Introduction:
The skin is the body’s largest organ, made up of water, proteins, fats, and minerals. It protects the body from germs, regulates body temperature, and contains sensory nerves that allow us to feel sensations like heat and cold. The skin, along with hair, nails, oil glands, and sweat glands, forms the integumentary system, which serves as the body’s outer covering.
Layers of the Skin:
The skin is composed of three main layers:
- Epidermis – the outermost layer
- Dermis – the middle layer
- Hypodermis – the innermost fatty layer
1) Epidermis:
The epidermis is made up of stratified epithelium. Its thickness varies depending on the body part — thicker on the soles and palms. It contains no blood vessels; instead, it receives oxygen and nutrients from the interstitial fluid of the dermis. The stratum corneum (outermost layer) constantly sheds and is replaced by deeper layers approximately every 40–45 days.
Skin color depends on oxygen levels in the blood, bile pigments, and the amount of melanin produced by melanocytes in the basal layer. Fingerprints arise from ridges formed by dermal papillae beneath the epidermis.
The epidermis has several distinct layers:
- Stratum corneum – upper layer of flat, keratinized cells without nuclei
- Stratum lucidum – translucent layer with distinct nuclei
- Stratum granulosum – contains fine granules
- Stratum spinosum – prickle cell layer with thorn-like processes
- Stratum basale – basal cell layer responsible for cell division and new cell formation
The top three layers form the horny zone, while the bottom two make up the terminative zone.
2) Dermis:
The dermis lies beneath the epidermis and is made up of elastic and collagen fibers, providing strength and flexibility. Overstretching of the dermis can rupture elastic fibers (as seen during pregnancy), and aging reduces its water-binding ability, leading to wrinkles.
The dermis contains several structures, including:
- Lymph vessels and blood vessels
- Sensory nerve endings
- Sweat glands and ducts
- Hair roots and follicles
- Erector pili muscles
- Sebaceous glands
Blood Vessels:
The dermis has a network of blood vessels that nourish sweat glands, sebaceous glands, and hair follicles. The epidermis receives nutrients and oxygen via interstitial fluid from these vessels. Sensory nerve endings in the dermis detect touch, pressure, temperature, and pain.
Sweat Glands:
Sweat glands are coiled tubular glands found throughout the skin, especially on the palms, soles, groin, and axillae. They help regulate body temperature by releasing sweat, which cools the body through evaporation. Excessive sweating can lead to dehydration and sodium loss, which can be corrected by water and salt intake. Sweat decomposition by bacteria in the axilla causes body odor.
Hair Follicle and Hair Structure:
The hair follicle is a downward growth of epidermal cells, with the lower widened part forming the hair bulb. Hair color is determined by melanin content. Each follicle is attached to an involuntary muscle called the arrector pili, which contracts under sympathetic stimulation, making hair stand erect (“goosebumps”).
Sebaceous Glands:
Sebaceous glands are found throughout the skin except on the palms and soles. They secrete sebum, an oily substance that lubricates and waterproofs the skin and hair, keeping them soft and preventing bacterial and fungal infections.
3) Hypodermis:
The hypodermis or subcutaneous layer is composed mainly of adipose tissue (fat). It acts as an insulator against heat and cold, cushions internal organs and muscles, and stores energy. The thickness of this layer varies depending on the body region (e.g., thinner near the eyes).
Functions of the Skin:
The skin serves as a protective barrier against mechanical injury, pathogens, and harmful chemicals. It prevents dehydration and contains sensory receptors that alert the body to environmental stimuli.
1) Temperature Regulation:
The normal body temperature is about 36.5°C. The hypothalamus acts as the body’s thermostat, regulating heat through dilation or constriction of dermal blood vessels. Sweat glands also play a role in thermoregulation by releasing sweat for evaporative cooling.
- In hot conditions – blood vessels dilate, and sweat production increases to release heat.
- In cold conditions – blood vessels constrict, and sweat production decreases to conserve heat.
2) Blood Reservoir:
The dermis contains 8–10% of the body’s total blood flow at rest, allowing the skin to function as a blood reservoir.
3) Protection:
The skin protects the body through several mechanisms:
- Keratin prevents entry of microbes and harmful chemicals.
- Sebum keeps skin supple and kills surface bacteria.
- Melanin protects against ultraviolet (UV) radiation.
- Lipids prevent dehydration and limit water absorption.
4) Cutaneous Sensation:
Cutaneous sensations include touch, pressure, vibration, temperature, and pain. Specialized receptors in the skin, such as tactile discs, corpuscles, and hair root plexuses, detect these stimuli and transmit signals to the brain.
5) Excretion and Absorption:
The skin assists in excretion of waste products like water, salts, urea, and ammonia through sweat. It also aids in limited absorption of lipid-soluble substances, including vitamins (A, D, E, and K), gases, and some medications (like corticosteroid creams). Certain toxins and heavy metals can also be absorbed through the skin.
6) Synthesis of Vitamin D:
When exposed to sunlight, the skin converts a precursor molecule into vitamin D. The liver and kidneys further process it into calcitriol, the active form of vitamin D, which aids in calcium absorption from the diet.
Detailed Notes:
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