is surplus to present needs is put away in a form that can be brought out later.

There is a common way of describing this that is worth correcting now, because it will cause confusion later. It is wrong to say that insulin converts glucose into glycogen. Insulin does not carry out the conversion. It enables the cells to absorb glucose, and once inside, the cell either uses that glucose or converts it into glycogen. The hormone gives the instruction; the cell does the chemistry.

12.5.4 Glucagon

Glucagon is secreted by the alpha cells, and it does the opposite work. It stimulates the liver to break its stored glycogen down into glucose, which passes into the blood. The blood sugar level therefore rises.

Insulin and glucagon are set against each other in Table 12.6. Between them they hold the blood sugar within a narrow range, one hormone pulling it down when it climbs and the other pushing it up when it falls, as FC 12.3 shows.

Compare Table

Flowchart

FC 12.3 — How Blood Sugar Is Held Steady Blood sugar rises after a meal → beta cells release insulin → body cells take up glucose and the liver stores the surplus as glycogen → blood sugar falls back to normal → later, blood sugar falls during a long gap between meals → alpha cells release glucagon → liver breaks glycogen down to glucose and releases it → blood sugar rises back to normal → the cycle begins again at the next meal

12.5.5 Under-Secretion Of Insulin

If the pancreas does not secrete enough insulin, glucose is neither taken up by the cells nor stored, and it accumulates in the blood. The condition that results is diabetes mellitus.

The signs follow from that one fact. The concentration of sugar in the blood is high, a state called hyperglycaemia (hyper: excess, glyce: sugar, emia: blood). Sugar begins to appear in the urine, and a great deal of urine is passed. Because so much water is lost in that urine, the person feels thirsty. Weight is lost and the body grows weaker, and in some cases the eyesight is affected.

Definition

Diabetes Mellitus A disorder caused by the hyposecretion of insulin, in which the blood sugar level remains abnormally high and glucose is passed out in the urine.

The usual treatment is to supply the hormone the pancreas is not producing, by injection. This controls the condition but does not cure it, because the beta cells are not repaired by it.

Side Note

Why ‘Mellitus’? The word mellitus means honey. It records how the disease was recognised long before blood could be tested — by the sugar carried out in the urine. Its partner disorder, diabetes insipidus in Section 12.7.3.1, is named for the opposite reason.

12.5.6 Over-Secretion Of Insulin

Too much insulin has the reverse effect. Glucose is pulled out of the blood faster than it is replaced, and the sugar level falls below normal. This is hypoglycaemia (hypo: below).

The brain is the organ that suffers first, because it depends on a steady supply of glucose from the blood and stores almost none itself. If the level falls very low, even for a few minutes, the brain may enter a state of coma.

The same thing can happen to a person with diabetes who receives an overdose of insulin, and they may become unconscious. This is called insulin shock. Taking something sweet at once — a sugar sweet or a sweet biscuit — raises the blood sugar again and is of immediate help.

Do Not Mix Up

Hyperglycaemia And Hypoglycaemia Hyper is above: too much sugar in the blood, which follows too little insulin. Hypo is below: too little sugar in the blood, which follows too much insulin. The prefix tells you about the sugar, not about the hormone.

12.6 The Thyroid Gland

12.6.1 Position And Shape Of The Thyroid

The thyroid lies in the front of the neck, just below the larynx or voice box, wrapped around the front of the windpipe. It is a bilobed gland, which is to say it has two lobes, one on each side, joined across the middle by a narrow bridge called the isthmus. The whole gland is often described as butterfly-shaped. Fig. 12.6-1 shows it from the front and from behind.

Diagram

Fig. 12.6-1 — The Thyroid Gland Two line drawings side by side. Left, front view: the larynx above, the two lobes of the thyroid gland lying one on each side of the windpipe and joined across the middle by the isthmus, and the trachea passing down below; label larynx, thyroid gland, isthmus, trachea. Right, back view of the same region: the oesophagus running down behind the trachea, and four small parathyroid glands embedded on the rear surface of the two thyroid lobes; label oesophagus, thyroid gland, parathyroid glands, trachea. Labels on straight, non-crossing lead lines.

The gland secretes two hormones. One is thyroxine, which the rest of this section is about. The other is calcitonin, which moves calcium ions out of the blood and into the bones.

Side Note

The Four Small Neighbours The back view of Fig. 12.6-1 shows four tiny parathyroid glands set into the rear surface of the thyroid. They sit on the thyroid but are a separate gland with a hormone of their own, which controls the calcium level of the blood.

Try Yourself

Find The Position Run a finger down the front of your own neck until you feel the firm ridge of the larynx. The thyroid lies just below that ridge, with one lobe on either side of the windpipe.

12.6.2 Thyroxine And Its Actions

Thyroxine sets the speed at which the body’s chemistry runs. Its central action is on basal metabolism — the rate at which your cells oxidise fuel, and so produce heat, when you are completely at rest.

Definition

Basal Metabolism The rate of cellular oxidation in the body, and the heat production that results from it, measured while the body is at rest.

The relation is a simple one and runs both ways. An increase in the secretion of thyroxine increases metabolism; a decrease in secretion lowers it. Every cell in the body is affected, because every cell is oxidising fuel.

From this one action several others follow. Thyroxine influences the general growth of the body, the ossification of bones — the laying down of bone as a child grows — the maintenance of body temperature, and mental development. A hormone that governs the rate of cellular chemistry is bound to govern anything that chemistry builds.

Two further points are worth holding on to. The first is that thyroxine cannot be made without iodine, which the body must obtain from food. The second is that both too little and too much of it cause well-recognised conditions, and these are the subject of the next two subsections.

12.6.3 Hypothyroidism

Insufficient secretion of thyroxine is called hypothyroidism. It may lead to three conditions: simple goitre, cretinism and myxoedema.

Simple Goitre

Simple goitre is an enlargement of the thyroid gland, visible as a swelling in the neck. Its usual cause is an insufficient quantity of iodine in the food.

This makes it a disease of particular places rather than of particular people. Where the soil is poor in iodine, the crops grown in that soil are poor in it too, and so is the diet of everyone eating them. The condition has therefore long been common among people living in hilly regions, where rain has washed iodine out of the soil over a very long time. FC 12.4 sets out the chain.

Flowchart

FC 12.4 — How A Shortage Of Iodine Leads To Simple Goitre Iodine scarce in the soil → little iodine in the crops grown there → little iodine in the food eaten → thyroid cannot make enough thyroxine → the gland is driven to work harder and grows larger → visible swelling in the neck, which is simple goitre

The last step of that chain raises a fair question: what drives the gland to grow when it is short of the raw material it needs? The answer lies in the control system described in Section 12.8, which keeps ordering the thyroid to produce more.

Because the cause is a missing element, the remedy is to supply it. Thyroid deficiencies are common in many parts of India, and the use of iodised salt in food is recommended for that reason.

Curiosity Corner

Q5. Why is the packet of salt in your kitchen sold with iodine added to it?

A. Because iodine is the active ingredient in the making of thyroxine, and in many regions the soil, and therefore the food, does not supply enough of it. Salt is eaten in small, steady amounts by almost everybody, every day, which makes it a good carrier for a nutrient that has to reach a whole population. Adding iodine to salt prevents simple goitre at its source.

Do Not Mix Up

Simple Goitre And Exophthalmic Goitre Both are swellings of the thyroid in the neck, and their causes are opposite. Simple goitre comes from too little thyroxine, usually for want of iodine. Exophthalmic goitre comes from too much thyroxine, and the eyes protrude.

Cretinism And Myxoedema

The other two conditions depend on when in life the shortage occurs, and the difference between them is set out in Table 12.8.

Cretinism affects children. It arises when the thyroid has developed defectively, or has degenerated early, so that thyroxine is short from the beginning. A child with the condition shows stunted physical growth and delayed mental development, since thyroxine is needed for both.

Myxoedema affects an adult whose thyroid stops working properly after growth is complete. The person becomes sluggish in body and mind, and the face and hands swell.

Compare Table

12.6.4 Hyperthyroidism

Excessive secretion of thyroxine is called hyperthyroidism, and it too may produce a goitre — a kind known as exophthalmic goitre (exo: outward, ophthalmos: eye), named from its most noticeable sign.

A person with this condition shows a recognisable set of changes.

Protrusion of the eyes.
Formation of a goitre, the swelling in the neck.
A marked increase in the metabolic rate.
Rapid heart beat, and shortness of breath.
Restlessness.

Most of that list follows from a single fact. If thyroxine sets the speed of cellular oxidation, then an excess of it makes every cell burn fuel faster than it should. The body must then supply more oxygen and carry away more carbon dioxide, so the heart beats faster and the breathing becomes laboured, and the person feels restless while sitting still.

12.7 The Pituitary Gland

12.7.1 Position And Lobes Of The Pituitary

The pituitary is a small projection, about the size of a pea seed, hanging from the base of the brain just below the hypothalamus. A short stalk joins the two. The entire gland weighs only about one-half gram, which makes it the smallest gland in this chapter and, by a wide margin, the most powerful.

It is popularly called the master gland, because it appears to control practically all the other endocrine glands of the body. You will see in the next subsection exactly how it does this.

The gland has two distinct lobes, the anterior pituitary in front and the posterior pituitary behind, and they release quite different hormones. Between them lies a narrow region called the intermediate lobe, which is almost absent in human beings although it is larger and more active in some other animals. Fig. 12.7-1 shows the arrangement.

Diagram

Fig. 12.7-1 — The Pituitary Gland And Its Lobes Vertical section line drawing of the pituitary gland hanging below the base of the brain. Show the hypothalamus above, the hypophysial stalk descending from it, and the gland below divided into a larger anterior lobe in front and a posterior lobe behind, with a narrow intermediate lobe between them; show the blood vessels running down the stalk into the anterior lobe. Label hypothalamus, hypophysial stalk, anterior pituitary, intermediate lobe, posterior pituitary. Labels on straight, non-crossing lead lines.

Structure and Function

The Pituitary Stalk The gland hangs directly beneath the hypothalamus, joined to it by a short stalk carrying nerve fibres and blood vessels → orders from the brain reach the pituitary over the shortest possible route → the nervous system can take charge of the endocrine system through this one small gland.

12.7.2 Hormones Of The Anterior Pituitary

12.7.2.1 Growth Hormone

Growth hormone, abbreviated GH, is essential for normal growth. It is also called somatotropin (somatic: body, tropic: stimulating), a name that describes it as the hormone that stimulates the body to grow.

Its effects are seen most clearly when the amount released is wrong, and what happens then depends on when in life it goes wrong. Bones can only lengthen while a person is still growing; once the growing ends, extra growth hormone has to act on bone in some other way. That single fact explains the three conditions below, which are drawn in Fig. 12.7-2.

Deficiency in childhood results in dwarfism. A person with this condition is fully developed but small, and retains the body proportions of a child.

Oversecretion in childhood results in gigantism. The long bones lengthen far beyond the normal, and heights of up to about 2.7 metres have been recorded.

Oversecretion in an adult results in acromegaly (acro: extremity, megaly: large). The long bones can no longer lengthen, so the excess shows as a thickening of bones at the ends of the body — the jaws in particular, and the hands and feet. A person with acromegaly develops a large nose and thick lips.

Diagram

Fig. 12.7-2 — Effects Of Abnormal Growth Hormone Secretion In Childhood Three plain human outline silhouettes standing side by side on a common ground line, with no facial features and no identifiable individual shown, drawn against a vertical height scale in metres. Left figure noticeably short, centre figure of average adult height, right figure noticeably tall. Label the three, in order, undersecretion of growth hormone in childhood (dwarfism), normal secretion, oversecretion of growth hormone in childhood (gigantism). Labels on straight, non-crossing lead lines.

Do Not Mix Up

Gigantism And Acromegaly Both come from too much growth hormone. Gigantism begins in childhood, while the long bones can still lengthen, and the whole body becomes unusually tall. Acromegaly begins in adulthood, when they cannot, and the jaws, hands and feet enlarge instead.

12.7.2.2 The Tropic Hormones

The anterior pituitary makes a second kind of hormone altogether. These do not act on ordinary body tissue at all. They act on other endocrine glands, telling them to release their own hormones, and they are called tropic hormones for that reason: ‘tropic’ means influencing the activity of the organ named in the word. Thyrotropic means acting on the thyroid; adrenocorticotropic means acting on the adrenal cortex.

This is what makes the pituitary a master gland. A single small structure at the base of the brain can raise or lower the output of the thyroid, of the adrenal cortex and of the gonads, without touching those organs itself. The arrangement is a hierarchy, set out in FC 12.5.

Flowchart

FC 12.5 — The Chain Of Command From Brain To Target Organ Hypothalamus (part of the brain) → anterior pituitary → tropic hormone released into the blood → branches to three target glands → thyroid (on TSH), adrenal cortex (on ACTH), gonads (on FSH and LH) → each gland releases its own hormone → that hormone acts on the tissues of the body

The main hormones of the anterior lobe are collected in Table 12.9. Growth hormone, already described, is included so that the list is complete.

Compare Table

FSH and LH are together called the gonadotropic hormones, because they regulate the activities of the gonads — the testes and the ovaries. The corpus luteum named in the table is the structure left behind in the ovary after an egg has been released.

Side Note

Master, Or Foreman? The pituitary controls the other glands, but it is not the top of the chain. It takes its own orders from the hypothalamus above it, and, as Section 12.8 shows, it is also answerable to the hormones of the glands it commands.

12.7.3 Hormones Of The Posterior Pituitary

The posterior lobe releases two hormones only: vasopressin and oxytocin.

12.7.3.1 Antidiuretic Hormone

Antidiuretic hormone, or ADH, is also called vasopressin. Both names describe something it does. Vasopressin refers to its action on blood vessels, which it constricts, causing a rise in blood pressure. Antidiuretic refers to its action on the kidney: anti means against and diuresis means the passing of urine, so an antidiuretic hormone is one that reduces the amount of urine formed.

It does this by increasing the reabsorption of water from the kidney tubules. Water that would otherwise have left the body in the urine is returned to the blood instead.

Deficiency of ADH therefore causes a condition in which urination is frequent and copious. Large quantities of water are lost from the body, and the person becomes thirsty. The condition is called diabetes insipidus, or water diabetes.

The word diabetes, on its own, simply means passing out excessive amounts of urine, which is why it appears in the name of two unrelated conditions. Table 12.10 separates them.

Compare Table

12.7.3.2 Oxytocin

Oxytocin (Greek oxys: quick, tokos: childbirth) stimulates vigorous contractions of the uterus in a pregnant mother, and these contractions bring about the birth of the baby. Its second action is on the mammary glands, where it stimulates the ejection of milk.

The two actions belong together. One hormone prepares for the arrival of the infant and the other for feeding it, and the name records the first of them.

12.8 Control Of Hormonal Secretions: The Feedback Mechanism

You know from Section 12.2 that hormones are not stored. The amount of a hormone in your blood at any moment therefore depends on what its gland is doing now, and a question follows immediately: how does the gland know how much to make?

Think of the thermostat in a geyser. It does not run continuously. It heats the water, and when the temperature rises past the mark it has been set to, it switches itself off. When the water cools below that mark, it switches on again. Nobody watches it; the temperature it produces is what controls it.

The endocrine glands are regulated in the same way, by what is called the feedback mechanism. The body has a normal state that it works to maintain, and whenever it departs from that state, messages pass through the body systems ordering the opposite — increase if the level has fallen below normal, decrease if it has risen above.

12.8.1 Negative Feedback And The TSH Level

An arrangement that answers a rise with a command to decrease, and a fall with a command to increase, is called negative feedback. The word negative refers to this reversing of direction. Most of the feedback mechanisms in the body are of this type.

Definition

Negative Feedback A control mechanism in which a departure from the normal state produces a message ordering the opposite change, so that the normal state is restored.

The clearest example is the control of thyroxine, and it is worth following in full because it uses the chain of command from Section 12.7.2.2.

The hypothalamus secretes thyrotropin-releasing hormone, or TRH. TRH stimulates the anterior pituitary to secrete thyroid stimulating hormone, TSH. TSH in turn stimulates the thyroid gland to secrete thyroxine. So far this is a straight line from brain to gland.

The line then closes into a loop. When the concentration of thyroxine in the blood exceeds the normal level, thyroxine itself inhibits the secretion of TRH by the hypothalamus and the secretion of TSH by the anterior pituitary. Less TSH reaches the thyroid, the thyroid slows down, and the thyroxine level falls back. Should it fall too far, the inhibition is lifted, TRH and TSH rise again, and the thyroid is pushed back up. FC 12.6 shows both halves.

Flowchart

FC 12.6 — Negative Feedback Control Of The Thyroxine Level Outward chain: hypothalamus secretes TRH → TRH stimulates the anterior pituitary → anterior pituitary secretes TSH → TSH stimulates the thyroid gland → thyroid secretes thyroxine into the blood Return chain, when the thyroxine level rises above normal: raised thyroxine in blood → inhibits secretion of TRH by the hypothalamus and inhibits secretion of TSH by the anterior pituitary → less TSH reaches the thyroid → thyroxine output falls → level returns to normal, at which the outward chain begins again

This loop also settles a question left open in Section 12.6.3: why the thyroid enlarges in simple goitre. When the diet lacks iodine, the gland cannot make enough thyroxine, however hard it works. The thyroxine level in the blood stays low, so nothing inhibits the hypothalamus or the pituitary, and TRH and TSH keep arriving. The thyroid is stimulated continuously, and a gland urged to work without pause grows larger. The swelling in the neck is the visible result of a feedback loop that can never be satisfied.

Curiosity Corner

Q4. Once a gland has made enough of its hormone, what tells it to stop?

A. The hormone itself does. As its concentration in the blood rises past the normal level, it acts back on the hypothalamus and the pituitary and inhibits the very messages that were ordering its production. No separate supervisor is needed: the product switches off its own manufacture, and starts it again when the level falls.

Do Not Mix Up

‘Negative’ Feedback Is Not A Bad Thing Negative here describes the direction of the correction, not its value. The response opposes the change that caused it. This is the mechanism that keeps your body steady, so its effects are entirely positive for you.

Try Yourself

What Would Happen If Suppose the return chain of FC 12.6 were broken, so that thyroxine could no longer inhibit the hypothalamus and the pituitary. Predict what would happen to the thyroxine level in the blood, and which of the conditions in Section 12.6.4 the person would come to show.

12.8.2 Positive Feedback

Positive feedback works the other way. Instead of ordering the opposite, the response strengthens the very change that produced it, and the process builds on itself. Mechanisms of this kind are very few in the body.

The standard example is the contraction of the uterus during childbirth. The normal state of the uterus is uncontracted, so negative feedback would answer a contraction with a message to relax. What happens instead is that one contraction gives the message to contract further, and each contraction makes the next one stronger.

Such a loop cannot correct itself, and it does not stop until the situation that started it has ended — in this case, when delivery is complete.

Side Note

Why Positive Feedback Is Rare A loop that amplifies itself has no built-in stopping point. It is useful only where the process is meant to run to completion and then finish, as childbirth and the clotting of blood do. For everything the body must simply hold steady, negative feedback is the safer design.

12.9 The Hormones At A Glance

Everything the previous five sections established is collected in Table 12.11. It gives, for each of the four major endocrine glands, the hormones it secretes, what those hormones do, and the conditions that follow when too little or too much is released. One term in it needs a word of explanation: differentiation means the process by which cells become specialised for particular jobs as an organism develops.

Read down the last two columns and the shape of the whole chapter appears. Each hormone has a normal amount, and a departure from that amount in either direction has a name and a set of consequences. A dash means that the chapter has not treated a named condition for that entry, not that the hormone is unimportant.

Compare Table

Three patterns in the table are worth naming before you leave it. Some hormones work in opposing pairs, so that between them they hold a level steady, as insulin and glucagon hold the blood sugar. Some do not act on the body’s tissues at all, but on other glands, which is what makes the anterior pituitary the master gland. And several conditions differ from one another only in when they begin — gigantism from acromegaly, cretinism from myxoedema — because a hormone acts on whatever the body happens to be doing at the time it arrives.

12.10 Summary

The body’s many activities must take place at the proper time and in the correct sequence, and two systems regulate them: the nervous system and the endocrine system.
A hormone is a chemical secretion from a glandular part of the body, poured directly into the blood, which acts on the target organs or cells of the same individual and coordinates distant parts of the body.
A hormone is carried to every part of the body by the blood, but only the cells of its target organ respond to it.
Hormonal control is slow, widespread, sometimes long-lasting, can affect growth, and regulates metabolism. Nervous control is rapid, local, short-lived, cannot affect growth, and does not regulate metabolism.
Endocrine glands are ductless and pour their secretion straight into the blood. Exocrine glands have ducts, which carry their secretion to a body surface or cavity.
The endocrine glands together form the endocrine system, whose overall function is chemical coordination.
Hormones regulate physiological processes by chemical means, acting on the enzyme systems of their target cells. They are produced in very small quantities and are biologically very active.
Chemically, hormones are peptides, amines or steroids. A hormone produced in one species usually has a similar influence in another.
Both hypersecretion and hyposecretion of a hormone lead to serious consequences.
Hormones are not stored in the body; they are broken down or excreted after use, so a gland must keep producing them.
The principal endocrine glands are the adrenals, pancreas, thyroid, pituitary, parathyroids, thymus and gonads. The first four are studied in this chapter.
Each adrenal gland caps a kidney and has two parts with different functions: an inner medulla and an outer cortex.
The adrenal medulla secretes adrenaline, the emergency hormone of fight or flight. It is stimulated directly by nerve endings of the autonomic nervous system.
Adrenaline quickens the heart and the breathing, raises blood pressure, constricts the arterioles of the skin and digestive organs so that more blood reaches the muscles, and releases glucose from the glycogen of the liver.
The adrenal cortex secretes mineralocorticoids, glucocorticoids and sex corticoids. Together they raise blood glucose, influence fat and protein metabolism, regulate salt and water balance, and adapt the body to stresses.
Hyposecretion of cortisol causes Addison’s disease and hypersecretion causes Cushing’s syndrome; an excess of the sex corticoids causes adrenal virilism.
The pancreas is both a duct gland, secreting pancreatic juice into the duodenum, and a ductless gland, whose islets of Langerhans secrete hormones into the blood.
Insulin, from the beta cells, lowers the blood sugar level by enabling body cells to take up and use glucose and by having the surplus stored as glycogen in liver and muscles.
Glucagon, from the alpha cells, raises the blood sugar level by stimulating the liver to convert glycogen back into glucose. The two hormones together hold the level steady.
Under-secretion of insulin causes diabetes mellitus, with a high blood sugar level, copious urine loaded with sugar, thirst, loss of weight and weakness. Administering insulin supplies the missing hormone but does not cure the condition.
Over-secretion of insulin causes hypoglycaemia. Too low a blood sugar level, even for a few minutes, may bring on a coma; when it follows an overdose of insulin it is called insulin shock.
The thyroid is a bilobed gland in the front of the neck below the larynx, its two lobes joined by an isthmus. It secretes thyroxine and calcitonin.
Thyroxine regulates basal metabolism, the rate of cellular oxidation at rest, and also influences general growth, the ossification of bones, body temperature and mental development. Iodine is needed for its production.
Hypothyroidism leads to simple goitre, which follows a shortage of iodine in the food, to cretinism in children, and to myxoedema in adults.
Hyperthyroidism leads to exophthalmic goitre, with protruding eyes, a swelling in the neck, a raised metabolic rate, rapid heart beat, shortness of breath and restlessness.
The pituitary is a pea-sized gland hanging from the base of the brain below the hypothalamus. It is called the master gland and has an anterior and a posterior lobe.
Deficiency of growth hormone in childhood causes dwarfism; oversecretion in childhood causes gigantism; oversecretion in an adult causes acromegaly.
The anterior pituitary also secretes the tropic hormones TSH, ACTH, FSH and LH, which act on other endocrine glands, and prolactin, which promotes the formation and secretion of milk.
The posterior pituitary releases antidiuretic hormone, or vasopressin, which increases the reabsorption of water from the kidney tubules and raises blood pressure. Its deficiency causes diabetes insipidus.
The posterior pituitary also releases oxytocin, which causes contraction of the uterus during childbirth and stimulates milk ejection.
The quantity of a hormone produced is controlled by a feedback mechanism. In negative feedback, a rise above the normal level produces a message to decrease and a fall below it a message to increase, so the normal state is restored.
Thyroxine is controlled in this way: when its level in the blood rises above normal, it inhibits the secretion of TRH by the hypothalamus and of TSH by the anterior pituitary.
Positive feedback, in which the response strengthens the change that caused it, is very rare. The contraction of the uterus during childbirth is one example.

Flowchart

FC 12.7 — The Chapter In One Chain Change from the normal state of the body → endocrine gland stimulated → hormone secreted directly into the blood → carried to all parts of the body → only the target organ responds → its activity changes and the normal state is restored → the restored level acts back on the gland and its output is reduced → the loop waits for the next change

12.11 Key Terms

Acromegaly — the condition caused by oversecretion of growth hormone in an adult, in which the bones of the jaws, hands and feet enlarge.
Addison’s disease — the condition caused by hyposecretion of cortisol from the adrenal cortex.
Adrenal virilism — the condition in which an excess of sex corticoids gives an adult physical characters of the other sex.
Adrenaline — the hormone of the adrenal medulla, which prepares the body to meet an emergency.
Adrenocorticotropic hormone (ACTH) — the tropic hormone of the anterior pituitary that stimulates secretion from the adrenal cortex.
Antidiuretic hormone (ADH) — the posterior pituitary hormone that increases the reabsorption of water from the kidney tubules.
Basal metabolism — the rate of cellular oxidation, and the heat it produces, when the body is at rest.
Calcitonin — the thyroid hormone that moves calcium ions from the blood into the bones.
Chemical coordination — the keeping of the body’s activities in step with one another by hormones rather than by nerves.
Cortex — the outer region of an organ; in the adrenal gland, the part that secretes the cortical hormones.
Cortisol — the chief glucocorticoid of the adrenal cortex, called cortisone in older accounts.
Cretinism — the condition in a child caused by hyposecretion of thyroxine, with stunted growth and delayed mental development.
Cushing’s syndrome — the condition caused by hypersecretion of cortisol from the adrenal cortex.
Diabetes insipidus — the condition caused by a deficiency of ADH, in which large quantities of urine, free of sugar, are passed.
Diabetes mellitus — the condition caused by insufficient insulin, in which the blood sugar level stays high and sugar appears in the urine.
Ductless gland — another name for an endocrine gland, which has no duct.
Dwarfism — the condition caused by deficiency of growth hormone in childhood, in which a fully developed person remains small.
Endocrine gland — a gland that pours its secretion directly into the blood instead of through a duct.
Endocrine system — all the endocrine glands of the body, working together to bring about chemical coordination.
Exocrine gland — a gland that carries its secretion through a duct to a body surface or cavity.
Exophthalmic goitre — the condition caused by oversecretion of thyroxine, with a swelling in the neck and protruding eyes.
Feedback mechanism — the arrangement by which the result of a gland’s activity controls how much more of the hormone that gland releases.
Follicle stimulating hormone (FSH) — the anterior pituitary hormone that stimulates egg formation in females and sperm formation in males.
Gigantism — the condition caused by oversecretion of growth hormone in childhood, in which the long bones lengthen beyond the normal.
Glucagon — the hormone of the alpha cells of the pancreas, which raises the blood sugar level.
Glucocorticoids — the group of cortical hormones that regulate carbohydrate, fat and protein metabolism.
Glycogen — the storage form of glucose, kept mainly in the liver and the muscles.
Gonadotropic hormones — FSH and LH together, the anterior pituitary hormones that regulate the activity of the gonads.
Gonads — the sex glands, that is, the ovaries in females and the testes in males.
Growth hormone — the anterior pituitary hormone essential for the normal growth of the body.
Hormone — a chemical secretion poured directly into the blood, which acts on target organs of the same individual and coordinates distant parts of the body.
Hyperglycaemia — a blood sugar level higher than normal.
Hypersecretion — the release of a hormone in excess.
Hyperthyroidism — excessive secretion of thyroxine by the thyroid gland.
Hypoglycaemia — a blood sugar level lower than normal.
Hyposecretion — the release of a hormone in short supply.
Hypothalamus — the region at the base of the brain that controls the pituitary and links the nervous and endocrine systems.
Hypothyroidism — insufficient secretion of thyroxine by the thyroid gland.
Insulin — the hormone of the beta cells of the pancreas, which lowers the blood sugar level.
Insulin shock — the unconsciousness that follows an overdose of insulin, caused by a very low blood sugar level.
Islets of Langerhans — the patches of hormone-secreting cells scattered through the pancreas (islets: little islands).
Isthmus — the narrow bridge of tissue joining the two lobes of the thyroid gland.
Luteinizing hormone (LH) — the anterior pituitary hormone that brings about ovulation and the formation of the corpus luteum in females, and testosterone secretion in males.
Master gland — the popular name for the pituitary, which controls practically all the other endocrine glands.
Medulla — the inner region of an organ; in the adrenal gland, the part that secretes adrenaline.
Metabolism — the whole set of chemical reactions taking place inside the body.
Mineralocorticoids — the group of cortical hormones that regulate mineral metabolism, particularly sodium and potassium ions.
Myxoedema — the condition in an adult caused by hyposecretion of thyroxine, with sluggishness and swelling of the face and hands.
Negative feedback — control in which a departure from the normal state produces a message ordering the opposite change.
Oxytocin — the posterior pituitary hormone that causes contraction of the uterus during childbirth and the ejection of milk.
Positive feedback — control in which the response strengthens the change that caused it, so the process builds on itself.
Prolactin — the anterior pituitary hormone that promotes the formation and secretion of milk.
Sex corticoids — the cortical hormones that behave like sex hormones, made in both sexes.
Simple goitre — the enlargement of the thyroid, seen as a swelling in the neck, usually caused by a shortage of iodine in the food.
Somatotropin — another name for growth hormone (somatic: body, tropic: stimulating).
Target organ — the organ or cells on which a particular hormone produces its effect.
Thyroid stimulating hormone (TSH) — the tropic hormone of the anterior pituitary that activates the thyroid to secrete thyroxine.
Thyroxine — the thyroid hormone that regulates basal metabolism and influences growth and mental development.
Tropic hormone — a hormone that acts on another endocrine gland, stimulating it to release its own hormone.
Vasopressin — another name for antidiuretic hormone, from its action in constricting blood vessels.

12.12 Points To Ponder

Adrenaline switches digestion down and pushes fuel and blood to the muscles, and the whole response is built for an emergency lasting a few seconds. Much of what troubles a person today — a crowd, a difficult conversation, a long stretch of worry — lasts far longer than that. What might it cost the body to run a seconds-long response for hours, or to run it several times a day?
The blood sugar level is held steady by two hormones that oppose each other rather than by one hormone turned up and down. What advantages might a pair of opposing controls have over a single one? Can you think of another place in this chapter where the body uses the same idea?
Hormones are destroyed after use, while enzymes are used over and over again. What does that difference suggest about the kind of work each type of molecule is built to do?
The same hormone produces gigantism in a child and acromegaly in an adult. Does this mean the hormone is carrying two different messages, or that the two bodies are answering the same message differently? What does your answer imply about where the specificity of a hormone’s effect really lies?
The pituitary is called the master gland, yet it takes its orders from the hypothalamus and is inhibited by hormones from the very glands it commands. Is ‘master’ a fair description? Suggest a better one, and defend it.
A hormone produced in one species usually has a similar effect in another. What does that shared chemistry suggest about when chemical coordination first appeared among animals, and about how much of it has changed since?

12.13 Objective Questions

Multiple Choice

The hormone that enables the body cells to take up glucose from the blood is: (1 mark)
1. glucagon
2. insulin
3. somatostatin
4. thyroxine
The two lobes of the thyroid gland are joined by the: (1 mark)
1. hypophysial stalk
2. islet
3. isthmus
4. medulla
Constriction of the arterioles of the digestive system by adrenaline is felt as: (1 mark)
1. panting
2. a thumping heart
3. shivering
4. a dry mouth
A hormone that acts on another endocrine gland rather than on ordinary body tissue is called: (1 mark)
1. a steroid hormone
2. a tropic hormone
3. an antidiuretic hormone
4. a mineralocorticoid
Oversecretion of growth hormone after growth is complete results in: (1 mark)
1. gigantism
2. dwarfism
3. acromegaly
4. cretinism
Reabsorption of water from the kidney tubules is increased by: (1 mark)
1. oxytocin
2. prolactin
3. calcitonin
4. vasopressin

Assertion And Reason

For each of the following, an Assertion (A) is followed by a Reason (R). Choose the correct option from the codes given below. (1) Both A and R are true, and R is the correct explanation of A. (2) Both A and R are true, but R is not the correct explanation of A. (3) A is true but R is false. (4) A is false but R is true.

Assertion (A): In simple goitre the thyroid gland becomes enlarged. Reason (R): A shortage of iodine keeps the thyroxine level low, so the gland continues to be stimulated. (1 mark)
Assertion (A): Insulin is injected into the body and not swallowed. Reason (R): Insulin is a steroid hormone and dissolves only in fat. (1 mark)
Assertion (A): Nervous control brings about long-lasting changes in the metabolism of the body. Reason (R): A nerve impulse travels electrochemically along the nerve fibre. (1 mark)

Match The Following

Match each item in Column A with the correct item in Column B. (5 marks)

Column A	Column B
a. Alpha cells of the pancreas	i. Adrenaline
b. Adrenal medulla	ii. Oxytocin
c. Posterior pituitary	iii. Glucagon
d. Thyroid	iv. Aldosterone
e. Adrenal cortex	v. Calcitonin

Fill In The Blanks

Fill in each blank with a single suitable word. (5 marks)
1. The islets of Langerhans are found in the ____.
2. The narrow bridge joining the two lobes of the thyroid gland is called the ____.
3. The hormone secreted by the alpha cells of the pancreas is ____.
4. A shortage of ____ in the food leads to simple goitre.
5. Most feedback mechanisms in the human body are of the ____ type.

True Or False

State whether each statement is true or false. Rewrite the false statements correctly. (4 marks)
1. The adrenal cortex lies inside the adrenal medulla.
2. Hormones are stored in the gland that makes them until they are needed.
3. Adrenocorticotropic hormone is secreted by the anterior pituitary.
4. Diabetes insipidus is caused by a deficiency of insulin.

Name The Following

Name the following. (5 marks)
1. The gland that is both a duct gland and a ductless gland.
2. The hormone whose oversecretion in a child causes gigantism.
3. The region of the brain that controls the pituitary gland.
4. The condition caused by hyposecretion of cortisol from the adrenal cortex.
5. The hormone that stimulates the ejection of milk.

Odd One Out

Choose the odd one out from each group and give a reason for your choice. (4 marks)
1. Insulin, glucagon, somatostatin, thyroxine
2. Gigantism, acromegaly, dwarfism, myxoedema
3. TSH, ACTH, FSH, oxytocin
4. Saliva, sweat, tears, adrenaline

12.14 Check Your Concepts

Define a hormone. (2 marks)
State the exact location of the adrenal glands and of the pituitary gland in the human body. (2 marks)
Give a reason why adrenaline is described as the emergency hormone. (2 marks)
Name the three conditions that may result from hyposecretion of thyroxine, and state which of them affects adults. (2 marks)
Differentiate between endocrine glands and exocrine glands, stating the basis of difference in each case. (3 marks)
State the two principal ways in which insulin lowers the blood sugar level, and explain why it is wrong to say that insulin converts glucose into glycogen. (3 marks)
Give reasons for the following. (3 marks)
1. Simple goitre has long been common in regions where the soil is poor in iodine.
2. A person with hyperthyroidism may lose weight although the diet has not changed.
3. Glucagon is needed even though insulin already controls the blood sugar level.
Differentiate between diabetes mellitus and diabetes insipidus, stating the basis of difference in each case. (3 marks)
Explain with an example what is meant by a tropic hormone, and state why the pituitary is called the master gland. (4 marks)
What would happen if the adrenal cortex of an adult secreted far more cortisol than normal? Name the condition produced and describe its chief effects. (4 marks)
Why does the body need hormonal control as well as nervous control? Answer with reference to the speed, the reach and the duration of the effects produced. (4 marks)
Describe the negative feedback control of the thyroxine level in the blood, naming every gland and hormone involved, and explain how this mechanism accounts for the enlargement of the thyroid gland when iodine is short in the diet. (5 marks)
Trace what happens in the body from the moment a person eats a meal rich in starch to the moment the blood sugar level returns to normal. Name the cells and the hormone responsible at each stage. (5 marks)

12.15 Figure-Based Questions

Study Fig. 12.4-1 and answer the following.
1. Name the organ on which the gland shown rests. (1 mark)
2. Name the outer region and the inner region of the gland. (1 mark)
3. Name one hormone secreted by each of these two regions. (2 marks)
Study Fig. 12.6-1 and answer the following.
1. Name the gland shown and state its exact position in the body. (2 marks)
2. Name the structure that joins its two lobes. (1 mark)
3. Name the four small glands seen in the back view, and state the ion whose level in the blood they control. (2 marks)
Study Fig. 12.7-1 and answer the following.
1. Name the region of the brain lying immediately above the gland shown. (1 mark)
2. Name the two distinct lobes of the gland. (1 mark)
3. Name the two hormones released by the lobe lying behind, and state one action of each. (3 marks)
Study Fig. 12.5-1 and answer the following.
1. Name the organ from which this section has been taken. (1 mark)
2. Name the pale patch of cells seen in the middle of the field, and name the two hormones it produces that control the blood sugar level. (2 marks)
3. Redraw the patch and label three of its parts, keeping all label lines straight and free of crossings. (3 marks)