Human Organ Systems - Integumentary System (Skin, Hair and Nails)
The integumentary system (From Latin integumentum, from integere 'to cover'; from in- + tegere 'to cover') is the organ system that protects the body from damage, comprising the skin and its appendages (including hair, scales, feathers, and nails). The integumentary system has a variety of functions; it may serve to waterproof, cushion, and protect the deeper tissues, excrete wastes, and regulate temperature, and is the attachment site for sensory receptors to detect pain, sensation, pressure, and temperature. In humans the integumentary system also provides vitamin D synthesis.
The integumentary system is the largest organ system. In humans, this system accounts for about 16 percent of total body weight and covers 1.5-2m2 of surface area. It distinguishes, separates, protects and informs the animal with regard to its surroundings. Small-bodied invertebrates of aquatic or continually moist habitats respire using the outer layer (integument). This gas exchange system, where gases simply diffuse into and out of the interstitial fluid, is called integumentary exchange.
Skin is a soft outer covering of an animal, in particular a vertebrate. Other animal coverings such as the arthropod exoskeleton or the seashell have different developmental origin, structure and chemical composition. The adjective cutaneous literally means "of the skin" (from Latin cutis, skin). In mammals, the skin is the largest organ of the integumentary system made up of multiple layers of ectodermal tissue, and guards the underlying muscles, bones, ligaments and internal organs. Skin of a different nature exists in amphibians, reptiles, and birds. All mammals have some hair on their skin, even marine mammals which appear to be hairless. Because it interfaces with the environment, skin plays a key role in protecting (the body) against pathogens and excessive water loss. Its other functions are insulation, temperature regulation, sensation, and the protection of vitamin D folates. Severely damaged skin will try to heal by forming scar tissue. This is often discoloured and depigmented.
Hair with sufficient density is called fur. The fur mainly serves to augment the insulation the skin provides, but can also serve as a secondary sexual characteristic or as camouflage. On some animals, the skin is very hard and thick, and can be processed to create leather. Reptiles and fish have hard protective scales on their skin for protection, and birds have hard feathers, all made of tough β-keratins. Amphibian skin is not a strong barrier to passage of chemicals and is often subject to osmosis. A frog sitting in an anaesthetic solution could quickly go to sleep.
Hair is a filamentous biomaterial that grows from follicles found in the dermis. The human body, apart from its glabrous skin, is covered in follicles which produce thick terminal and fine vellus hair. Most common interest in hair is focused on hair growth, hair types and hair care, but hair is also an important biomaterial primarily composed of protein, notably keratin. There are all different shades of blonde, brunette, black, and red hair.
Found exclusively in mammals, hair is one of the defining characteristics of the mammalian class. Hair-like structures, called cilia, but which are not classified as hair, are visible in organs such as the nose and ear and these structures also occur in many other organs of mammals, other animals, and plants. Although non-mammals, especially insects, show filamentous outgrowths, these are not considered "hair" either, in the scientific sense. These so-called "hairs" (trichomes) also are found on plants. The projections on arthropods, such as insects and spiders, are classified as bristles, which are composed of a polysaccharide called chitin. There are varieties of cats, dogs, and mice bred to have little or no visible hair (fur). In some species, hair is absent at certain stages of life.
Hair often refers to two distinct structures: 1) the part beneath the skin, called the hair follicle or when pulled from the skin, called the bulb. This organ is located in the dermis and maintains stem cells which not only re-grow the hair after it falls out, but also are recruited to regrow skin after a wound; and 2) the shaft, which is the hard filamentous part that extends above the skin surface. A cross section of the hair shaft may be divided roughly into three zones. Starting from the outside: 1) the cuticle which consists of several layers of flat, thin cells laid out overlapping one another as roof shingles, 2) the cortex which contain the keratin bundles in cell structures that remain roughly rod-like and in some cases, 3) the medulla, a disorganized and open area at the fibre’s center.
A nail is a horn-like envelope covering the dorsal aspect of the terminal phalanges of fingers and toes in humans, most non-human primates, and a few other mammals. Nails are similar to claws, which are found on numerous other animals. In common usage, the word nail often refers to the nail plate only. Fingernails and toenails are made of a tough protein called keratin, as are animals' hooves and horns. Along with hair they are an appendage of the skin.
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