Skin: The Jack of All Trades — ICSE Class 9 Biology Important Questions

Answer: (b) Melanin.

Melanin is produced by cells in the Malpighian layer of the epidermis and determines skin colour; it also protects deeper cells from harmful ultraviolet rays.

Answer: (b) Sweat glands.

Sweat glands lie in the dermis and pour out sweat (water, salts and urea) through pores onto the skin surface, which cools the body as it evaporates.

Answer: (c) Vitamin D.

Under ultraviolet light from the sun, the skin synthesises vitamin D, which is needed for the absorption of calcium and healthy bones and teeth.

Answer: (b) Skin blood vessels constrict, reducing blood flow to the surface.

In cold conditions the skin arterioles constrict so less warm blood reaches the surface (reducing heat loss and making the skin pale), while shivering generates heat.

Answer: (a) When sweat evaporates it takes latent heat from the skin surface, and it is precisely this heat loss that cools the body, so R correctly explains A.

The two main layers of the skin are the epidermis and the dermis. The epidermis is the outer, thinner, protective layer, while the dermis is the inner, thicker layer that contains the glands, blood vessels, nerve endings and hair follicles.

(i) Sebaceous glands: secrete an oily substance called sebum that keeps the skin and hair soft, supple and waterproof, and helps prevent the growth of germs.

(ii) Cornified (horny) layer: the outermost layer of dead, flattened cells that protects the underlying living cells from injury, germs, water loss and the entry of harmful substances.

A vertical section of skin shows the thin outer epidermis over the thicker dermis, which contains a coiled sweat gland, a hair with its follicle, and blood capillaries and nerve endings.

The skin is called the 'jack of all trades' because a single organ performs many different, unrelated functions:

(i) Protection - it forms a barrier against injury, germs, harmful rays and water loss. (ii) Sensation - its nerve endings detect touch, pain, pressure, heat and cold. (iii) Excretion - sweat glands remove water, salts and urea in sweat. (iv) Temperature regulation - by sweating and by altering blood flow it keeps the body temperature steady. In addition it makes vitamin D and stores fat, so one organ does the work of many.

Sweat glands open directly onto the skin surface through pores, whereas sebaceous glands usually open into the hair follicles.

The human body must keep its temperature close to $37^{\circ}\text{C}$37^C. The skin is the main organ that controls this.

On a hot day (to lose heat):
(i) The sweat glands become very active and secrete a large amount of sweat. As the sweat evaporates from the skin surface it absorbs latent heat, cooling the body.
(ii) The blood vessels (arterioles) of the skin dilate (vasodilation), so more warm blood flows near the surface and heat is lost to the surroundings by radiation. The skin looks flushed/red.
(iii) Hairs lie flat, so less air is trapped and heat escapes easily.

On a cold day (to conserve heat):
(i) Sweating is greatly reduced, so little heat is lost by evaporation.
(ii) The skin blood vessels constrict (vasoconstriction), so less blood reaches the surface and less heat is lost; the skin looks pale.
(iii) The tiny erector muscles pull the hairs erect, trapping a layer of insulating air (goose bumps).
(iv) Shivering (rapid muscle contraction) generates extra heat.

Through these opposite responses the skin keeps the internal body temperature nearly constant.

The skin has two main layers - the epidermis (outer) and the dermis (inner).

Epidermis (outer layer):
(i) Cornified (horny) layer - dead, flattened cells forming a tough outer covering; protects against injury, germs and water loss.
(ii) Granular layer - transition zone where cells are dying and being pushed outward.
(iii) Malpighian (germinative) layer - the innermost living layer; its cells continuously divide to form new epidermal cells and produce the pigment melanin, which gives colour and screens ultraviolet rays.

Dermis (inner layer):
(i) Sweat glands - coiled tubes that secrete sweat, helping in cooling and excretion.
(ii) Sebaceous glands - secrete oily sebum that keeps skin and hair soft and waterproof.
(iii) Hair follicle with hair - hair provides protection and, when erect, insulation.
(iv) Blood vessels - supply nutrients and oxygen and help in temperature regulation by dilating or constricting.
(v) Nerve endings (receptors) - detect touch, pressure, pain, heat and cold.
(vi) Fat (adipose) layer at the base - stores energy and acts as an insulator against heat loss.

Together these structures make the skin a protective, sensory, excretory and heat-regulating organ.

(i) During exercise the body produces extra heat. The skin blood vessels dilate (vasodilation) so that more warm blood flows near the surface to lose heat; this extra blood makes the skin look red/flushed.

(ii) The sweat glands pour out sweat onto the skin. As this sweat evaporates it absorbs latent heat from the body surface, thereby cooling the body.

(iii) The sweat glands, situated in the dermis, produce sweat.

(iv) In a cold room the skin blood vessels would constrict (vasoconstriction), reducing blood flow to the surface so that less heat is lost and body heat is conserved; the skin would look paler.