The skin and its accessory structures (glands, nails, and hair) constitute the integumentary system.
The skin is an organ because it consists of several kinds of tissues that are structurally arranged to function together. It covers approximately 3000 sq. in, making it the largest organ. It is of variable thickness, averaging 1.5 mm. It is thickest on area of high friction, such as the soles and palms, where it is 6 mm thick.
It is thinnest on the eyelids, external genitalia, and tympanic membrane, where it is .5 mm thick. It variable texture as well: it can be rough and callous, such as on the elbows and knees; it can be soft and sensitive, such as on the genitalia.
It has three main layers, with the outermost being the epidermis, the middle one being the dermis, and the innermost one being the hypodermis aka subcutaneous layer.
Integumentary system cells
|Keratinocytes||Keratinocytes produce keratin, which toughens and waterproofs the skin.|
Integumentary system structures
Integumentary system functions
|Protection||The skin is a barrier to microorganisms, water, and excessive sunlight. Oily secretions onto skin surface form an acidic protective film (pH 4.0 - 6.8) that waterproofs body and retards growth of most pathogens. Keratin also waterproofs the skin, and cornified outer layer resusts scraping and keeps out microbes.|
|Hydroregulation||The thickened, keratinized, and cornified epidermis of the skin is adapted to continuous exposure to the air. The outer-layers are dead and scalelike, and a protein-polysaccharide basement mebrane adheres the stratum basale to the dermis. Human skin is virtually waterproof, protecting the body from desiccation on dry land and from absorption when immersed in water.|
|Thermoregulation||Skin regulates body temperature. Excessive heat loss triggers shivering in muscles, and tiny smooth muscles called arrectores pilorum (which are attached to hair) contract involuntarily to cause goose bumps. Overheating is prevented by:
|Absorption||Some gases, such as O2 and CO2, can pass through the skin and into the blood very easily. This is called cutaneous absorption. Small amounts of UV, necessary for Vitamin D synthesis, can also infiltrate the skin. Unfortunately, though, lipid-soluble toxins and pesticides can also enter easily.|
|Synthesis||The integument syntehsizes melanin, keratin, and Vitamin D.|
|Sense||Sesnsory receptors in the dermis are called cutaneous receptors. They are especially abundant in the face, palms, fingers, soles and genitalia. The thinner the skin, the greater the sensitivity.|
|Communication||Contraction of facial muscles, and blushing are ways that emotions can be communicated through the skin. Also, certain integumentary glands have odors that elicit subconscious responses.|
Features of human integument
Congenital lines are fingerprints (friction ridges). They were formed by the pull of eleastic fibers within dermis, and are present on palms and soles. The four basic combinations are: arch, whorl, loop, and combination.
Acquired lines include deep flexion creases on palms and flexion lines that are seen on surface of joints (such as knuckles).
There are 3 important pigments which give the skin color and protection:
- Melanin is a brown-black pigment produce by melanocutes of stratum basale. It protects against the ultraviolet (UV) rays of the sun.
- Carotene is a yellowish pigment found in certain plants, such as carrots. It accumulates in the cells of the stratum corneum and fatty parts of the dermis. It is not the cause of the yellow-tan skin of Asian people; melanin variants are.
- Hemoglobin in oxygenated blood flowing through the dermis gives the skin pinkish tones.
|Albinism||Albinos a normal number of melanocytes, but are unable to produce the enzyme tyrosinase to convert tyrosine to melanine. Albinism is a hereditary recessive condition.|
|Freckles and vitiligo||Freckles are caused by aggregated patches of melanin. A lack of melanocytes causese white patches called vitiligo.|
|Sebborheic hyperkeratoses||Seborrheic hyperkeratoses are brown plaquelike growths commonly called liver spots. They are benign growths of melanocytes.|