What the skin does
The integumentary system consists of the skin and its accessory structures: hair, nails and glands. Its functions are protection against water loss, germs and ultraviolet light; temperature regulation; sensation; excretion of small amounts of wastes in sweat; and the production of vitamin D when skin is exposed to ultraviolet radiation.
Three layers
- Epidermis: the outer layer, made of stratified squamous epithelium. It has no blood vessels.
- Dermis: a thick connective tissue layer. The upper papillary layer is loose and holds capillaries and touch receptors. The deeper reticular layer is dense irregular connective tissue, giving strength and stretch.
- Hypodermis (subcutaneous layer): mostly adipose tissue that insulates, cushions and anchors the skin. Strictly it lies beneath the skin rather than being part of it.
Layers of the epidermis
From deepest to most superficial: stratum basale (cells divide here), stratum spinosum, stratum granulosum, stratum lucidum (only in thick skin such as palms and soles) and stratum corneum (dead, flattened, keratin-filled cells that are shed). New cells are produced at the bottom and are pushed upward, dying as they go.
Epidermal cells
- Keratinocytes: the majority; they make the tough protein keratin.
- Melanocytes: produce the pigment melanin, which absorbs ultraviolet light and protects DNA.
- Langerhans cells: immune cells that detect invaders.
- Merkel cells: associated with touch sensation.
Accessory structures
Sebaceous glands release oily sebum into hair follicles. Eccrine sweat glands open directly onto the skin and cool the body as sweat evaporates. Apocrine sweat glands open into follicles in areas such as the armpit. Hairs grow from follicles, and small arrector pili muscles pull them upright.
Worked example: estimating surface area with the rule of nines
Clinicians estimate the proportion of body surface in adults by dividing it into regions of about 9 percent. In this simplified adult model: head and neck 9 percent, each whole arm 9 percent, front of the trunk 18 percent, back of the trunk 18 percent, each leg 18 percent, and the perineum 1 percent. These add to 100 percent.
Suppose we want the surface represented by the head, one whole arm and the front of the trunk.
- Head and neck: 9 percent.
- One whole arm: 9 percent.
- Front of the trunk: 18 percent.
- Sum: 9 + 9 + 18 = 36 percent of total body surface.
This is only a rough anatomical estimate; children have larger heads in proportion and use different tables. The calculation illustrates how the skin is the body's largest organ in surface area, and why a method that splits it into simple fractions is useful for quick comparison.