Integumentary System

The skin is the largest organ in the body: 12-15% of body weight, with a surface area of 1-2
meters. Skin is continuous with, but structurally distinct from mucous membranes that line the
mouth, anus, urethra, and vagina. Two distinct layers occur in the skin: the dermis and epidermis.
The basic cell type of the epidermis is the keratinocyte, which contain keratin, a fibrous protein.
Basal cells are the innermost layer of the epidermis. Melanocytes produce the pigment melanin, and
are also in the inner layer of the epidermis. The dermis is a connective tissue layer under the
epidermis, and contains nerve endings, sensory receptors, capillaries, and elastic fibers.

The integumentary system has multiple roles in homeostasis, including protection, temperature
regulation, sensory reception, biochemical synthesis, and absorption. All body systems work in an
interconnected manner to maintain the internal conditions essential to the function of the body.

Follicles and Glands

Hair follicles are lined with cells that synthesize the proteins that form hair. A sebaceous gland
(that secretes the oily coating of the hair shaft), capillary bed, nerve ending, and small muscle
are associated with each hair follicle. If the sebaceous glands becomes plugged and infected, it
becomes a skin blemish (or pimple). The sweat glands open to the surface through the skin pores.
Eccrine glands are a type of sweat gland linked to the sympathetic nervous system; they occur all
over the body. Apocrine glands are the other type of sweat gland, and are larger and occur in the
armpits and groin areas; these produce a solution that bacteria act upon to produce “body odor”.

Hair and Nails

Hair, scales, feathers, claws, horns, and nails are animal structures derived from skin. The hair
shaft extends above the skin surface, the hair root extends from the surface to the base or hair
bulb. Genetics controls several features of hair: baldness, color, texture.

Nails consist of highly keratinized, modified epidermal cells. The nail arises from the nail bed,
which is thickened to form a lunula (or little moon). Cells forming the nail bed are linked
together to form the nail.

Skin and Homeostasis

Skin functions in homeostasis include protection, regulation of body temperature, sensory
reception, water balance, synthesis of vitamins and hormones, and absorption of materials. The
skin’s primary functions are to serve as a barrier to the entry of microbes and viruses, and to
prevent water and extracellular fluid loss. Acidic secretions from skin glands also retard the
growth of fungi. Melanocytes form a second barrier: protection from the damaging effects of
ultraviolet radiation. When a microbe penetrates the skin (or when the skin is breached by a cut)
the inflammatory response occurs.

Heat and cold receptors are located in the skin. When the body temperature rises, the hypothalamus
sends a nerve signal to the sweat-producing skin glands, causing them to release about 1-2 liters
of water per hour, cooling the body. The hypothalamus also causes dilation of the blood vessels of
the skin, allowing more blood to flow into those areas, causing heat to be conducted away from the
skin surface. When body temperature falls, the sweat glands constrict and sweat production
decreases. If the body temperature continues to fall, the body will engage in thermogenesis, or
heat generation, by raising the body’s metabolic rate and by shivering.

Water loss occurs in the skin by two routes.
 1. evaporation
 2. sweating

In hot weather up to 4 liters per hour can be lost by these mechanisms. Skin damaged by burns is
less effective at preventing fluid loss, often resulting in a possibly life threatening problem if
not treated.

Skin and Sensory Reception

Sensory receptors in the skin include those for pain, pressure (touch), and temperature. Deeper
within the skin are Meissner’s corpuscles, which are especially common in the tips of the fingers
and lips, and are very sensitive to touch. Pacinian corpuscles respond to pressure. Temperature
receptors: more cold ones than hot ones.

Skin and Synthesis

Skin cells synthesize melanin and carotenes, which give the skin its color. The skin also assists
in the synthesis of vitamin D. Children lacking sufficient vitamin D develop bone abnormalities
known as rickets.

Skin Is Selectively Permeable

The skin is selectively soluble to fat-soluble substances such as vitamins A, D, E, and K, as well
as steroid hormones such as estrogen. These substances enter the bloodstream through the capillary
networks in the skin. Patches have been used to deliver a number of therapeutic drugs in this
manner. These include estrogen, scopolamine (motion sickness), nitroglycerin (heart problems), and
nicotine (for those trying to quit smoking).