The skin, hair, nails and glands
The integumentary system is the skin together with its accessory organs, the hair, the nails and the skin glands. The skin is built in layers, its colour can point to disease, and the hair, nails and glands each have their own functions.
The skin and what it does
The integumentary system includes the skin, also called the integument, and the accessory structures of hair, nails and skin glands. Skin is the principal organ of the integumentary system. Skin is called a cutaneous membrane because it is one of the membranes, a group of simple but functionally important sheet-like organs. The skin functions as the first line of defence against infection by microbes, protects underlying tissue from the sun's ultraviolet rays and from harmful chemicals, is involved in temperature regulation, and functions as an enormous sense organ.
The layers of the skin
The skin is composed of two primary layers, the epidermis and the dermis, as well as a small subcutaneous layer. The epidermis is the outermost, nonvascular layer of the skin. It comprises epithelial cells filled with keratin, a tough, waterproof protein that provides a heavy, abrasion-resistant and protective quality. The outermost layer of the epithelium consists of dead cells. Millions of epithelial cells are reproduced daily to replace the millions shed onto clothes, bed sheets and the things touched, and into bathwater. Mitoses in the stratum germinativum, or basal layer, replace the superficial cells lost at the surface of the epidermis, in the corneal layer, also called the stratum corneum. The thickness and contour of the surface vary with the location in the body, as between the eyelids and the palms of the hands, and with the environmental stresses encountered, such as tight shoes.
The dermis, the second and innermost layer of the skin, lies under the epidermis, is deeper and thicker than the epidermis, and is composed of connective tissue. It contains blood vessels, lymphatic vessels, hair follicles, sweat glands, sebaceous glands and sensory nerves. The dermis supports and nourishes the overlying epidermis. Ridges and grooves in the dermis form the fingerprints, patterns unique to each individual.
Skin colour
In general, normal skin colour is based on the amount of melanin in the skin. Melanin is a substance produced by melanocytes that gives dark pigment to the skin. Skin described as white has little melanin, allowing the pink of the blood vessels to show, whereas skin described as black has melanin in all layers, masking the blood vessels. Skin described as yellow contains a large amount of carotene, the yellow colouring of carrots and pumpkins, in the fatty dermis. Carotene is a vitamin precursor that is naturally produced in orange vegetables and fruits and is stored in dermis fat.
Skin colour can be a useful diagnostic tool. Abnormal skin colour may indicate a specific condition or disease. Cyanosis is the bluish colour of the skin and mucous membranes resulting from deficient oxygenation of the blood. Cyanosis can mean that the body's cells are not receiving enough oxygen because of heart or lung problems.
Hair, nails and glands
Hair, nails and glands are the accessory organs of the integumentary system. Keratin is the principal constituent of hair and nails. Hair is formed from cells filled with keratin. Humans are born with all the hair follicles they will ever have. The follicles are formed during the embryonic period and, if destroyed, cannot be replaced. Hair protects: hair on the head cushions against blows, and hair in the nose keeps out dust and insects. Hair also acts as a sensory organ, as when an insect crawls on the arm. A tiny smooth, involuntary muscle, the arrector pili, attaches to a dermal papilla above and to the side of a hair follicle. The arrector pili muscles contract when a person is cold or frightened and produce goose bumps. Nails are produced by cells in the epidermis. Like hair, they are filled with keratin and function as protection.
Several types of gland are found in the integumentary system. Sebaceous glands produce a lipid mixture called sebum, which is discharged into hair follicles or directly onto the surface of the skin at sebaceous follicles. Sebum is composed of fat and epithelial debris. Sebum lubricates the epidermis to keep it intact as a protection from microbes. Apocrine sweat glands produce an odorous secretion in response to neural and hormonal stimuli. They are mainly present in the armpits and around the genital area, are the main cause of sweat odour, and produce sweat that contains fatty materials. Exocrine sweat glands produce a watery secretion that is important in the regulation of body temperature. They are distributed over the entire body surface but are particularly abundant on the palms of the hands, the soles of the feet and the forehead, and their sweat is composed chiefly of water with various salts. Sweat may be an important route for the excretion not only of water but also of electrolytes, wastes and foreign compounds. Ceruminous glands of the ear produce cerumen, also called earwax, which prevents dehydration of the ear canal and keeps out insects and foreign materials.