Skin: The Jack of All Trades | ICSE Class 9 Biology Notes

This note covers the ICSE Class 9 Biology topic Structure and Functions of Skin: the layers of the skin and what each part does, the special derivatives of the skin (sweat glands, sebaceous glands, hair, nails and mammary glands), and how the skin helps to keep body temperature constant through vasodilation and vasoconstriction.

Why is the skin called the jack of all trades?

The skin is the outer covering of the body and its largest organ, with a surface area of about 1.5 to 2 square metres in an adult.

It is called the "jack of all trades" because it does so many different jobs: it protects, senses, regulates temperature, excretes, and even makes a vitamin.

Because it is in direct contact with the surroundings, the skin is the body's first line of defence and also one of its main links with the outside world, sending information about touch and temperature to the brain.

What are the layers of the skin?

The skin has two main layers: an outer, thinner epidermis and an inner, thicker dermis. Below the dermis lies a layer of fatty tissue, the subcutaneous layer (hypodermis), which joins the skin to the muscles beneath.

Draw the thin epidermis on top, labelling the cornified layer at the surface, the granular layer and the Malpighian layer below it.

Beneath it draw the thick dermis containing: a hair in its follicle with a sebaceous gland opening into the follicle and a small arrector pili muscle attached to it; a coiled sweat gland with its duct running up to a sweat pore on the surface; blood capillaries and nerve endings. At the bottom, draw the subcutaneous fat layer.

The epidermis has no blood vessels. Its living cells get food and oxygen by diffusion from the blood vessels of the dermis. This is why a shallow scratch that grazes only the epidermis does not bleed, while a deeper cut that reaches the dermis does.

What does the epidermis do?

The epidermis is the protective outer layer of the skin. Its structure suits it to its jobs:

1. Constant renewal: the germinative cells at the base of the Malpighian layer divide all the time. The new cells are pushed upwards, gradually flatten, fill with keratin and die.
2. A tough, dead surface: by the time the cells reach the surface they form the cornified layer, a tough, horny, waterproof sheet of dead cells.
3. Shedding: the outermost dead cells are constantly rubbed off as tiny flakes, and are replaced from below, so the skin surface is always renewed.
4. Protection: the unbroken cornified layer stops germs from entering, prevents the loss of water from the body, and protects the inner tissues from mechanical injury.

The cornified layer is thickest where the skin is most rubbed, as on the palms and the soles of the feet. Repeated rubbing thickens it further, forming corns and calluses.

Melanin and skin colour

Special cells in the Malpighian layer produce a dark brown pigment called melanin. Melanin absorbs ultraviolet (UV) rays of sunlight and so protects the deeper, living cells from damage.

The amount of melanin decides the colour of the skin: more melanin gives darker skin. Exposure to strong sunlight makes the skin produce more melanin, which causes tanning.

In the inherited condition albinism, melanin is absent or greatly reduced, so the skin and hair are very pale and the eyes are sensitive to light.

What does the dermis contain, and what do its parts do?

The dermis is the thick, living layer of the skin. Its connective tissue, with elastic and collagen fibres, makes the skin strong and elastic, so it can stretch and return to shape. It contains:

• Blood vessels: a network of capillaries supplies food and oxygen to the skin and plays a key role in temperature regulation.
• Nerve endings (sense receptors): these detect touch, pressure, pain, heat and cold, making the skin a sense organ. They are especially numerous in the fingertips and lips.
• Sweat glands: coiled tubes that produce sweat.
• Sebaceous glands: small glands that open into the hair follicles and produce oily sebum.
• Hair follicles and hair: pits in which hairs grow, each with a small arrector pili muscle.
• Lymph vessels: they drain tissue fluid from the skin.

Below the dermis, the subcutaneous fat stores food, acts as an insulating layer that reduces heat loss, and cushions the body against knocks. People and animals in cold regions often have a thicker layer of fat under the skin.

Vitamin D

When the skin is exposed to sunlight, the ultraviolet rays convert a substance present in the skin into vitamin D. Vitamin D is needed for the absorption of calcium and the formation of strong bones, so some exposure to sunlight is important, especially for growing children.

A lack of vitamin D in children causes rickets, in which the bones stay soft and may bend, as in bow legs. Children who spend almost all their time indoors, or whose skin is always fully covered, are more at risk. Vitamin D is also found in some foods, such as fish liver oil, egg yolk and fortified milk, so a good diet and some time in the sun both help.

What are the sweat glands and the sebaceous glands?

Sweat glands

Sweat glands are coiled, tube-like glands lying deep in the dermis. Each has a long duct that runs up through the epidermis and opens on the surface at a tiny sweat pore.

The coiled part is surrounded by blood capillaries, from which it takes water and dissolved substances to make sweat.

Sweat is a watery fluid containing sodium chloride (common salt), small amounts of urea, lactic acid and other substances. Its primary function is to cool the body: as sweat evaporates from the skin, it takes heat from the body.

Sweating also helps to remove some wastes, so the skin acts as a minor excretory organ.

Sebaceous glands

Sebaceous glands are small glands in the dermis that open into the hair follicles. They secrete an oily substance called sebum, which contains substances such as sterols, hydrocarbons and waxes. Sebum:

• forms a protective oily covering for the skin;
• keeps the skin and hair soft and flexible, and stops them from drying and cracking;
• makes the skin surface waterproof;
• helps to check the growth of some bacteria and fungi.

What are hair, nails and mammary glands?

Hair, nails and the mammary glands, like the sweat and sebaceous glands, are called derivatives of the skin because they develop from it.

Hair

Each hair grows from a hair follicle, a deep pit of the epidermis that dips into the dermis.

At the base of the follicle is a swelling, the hair papilla, supplied with blood vessels; the living cells covering it divide to make the hair grow.

The part of the hair above the skin, the shaft, is made of dead cells filled with keratin, which is why cutting hair is painless.

A tiny muscle, the arrector pili, joins each follicle to the upper part of the dermis, just below the epidermis. When it contracts, as in cold or fear, the hair stands erect, and the skin around it rises in small bumps called goose pimples.

In furry mammals, erect hairs trap a thicker layer of air, which insulates the body. Hair on the head also shields the scalp from the sun, and eyelashes and eyebrows help keep dust and sweat out of the eyes.

Draw a hair rising out of the skin surface from a deep, narrow pit, the follicle, that dips through the epidermis into the dermis.

At its base show the swollen hair papilla with a small blood vessel. Show a sebaceous gland opening into the side of the follicle, and a small arrector pili muscle running from the follicle wall up to the base of the epidermis.

Nails

Nails are hard plates of keratin formed by the epidermis at the tips of the fingers and toes. They protect the sensitive tips and help in picking up small objects and scratching.

Like hair, nails are made of dead cells, so cutting them does not hurt; they grow from living cells at their base.

Mammary glands

Mammary glands are modified sweat glands. They are well developed in females, where they produce milk to feed the young. The presence of mammary glands is a feature of mammals, which take their name from these glands.

How does the skin regulate body temperature?

Humans are warm-blooded: the body temperature stays nearly constant at about 37°C (98.6°F), even when the temperature outside changes. Heat is produced by the body's chemical activities, especially in the liver and the muscles.

The skin controls how much of this heat is lost. A heat-regulating centre in the brain, the hypothalamus, senses changes in the temperature of the blood and adjusts the skin's responses.

In hot conditions or during exercise

1. Vasodilation: the small blood vessels in the dermis widen (dilate), so more blood flows close to the skin surface.
2. More heat is lost from this blood to the air by radiation, and the skin looks flushed.
3. The sweat glands produce more sweat. As the sweat evaporates, it takes heat from the skin (latent heat of evaporation) and cools the body.
4. The hairs lie flat, so little air is trapped near the skin.

In cold conditions

1. Vasoconstriction: the small blood vessels in the dermis narrow (constrict), so less blood flows near the surface.
2. Less heat is lost from the blood to the air, and the skin looks pale.
3. Sweating is reduced to a minimum.
4. The arrector pili muscles contract, making the hairs stand erect (goose pimples).
5. The muscles may contract rapidly and involuntarily, which is shivering; this produces extra heat.

Draw two small sections of skin side by side.

In the "hot" section, show a wide blood vessel lying close under the epidermis, arrows of heat leaving the surface, and drops of sweat at the sweat pores.

In the "cold" section, show a narrow blood vessel with little blood near the surface, few heat arrows, no sweat, and an erect hair with its contracted arrector pili muscle.

Sweating cools the body well only when the sweat can evaporate. On a hot, humid day the air is already full of moisture, sweat evaporates slowly, and we feel sticky and uncomfortable. A fan helps by moving the humid air away from the skin, so that sweat evaporates faster.

How does the skin act as a sense organ?

The skin is one of the body's sense organs. The skin, mainly its dermis, contains many sensory receptors, the endings of nerves that respond to different kinds of stimuli and send messages to the brain.

• Touch receptors lie close under the epidermis and respond to light contact.
• Pressure receptors lie deeper in the dermis and respond to firm pressure.
• Pain receptors are free nerve endings that respond to injury, such as a cut or a burn.
• Heat and cold receptors respond to changes in temperature.

The receptors are not spread evenly. They are packed closely in the fingertips and the lips, which are very sensitive, and are far fewer on the back. This is why visually impaired people read Braille, a script of raised dots, with their fingertips.

The sense of touch protects the body. If the hand touches a very hot object, pain and heat receptors send messages that make us pull the hand away at once, before serious damage is done. The skin also tells us about the texture, shape and temperature of things around us.

How can the skin be kept healthy?

Because the skin is always exposed, it needs care to do its many jobs well.

1. Keep it clean: regular bathing removes sweat, excess sebum, dead cells, dust and germs. If these collect, they can cause body odour and skin infections.
2. Clean cuts and wounds: broken skin lets germs in, so cuts should be washed and covered to prevent infection.
3. Replace lost water and salt: heavy sweating on hot days or during exercise removes water and salt from the body. Drinking enough water, with a little salt (for example in oral rehydration solution or lemon water) when sweating is heavy, helps to prevent dehydration and cramps.
4. Protect it from too much sun: some sunlight is needed for vitamin D, but long exposure to strong ultraviolet rays can cause sunburn and damage the skin.

During adolescence the sebaceous glands become more active. If their openings in the hair follicles get blocked with sebum and dead cells, pimples or acne may form. Washing the face gently and regularly helps to keep the openings clear.

Some changes in the skin are signs of illness elsewhere in the body, such as a yellow tinge in jaundice or a rash in measles, which is one more way in which this jack of all trades is useful.

Glossary

• Epidermis — The outer layer of the skin, without blood vessels, made of a dead cornified layer over living cells.
• Dermis — The inner, thick layer of the skin, containing blood vessels, nerves, glands and hair follicles.
• Cornified layer — The outermost layer of dead, keratin-filled cells that forms a tough, waterproof covering.
• Malpighian layer — The innermost living layer of the epidermis, whose deepest cells divide to renew the skin.
• Keratin — A tough protein that fills the dead cells of the skin surface, hair and nails.
• Melanin — A dark pigment made in the Malpighian layer that colours the skin and absorbs ultraviolet rays.
• Sebum — The oily secretion of the sebaceous glands that keeps the skin and hair soft and waterproof.
• Hair follicle — A pit of the epidermis dipping into the dermis, from which a hair grows.
• Arrector pili — A tiny muscle attached to a hair follicle that makes the hair stand erect.
• Vasodilation — The widening of the blood vessels in the skin, which increases heat loss.
• Vasoconstriction — The narrowing of the blood vessels in the skin, which reduces heat loss.
• Mammary glands — Modified sweat glands that produce milk in female mammals.

Common errors and misconceptions

• Misconception: The epidermis has blood vessels. Correct: The epidermis has no blood vessels; it is nourished by diffusion from the dermis.
• Misconception: Sweat is pure water. Correct: Sweat contains salt (NaCl), small amounts of urea and lactic acid, so it also removes some wastes.
• Misconception: Sweating cools the body by itself. Correct: Cooling comes from the evaporation of sweat, which takes heat from the skin.
• Misconception: In the cold, blood vessels in the skin widen to warm it. Correct: They narrow (vasoconstriction), so less blood reaches the surface and less heat is lost.
• Misconception: Sebaceous glands open directly onto the skin. Correct: They open into the hair follicles.
• Misconception: Hair and nails are living tissue throughout. Correct: The visible parts are dead, keratin-filled cells; only the cells at their base are alive.
• Misconception: Mammary glands are modified sebaceous glands. Correct: They are modified sweat glands.

Exam-style questions with model answers

Q1. Name the pigment that gives colour to the skin and state one function. [1 mark]

1. Melanin gives colour to the skin; it absorbs ultraviolet rays of sunlight and protects the deeper living cells.

Q2. Give the location and one function of (a) sebaceous glands, (b) the Malpighian layer. [2 marks]

1. (a) Sebaceous glands lie in the dermis and open into the hair follicles; they secrete sebum, which keeps the skin and hair soft and waterproof.
2. (b) The Malpighian layer is the innermost layer of the epidermis; its cells divide constantly to replace the worn-out cells of the surface.

Q3. State three functions of the skin other than temperature regulation. [3 marks]

1. Protection: its tough cornified layer guards against injury, germs and water loss, and melanin screens out ultraviolet rays.
2. Sensation: nerve endings in the dermis detect touch, pressure, pain, heat and cold.
3. Excretion and synthesis: sweat removes salt and small amounts of urea, and in sunlight the skin makes vitamin D.

Q4. Distinguish between sweat glands and sebaceous glands. [3 marks]

1. Sweat glands are long, coiled tubes that open directly on the skin surface through sweat pores; sebaceous glands are small, sac-like glands that open into the hair follicles.
2. Sweat glands secrete watery sweat containing salt, urea and lactic acid; sebaceous glands secrete oily sebum with sterols, hydrocarbons and waxes.
3. Sweat mainly cools the body and removes some wastes; sebum keeps the skin and hair soft, oily and waterproof.

Q5. Describe the structure of the epidermis and explain how it protects the body. [4 marks]

1. The epidermis has an outer cornified layer of flat, dead cells filled with keratin, a granular layer below it, and an innermost living Malpighian layer.
2. The deepest cells of the Malpighian layer divide constantly; the new cells are pushed up, fill with keratin and die, replacing the dead cells that are rubbed off the surface.
3. The unbroken, waterproof cornified layer stops germs from entering, prevents water loss and protects the inner tissues from injury.
4. Melanin made in the Malpighian layer absorbs ultraviolet rays and protects the living cells beneath.

Q6. Explain how the skin helps to keep the body temperature constant (a) on a hot day, (b) on a cold day. [5 marks]

1. On a hot day, the blood vessels in the dermis dilate (vasodilation), so more blood flows near the surface and more heat is lost to the air by radiation; the skin looks flushed.
2. The sweat glands secrete more sweat, and its evaporation from the skin takes away heat, cooling the body.
3. On a cold day, the blood vessels in the dermis constrict (vasoconstriction), so less blood flows near the surface and less heat is lost; the skin looks pale.
4. Sweating is reduced, and the arrector pili muscles contract so that the hairs stand erect, forming goose pimples.
5. Shivering, the rapid contraction of muscles, produces extra heat. These responses, controlled by a heat-regulating centre in the brain, keep the temperature near 37°C.

Key takeaways

• The skin is the body's largest organ and is called the jack of all trades because it protects, senses, excretes, regulates temperature and makes vitamin D.
• The epidermis has a dead, keratin-filled cornified layer over a living Malpighian layer whose cells divide to renew it; it has no blood vessels.
• The dermis contains blood vessels, nerve endings, sweat glands, sebaceous glands and hair follicles, over a layer of subcutaneous fat.
• Melanin in the Malpighian layer colours the skin and absorbs ultraviolet rays; its absence or severe shortage causes albinism.
• Sweat glands open at sweat pores and secrete sweat with salt, urea and lactic acid; its evaporation cools the body.
• Sebaceous glands open into hair follicles and secrete sebum, which keeps the skin and hair soft and waterproof.
• Hair and nails are made of dead keratin cells, and the mammary glands are modified sweat glands.
• In heat, vasodilation and sweating increase heat loss; in cold, vasoconstriction, erect hairs and shivering conserve and produce heat.

Test yourself

Why does a shallow scratch not bleed?

A shallow scratch reaches only the epidermis, which has no blood vessels; bleeding begins when a cut reaches the dermis.

What is the normal human body temperature?

The normal human body temperature is about 37°C, or 98.6°F.

What causes goose pimples?

Contraction of the arrector pili muscles in cold or fear makes the hairs stand erect and raises small bumps.

Why is cutting the hair and nails painless?

The visible parts of hair and nails are made of dead, keratin-filled cells without nerves.

Why do we feel sticky and hot on a humid day?

Humid air is full of moisture, so sweat evaporates slowly and cannot cool the body well.

How does the skin help in making bones strong?

In sunlight the skin makes vitamin D, which is needed for calcium absorption and strong bones.

What is the function of the subcutaneous fat?

It stores food, insulates the body against heat loss and cushions it against knocks.

Where does the cornified layer become thickest?

On the palms and soles, where the skin is most rubbed; repeated rubbing forms corns and calluses.