Skin: Structure and Functions ICSE Class 9: Layers, Derivatives and Heat Regulation

What Skin: Structure and Functions is about

Weightage: CISCE publishes no chapter weightage for ICSE Biology. Skin: Structure and Functions is topic 6, Human Anatomy and Physiology (iii), of the Class IX part of the 2028 syllabus; the Class IX paper (80 marks, 2 hours) is set and marked by the school.Source: CISCE ICSE Biology (Science Paper 3) syllabus, examination year 2028 (Class IX)

Key concepts

Epidermis

The epidermis is the outer, thinner layer, made of stratified epithelium. It has no blood vessels; it gets nourishment from the dermis below.

Layers (outside to inside):

• Cornified layer (stratum corneum): several layers of dead, flat cells filled with the tough protein keratin. It is waterproof, prevents water loss and keeps out germs; its cells are constantly rubbed off and replaced.
• Granular layer: living cells becoming keratinised and flat as they move up.
• Malpighian (germinative) layer: the innermost layer of living, actively dividing cells that replace the cells lost from the surface. It contains melanin, the pigment that gives skin colour and protects against ultraviolet rays.

Worked example

Why does a small scratch on the surface of the skin not bleed?

Try it first, then show the solutionSolution

1. The scratch damages only the epidermis.
2. The epidermis has no blood vessels; they are in the dermis below.

Answer: The epidermis has no blood vessels.

Dermis and subcutaneous layer

The dermis is the inner, thicker layer of connective tissue with elastic and collagen fibres. It contains:

• Blood capillaries: supply food and oxygen; help regulate temperature.
• Nerve endings (sense receptors) for touch, pressure, pain, heat and cold.
• Sweat glands with their ducts.
• Sebaceous (oil) glands.
• Hair follicles with hair roots and tiny arrector pili muscles.
• Papillae: finger-like projections into the epidermis (forming fingerprint ridges).

Below the dermis lies the subcutaneous layer of adipose tissue (fat), which stores food, insulates the body against heat loss and cushions it against shocks.

Worked example

Why do people with more fat under the skin feel less cold in winter?

Try it first, then show the solutionSolution

1. The subcutaneous fat is a poor conductor of heat.
2. It reduces heat loss from the body to the cold air.

Answer: Fat insulates the body.

Functions of the skin

1. Protection: a barrier against injury, germs and harmful chemicals; melanin protects against UV rays; the waterproof keratin layer prevents loss of water from the body.
2. Temperature regulation: through sweating and changes in blood flow (see below); fat under the skin insulates.
3. Excretion: sweat removes water, salts (mainly sodium chloride) and a little urea.
4. Sensation: receptors for touch, pressure, pain, heat and cold make the skin a sense organ.
5. Synthesis of vitamin D when exposed to sunlight.
6. Storage of fat in the subcutaneous layer.
7. Absorption of small amounts of some substances (such as some ointments), though limited.
8. Secretion of sebum (oil) that keeps the skin and hair soft and resists germs.

Worked example

Give two ways in which the skin protects the body from infection.

Try it first, then show the solutionSolution

1. The dead keratinised outer layer is a physical barrier germs cannot easily cross.
2. Sebum and sweat on the surface contain substances that discourage the growth of bacteria.

Answer: A keratin barrier and antibacterial secretions.

Sweat glands and sebaceous glands

Sweat glands:

• Coiled, tubular glands lying deep in the dermis, surrounded by blood capillaries.
• They take water, salts and a little urea from the blood and secrete sweat, which reaches the surface through a long duct opening at a sweat pore.
• Functions: cooling the body by evaporation of sweat; excretion of salts and urea.
• Most numerous on the palms, soles, forehead and armpits.

Sebaceous (oil) glands:

• Small, sac-like glands that open into the hair follicles.
• Secrete an oily substance, sebum.
• Functions: keeps the hair and skin soft and oily, prevents them from drying and cracking, makes the skin waterproof, and has a mild antibacterial effect.

Worked example

Why does skin become dry and cracked after washing too often with strong soap?

Try it first, then show the solutionSolution

1. Soap removes sebum from the skin surface.
2. Without this oily layer, water evaporates from the skin and it dries and cracks.

Answer: Loss of sebum.

Hair, nails and mammary glands

Hair:

• Each hair grows from a hair follicle (a pit of epidermis sunk into the dermis). The root has a swollen bulb over a papilla with blood vessels that nourish it; the shaft projects above the surface.
• Hair is made of dead, keratinised cells; only the root is living.
• A tiny arrector pili muscle attached to the follicle raises the hair when it contracts (goosebumps), and a sebaceous gland oils it.
• Functions: insulation (in animals, raised hair traps air), protection (eyelashes, nostril hairs keep out dust), sensation.

Nails:

• Hard, keratinised plates on the upper surface of the ends of fingers and toes, growing from a nail root under the skin and resting on the nail bed.
• Functions: protect the tips of fingers and toes; help in picking up small objects and scratching.

Mammary glands:

• Large, modified sweat glands found in the breasts of mammals; well developed in females.
• They secrete milk to nourish the young, and are the defining feature of mammals.

Worked example

Why does cutting hair or nails not hurt?

Try it first, then show the solutionSolution

1. The visible parts of hair and nails are made of dead, keratinised cells.
2. They have no nerves or blood vessels.

Answer: They are dead tissue.

Heat regulation: vasodilation and vasoconstriction

Humans are warm-blooded: we keep our body temperature steady at about 37 °C whether the surroundings are hot or cold. The hypothalamus of the brain acts as the thermostat and controls the skin's responses.

When the body is too hot (hot day, exercise):

• Vasodilation: the blood vessels (arterioles) of the skin widen, so more blood flows near the surface and more heat is lost to the air by radiation. The skin looks flushed (red).
• Sweating increases: the evaporation of sweat takes latent heat from the skin, cooling it.
• Hairs lie flat.

When the body is too cold:

• Vasoconstriction: the skin's blood vessels narrow, so less blood flows near the surface and less heat is lost; the skin looks pale.
• Sweating decreases or stops.
• Arrector pili muscles contract: hairs stand up (goosebumps), trapping a layer of air (very effective in furry animals).
• Shivering: rapid muscle contractions produce extra heat.
• Subcutaneous fat insulates.

Worked example

Why does your face turn red after a run, and pale when you stand in an icy wind?

Try it first, then show the solutionSolution

1. After running, the body is hot: skin vessels dilate (vasodilation), bringing more blood to the surface to lose heat.
2. In cold wind, skin vessels constrict (vasoconstriction), reducing blood flow to the surface to keep heat in.

Answer: Vasodilation (red) and vasoconstriction (pale).

Skin: Structure and Functions in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• Structure of the Skin, Glands and Heat Regulation

Skin: Structure and Functions: key facts

Layers of skin

$\text{epidermis (cornified, granular, Malpighian)}+\text{dermis}+\text{subcutaneous fat}$

Outside to inside.

Hot body

$\text{vasodilation}+\text{more sweating}\to \text{more heat lost}$

Skin flushed.

Cold body

$\text{vasoconstriction}+\text{less sweat}+\text{goosebumps}+\text{shivering}\to \text{heat kept}$

Skin pale.

Sweat

$\text{water}+\text{salts (NaCl)}+\text{a little urea}$

Excretion and cooling.

Skin: Structure and Functions: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Skin: Structure and Functions

The question patterns to expect, and the method that works for each.

Tricks to remember Skin: Structure and Functions

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Epidermis from top: 'Corn, Grain, Malpighi'

  Cornified (dead), Granular, Malpighian (dividing, melanin).

  Use it when: Layers of the epidermis.

• Dilate = Dump heat; Constrict = Conserve

  Vasodilation loses heat; vasoconstriction keeps it.

  Use it when: Heat regulation.

• Sebum Softens, Sweat Sheds heat

  Sebaceous glands keep skin soft; sweat glands cool and excrete.

  Use it when: Two skin glands.

• Skin's 'PET SVS'

  Protection, Excretion, Temperature, Sensation, Vitamin D, Storage of fat.

  Use it when: Functions of skin.

Skin: Structure and Functions: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section B style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 5-mark question split into 2 + 2 + 1 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Skin: Structure and Functions

The slips examiners see most often, and what to write instead.

• ✗Mistake: Vasodilation and vasoconstriction swapped

  Students link 'constriction' with heat.

  Wrong working

  On a hot day the blood vessels of the skin constrict.

  Right working

  Hot: vasodilation (lose heat). Cold: vasoconstriction (keep heat).

  ✓Do this: Dilate to Dump heat.

  Usually costs: 1 mark.

• ✗Mistake: Blood vessels placed in the epidermis

  The layers are mixed up.

  Wrong working

  The epidermis is supplied with blood capillaries.

  Right working

  The epidermis has no blood vessels; capillaries are in the dermis.

  ✓Do this: Epidermis is fed from below.

  Usually costs: 1 mark.

• ✗Mistake: Sweat said to cool by itself

  Students miss the role of evaporation.

  Wrong working

  Sweat is cold, so it cools the skin.

  Right working

  Evaporation of sweat takes latent heat from the skin.

  ✓Do this: Always say 'evaporation'.

  Usually costs: 1 mark.

• ✗Mistake: Sebaceous glands opening on the skin surface

  They are confused with sweat glands.

  Wrong working

  Sebaceous glands open through pores on the skin.

  Right working

  Sebaceous glands open into hair follicles.

  ✓Do this: Oil goes along the hair.

  Usually costs: 1 mark.

Practice questions on Skin: Structure and Functions

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   Sebaceous glands open into the:
   (a) skin surface
   (b) hair follicle
   (c) sweat duct
   (d) blood vessels

   Hint

   Oil the hair.

   Answer

   (b)

2. Q2 · 1 mark · MCQExam level

   Which of these helps to conserve heat?
   (a) vasodilation
   (b) sweating
   (c) vasoconstriction
   (d) flat hairs

   Hint

   Less blood to the surface.

   Answer

   (c)

3. Q3 · 3 marks · Very short answerExam level

   Name: (a) the protein in hair and nails, (b) the pigment that protects against UV, (c) the oily secretion of the skin.

   Hint

   K, M, S.

   Answer

   (a) Keratin (b) Melanin (c) Sebum

4. Q4 · 2 marks · Short answerExam level

   Why is the skin called a sense organ? Name three sensations it detects.

   Hint

   Receptors in dermis.

   Answer

   It has nerve endings (receptors) in the dermis that detect stimuli and send impulses to the brain. Touch, pain, heat/cold (also pressure).

5. Q5 · 2 marks · Short answerHarder exam level

   Explain why we feel cold when we come out of a swimming pool on a windy day.

   Hint

   Evaporation.

   Answer

   Water on the skin evaporates quickly in the wind, taking latent heat from the skin, so we lose heat fast and feel cold.

6. Q6 · 3 marks · Short answerHarder exam level

   Give the role of (a) the cornified layer, (b) subcutaneous fat, (c) sweat in excretion.

   Hint

   Barrier, insulator, salts.

   Answer

   (a) Waterproof, protective barrier against germs, injury and water loss. (b) Insulates, stores food, cushions. (c) Removes excess water, salts and a little urea.

7. Q7 · 2 marks · Short answerHarder exam level

   What is the function of the arrector pili muscle, and why are goosebumps of little use to humans compared with cats?

   Hint

   Hair density.

   Answer

   It raises the hair to trap a layer of insulating air. Humans have sparse body hair, so little air is trapped; a cat's thick fur traps a lot.

8. Q8 · 3 marks · Short answerHarder exam level

   Describe three ways in which the body responds to a cold environment.

   Hint

   Vessels, sweat, muscles.

   Answer

   Vasoconstriction reduces blood flow to the skin, conserving heat; sweating stops; shivering produces heat (also hair erection traps air).

9. Q9 · 2 marks · Short answerHarder exam level

   Why do mammary glands count as a skin derivative, and why are they important for classifying animals?

   Hint

   Modified sweat glands.

   Answer

   They develop from the skin as modified sweat glands. Only mammals have them, so they are a defining feature of class Mammalia.

10. Q10 · 3 marks · Short answerHOTS / competency

    A person with severe burns over a large area of the body is at risk of dehydration and infection, and of losing body heat. Explain each risk with reference to skin functions.

    Hint

    Barrier lost.

    Answer

    Dehydration: the waterproof cornified layer is lost, so water evaporates freely. Infection: the barrier against germs is gone. Heat loss: insulation and control of blood flow and sweating are damaged, so temperature cannot be regulated.

11. Q11 · 3 marks · Short answerHOTS / competency

    During a 2-hour game a player loses 1.2 kg of sweat, of which 0.9 kg evaporates. Taking 2.4 kJ per gram of sweat evaporated, how much heat was removed by evaporation? Why did the remaining 0.3 kg not help cooling?

    Hint

    Only evaporated sweat counts.

    Answer

    900 g × 2.4 kJ/g = 2160 kJ. The 0.3 kg dripped off or was wiped away without evaporating, so it took no latent heat from the skin.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Outer layer of the epidermis?Show answer

   Cornified layer (dead, keratinised).

2. 2.Layer of the epidermis that produces new cells?Show answer

   Malpighian (germinative) layer.

3. 3.Secretion of sebaceous glands?Show answer

   Sebum.

4. 4.Mammary glands are modified forms of?Show answer

   Sweat glands.

5. 5.Normal human body temperature?Show answer

   About 37 °C.

6. 6.Muscle that raises a hair?Show answer

   Arrector pili.

7. 7.Vitamin made in the skin in sunlight?Show answer

   Vitamin D.

8. 8.Which part of the brain controls body temperature?Show answer

   Hypothalamus.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you name the three epidermal layers and their features?
• ☐Can you list the structures found in the dermis?
• ☐Can you give six functions of skin?
• ☐Can you compare sweat and sebaceous glands?
• ☐Can you explain vasodilation and vasoconstriction with effects?
• ☐Can you draw and label the V.S. of skin?

Skin: Structure and Functions: frequently asked questions