The skin's functions are to protect deeper tissues from: mechanical, chemical, bacterial, and thermal damage, ultraviolet radiation, and desiccation. It aids in heat regulation, aids in excretion of urea and uric acid, and synthesize vitamin D. Sweat glands, oil glands, hairs and nails all work to maintain the functions of the skin. The epidermis (outer layer) has five layers: stratum basale (where cells undergo mitosis and it lies next to the dermis), stratum spinosum, stratum granulosum, stratum lucidum (only occurs in thick skin), and stratum corneum (outermost layer and has shingle-like dead cells). The dermis is made up of dense connective tissue which has collagen fibers and it has two layers: papillary layer (dermal papillae) has pain receptors and capillary loops and reticular layer contains blood vessels, glands, and nerve receptors. Hypodermis is deep to the dermis, it's not part of the skin, it anchors to underlying organs, and it's composed mostly of adipose and areolar tissue. Sebaceous glands produce oil, which lubricate the skin and kills bacteria, also are activated at puberty. Sweat glands are widely distributed in the skin and there are two types: eccrine opens via duct to pore on skin surface and apocrine has ducts empty into hair follicles. All the glands are separated from the dermis by a layer of epidermal cells. The secretion of sebaceous glands is called sebum, it's a combination of fats, proteins, cholesterol, alcohol, and salts. It lubricates and conditions the hair and skin, it's combined with sweat, creates an acidic barrier on the skin, and it breaks down products of dead cells. The secretion of sweat glands is called sweat, it's mostly made of water, some metabolic waste, fatty acids and proteins. It helps dissipate excess heat, excretes waste products, and the acidic nature inhibits bacteria growth. Hair is produced by hair bulb and it consists of hard kerantinized epithelial cells, and the melanocytes provide pigment for hair color. The hair follicle is the dermal and epidermal sheath surround hair root. Arrector pilli is attached to the skin and it's smooth muscle. The nail is an extension of the epidermis, it is heavily kerantinized, stratum basale extends beneath the nail bed and it's responsible for growth.
The physiology of the skin is to protect, excrete, sensation, synthesize, thermoregulation, produce heat, and dissipate heat. For protection there are three forms: physical barrier is a continuous surface with all openings (mouth, anus, urethra, and vagina), and the cells contain keratin and attached to each other by desmosomes, this is called the kerantinization process. Chemical/Biological barrier is acidic secretions from glands retards growth of fungi, microbes, and viruses (dermicidin). It flushes the microbes from the surface, the melanin stops UV from penetrating to deeper tissues. Water barrier is hydrophobic barrier (carotene), and it's impacting loss and gain. For excretion there is a fluid and electrolyte balance, the sweat releases water, salts, and ammonia; and the oil glands release lipids and acids. For sensation it is basically touch, press, heat/cold, and pain. For synthesis vitamin D precursor passes through capillaries in the skin and light converts it to vitamin D. For thermoregulation there is maintenance of constant body temperature and the associated changes in glandular secretions, accessory organs, vasodilation, and vasoconstriction to produce or dissipate heat. For the production of heat it involves vasoconstriction and shivering. For dissipating heat there is the evaporation of sweat and vasodilation.
