Chapter 14: Skin
In the 1660s, in the Italian city of Bologna, a physician named Marcello Malpighi spent his evenings looking at scraps of tissue through one of the earliest microscopes. The instrument was little more than a small lens held in a brass mount, and the images it gave were blurred at the edges. Still, it showed him things no doctor had ever seen.
One of the tissues he studied was skin. Everybody at the time thought of skin as a single covering, rather like a leather sheet stretched over the body. Malpighi cut thin pieces of it, held them up to the light and found something quite different. The skin was built in layers. Its surface was a crust of flattened, lifeless cells. Below that crust lay a deeper layer whose cells were plump and alive, and which appeared to be making new cells all the time. Below that again, the tissue was raised into rows of tiny mounds, each carrying fine threads of nerve running into it.
Malpighi had found, without knowing it, the answer to a very ordinary puzzle: why the surface of the body wears away every day and yet never wears out, and why a covering of dead cells can still feel the lightest touch. The living layer he described is still called the malpighian layer today.
Diagram
Fig. 14.0-1 — Malpighi’s View Of Human Skin Line drawing in two parts: on the left, a simple seventeenth-century single-lens microscope on a table with a thin slice of skin mounted on it; on the right, a circular magnified inset of that slice showing the layered structure; label lens, thin slice of skin, flattened dead cells at the surface, living dividing cells below, raised mounds of the deeper layer, nerve fibre entering a mound. Labels on straight, non-crossing lead lines.
14.1 What Is Skin?
Run a finger along your forearm. Everything your finger touches, and the finger itself, is covered by the same continuous sheet of living tissue. That sheet is the skin — the outermost covering of the body, stretched over it without a break from scalp to sole.
The skin is not a wrapper laid on top of the body. It is an organ, and an organ is a structure built of several tissues working together to do one job. The skin has covering tissue on the outside, connective tissue below it, blood vessels running through it, and nerve endings woven into it.
It is also the largest organ we have. In an adult, the skin accounts for about 15 per cent of the total body weight — more than the liver, the lungs and the brain put together.
An Almost Impermeable Barrier
The chief quality of this covering is that very little gets through it in either direction. Water inside the body does not seep out through intact skin, and germs and harmful substances outside it do not seep in. A barrier of this kind is described as almost impermeable, meaning that substances pass through it only with great difficulty.
That single property is worth pausing on. Without it, the body would dry out in a few hours in ordinary air, in the way a peeled potato does.
The skin, together with everything that grows out of it, forms the integumentary system of the body. The skin itself is sometimes called the integument.
Structures Grown From The Skin
The skin does not work alone. Hair, nails, sweat glands, oil glands and mammary glands are all developed from the skin during the growth of an embryo, and all are still rooted in it. They are dealt with in section 14.5.
So the skin is worth thinking about as a base as well as a barrier: a covering that protects, and a bed in which several other useful structures are set.
14.2 Functions Of The Skin
A structure that merely covered the body could be made of anything — cloth, leather, plastic. The skin is living tissue, supplied with blood and nerves, and it earns that supply by doing several different jobs at once. Seven of them are worth knowing.
14.2.1 Protection
Protection is the primary function of the skin, and it works in four different ways. These are drawn together in Fig. 14.2-1.
First, the skin protects the tissues under it from mechanical shocks. A knock that would bruise a bare muscle is spread and softened by the tough, springy covering above it.
Second, it holds the body fluids inside. Your tissues are mostly water, and water evaporates from any wet surface exposed to air. The skin seals that surface and prevents excessive loss of water by evaporation.
Third, it keeps harmful substances and disease-causing germs from entering the body. Unbroken skin is one of the body’s most reliable defences; this is exactly why a cut or a burn carries a risk of infection while the skin around it does not.
Fourth, it protects the body against excess ultraviolet light. Ultraviolet light is an invisible part of sunlight, and in large amounts it damages the living cells beneath the surface. The skin absorbs much of it before it can reach them.
Diagram
Fig. 14.2-1 — The Four Ways In Which Skin Protects Simplified block diagram of a piece of skin in section with the tissues below it; four labelled arrows directed at the skin surface from outside and inside — a blunt object striking the surface (mechanical shock absorbed), water molecules held inside the body (loss of water prevented), germs stopped at the surface (entry of germs prevented), and ultraviolet rays of sunlight absorbed at the surface. Labels on straight, non-crossing lead lines.
14.2.2 The Other Functions Of The Skin
Sensation
The skin is a sense organ. Nerve endings within it respond to touch, pain, pressure, heat and cold, and send that information to the brain. This is the only sense organ that covers the whole body, which is why you can feel an insect land anywhere on you.
Temperature Regulation
The skin prevents the loss of heat in cold weather and helps the body lose heat in hot weather. It is the main organ by which the body holds its temperature steady. How it manages this is set out in section 14.6.
Storage Of Food
The skin stores reserve food in the form of a layer of fat, held inside special cells that lie beneath it. When food is short, this reserve can be drawn upon.
Excretion
Sweat carries out of the body a little water, some salts and, to a very limited extent, urea. In that sense the skin assists in excretion — the removal of waste products made by the body.
The help is small, though. Sweating happens mainly to control temperature, not to remove waste, and the kidneys deal with almost all the urea. The skin is therefore not counted as an excretory organ.
Synthesis Of Vitamin D
When exposed to sunshine, the skin can make vitamin D. This is a minor function, and the mechanism behind it is not yet fully understood. Too much sunshine, on the other hand, causes tanning and other ill-effects, including skin cancers.
Grip
On prolonged exposure to water, the skin of the fingertips wrinkles or puckers up. The wrinkled surface grips better on wet, slippery objects, in the way that the grooves cut into a tyre grip a wet road.
14.3 The Components Of The Skin
Before looking at the skin closely, it helps to have its plan in mind. The human skin — and mammalian skin in general — falls into two parts.
The first part is the skin proper, the sheet of tissue itself. It is built of two layers: an outer epidermis, the thin surface layer of covering cells, and an inner dermis, the thicker layer beneath it that carries elastic fibres, blood vessels and nerves.
The second part consists of the derivatives of the skin — structures that grow out of the skin and remain rooted in it. There are five of them: hair, nails, mammary glands, sweat glands and sebaceous glands.
The whole scheme is set out in FC 14.1. Everything in the rest of the chapter fits somewhere on it.
Flowchart
FC 14.1 — The Principal Components Of Human Skin Human (mammalian) skin → branches into two: (A) Skin proper → branches into epidermis (cornified, granular and germinative layers) and dermis (elastic fibres, blood vessels, nerves); (B) Derivatives of the skin → branches into hair, nails, mammary glands, sweat glands, sebaceous glands
Notice how the two parts differ. The skin proper is a continuous covering that runs over the whole body. The derivatives are separate structures, each set into that covering at particular places and each doing one special job.
14.4 The Skin Proper
Under a microscope, the skin proper shows its two layers clearly. The epidermis lies above and is the thinner of the two; the dermis lies below and is much the thicker. The two are pressed tightly together, but they are made of different tissues and do different work, as Table 14.1 shows.
A vertical section through the skin, cut straight down like a slice through a cake, is the standard way of showing this arrangement. Study Fig. 14.4-1 and Fig. 14.4-2 together as you read this section; the first shows the layers in section, the second shows the same structures in three dimensions.
Diagram
Fig. 14.4-1 — Vertical Section Of Human Skin Diagrammatic vertical section through human skin. Mark the two main regions with brackets on the left: epidermis and dermis. Label stratum corneum, granular layer, stratum malpighi, sweat pore, hair shaft, free nerve ending, blood capillaries, sensory corpuscle, sebaceous gland, erector muscle of hair, hair follicle, hair bulb, hair papilla, sweat gland, nerve, fat cells, artery, vein. Labels on straight, non-crossing lead lines.
Diagram
Fig. 14.4-2 — Three-Dimensional Sketch Of Human Skin Diagrammatic three-dimensional block of skin cut open at two faces to show depth. Label epidermis, dermis, dead cells at the surface, papillae, hair shaft, sebaceous gland, erector muscle, hair follicle, free nerve ending, nerve, sweat gland, blood vessels, fat cells. Labels on straight, non-crossing lead lines.
14.4.1 The Epidermis
The epidermis is the outer, thinner part of the skin. It is formed of stratified epithelium — covering tissue whose cells are piled up layer after layer, rather than laid out in a single sheet.
Its thickness is not the same everywhere. At places that take hard use, such as the palms, the soles and especially the heels, the epidermis becomes thick and hard.
One fact about the epidermis explains a great deal of what follows: it is devoid of blood vessels at all places. No capillary reaches into it. Its cells must therefore take what they need from the tissue below, and the cells pushed furthest from that supply cannot survive.
Cut depthwise, the epidermis shows three regions, one below the other.
14.4.1.1 The Cornified Layer
The cornified layer, also called the stratum corneum, is the outermost layer. It consists of several piled-up layers of flattened dead cells.
These cells are made of keratin, a fibrous structural protein — the same protein found in nails, hair, horns, hooves and silk. Keratin is tough, insoluble in water and not easily broken down, which is exactly what a surface exposed to the world needs.
The cornified layer is worn away or shed continually. You leave a little of it behind on every towel you use. It is replaced from beneath by cells arising from the deeper malpighian layer.
Because it is tough, the cornified layer offers resistance to three things: mechanical damage, bacterial infection, and loss of water by evaporation.
14.4.1.2 The Granular Layer
The granular layer is a very thin middle layer. It consists of only two or three sub-layers of flattened cells.
It is best thought of as a stage rather than a structure. Cells moving outwards flatten here and gradually give way to the cornified layer above.
14.4.1.3 The Malpighian Layer
The malpighian layer — also called the stratum malpighi or the germinative layer — is the innermost region of the epidermis. Its name comes from the physician who first described it, and ‘germinative’ means giving rise to new growth.
Its cells divide actively. Each new cell produced presses the cells above it outward, so the whole epidermis creeps slowly towards the surface. As a cell is pushed further from the blood supply of the dermis, it flattens, fills with keratin and dies, and finally reaches the surface as part of the cornified layer to be shed.
The skin therefore replaces itself from below at about the rate it wears away from above. Follow one cell through the process in the trace below.
14.4.1.4 Melanin And Skin Colour
The colour of the skin is due to a pigment called melanin, contained in the cells of the malpighian layer.
Melanin is brown-black. The more of it the cells hold, the darker the skin; the less they hold, the lighter it is. Every shade from very light brown to deep dark is produced by nothing more than the quantity of this one pigment. How much a person makes depends partly on the ancestry they inherit it from and partly on how much sunlight their skin has met.
Melanin is not merely a colouring. It absorbs ultraviolet rays and acts as an umbrella over the living cells beneath, shielding them from radiation that can damage them and cause skin cancer.
This is why skin tans. On continued exposure to sunlight, the face or other exposed parts of a lighter-skinned person darken, as more melanin is laid down where the light falls. When the exposure is cut down, the skin reverts to its original condition.
14.4.1.5 Two Conditions Of Skin Pigmentation
Two conditions are known in which this pigment is missing.
In leukoderma, also called vitiligo, melanin is lost from smaller or larger patches at different regions of the body, so those patches appear much paler than the skin around them. The exact cause of the condition is not yet known. It is neither infectious nor painful, and a person with vitiligo is in every other way in normal health.
In albinism, pigmentation is absent from the skin all over the body, and also from the hair, eyebrows, eyelashes and even the iris of the eye. The skin of a person with albinism appears pinkish, because with no pigment to mask them the blood capillaries beneath show through.
Albinism is a recessive trait, and it is inherited. A recessive trait appears only when a person receives the factor for it from both parents, which is why it can pass unseen through a family. If both parents have albinism, neither has the normal factor to pass on, so all their children have albinism.
14.4.2 The Dermis
The dermis is the inner, thick layer of the skin. It is a connective tissue made largely of elastic fibres, and these give it its two chief qualities: it is tough, and it is flexible. Pinch the back of your hand and let go — the dermis pulls the skin flat again.
Its thickness varies with the work it does. The dermis is very thick where the skin takes hard wear, as on the palms and soles, and very thin where the skin must fold and move freely, as in the eyelids.
The dermis is also where nearly everything else in the skin is housed. It contains blood vessels, nerve fibres, sensory organs, hair follicles and sweat glands. The epidermis above has none of these.
14.4.2.1 Papillae And The Sense Receptors Of The Skin
The outer region of the dermis, lying next to the epidermis, is not flat. It is raised into numerous small processes called papillae, each containing blood capillaries and nerve endings.
The papillae matter for both supply and sensation. They push the blood supply as close as possible to an epidermis that has no vessels of its own, and they bring nerve endings close to the surface where a touch can reach them.
The nerve endings and sense organs of the papillae are concerned with the sensations of touch and pain. The sense of touch in the fingertips is sharp enough for a person who is blind to read Braille — raised dots so small and so close together that the eye can barely separate them.
Deeper in the dermis lie other receptors, concerned with the sensations of pressure, pain, heat and cold. Different receptors for different sensations is the rule; no single ending reports everything.
14.4.2.2 The Fat Layer Beneath The Dermis
The layers beneath the dermis contain numerous fat cells, which together make up adipose tissue. Because it lies under the skin, this fat is described as subcutaneous — sub, under, and cutis, skin.
The subcutaneous fat does three jobs at once. It serves as a food reserve, drawn upon when food is short. It works as a heat-insulating layer, slowing the escape of body heat. And it acts as a shock absorber, cushioning the parts beneath from blows.
This layer explains a point met earlier: storage of food was listed in section 14.2 as one of the functions of the skin, and this is the store meant.
14.5 Derivatives Of The Skin
Five structures develop out of the skin and stay rooted in it: hair, nails, sebaceous glands, sweat glands and mammary glands. Each began as a downgrowth of the epidermis into the dermis, which is why every one of them is set in the dermis but opens at the surface.
14.5.1 Hair
14.5.1.1 The Parts Of A Hair
A hair consists of three parts, shown in Fig. 14.5-1.
The hair shaft is the part that projects from the skin, and it may extend slightly below the surface of the epidermis. In normal situations, hairs lie obliquely on the skin rather than standing straight out of it.
The hair root is the part embedded within the dermis. The lowest part of the root is expanded to form the hair bulb, and into the bulb pushes a small projection of the dermis called the hair papilla, which carries a capillary blood supply.
That blood supply is the reason the bulb sits so deep. A hair grows by the addition of new cells at its base, and dividing cells need food and oxygen. The cells added at the base soon die, harden with keratin and are pushed up as part of the shaft — which is why cutting a hair is painless.
The hair follicle is the structure enclosing the hair root. It is composed of an epithelial and a connective tissue sheath. The hair bulb and the hair follicle together are responsible for the growth and elongation of the hair.
Diagram
Fig. 14.5-1 — Structure Of A Hair In Its Follicle Vertical section through skin showing one hair; label hair shaft, epidermis, dermis, hair root, hair follicle, hair bulb, hair papilla with blood capillaries, sebaceous gland opening into the follicle, erector muscle of hair. Labels on straight, non-crossing lead lines.
Hair follicles contain pigment cells that produce melanin, and the colour of hair is due to the varying quantities of this pigment. When melanin production decreases, hair becomes grey, silvery or even completely white.
14.5.1.2 The Erector Muscle And Goose Flesh
Everyone has felt ‘goose flesh’ at some time, during winter or during a strong emotion. The hairs are lifted, and the surface of the skin looks contracted and puckered. The lifting of hair in this way is called piloerection (pilo : hair).
It is caused by the erector muscle of the hair, also called the arrector muscle. This small muscle runs obliquely between the hair follicle and the outer part of the dermis.
When it contracts, it pulls at both its ends at once. At one end it pulls the hair into a somewhat vertical position; at the other end it depresses the epidermis. The lifted hairs and the pitted surface between them are what you see and feel.
14.5.1.3 Hair Colour, Growth And Kinds Of Hair
Hair in human beings is continuously lost and regrown. The duration of scalp hair is 2 to 5 years, and that of the eyebrows and eyelashes 3 to 5 months. Eyelashes are short because they are shed long before they can grow long.
Hairs from different parts of the body — head, chest, armpit, beard, nose and others — show subtle differences from one another. These differences make hair useful in forensic investigations, where a hair found at a scene can be matched to the region of the body it came from.
Hairs also give a sensation of touch, because nerve fibres extend up to their bases. The hair itself is dead, but it works as a lever: bend it, and the movement is felt at the follicle. This is how you notice an insect on your arm before it reaches your skin.
Particular hairs do particular jobs. The hairs along the edges of the eyelids, the eyelashes, help prevent the entry of foreign particles into the eye. The hairs in the nose prevent dust particles from entering the nasal passages. In human males, facial hair — the moustache and the beard — is one of the features distinguishing the sexes, an example of sexual dimorphism, which means a visible difference in form between males and females of the same species.
14.5.2 Nails
Nails are hardened keratinous plate-like structures. They grow as dead cells from the nail root, which lies below the skin at the base of the nail.
A nail has three parts, shown in Fig. 14.5-2.
The nail plate is the hard, outer part — the part you cut. It is made up of dead, keratinised cells, which is why cutting it does not hurt.
The nail bed, or root, lies below the nail plate.
The nail matrix lies just below the skin surface at the base of the nail. It is usually visible as the whitish half-moon there. The matrix produces new cells, and these, on maturing, push the older cells towards the tip of the nail. This is what causes the growth of the nail.
Diagram
Fig. 14.5-2 — Structure Of A Human Nail Two views of a finger nail: a surface view of the finger tip and a longitudinal section through it. Label nail plate, nail bed, nail matrix, whitish half-moon at the base, skin fold at the base, finger bone. Labels on straight, non-crossing lead lines.
14.5.3 Sebaceous Glands
Sebaceous glands are branched glands that usually open into a hair follicle, though sometimes they open directly to the outside.
They give out an oily secretion called sebum. Sebum makes the hair and the outer surface of the skin oily and waterproof. This keeps the epidermis supple and prevents loss of moisture by evaporation.
Weather changes the amount produced. In cold and dry weather, the skin may become rough and leave a powdery surface when scratched; this is due to a reduced secretion of oil from the sebaceous glands. In hot and humid weather, the skin becomes extra oily because the secretion of sebum increases.
Three common problems are related to these glands.
Pimples form when sebum accumulates. Sebum is nutritive, so bacteria grow in it; the accumulation gets infected and results in the formation of boils and pimples.
Acne occurs when the sebaceous glands become inflamed under hormonal influence. It is one of the commonest problems in adolescents, and it is a matter of hormones, not of cleanliness.
Blackheads form when an open pore in the skin gets clogged with dead skin cells and sebum. The plug turns black upon oxidation by the air — the darkening comes from contact with air, not from dirt.
14.5.4 Sweat Glands
Each sweat gland is a simple coiled tube. It consists of a deeper secretory part and an excretory part that runs upwards to open on the surface. The outer openings are called the sweat pores.
The total number of sweat pores in the body is estimated at about two million. They are not spread evenly: there are 60 to 80 per square centimetre on the back, but about 400 per square centimetre on the palms of the hands. The dampness of the palms raises the surface tension between skin and object and improves grip — a slightly moist hand holds a glass better than a bone-dry one.
The secretory part absorbs fluid from the surrounding cells and from the blood capillaries of the dermis, and passes it into the excretory sweat duct, which pours it out on the surface. The path is set out in FC 14.2.
Flowchart
FC 14.2 — Path Of Sweat From Blood To Skin Surface Blood capillaries and surrounding cells of the dermis → secretory part of sweat gland (fluid absorbed) → sweat duct → sweat pore → film of sweat on skin surface → evaporation, taking heat from the body
Sweating, or perspiration, goes on at all times in minute quantities, so small that it is not noticed; this is called incipient or invisible perspiration. It occurs in large quantities during strenuous exercise, or during hot and humid weather. The major function of sweating is to lose body heat by evaporation.
Sweat consists of about 99 per cent water, 0.2 to 0.5 per cent salts, mainly sodium chloride, and traces of urea, about 0.08 per cent. The urea lost through sweat is about 1 per cent of the total urea excreted by the body — the remaining 99 per cent leaves through the kidneys.
‘Cold sweat’ may be due to psychic influences such as fright and nervousness, when the skin is not hot at all. Sweating may also accompany nausea and severe pain, and heavy sweating causes a loss of body salts along with water.
14.5.5 Mammary Glands
The mammary glands, or milk glands, are modified sweat glands. Fig. 14.5-3 shows their arrangement.
These glands are present in both males and females. In males they persist only in a rudimentary state, that is, present but undeveloped. In females they enlarge at puberty, in the form of a pair of breasts.
Each breast carries a central conical projection called the nipple. Fifteen to twenty milk ducts open on the nipple. Each milk duct continues inward in a branching manner to join a cluster of 15 to 20 lobes of the mammary gland.
The activity of the mammary glands is related to the reproductive hormones, particularly prolactin, and to pregnancy. The glands enlarge and begin secreting only when these hormones act on them. The milk they secrete is highly nutritious for the newborn baby, and is its complete food in the first months of life.
Diagram
Fig. 14.5-3 — Mammary Glands Two views side by side. A: front view of a breast showing mammary glands, milk ducts, fat, areola and nipple. B: sectional side view showing fat, mammary glands, mammary duct and milk duct converging on the nipple. Label all named parts on straight, non-crossing lead lines.
14.5.6 Meibomian And Ceruminous Glands
Two further glands are worth naming, because each is one of the derivatives above, altered for a special place.
Meibomian glands are modified sebaceous glands that open on the margins of the eyelids. Their secretion is oily, and it lubricates the margins of the lids and prevents the overflow of tears down the face.
Ceruminous glands are modified sweat glands found in the auditory canal, the passage leading to the eardrum. They secrete a wax-like substance called cerumen, or earwax, which lubricates the canal and protects the delicate eardrum from dust particles and germs.
14.6 The Skin And Heat Regulation Of The Body
Section 14.2 listed temperature regulation among the functions of the skin and left the mechanism for later. This is the later.
14.6.1 Warm-Blooded And Cold-Blooded Animals
All mammals, including humans, are warm-blooded, or endothermal, meaning that their body heat is generated from inside. Birds are warm-blooded too. They all maintain a more or less constant body temperature even if it is very cold or very hot outside.
The body temperature of cold-blooded, or ectothermal, organisms fluctuates with the temperature of their surroundings. Such an animal has no way of holding its own temperature steady, so it must escape conditions that are too hot or too cold. This is why frogs hibernate, passing the cold season in winter sleep, and aestivate, passing the hottest season in summer sleep.
The older pair of terms has now been replaced. Cold-blooded is properly poikilothermal (poikilo : varying) and warm-blooded is homeothermal (homeo : same). The newer names describe the animals better, because the blood of a lizard on a hot afternoon is not cold at all — it is simply as warm as the air around it.
14.6.2 Normal Body Temperature
Human body temperature is usually about 37°C when measured in the mouth. It is about 1°C higher in the rectum and about 1°C lower in the armpits, so the site of measurement must always be known before a reading means anything.
The temperature also shifts a little through the day. It varies by 0.3°C to 0.5°C in the course of 24 hours, being lowest in the early morning and highest in the late afternoon.
A rise in body temperature above normal means fever and sickness, and a fall in temperature may be dangerous. A suitable, or optimum, temperature of 35°C to 40°C is essential for normal body activities.
The reason lies with the enzymes. Enzymes are proteins that speed up the chemical reactions of the body, and they work well only within a narrow range of temperature. The working of enzymes is slowed down when the temperature falls, while higher temperatures destroy them altogether. Holding the body near 37°C is therefore not a preference but a necessity.
14.6.3 Heat Production
The body has several sources of heat.
Chemical reactions occurring in all body cells produce heat, especially in the liver, where glucose is oxidised. Every reaction that releases energy releases a part of it as heat.
Most of the heat produced in the body comes from the activity of the muscles. Vigorous activity makes you warm on a cold day and overheats you on a hot one.
A small amount of heat comes from the ingestion of hot foods and beverages — a cup of hot tea adds a little heat, though far less than a brisk walk.
14.6.4 Heat Loss
Heat leaves the body through four channels.
The skin accounts for about 85 per cent of the heat lost. It loses heat by convection, conduction and radiation, and through the evaporation of sweat. This is the channel the body can control, and the rest of this section is about how.
The lungs lose heat in the warm air breathed out. You can feel this by gently blowing on the back of your hand with the mouth wide open. Some heat is also lost during the vaporisation of water from the lungs.
Urine and faeces carry heat away, because these substances are eliminated at body temperature.
Foods take heat when cold food, water or cold beverages are taken into the body, since the body must warm them to its own temperature.
14.6.5 Temperature Regulation By The Hypothalamus
The principal heat-regulating centre is located in the hypothalamus, a portion of the fore-brain. This part acts like a thermostat — the device in a refrigerator or an oven that switches the machinery on when the temperature drifts one way and off when it drifts the other.
When the body tends to cool below the normal temperature, the hypothalamus switches on or speeds up the heat-producing processes. When the body tends to get overheated, it accelerates the cooling processes and switches off the heat-producing ones.
Notice that the control works in both directions from a fixed point. FC 14.3 sets out the two paths.
Flowchart
FC 14.3 — The Hypothalamus As A Thermostat Body temperature moves away from 37°C → hypothalamus detects the change → branches into two: (A) body too cold → vasoconstriction, less sweat, increased metabolic rate, shivering → heat conserved and produced → temperature returns to 37°C; (B) body too hot → vasodilation, more sweat, evaporation → heat lost → temperature returns to 37°C
14.6.5.1 In Cold Weather — Vasoconstriction
When the outside temperature is low, the blood vessels of the skin get narrowed. This narrowing is called vasoconstriction, and it is shown in Fig. 14.6-1 A.
Narrowed vessels reduce the blood supply to the skin. With less warm blood at the surface, there is less loss of heat by convection, conduction and radiation, and less loss through the vaporisation of sweat. As the blood supply to the skin is reduced, the sweat glands also secrete less sweat. This is the body’s natural method of conserving heat.
Vasoconstriction is visible. It makes a person look pale, or bluish at the lips and fingertips, because so little blood is reaching the skin.
At the same time, heat production is increased. The metabolic rate rises, and muscular activity increases — sometimes in the form of shivering, which is rapid involuntary contraction of muscles whose only purpose is to release heat.
14.6.5.2 In Hot Weather — Vasodilation
When the outside temperature is high, or when a person is engaged in strenuous physical work, heat is being overproduced within the body. The blood supply to the skin is then increased by the dilation of the blood vessels of the skin. This widening is called vasodilation, shown in Fig. 14.6-1 B.
More blood at the surface means greater loss of heat by radiation and the other direct routes. It also means greater loss by the vaporisation of sweat, which is now produced in larger quantities because of the rich supply of blood to the sweat glands.
Evaporation cools because water needs heat to turn into vapour, and it takes that heat — the latent heat — from the skin it is sitting on. The skin is left cooler than before.
Air movements over the body help speed up the evaporation of sweat. This is why a fan cools you although it does not cool the air of the room at all. It cools you by removing the moist air lying against your skin and letting more sweat evaporate.
Diagram
Fig. 14.6-1 — Vasoconstriction And Vasodilation In The Skin Two sectional diagrams of skin, one above the other. A: arterioles constricted, less blood flowing in the capillaries near the surface, few arrows of heat radiated — caption side label ‘vasoconstriction to cut down heat loss’. B: arterioles dilated, more blood flowing in the capillaries near the surface, many arrows of heat radiated — side label ‘vasodilation to increase heat loss’. In both, label epidermis, dermis, artery, capillaries. Labels on straight, non-crossing lead lines.
14.6.5.3 Heatstroke
Heatstroke, also called sunstroke, is a condition in which sweat production is unable to keep pace with the demand for cooling in very hot winds. Sweat is carried off faster than the glands can replace it, cooling fails, and the body temperature rises.
The rise may be severe, and heatstroke may sometimes be fatal. It is a medical emergency, and a person who collapses in strong heat needs shade, cooling and medical help at once.
14.7 Summary
The skin is the outermost covering of the body and its largest organ, accounting for about 15 per cent of the body weight of an adult. It is an almost impermeable barrier.
The skin, with the structures that grow from it, forms the integumentary system.
Protection is the primary function of the skin. It guards against mechanical shocks, prevents excessive loss of water, keeps out harmful substances and disease-causing germs, and absorbs excess ultraviolet light.
The skin also provides sensation, regulates body temperature, stores food as fat, assists slightly in excretion, synthesises vitamin D in sunlight, and gives grip through the wrinkling of wet fingertips.
The skin has two parts: the skin proper, made of the outer epidermis and the inner dermis, and five derivatives — hair, nails, mammary glands, sweat glands and sebaceous glands.
The epidermis is the thinner outer part, made of stratified epithelium and devoid of blood vessels. It has three regions: the outermost cornified layer, the middle granular layer and the innermost malpighian layer.
The cornified layer is made of flattened dead cells containing keratin. It resists mechanical damage, bacterial infection and loss of water, and is continually shed.
The malpighian, or germinative, layer divides actively and pushes new cells outward to replace those shed from the surface.
Skin colour is due to melanin, a pigment held in the cells of the malpighian layer. Melanin absorbs ultraviolet rays and shields the cells beneath; extra melanin laid down in sunlight shows as a tan.
In leukoderma, or vitiligo, melanin is lost from patches of skin; the cause is not yet known. In albinism, pigment is absent from the skin, hair and iris; it is a recessive inherited trait.
The dermis is the thick inner layer of connective tissue with elastic fibres. It is tough and flexible, thick on palms and soles and thin in the eyelids, and it holds blood vessels, nerve fibres, sensory organs, hair follicles and sweat glands.
The outer dermis is raised into papillae carrying blood capillaries and nerve endings concerned with touch and pain. Deeper receptors respond to pressure, pain, heat and cold.
Beneath the dermis lies a layer of fat cells, or adipose tissue, which serves as a food reserve, a heat insulator and a shock absorber.
A hair has three parts — shaft, root and bulb — with the hair papilla and its capillaries pushing into the bulb. The follicle encloses the root, and follicle and bulb together bring about growth.
Hair colour is due to melanin from pigment cells in the follicle; less melanin gives grey or white hair. The erector muscle lifts the hair in piloerection, seen as goose flesh.
Scalp hair lasts 2 to 5 years and eyebrow and eyelash hair 3 to 5 months. Hairs give a sense of touch through nerve fibres at their bases, and hairs of different body regions differ, which is useful in forensic work.
A nail has a plate of dead keratinised cells, a bed below the plate, and a matrix at the base, visible as a whitish half-moon, which produces the cells that make the nail grow.
Sebaceous glands open into hair follicles and secrete sebum, which keeps skin and hair oily, supple and waterproof. Pimples, acne and blackheads are the common problems related to them.
A sweat gland is a coiled tube with a secretory part in the dermis and a duct opening at a sweat pore. About two million pores are present, more densely on the palms than on the back.
Sweat is about 99 per cent water, with 0.2 to 0.5 per cent salts and about 0.08 per cent urea. Its major function is to lose body heat by evaporation.
Mammary glands are modified sweat glands, rudimentary in males and enlarging in females at puberty. Their activity depends on reproductive hormones, particularly prolactin, and on pregnancy.
Meibomian glands are modified sebaceous glands of the eyelid margins; ceruminous glands are modified sweat glands of the auditory canal that secrete cerumen.
Mammals and birds are endothermal, or homeothermal, and hold a nearly constant body temperature. Ectothermal, or poikilothermal, animals vary with their surroundings and escape extremes by hibernation or aestivation.
Human body temperature is about 37°C in the mouth, about 1°C higher in the rectum and about 1°C lower in the armpits, varying 0.3°C to 0.5°C through the day. A range of 35°C to 40°C is needed for enzymes to work.
Heat is produced by chemical reactions in the cells, especially oxidation of glucose in the liver, chiefly by muscular activity, and slightly by hot food and drink.
Heat is lost through four channels: the skin, which accounts for about 85 per cent, the lungs, urine and faeces, and cold food and drink taken in.
The hypothalamus of the fore-brain is the principal heat-regulating centre and acts as a thermostat, switching heat-producing processes on when the body cools and off when it overheats.
In cold weather, vasoconstriction reduces the blood supply to the skin and sweat secretion falls, while shivering and a raised metabolic rate produce more heat.
In hot weather, vasodilation increases the blood supply to the skin and more sweat is produced, so heat is lost by radiation and by the latent heat taken up in evaporation.
Heatstroke occurs when sweating can no longer keep pace with the cooling needed in very hot winds, so body temperature rises; it may be fatal, and plenty of water with a little extra salt is a good precaution.
14.8 Key Terms
Adipose tissue — the tissue of fat cells lying beneath the dermis, serving as a food reserve, a heat insulator and a shock absorber.
Aestivate — to pass the hottest, driest season in summer sleep, as frogs do.
Albinism — an inherited recessive condition in which pigment is absent from the skin, hair and iris.
Body temperature — the internal temperature of the body, about 37°C in the mouth in humans.
Cerumen — the wax-like secretion of the ceruminous glands of the auditory canal; earwax.
Ceruminous glands — modified sweat glands of the auditory canal that secrete cerumen.
Cold-blooded — see ectothermal.
Cornified layer — the outermost layer of the epidermis, made of flattened dead cells filled with keratin; also called the stratum corneum.
Dermis — the thick inner layer of the skin proper, made of connective tissue with elastic fibres.
Derivatives of the skin — the five structures that grow from the skin: hair, nails, mammary glands, sweat glands and sebaceous glands.
Ectothermal — describing an animal whose body temperature varies with its surroundings; cold-blooded.
Endothermal — describing an animal whose body heat is generated inside it, so its temperature stays nearly constant; warm-blooded.
Epidermis — the thin outer layer of the skin proper, made of stratified epithelium and having no blood vessels.
Erector muscle — the small muscle running obliquely from a hair follicle to the dermis, whose contraction lifts the hair.
Germinative layer — see malpighian layer.
Granular layer — the very thin middle region of the epidermis, of two or three sub-layers of flattened cells.
Hair bulb — the expanded lowest part of the hair root, into which the hair papilla projects.
Hair follicle — the epithelial and connective tissue sheath enclosing the hair root.
Hair papilla — the small projection of the dermis, carrying capillaries, that pushes into the hair bulb.
Hair root — the part of a hair embedded within the dermis.
Hair shaft — the part of a hair that projects from the surface of the skin.
Heatstroke — a dangerous rise in body temperature caused when sweating can no longer keep pace with the cooling the body needs; also called sunstroke.
Hibernate — to pass the cold season in winter sleep, as frogs do.
Homeothermal — the modern term for warm-blooded: keeping the body temperature the same whatever the surroundings.
Hypothalamus — the part of the fore-brain that acts as the body’s heat-regulating centre.
Integument — another name for the skin.
Integumentary system — the skin together with hair, nails and the glands derived from the skin.
Keratin — a tough fibrous structural protein found in the cornified layer, nails, hair, horns, hooves and silk.
Leukoderma — a condition in which melanin is lost from patches of skin; also called vitiligo.
Malpighian layer — the innermost, actively dividing region of the epidermis, which produces new cells and holds melanin; also called the stratum malpighi or germinative layer.
Mammary glands — modified sweat glands that secrete milk; the milk glands.
Meibomian glands — modified sebaceous glands opening on the margins of the eyelids.
Melanin — the brown-black pigment of the skin and hair, which absorbs ultraviolet rays.
Nail bed — the layer lying below the nail plate.
Nail matrix — the growing region at the base of the nail, visible as a whitish half-moon.
Nail plate — the hard outer part of the nail, made of dead keratinised cells.
Nipple — the central conical projection of the breast, on which the milk ducts open.
Papillae — the small processes of the outer dermis, containing blood capillaries and nerve endings.
Perspiration — the secretion of sweat onto the skin surface; sweating.
Piloerection — the lifting of hairs by the erector muscles, seen as goose flesh.
Poikilothermal — the modern term for cold-blooded: having a body temperature that varies with the surroundings.
Prolactin — the reproductive hormone concerned with the activity of the mammary glands.
Sebaceous glands — branched glands, usually opening into hair follicles, that secrete sebum.
Sebum — the oily secretion of the sebaceous glands, which keeps skin and hair supple and waterproof.
Skin — the outermost covering of the body, made of the epidermis and the dermis.
Skin proper — the sheet of the skin itself, that is, the epidermis together with the dermis.
Stratum corneum — see cornified layer.
Stratum malpighi — see malpighian layer.
Sweat duct — the tube that carries sweat from the secretory part of the gland to a sweat pore.
Sweat gland — a simple coiled tube in the dermis that secretes sweat.
Sweat pores — the openings of the sweat ducts on the surface of the skin.
Vasoconstriction — the narrowing of the blood vessels of the skin, which cuts down heat loss.
Vasodilation — the widening of the blood vessels of the skin, which increases heat loss.
Vitiligo — see leukoderma.
Warm-blooded — see endothermal.
14.9 Points To Ponder
The surface of the body is made of dead cells. Is it an advantage or a disadvantage for a living animal to meet the world with dead tissue, and what would a skin of living surface cells have to cope with?
The epidermis has no blood vessels of its own, while the dermis is richly supplied. What problems does that arrangement create for the epidermis, and what does the skin do about each of them?
Sweating cools the body only when the sweat evaporates. What would you expect to happen to a person working hard on a day that is very hot and also very humid, and why is such weather harder to bear than dry heat at the same temperature?
Melanin protects the cells beneath it from ultraviolet rays, yet the skin also needs sunlight to make vitamin D. How might these two demands pull in opposite directions in a person living far from the equator?
A frog escapes extreme temperatures by hibernating or aestivating, while a human regulates temperature from within. What does each strategy cost the animal, and why might it suit the life the animal leads?
Shivering warms the body by making muscles contract without doing any useful work. Where does that heat come from, and what does this suggest about the efficiency of muscles in ordinary movement?
14.10 Objective Questions
- The layer of the epidermis in which new cells are formed is the: (1 mark)
- cornified layer
- granular layer
- malpighian layer
- dermis
- Keratin is a: (1 mark)
- pigment
- fibrous structural protein
- fatty secretion
- hormone
- Mammary glands are modified: (1 mark)
- sebaceous glands
- sweat glands
- salivary glands
- ceruminous glands
- Roughly what proportion of the body’s heat is lost through the skin? (1 mark)
- 25 per cent
- 50 per cent
- 85 per cent
- 100 per cent
- The principal heat-regulating centre of the body lies in the: (1 mark)
- thyroid gland
- hypothalamus
- spinal cord
- dermis
- Which one of the following is NOT a function of human skin? (1 mark)
- Synthesis of vitamin D
- Storage of fat
- Digestion of food
- Sensation of pressure
- For a. and b., choose from: (a) both assertion and reason are true and the reason explains the assertion; (b) both are true but the reason does not explain the assertion; (c) the assertion is true but the reason is false; (d) the assertion is false but the reason is true.
- Assertion: In cold weather the skin of a person may look pale. (1 mark) Reason: The blood vessels of the skin become narrowed, reducing the blood supply to the surface.
- Assertion: Cutting the nail plate causes no pain. (1 mark) Reason: The nail plate is made of dead, keratinised cells.
- Match the items in Column I with those in Column II. (5 marks)
| Column I | Column II |
|---|---|
| a. Sebum | i. Auditory canal |
| b. Cerumen | ii. Winter sleep |
| c. Melanin | iii. Sebaceous gland |
| d. Hibernation | iv. Erector muscle |
| e. Piloerection | v. Malpighian layer |
- Fill in the blanks. (5 marks)
- The outermost layer of the epidermis is the ______ layer.
- The pigment responsible for the colour of the skin is ______.
- The small processes of the outer dermis that carry capillaries and nerve endings are called ______.
- Sweat contains about ______ per cent water.
- Widening of the blood vessels of the skin in hot weather is called ______.
- Write True or False for each statement. (4 marks)
- The epidermis is richly supplied with blood vessels.
- Aestivation is summer sleep.
- The dermis is very thin in the eyelids.
- Albinism is an infectious disease of the skin.
- Name the following. (5 marks)
- The protein that makes the cornified layer tough.
- The part of a hair embedded within the dermis.
- The tissue of fat cells lying beneath the dermis.
- The hormone connected with the activity of the mammary glands.
- The modern term used in place of ‘warm-blooded’.
- Pick the odd one out in each group and give a reason for your choice. (3 marks)
- Hair, nails, sweat glands, cartilage
- Cornified layer, granular layer, malpighian layer, papillae
- Frog, lizard, sparrow, fish
14.11 Check Your Concepts
Define the term integumentary system. (2 marks)
State two functions of the subcutaneous fat layer. (2 marks)
Name the three parts of a nail. (2 marks)
Give a reason: cutting hair does not hurt, but pulling it out does. (2 marks)
Explain how the skin protects the body against the entry of germs and against loss of water. (3 marks)
Why is the skin not counted as an excretory organ, although sweat contains urea? (3 marks)
Differentiate between the epidermis and the dermis, stating the basis of difference in each case. (3 marks)
Give a reason for each: (3 marks)
- Hair turns grey with age.
- The skin feels rough and powdery in cold, dry weather.
- Fingertips wrinkle after a long time in water.
Describe the structure of a sweat gland and trace the path taken by sweat from the dermis to the surface of the skin. (4 marks)
Explain, with reasons, how the body loses less heat on a cold day and more heat on a hot day. (4 marks)
What would happen if the malpighian layer of the epidermis stopped dividing? Give three consequences. (4 marks)
Describe the role of the hypothalamus in maintaining body temperature, naming the changes it brings about in the skin in cold and in hot weather. (5 marks)
A person is doing heavy physical work in a hot, dry wind. Explain how the skin tries to keep the body temperature steady, and why the attempt may fail and lead to heatstroke. (5 marks)
14.12 Figure-Based Questions
Refer to Fig. 14.4-1, a vertical section of human skin. Name the two main regions marked by brackets and state one structure found in each. (3 marks)
Refer to Fig. 14.5-1. Identify the structure shown, name the part that supplies it with blood, and state the function of the muscle attached to the follicle. (3 marks)
Refer to Fig. 14.5-2. Name the part of the nail that is visible as a whitish half-moon and state its role in the growth of the nail. (2 marks)
Refer to Fig. 14.6-1. Name the condition shown in part A and the condition shown in part B, and state which of the two occurs on a cold day. (3 marks)
Draw a neat labelled diagram of a vertical section of human skin, showing the epidermis, the dermis, a hair with its follicle, a sebaceous gland, a sweat gland and the fat cells. Use straight, non-crossing lead lines. (5 marks)