Skin: The Jack of All Trades
ICSE Class 9 · Biology · Chapter 14
What this chapter covers
- The skin as an organ
- The epidermis
- The dermis
- The subcutaneous layer
- Functions of the skin
- Sweat glands and sebaceous glands
- Hair and nails
- Mammary glands
- Heat regulation
- Vasodilation and vasoconstriction
Key definitions
Melanin is the pigment that gives colour to the skin, hair and the iris of the eye and absorbs the harmful ultraviolet rays of sunlight.
Sweat is the watery fluid secreted by the sweat glands, made of about 99 per cent water with small amounts of common salt, urea and lactic acid.
Sebum is the oily secretion of the sebaceous glands that keeps the hair and the skin soft, smooth and waterproof and checks the growth of some bacteria.
Lactation is the secretion of milk by the mammary glands of a mother after the birth of her young.
Heat regulation is the process by which the body keeps its internal temperature nearly constant by balancing the heat it produces against the heat it loses.
Vasodilation is the widening of the arterioles that supply the skin, which increases the flow of blood near the surface so that more body heat is lost to the surroundings.
Vasoconstriction is the narrowing of the arterioles that supply the skin, which reduces the flow of blood near the surface so that less body heat is lost to the surroundings.
Key points
- The skin is the largest organ of the body and has two layers, the epidermis and the dermis, with a subcutaneous fat layer below them.
- The epidermis is made of stratified epithelium with no blood vessels; its layers from outside inwards are the cornified, granular and Malpighian layers.
- The cornified layer is made of dead, keratin-filled cells that are shed continuously; the Malpighian layer divides to replace them and contains melanocytes that make melanin.
- The dermis is dense connective tissue with collagen and elastic fibres, blood vessels, lymph vessels, sense receptors, sweat and sebaceous glands, hair follicles and arrector pili muscles.
- The subcutaneous fat insulates the body, stores food and cushions the deeper organs.
- The skin protects against injury, germs, water loss and ultraviolet rays; it senses touch, pressure, pain, heat and cold; it regulates temperature, excretes some wastes, makes vitamin D in sunlight, stores fat and secretes sebum.
- Sweat glands are coiled tubular glands that open through sweat pores; sweat is about 99 per cent water with salts and a little urea, and its evaporation cools the body.
- Sebaceous glands open into hair follicles and secrete oily sebum, which keeps skin and hair soft and waterproof; they are absent from the palms and soles.
- Hair grows from a hair follicle; the root is living and the shaft is dead keratin; the arrector pili muscle raises the hair.
- Nails are plates of keratin with a nail plate, root, bed, lunula and cuticle; they grow from the root.
- Mammary glands are modified sweat glands that secrete milk after childbirth; they are found only in mammals.
- Human body temperature is kept at about 37 °C by the hypothalamus, which balances heat production and heat loss.
- Vasodilation widens skin arterioles and increases heat loss when the body is hot; vasoconstriction narrows them and reduces heat loss when the body is cold.
- Other responses to heat are more sweating and hairs lying flat; other responses to cold are shivering, little sweating and goose bumps.
Questions answered
Name the three layers of the epidermis from the outside inwards.
Define the Malpighian layer and state its importance.
Why does a shallow scratch on the skin not bleed?
Name any four structures found in the dermis.
Give two functions of the subcutaneous fat.
What is melanin? Give one function.
Name the substance produced in the skin in the presence of sunlight and state why it is needed.
Give two differences between a sweat gland and a sebaceous gland.
Past board exam questions
ICSE 2019 sample paper · 1 mark
Mammary glands are modified:
- Sweat glands
- Gastric glands
- Tear glands
- Sebaceous glands