Skin Chapter Notes | Biology Class 9 ICSE PDF Download

Introduction

The skin is far more than a simple casing to contain the body’s contents; it is one of the body’s most dynamic organs, performing a wide range of functions. Our distinctive features, such as complexion and hair patterns, are largely shaped by the skin. The texture of the skin—whether loose and wrinkled in an elderly person, firm and smooth in a healthy youth, or soft and delicate in an infant—clearly reflects age and vitality. Often called the “Jack of all trades” by some and the “master of many tasks” by others, the skin has many fascinating roles to explore. As the body’s largest organ, it makes up about 15% of an adult human’s total body weight and serves primarily as a nearly impermeable barrier.  

What is Skin?

• Skin is the outermost layer covering the entire body.
• It is a flexible, stretched layer containing various structures and glands.

Functions of the Skin

Protection:

• Shields underlying tissues from mechanical shocks.
• Prevents excessive water loss by evaporation.
• Blocks entry of harmful substances and disease-causing germs.
• Protects against harmful ultraviolet (UV) light.

Sensation:

• Acts as a sense organ for touch, pain, pressure, and heat.

Temperature Regulation:

• Conserves heat in cold weather and promotes heat loss in hot weather.

Storage of Food:

• Stores reserve food in the form of fat in special cells.

Excretion:

• Eliminates water, salts, and small amounts of urea through sweating (though not a primary excretory organ).

Synthesis of Vitamin D:

• Produces vitamin D when exposed to sunlight, though excessive exposure can cause tanning or skin cancer.

Grip Enhancement:

• Ridges and grooves on fingers and palms improve grip efficiency.

Structure of the Skin

• Skin consists of two main layers: epidermis (outer) and dermis (inner).
• It also includes derivatives like hair, nails, and glands.

The Skin Proper

Microscopically, the skin consists of two main layers: the outer epidermis and the inner dermis.

Epidermis

• Outer, thinner layer made of stratified epithelium.
• Thicker in areas like palms, soles, and heels; lacks blood vessels.

Comprises three sublayers

Cornified Layer (Stratum Corneum):

• Outermost layer of flattened, dead cells made of keratin.
• Cells are shed and replaced by cells from the malpighian layer.
• Protects against mechanical damage, bacterial infection, and water loss.

Granular Layer:

• Thin middle layer with 2-3 sublayers of flattened cells.
• Transitions into the cornified layer.

Malpighian Layer (Stratum Malpighi/Germinative Layer):

• Innermost layer with actively dividing cells.
• Produces new cells that move outward to replace worn-out cornified layer cells.
• Contains melanin pigment, determining skin color (e.g., dark in Africans, light in Europeans, intermediate in Indians).
• Melanin protects inner body parts from UV rays.

Skin Pigmentation Abnormalities:

• Leucoderma (Vitiligo): Loss of melanin in patches; cause unknown.
• Albinism: Complete loss of pigmentation across body, hair, and eyes due to genetic recessive trait; skin appears pinkish.

Dermis

• Inner, thick layer of connective tissue with elastic fibers; tough and flexible.
• Thicker in palms and soles, thinner in eyelids.
• Contains blood vessels, nerve fibers, sensory organs, hair follicles, and sweat glands.
• Outer region has papillae with blood capillaries and nerve endings for touch and pain sensation.
• Deeper parts have receptors for pressure, pain, heat, and cold.
• Subcutaneous fat layer beneath dermis acts as food reserve, heat insulator, and shock absorber.

Derivatives of the Skin

Includes hair, nails, sebaceous glands, sweat glands, and mammary glands.

Hair

• Consists of:
  • Hair Shaft: Projects from skin, extends slightly below epidermis.
  • Hair Root: Embedded in dermis, with a hair bulb at the base containing a hair papilla with blood supply.
  • Hair Follicle: Encloses hair root, aiding growth with epithelial and connective tissue sheaths.
• Hair grows by adding cells at the base, which die as they move upward.
• Hair color depends on melanin; gray hair results from air spaces when pigment is lost.
• Piloerection ("Goose Flesh"): Erector muscle contracts, lifting hair and causing skin to wrinkle, often in cold or emotional states.
• Hair regrows continuously; scalp hair lasts 2-5 years, eyebrows/eyelashes 3-5 months.
• Hair provides touch sensation via nerve fibers at the base.
• Eyelashes and nasal hairs prevent entry of particles and dust.
• Facial hair (moustache, beard) in males aids sexual dimorphism.

Nails

Hard, keratinized plate-like structures growing from the nail root.

Parts:

• Plate: Hard, outer part of dead keratinized cells.
• Bed (Root): Lies beneath the plate.
• Matrix: Below skin at nail base, visible as a whitish half-moon, produces new cells for nail growth.

Sebaceous Glands

• Branched glands opening into hair follicles or directly to skin surface.
• Secrete sebum (oily substance) to keep hair and skin waterproof and supple.
• Sebum production decreases in cold/dry weather (causing rough skin) and increases in hot/humid weather (causing oily skin).
• Common issues:
  • Pimples: Sebum accumulation fosters bacterial growth, leading to boils and pimples.
  • Acne: Inflamed glands due to hormonal changes, common in adolescence.
  • Blackheads: Enlarged glands with oxidized sebum and melanin, appearing black.

Sweat Glands

• Simple coiled tubes with a secretory part and an excretory duct opening at sweat pores.
• Approximately 2 million sweat glands in the body; more in hotter climates (60-80 per cm² on back, 400 per cm² on palms).
• Palms have more sweat pores for better grip due to surface tension.
• Sweat (99% water, 0.2-0.5% salts, 0.08% urea) removes heat via evaporation.
• Sweating occurs constantly in small amounts (incipient perspiration) or heavily during exercise or hot weather (up to 1 kg/hour).
• "Cold sweat" may occur due to fright, nervousness, nausea, or severe pain.

Mammary Glands

• Modified sweat glands, present in both males (rudimentary) and females.
• In females, enlarge at puberty into breasts with a central nipple.
• Each breast has 15-20 milk ducts connected to mammary gland lobes, activated by prolactin and pregnancy.
• Milk is highly nutritious for newborns.
• Meibomian Glands: Modified sebaceous glands in eyelids, secrete oil to lubricate and prevent tear overflow.
• Ceruminous Glands: Modified sebaceous glands in the auditory canal, secrete earwax to protect the eardrum from dust and germs.

Skin and Heat Regulation of Body

• Mammals (including humans) and birds are warm-blooded (endothermal), maintaining constant body temperature (around 37°C in humans).
• Body temperature varies slightly: 1°C higher in rectum, 1°C lower in armpits, and 0.3-0.5°C daily fluctuation (lowest in early morning, highest in late afternoon).
• Optimal temperature (35-40°C) is crucial for enzyme function; fever or low temperature can disrupt bodily functions.

Heat Production

Heat is generated by:

• Chemical reactions (e.g., glucose oxidation in liver).
• Muscle activity (vigorous activity warms the body).
• Ingestion of hot foods and beverages.

Heat Loss

Heat is lost through:

• Skin: 85% of heat loss via convection, conduction, radiation, and sweat evaporation.
• Lungs: Heat lost in exhaled warm air and water vaporization.
• Urine and Faeces: Eliminated at body temperature.
• Cold Foods/Beverages: Absorb body heat when consumed.

Temperature Regulation

Hypothalamus in the forebrain acts as the body’s thermostat.

In Cold Weather:

• Vasoconstriction narrows blood vessels, reducing blood flow to skin, minimizing heat loss.
• Less sweat is produced due to reduced blood supply.
• Heat production increases via higher metabolism and shivering.
• Causes pale or bluish skin appearance.

In Hot Weather:

• Vasodilation widens blood vessels, increasing blood flow to skin for heat loss via radiation and sweat evaporation.
• Fans enhance evaporation, cooling the body.
• Heatstroke occurs when sweat production cannot match evaporation, raising body temperature (prevented by drinking water and consuming salt).

Points To Remember

• Skin functions include protection, sensation, temperature regulation, food storage, excretion, and vitamin D synthesis.
• Epidermis has three layers: cornified (protective dead cells), granular (transitional), and malpighian (cell-producing).
• Dermis contains elastic fibers, sensory cells, blood vessels, and glands.
• Subcutaneous fat layer stores food, insulates heat, and absorbs shocks.
• Skin derivatives: hair, nails, sebaceous glands, sweat glands, and mammary glands.
• Temperature regulation involves hair, fat, vasoconstriction (cold), and vasodilation/sweat (hot).