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Anatomy and Physiology Unit 7: Integumentary system

The skin is the body's largest organ: it protects, regulates temperature, senses the environment and makes vitamin D.

Covers: Layers of the skin, accessory structures (hair, nails, glands), functions of the skin, thermoregulation, burns and wound healing, skin disorders

Study guide

The integumentary system includes the skin, hair, nails and glands. The skin has two main layers. The epidermis is the outer layer of stratified squamous epithelium with no blood vessels; it is nourished by diffusion from below. Its layers, from deep to surface, are the stratum basale (where stem cells divide and melanocytes make the pigment melanin), stratum spinosum, stratum granulosum, stratum lucidum (only in thick skin of palms and soles) and stratum corneum (dead, keratin-filled cells that shed). Keratinocytes make keratin, a tough protein that waterproofs the skin. The dermis below is dense connective tissue with collagen and elastic fibers, blood vessels, nerves, hair follicles and glands. Its upper papillary layer forms fingerprints, and its deeper reticular layer gives strength. The hypodermis (subcutaneous layer) beneath the skin is mostly adipose tissue that insulates and anchors the skin.

The skin protects against microbes, water loss, UV radiation and injury. It helps regulate body temperature: when the body is hot, sweat glands release sweat that cools by evaporation and dermal blood vessels dilate to release heat; when cold, vessels constrict to save heat. The skin makes vitamin D when exposed to sunlight, which the body needs to absorb calcium. Sensory receptors in the dermis detect touch, pressure, pain and temperature. Eccrine sweat glands are widespread and help cool the body, apocrine glands in the armpits and groin become active at puberty, and sebaceous glands secrete oily sebum into hair follicles to soften skin and hair. Arrector pili muscles pull hairs upright, causing goose bumps.

Burns are classified by depth. First-degree burns affect only the epidermis (redness, pain), second-degree burns reach into the dermis (blisters), and third-degree (full-thickness) burns destroy the entire skin, often with little pain because nerve endings are destroyed. The rule of nines estimates the burned body surface area. The biggest dangers of severe burns are fluid loss and infection. Wound healing involves inflammation, formation of granulation tissue and scar tissue. Skin cancers include basal cell carcinoma (most common, least dangerous), squamous cell carcinoma and melanoma (least common but most deadly); the ABCDE rule (asymmetry, border, color, diameter, evolving) helps spot melanoma.

Key ideas

• The epidermis is avascular epithelium; the dermis holds vessels, nerves and glands.
• New epidermal cells form in the stratum basale and move up to the stratum corneum.
• Skin protects, regulates temperature, senses and makes vitamin D.
• Sweat glands cool the body; sebaceous glands oil skin and hair.
• Burns are classified by depth; fluid loss and infection are the main dangers.

Formulas and rules

Common mistakes

• Thinking the epidermis has blood vessels; it doesn't.
• Thinking third-degree burns are always the most painful; nerve endings are often destroyed.

Words to know

Integumentary system

The skin, hair, nails and glands.

Epidermis

The outer layer of the skin, made of stratified squamous epithelium.

Dermis

The connective-tissue layer of skin containing vessels, nerves and glands.

Hypodermis

The fatty layer beneath the skin that insulates and anchors it.

Stratum basale

The deepest epidermal layer, where new cells form.

Stratum corneum

The outermost layer of dead, keratin-filled cells.

Keratin

A tough protein that waterproofs and protects the skin.

Melanocyte

A cell that produces the pigment melanin.

Melanin

Pigment that colors skin and protects against UV radiation.

Eccrine sweat gland

A sweat gland that helps cool the body.

Sebaceous gland

A gland that secretes oily sebum into hair follicles.

Arrector pili

A small muscle that makes a hair stand up.

Vitamin D synthesis

Skin makes vitamin D when exposed to UV light.

First-degree burn

A burn affecting only the epidermis.

Second-degree burn

A burn reaching into the dermis, with blisters.

Third-degree burn

A full-thickness burn destroying the entire skin.

Rule of nines

Method for estimating the percent of body surface burned.

Melanoma

The most dangerous form of skin cancer.

Sample practice questions

5 more questions like these, from level 1 to 5, are in the practice set.