Did you know that you see the largest organ in your body every day? You wash it, dry it, cover it up to stay warm, and uncover it to cool off. Yes, your skin is your bodys largest organ. Your skin is part of your integumentary system ( Figure 1.1), which is the outer covering of your body. The integumentary system is made up of your skin, hair, and nails. Skin acts as a barrier that stops water and other things, like soap and dirt, from getting into your body. The skin has many important functions. The skin: Provides a barrier. It keeps organisms that could harm the body out. It stops water from entering or leaving the body. Controls body temperature. It does this by making sweat (or perspiration), a watery substance that cools the body when it evaporates. Gathers information about your environment. Special nerve endings in your skin sense heat, pressure, cold, and pain. Helps the body get rid of some types of waste, which are removed in sweat. Acts as a sun block. A pigment called melanin blocks sunlight from getting to deeper layers of skin cells, which are easily damaged by sunlight. Your skin is always exposed to your external environment, so it gets cut, scratched, and worn down. You also naturally shed many skin cells every day. Your body replaces damaged or missing skin cells by growing more of them. Did you know that the layer of skin you can see is actually dead? As the dead cells are shed or removed from the upper layer, they are replaced by the skin cells below them. Two different layers make up the skin: the epidermis and the dermis ( Figure 1.2). A fatty layer lies under the dermis, but it is not part of your skin. The epidermis is the outermost layer of the skin. It forms the waterproof, protective wrap over the bodys surface. Although the top layer of epidermis is only about as thick as a sheet of paper, it is made up of 25 to 30 layers of cells. The epidermis also contains cells that produce melanin. Melanin is the brownish pigment that gives skin and hair their color. Melanin-producing cells are found in the bottom layer of the epidermis. The epidermis does not have any blood vessels. The lower part of the epidermis receives blood by diffusion from blood vessels of the dermis. Skin is made up of two layers, the epider- mis on top and the dermis below. The tissue below the dermis is called the hy- podermis, but it is not part of the skin. The dermis is the layer of skin directly under the epidermis. It is made of a tough connective tissue. The dermis contains hair follicles, sweat glands, oil glands, and blood vessels ( Figure 1.2). It also holds many nerve endings that give you your sense of touch, pressure, heat, and pain. Do you ever notice how your hair stands up when you are cold or afraid? Tiny muscles in the dermis pull on hair follicles which cause hair to stand up. The resulting little bumps in the skin are commonly called "goosebumps" ( Figure 1.3). Glands and hair follicles open out into the epidermis, but they start in the dermis. Oil glands ( Figure 1.2) release, or secrete an oily substance, called sebum, into the hair follicle. Sebum waterproofs hair and the skin surface to prevent them from drying out. It can also stop the growth of bacteria on the skin. It is odorless, but the breakdown of sebum by bacteria can cause odors. If an oil gland becomes plugged and infected, it develops into a pimple. Up to 85% of teenagers get pimples, which usually go away by adulthood. Frequent washing can help decrease the amount of sebum on the skin. Sweat glands ( Figure 1.2) open to the skin surface through skin pores. They are found all over the body. Evaporation of sweat from the skin surface helps to lower skin temperature. The skin also releases excess water, salts, sugars, and other wastes, such as ammonia and urea,
Output: how does skin act as a sun block?