brasheranatomyintegument

THE INTEGUMENTARY SYSTEM

BODY MEMBRANES The membranes of the body are structures which cover surfaces, line body cavities and form protective sheets around organs. They are divided into two categories: //Mucous (mucosa)--//These membranes line all body cavities that open to the outside, including the digestive, respiratory, urinary, and reproductive tracts. Most produce mucus, a thick secretion which is lubricating and protective. The epithelial cells making up these membranes are usually thick and protective.
 * //Epithelial membranes
 * Cutaneous--**//skin
 * //Serous//--These membranes line body cavities that do not open to the outside. They consist of very thin epithelial cells, which are //not// very protective, but secrete a thin, serous fluid that helps to reduce friction between organs. These membranes exist in pairs. The //parietal// membrane lines the wall of the body cavity, while the //visceral// membrane lies on the surface of the organ itself.
 * //Connective Tissue Membranes//--Also called //synovial membranes,// these structures line the 'capsules' surrounding joints and secrete a lubricating fluid which reduces friction in the joints. They often form fluid-filled sacs //called bursae.//

THE SKIN

The skin serves as a protective covering for our bodies. It keeps certain things, such as bacteria and excess water, from entering the tissues underneath it. The skin also keeps in water and other molecules that are necessary to keep our body functioning. Mechanical damage, chemical damage, thermal damage, and ultraviolet radiation damage are felt by the skin first, therefore protecting delicate tissues that lie beneath it. Other functions of the skin include its role as a mini-excretory system, regulation of heat loss, synthesis of vitamin D, and serving as a monitor of external conditions. The skin consists of two layers: the epithelial layer called the //epidermis// and a layer of dense connective tissue called the //dermis.// A layer of adipose (fat) tissue called the //hypodermis// attaches the skin to tissues that lie underneath it. The hypodermis also serves to insulate the body. The epidermis consists of a layer of stratified squamous epithelium, where the outermost cells are very flat. Found within the epidermis are many //keratinocytes,// which produce a hard and waterproofing protein called //keratin.// Because there are no blood vessels in the epidermis, the outermost cells are dead and are rubbed off very easily. Millions of new cells are produced daily and completely replace the epidermis every 25-45 days. The epidermis is thickest on the palms of the hands and soles of the feet. Exposure to sunlight enhances the performance of special cells found in the epidermis, called //melanocytes.// These cells produce a protective pigment called //melanin,// which causes tanning. Freckles and moles are concentrated areas of melanin. Specialized structures within the skin come from cells of the epidermis. Millions of hairs arise from //hair follicles.// The hair itself consists of dead protein material. The shape of the follicle determines the characteristics of the hair: an oval follicle produces wavy hair, a flat follicle produces curly hair, and a round follicle produces straight hair. A tiny muscle attached to the follicle, called the //arrector pili muscle,// causes the hair to stand on end (goose bumps) when it contracts. Several glands arise from the epidermis. All of these open onto the surface of the skin, so are called //exocrine glands. Sebaceous glands,// which produce an oily substance called //sebum,// usually open into a hair follicle that will allow the sebum to empty onto the skin's surface. The sebum is lubricating and also kills bacteria. These glands are typically very active during adolescence. There are also two types of sweat glands found in the skin. //Eccrine glands// produce a watery sweat when you are overheated. //Apocrine glands// are concentrated in the armpits and genital areas, and become active at puberty. The sweat itself is odorless, but bacteria that feed on it cause an unpleasant smell. The nails are heavily keratinized extensions of the epidermis. The innermost layer of the skin is the dermis. Leather is made from the dermis of animal hides. The dermis is highly vascularized, which means that it contains many blood vessels. Projections of the dermis form your fingerprints. Many sensory receptors are found in the dermis. Elastic fibers found in the dermis keep it stretchy when we are young. The skin can develop more than 1000 different problems. Some of the most common are: The skin is often damaged by burns. Because many cells are often destroyed, burns can be life-threatening. Fluid is lost and dehydration often follows. After about 24 hours, infection becomes a major threat and often causes death. First degree burns only involve the epidermis and cause redness. Sunburns are usually 1st degree burns. Second degree burns cause damage to the epidermis and the upper dermis. Blistering occurs, but usually no scarring. Third degree burns occur when the entire thickness of the skin is destroyed. Because nerve endings in the skin are destroyed, no pain is felt. Skin cancers are tumors that form in the skin. Typically, they are benign and do not spread. Basal cell carcinoma is the most common and least dangerous type. Malignant melanoma is often deadly. When a baby is born, its skin is covered with a white, oily substance called the //vernix caseosa//. This layer protects the baby's skin while it is covered in amniotic fluid during development. During adolescence, acne is a common problem. The skin is healthiest when we are in our 20s and 30s. As we get older, the amount of fat underneath the skin decreases, the skin becomes drier, thins, becomes less elastic, and forms wrinkles.
 * Decubitus ulcers (bedsores) form when pressure on the skin restricts blood flow.
 * Athlete's foot is a fungal infection of the feet, also called //tinea pedis.
 * //Boils and carbuncles are inflammations of the hair follicles and sebaceous glands.
 * Cold sores (fever blisters) are caused by a //herpes simplex// infection, and are often activated by emotional upset, fever, or UV radiation.
 * Contact dermatitis is an allergic reaction to chemicals which results in itching and redness.
 * Impetigo is a highly contagious bacterial infection which results in yellowish, runny sores.
 * Psoriasis causes chronic reddened patches which are covered by dry, scaly skin.