Introduction

The skin, together with its associated structures such as hair, nails, and glands, forms the Integumentary System. It is the largest organ system of the body and plays a vital role in protection, regulation, and sensation. For NHPC students, understanding the structure and functions of the skin is essential because many diseases, burns, infections, and injuries involve this system.

The skin is the first line of defense between the body and the outside world. It helps regulate temperature, prevents water loss, allows sensation, and even helps produce Vitamin D. On average, an adult’s skin covers about 1.5 to 2 square meters and weighs nearly 15% of total body weight.

[Diagram Placeholder: Cross-section of skin showing epidermis, dermis, and hypodermis]

Structure of the Skin

The skin has three main layers — each with special structures and functions:

1. Epidermis (outer layer)

2. Dermis (middle layer)

3. Hypodermis (subcutaneous layer)

1. Epidermis

The epidermis is the outermost layer and is made up of stratified squamous epithelial cells. It provides a waterproof, protective covering and is responsible for skin color. The epidermis does not have blood vessels; instead, it receives nutrients by diffusion from the dermis below.

Cells in the epidermis include:- Keratinocytes: Most abundant cells that produce keratin, a tough protein that protects and waterproofs the skin.- Melanocytes: Produce melanin, the pigment that gives skin its color and protects against UV rays.- Langerhans cells: Immune cells that fight infections.- Merkel cells: Specialized cells that detect touch and pressure.

Layers of the Epidermis (from deep to surface):1. Stratum basale (germinativum): Contains dividing cells that form new skin cells. Also contains melanocytes.2. Stratum spinosum: Several layers of keratinocytes joined by desmosomes (“spiny” appearance).3. Stratum granulosum: Cells begin to flatten and die; keratin formation starts here.4. Stratum lucidum: Found only in thick skin (palms, soles).5. Stratum corneum: The outermost layer of dead, keratin-filled cells that are continuously shed and replaced.

Did You Know? The epidermis completely renews itself approximately every 27 days!

2. Dermis

The dermis is a thicker layer made of connective tissue. It provides strength, elasticity, and nourishment to the skin. It contains blood vessels, lymph vessels, nerve endings, sweat glands, sebaceous glands, and hair follicles.

The dermis has two layers:- Papillary Layer: The upper layer with finger-like projections called dermal papillae (responsible for fingerprints).- Reticular Layer: The deeper layer made of dense connective tissue containing collagen and elastin fibers that provide strength and flexibility.

Exam Focus: NHPC questions often ask about which skin layer contains sweat glands, blood vessels, and nerve endings – the answer is the dermis.

3. Hypodermis (Subcutaneous Tissue)

The hypodermis lies beneath the dermis and is composed mainly of fat and connective tissue. It connects the skin to underlying muscles and organs, cushions the body, stores energy, and helps maintain body temperature.

Clinical Insight: The hypodermis is the layer where most injections (like insulin) are given because it has good blood supply and absorbs medicines slowly.

Accessory Structures of the Skin

The integumentary system also includes hair, nails, and glands. These structures originate from the epidermis but perform additional functions.

1. Hair

Hair protects the scalp, eyes, and body from external injury and helps in temperature control. Each hair grows from a hair follicle located in the dermis.

Parts of a hair:- Shaft: Visible part above the skin.- Root: Below the surface, extends into the dermis.- Bulb: Base where new cells are produced.- Arrector pili muscle: Small muscle attached to each follicle that causes “goosebumps.”

2. Nails

Nails are made of hard keratin and protect the tips of fingers and toes. They also help us grasp objects and provide support for touch sensation.

Parts of a nail include:- Nail plate: Visible hard part.- Nail bed: Skin under the nail.- Matrix: Area of active cell division that produces new nail cells.

3. Glands of the Skin

The skin contains several types of glands:1. Sebaceous glands: Secrete oily sebum that keeps the skin and hair soft and waterproof.2. Eccrine sweat glands: Found all over the body; produce watery sweat for cooling.3. Apocrine sweat glands: Found in armpits and groin; become active during puberty and can produce body odor.4. Ceruminous glands: Found in the ear canal; produce ear wax (cerumen) that protects against dust and bacteria.5. Mammary glands: Modified sweat glands that produce milk in females.

Functions of the Skin

The skin performs several essential functions that help maintain health and homeostasis:1. Protection: Shields the body from injury, chemicals, UV radiation, and microbes.2. Sensation: Detects touch, temperature, pain, and pressure through sensory receptors.3. Temperature Regulation: Sweat and blood vessel dilation help cool the body; vasoconstriction retains heat.4. Excretion: Sweat removes salts and small amounts of waste.5. Vitamin D Production: UV light helps synthesize Vitamin D, important for calcium absorption.6. Immunity: Langerhans cells provide local immune defense.7. Storage and Insulation: Fat in the hypodermis stores energy and insulates the body.

[Diagram Placeholder: Major functions of skin – protection, sensation, temperature control, excretion, Vitamin D production]

Clinical Importance

Common disorders of the skin include:- Acne: Inflammation of sebaceous glands due to excess sebum and bacteria.- Dermatitis: Skin inflammation caused by allergies or irritants.- Burns: Damage to skin layers; classified as first-, second-, or third-degree.- Vitiligo: Loss of pigment due to destruction of melanocytes.- Skin Cancer: Uncontrolled growth of abnormal skin cells, often due to UV exposure.

Did You Know? The skin can show signs of internal diseases — for example, yellow skin (jaundice) indicates liver problems.

Exam Focus: Know the difference between thick skin (palms, soles) and thin skin (rest of the body). Thick skin has an extra layer – the stratum lucidum.

Summary Table

| Layer | Composition | Main Functions ||--------|--------------|----------------|| Epidermis | Keratinized epithelium | Protection, waterproofing || Dermis | Connective tissue with nerves, vessels, glands | Strength, flexibility, sensation || Hypodermis | Fat and connective tissue | Insulation, energy storage |