The integumentary System ITouch your fingertips, your arm, and your face. The soft tissue

you feel is the outermost layer of your skin. Skin is the largest

organ of your body. It is part of the integumentary (ihn teh gyuhMEN tuh ree) system, which includes all the external coverings

of the body, including the skin, nails, and hair.

2'

gland

e twoys';skinhelps

I control body

k temperature.

452 Chapter 11

Figure 10 During exercise, sweatevaporates and blood vesselsenlarge. This releases thermalenergy.

Sweat

Functions of the Integumentary SystemRecall that skin is part of your body's first line of defense. It

helps protect your body from harmful substances in the external

environment. It keeps your body from drying out in sunlight and

wind. Skin also protects the cells and tissues under the skin from

damage. Too much exposure to the Sun can damage skin. The

damage can result in sun burns or skin cancer. Skin can be

protected by using sun block and regulating the amount of timeit is exposed to the Sun.

Close your eyes and feel the surface of your desk and the

objects on top of it. Even with your eyes closed, you can tell the

difference between the desk, a book, paper, and pencils. You can

feel these differences because your skin has special cells called

sensory receptors that detect texture. Sensory receptors also

detect temperature and sense pain. The more sensory receptors

there are in an area of skin, the more sensitive it is.

Skin helps control body temperature. When you exercise, sweat

comes from tiny holes, or pores, on the skin's surface. Sweating is

one way skin lowers your body temperature, as shown in Figure 10.

As sweat evaporates, excess thermal energy leaves the body and

the skin cools. Another way that skin lowers body temperature is

by releasing thermal energy from blood vessels. Has your face

ever tffrned red while exercising? This happens because blood

vessels near the skin's surface dilate, or enlarge.

If your skin is exposed to sunlight, it can make vitamin D. Your

body needs vitamin D to help it absorb calcium and phosphorus,

which are needed for healthy bones. Your skin is not the only

source of vitamin D. Vitamin D is usually added to milk, and it is

found naturally in certain types of fish.

Normal cellular processes produce waste products. The skin

helps eliminate these wastes. Water, salts, and other waste prod-

ucts are removed through the pores. This removal occurs all the

time, but you might only notice it when you sweat duringexercise.

Pore

u

o

_=

0

You shed about 600,000 flakes of skin everYou get an entirely new layer of skin about every20-30 days.

Epidermis

Dermis

tFatty layer[

Figure 11 Skin has three layers, each with different functions.

Structures of the Skin

The skin that you see and feel on your body is the outermost

layer of skin. Below it are two other layers of skin. These layers,

shown in Figure 11, differ in structure and function.12. Why is fiexib ty of

the skin important?

Epidermis

The epidermis (eh pub DUR mus) is the outermost layer of skinand the only layer that comes into direct contact with the outside

environment. The epidermis is tough but thin. The epidermis on

your eyelids is thinner than a sheet of paper. Cells of the epider-

mis are constantly shed and replaced by new cells. One impor-

tant function of the epidermis is the production of melanin

(MEH luh nun). Melanin is a pigment that protects the body byabsorbing some of the Sun's damaging ultraviolet rays.

Dermis

Below the epidermis is the dermis. The dermis is a thick layer

of skin that gives skin strength, nourishment, and flexibility. The

dermis contains sweat glands, blood vessels, nerves, hair follicles,

and muscles, as shown in Figure 11. When the muscles in thedermis contract, you get goose bumps.

\

Fatty Layer

The innermost layer of skin insulates the body, acts as a pro-

tective padding, and stores energy. This layer is sometimes called

the fatty layer. It can be very thin or very thick, depending on itslocation on the body. ,.

Lesson 11:1 Transport and Defense A53