Summary
Highlights
Hair is found everywhere except palms, soles, lips, nipples, and specific parts of genitalia and fingertips/toes. Each hair arises from a hair follicle, with a hard cortex and soft medulla. Hair growth occurs in cycles (growth and resting stages). Hair color is determined by the type and amount of melanin. Erector pilli muscles cause hair to stand perpendicular to the skin, creating 'goosebumps'.
Two types of skin glands exist: sebaceous and sweat glands. Sebaceous glands produce sebum, an oily material that lubricates hair and skin and protects against some bacteria. Sweat glands are of two types: eccrine (found everywhere, especially palms/soles, producing watery sweat for thermal regulation) and apocrine (found in armpits/genitalia, active from puberty, producing odorless secretions that cause body odor when acted upon by bacteria).
Nails are thin plates made of dead stratum corneum and hard keratin. The visible part is the nail body, while the part covered by skin is the nail root. The cuticle (eponychium) is stratum corneum extending onto the nail body. The nail matrix beneath the surface continuously produces cells, leading to continuous nail growth. The lunula is the white crescent moon-shaped area at the nail's base.
The integumentary system reduces water loss, acts as a barrier against microorganisms, protects underlying structures from abrasion, absorbs UV light (melanin), and aids in vitamin D production. Hair protects from heat/cold (head hair), keeps sweat out of eyes (eyebrows), shields eyes from foreign objects (eyelashes), and prevents entry of dust (nose/ear hair). Nails protect fingertips/toes and can be used for defense.
Sensory receptors in the epidermis and dermis detect pain, heat, cold, and pressure. Hair follicles also contain receptors detecting movement. UV light exposure causes the skin to produce a precursor molecule for vitamin D, which is then converted by the liver and kidneys into its active form. Active vitamin D stimulates the small intestine to absorb calcium and phosphate, crucial for many body functions.
The body regulates temperature to maintain homeostasis. When body temperature rises (e.g., exercise, fever, high environmental temperature), dermal blood vessels dilate to dissipate heat, and sweat glands produce sweat to cool the body. When temperature drops, dermal blood vessels constrict to conserve heat by reducing blood flow to the skin.
Though not a primary function, the skin excretes small amounts of waste products like urea, uric acid, and ammonia through sweat. The skin is useful for disease diagnosis; cyanosis (bluish skin) indicates decreased blood oxygen, jaundice (yellowish skin) suggests liver problems, and rashes/lesions can signal underlying systemic issues or infections.
Burns are classified by depth: first-degree (superficial, involves epidermis), second-degree (partial thickness, involves epidermis and dermis), and third-degree (full thickness, complete damage to epidermis and dermis). Second-degree burns can regenerate from hair follicles and sweat glands. Deep partial thickness and full thickness burns take longer to heal, may form scar tissue, and can be disfiguring/debilitating.
As skin ages, blood flow decreases, the skin thins due to reduced collagen, and sebaceous and sweat gland activity decreases. Loss of elastic fibers leads to skin sagging and wrinkling.
The integumentary system comprises the skin and accessory structures like hair, glands, and nails. Its appearance can indicate physiological imbalances within the body. The skin has three main layers: epidermis, dermis, and subcutaneous tissue (hypodermis), though the hypodermis is not technically part of the skin itself.
Key functions include protection against abrasion, UV light damage, and internal structures. It also provides sensation for detecting heat, cold, touch, pressure, and pain. Furthermore, it aids in vitamin D production through UV light exposure, regulates body temperature via blood flow and sweat glands, and facilitates minor excretion of waste products.
The epidermis is the most superficial skin layer, composed of keratinized stratified squamous epithelium. It consists of multiple strata: stratum corneum (topmost, dead cells filled with keratin), stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale (deepest, where mitosis occurs). The epidermis prevents water loss and resists abrasion.
Excessive shedding of the stratum corneum from the scalp results in dandruff. A callus forms when the stratum corneum thickens due to constant friction. A corn is a type of callus that forms over bony prominences and is cone-shaped.
The dermis is made of dense collagenous connective tissue, containing fibroblasts, adipocytes, macrophages, nerves, hair follicles, and glands. Collagen and elastic fibers provide structural strength and resistance to stretch. Cleavage lines, or tension lines, are areas where collagen fibers are more resistant to stretching, important for surgical incisions to minimize scarring. Dermal papillae are projections that create fingerprints and footprints.
Skin color is determined by pigments (melanin and carotene), blood circulation, and stratum corneum thickness. Melanin, produced by melanocytes, is responsible for skin, hair, and eye color and protects against UV light. Melanin molecules vary in color (brown, black, yellowish, reddish). Carotene, a yellow pigment from plants, can accumulate in the skin, giving a yellowish tint. All races have similar numbers of melanocytes; variations in skin color depend on the amount, kind, and distribution of melanin.
A genetic mutation preventing melanin production causes albinism. Excessive UV light exposure causes skin cancer, with fair-skinned individuals being more susceptible. UVA causes tanning and malignant melanoma, while UVB causes sunburns. Three main types are basal cell carcinoma, squamous cell carcinoma, and malignant melanoma (rare but fatal). The ABCDE rule (Asymmetry, Border, Color, Diameter, Evolving) helps identify skin cancer.
The subcutaneous tissue, or hypodermis, is beneath the dermis but not part of the skin. It attaches the skin to underlying bones and muscles, blood vessels, and nerves. Composed mainly of adipose tissue, it provides padding and insulation.