1. Skin, Hair, & Nails

2. Skin: The skin, which makes up about 15% of your total body weight, is the largest organ of the body. Many specialized structures are found in the skin, which along with the hair and nails, forms the integumentary system.

3. Skin: The skin protects the body from injury, provides the first line of defense against, helps regulate body temperature, and prevents the body from drying out through evaporation. The skin is made mostly of connective tissue and layers of epithelial tissue. The two primary layers of skin are epidermis and the dermis.

4. Epidermis The epidermis is the outermost layer of the skin. About as this thick as a sheet of paper, the epidermis is made of several layers of epithelial cells. The part of the epidermis you see when you look in a mirror is a thin layer of flattened, dead cells that contain keratin.

5. Epidermis Keratin is a protein that makes skin tough and waterproof. The cells of the epidermis are continuously damaged by the environment. They are scraped, ripped, worn away by friction, and dried out because of moisture loss. Your body deals with this damage not by repairing cells, but by replacing them.

6. The outermost cells of the skin are continuously shed and replaced by a layer of actively dividing cells at the base of the epidermis. As new skin cells form, they migrate upward and produce large amounts of keratin. These cells are shed about a month after they reach the surface.

7. The inner layer of the epidermis also contains cells that produce the pigment melanin. Melanin ranges in color from yellow to reddish brown to black, and it helps to determine skin color. People with more melanin tend to have darker skin, and people with less melanin usually have lighter skin.

8. Melanin also absorbs UV (ultraviolet) radiation protecting the skin from exposure to sunlight. Exposure to UV radiation increases the production of melanin. This is why some people become “tan” after exposure to UV radiation. However, UV radiation has also been shown to cause skin cancer, especially in people with light skin. Thus you should avoid excessive exposure to sunlight and wear sunscreen when outdoors.

9. Dermis The dermis is the functional layer of the skin that lies just beneath the epidermis. Connective tissue in the dermis makes the skin tough and elastic. The dermis contains many nerve cells, blood vessels, hair follicles, and specialized skin cells.

10. Dermis: Sensations of touch, temperature, and pain originate in nerve cells. The dermis also contains many tiny muscles that are attached to hair follicles in your skin. When you get cold, these muscles contract and pull the hair shafts upright, helping to insulate the body. These muscles also cause goose bumps on the skin surface.

11. Temperature regulation A network of blood vessels in the dermis provides nourishment to the living cells of the skin. These blood vessels also help regulate by temperature by either radiating heat into the air or conserving heat.

12. Temperature regulation If your body gets too hot, blood vessels just under the skin dilate so that blood flows near the surface, releasing heat from the body. This is why people with light complexions turn red during strenuous exercise. If your body gets too cold, the blood vessels constrict, keeping blood away from the surface and reducing heat loss.

13. Sweat glands in the dermis also help remove excess body heat. The evaporation of sweat from the skin’s surface removes heat more efficiently than the dilation of blood vessels. Most sweat is about 99% water and 1% dissolved salts and acids. Certain sweat glands located in body areas with dense hairs, such as the armpits, also secrete proteins and fatty acids. Because these substances provide a rich food source for bacteria, stale sweat often releases the offensive odor of bacterial waste products.

14. Subcutaneous tissue, located beneath the skin just under the dermis, is a layer of connective tissue made mostly of fat. Subcutaneous tissue acts as a shock absorber, provides additional insulation to help conserve body heat, and stores energy. Subcutaneous tissue also anchors the skin to underlying organs. The thickness of subcutaneous tissue varies in different parts of the body. For example, the eyelids have very little while the buttocks and thighs have a lot. The pads of subcutaneous tissue in the soles of the feet may be more than 6mm thick.

15. Hair and Nails: Hair and nails are derived from the epidermis. Hair follicles produce individual hairs, which help protect and insulate the body. Hair is made mostly of dead, keratin-filled cells. A shaft of hair grows up from the hair follicle and up from the skin’s surface. Each hair on your head grows for several years. Than the follicle enters a resting phase for several months and the hair is eventually shed.

16. Hair and Nails: Hair color is primarily determined by the presence of the pigment melanin. Blonde hair and red hair typically contain less melanin than brown hair or black hair. Remember that hair color is a polygenic trait, one controlled by multiple genes.

17. Nails: Nails are produced by specialized epidermal cells located in the light, semicircular area at the base of each nail. These cells become filled with keratin as they are pushed outwards by new cells. Nails protect the tips of the fingers and toes and continue to grow throughout life.

18. The skin is continuously exposed to damaging factors such as insect bites, microorganisms, and ultraviolet radiation. Injuries such as scrapes and blisters are often minor and usually heal rapidly without permanent scarring. Burns, however, can be very serious and can result in permanent scarring. Some skin disorders are a result of changes that occur within the body over time.

19. Acne: The most common skin problem for teenagers is acne, a chronic inflammatory condition that involves the skin’s oil-producing glands. Oil glands in the dermis release sebum, an oily secretion that lubricates the skin. Sebum is released through ducts, or pores, into nearby hair follicles. These oily glands are especially active during adolescents.

20. A cne: Acne is caused by excessive secretion of sebum, which blocks pores with oil, dirt, and bacteria. Makeup and other cosmetic tissues can contribute to clogging. As a result, the surrounding tissue becomes infected and inflamed, and the pores accumulate pus, producing pimples. Serious acne may need to be treated using antibiotics. Although acne can not be prevented, it can be managed with proper skin care.

21. Skin Cancer: Skin cancer can result from genetic mutations caused by overexposure to UV radiation. The most common types of skin cancer are carcinomas, which originate in skin cells that do not produce pigments. If they are detected early carcinomas can be treated.

22. Skin Cancer: A small percentage of skin cancers are caused by mutations that occur in pigment-producing cells. These cancers, called malignant melanomas, grow very quickly and spread easily to other parts of the body. About 8 out of 10 skin cancer deaths are from malignant melanomas. You can reduce the risk of skin cancer by avoiding over exposure to either natural or artificial UV radiation and by using protective sunscreens.