7.8 The Skin System

Learning Objectives

• List the functions of skin.
• Describe the structure of skin.
• Describe the structure of hair and nails.
• Identify two types of skin problems.
• Describe two ways to take care of your skin.

Introduction

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, or uncover it to cool off. In fact, you see it so often it is easy to forget the important role your skin plays in keeping you healthy.

Guided Learning

Your Skin and Homeostasis

Your skin is part of your integumentary system (Figure below), which is the outer covering of your body. The integumentary system is made up of your skin, hair, and nails. Your integumentary system has many roles in homeostasis, including protection, the sense of touch, and controlling body temperature.

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Skin acts as a barrier that stops water and other things, like soap and dirt, from getting into your body.

Functions of Skin

Your skin covers the entire outside of your body. Your skin is your body's largest organ, yet it is only about two millimeters thick. It has many important functions. The skin:

• Provides a barrier. It keeps organisms that could harm the body out. It stops water from leaving the body, and stops water from getting into the body.
• Controls body temperature. It does this by making sweat, 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 chemical called melanin is made by certain skin cells when they are exposed to sunlight. Melanin blocks sunlight from getting to deeper layers of skin cells, which are easily damaged by sunlight.

Structure of Skin

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? The dead cells are filled with a tough, waterproof protein called keratin. As the dead cells are shed or removed from the upper layer, they are replaced by the skin cells below them.

As you can see in Figure below, two different layers make up the skin — the epidermis and the dermis. A fatty layer, called subcutaneous tissue, lies under the dermis, but it is not part of your skin.

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Skin is made up of two layers, the epidermis on top and the dermis below. The tissue below the dermis is called the hypodermis, but it is not part of the skin.

The color, thickness, and texture of skin vary over the body. There are two general types of skin:

1. Thin and hairy, which is the most common type on the body.
2. Thick and hairless, which is found on parts of the body that experience a lot of contact with the environment such as the palms of the hands or the soles of the feet.

The Epidermis

The epidermis is the outermost layer of the skin. It forms the waterproof, protective wrap over the body's surface. The epidermis is divided into several layers of epithelial cells. The epithelial cells are formed by mitosis in the lowest layer. These cells move up through the layers of the epidermis to the top. 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.

The Dermis

The dermis is the layer of skin directly under the epidermis. It is made of a tough connective tissue that contains the protein collagen. Collagen is a long, fiber-like protein that is very strong. The dermis is tightly connected to the epidermis by a thin wall of collagen fibers.

As you can see in Figure above, the dermis contains hair follicles, sweat glands, oil glands, and blood vessels. 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," shown in Figure below.

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Goosebumps are caused by tiny muscles in the dermis that pull on hair follicles, which causes the hairs to stand up straight.

Oil Glands and Sweat Glands

Glands and follicles open out into the epidermis, but they start in the dermis. Oil glands release, or secrete, an oily substance, called sebum, into the hair follicle. An oil gland is shown in Figure above. 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 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. This is why sweat can help maintain homeostasis. The skin also releases excess water, salts, and other wastes in sweat. A sweat gland is shown in Figure above.

Nails and Hair

Nails and hair are made of the same types of cells that make up skin. Hair and nails contain the tough protein keratin.

Nails

Fingernails and toenails both grow from nail beds. A nail bed is thickened to form a lunula, or little moon, which you can see in Figure below. Cells forming the nail bed are linked together to form the nail. As the nail grows, more cells are added at the nail bed. Older cells get pushed away from the nail bed and the nail grows longer. There are no nerve endings in the nail, which is a good thing, otherwise cutting your nails would hurt a lot!

Nails act as protective plates over the fingertips and toes. Fingernails also help in sensing the environment. The area under your nail has many nerve endings, which allow you to receive more information about objects you touch. Nails are made up of many different parts, as shown in Figure below.

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The structure of fingernails is similar to toenails. The free edge is the part of the nail that extends past the finger, beyond the nail plate. The nail plate is what we think of when we say “nail,” the hard portion made of the tough protein keratin. The lunula is the crescent shaped whitish area of the nail bed. The cuticle is the fold of skin at the end of the nail.

Hair

Hair sticks out from the epidermis, but it grows from the dermis, as shown in Figure below. Hair is also made of keratin, the same protein that makes up skin and nails. Hair grows from inside the hair follicle. New cells grow in the bottom part of the hair, called the bulb. Older cells get pushed up, and the hair grows longer. Similar to nails and skin, the cells that make up the hair strand are dead and filled with keratin.

Hair color is caused by different types of melanin in the hair cells. In general, the more melanin in the cells, the darker the hair color; the less melanin, the lighter the hair color.

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Hair, hair follicle, and oil glands. The oil, called sebum, helps to prevent water loss from the skin.

Hair helps to keep the body warm. When you feel cold, your skin gets a little bumpy. These bumps are caused by tiny muscles that pull on the hair, causing the hair to stick out. The erect hairs help to trap a thin layer of air that is warmed by body heat. In mammals that have much more hair than humans, the hair traps a layer of warm air near the skin and acts like warm blanket. Hair also protects the skin from ultraviolet (UV) radiation from the sun.

Hair also acts as a filter. Nose hair helps to trap particles in the air that may otherwise travel to the lungs. Eyelashes shield eyes from dust and sunlight. Eyebrows stop salty sweat and rain from flowing into the eye.

Keeping Skin Healthy

Some sunlight is good for health. Vitamin D is made in the skin when it is exposed to sunlight. But getting too much sun can be unhealthy. A sunburn is a burn to the skin that is caused by overexposure to UV radiation from the sun's rays or tanning beds.

Light-skinned people, like the girl in Figure below, get sunburned more quickly than people with darker skin. This is because melanin in the skin acts as a natural sunblock that helps to protect the body from UV radiation. When exposed to UV radiation, certain skin cells make melanin, which causes skin to tan. Children and teens who have gotten sunburned are at a greater risk of developing skin cancer later in life.

Long-term exposure to UV radiation is the leading cause of skin cancer. About 90 percent of skin cancers are linked to sun exposure. UV radiation damages the genetic material of skin cells. This damage can cause the skin cells to grow out of control and form a tumor. Some of these tumors are very difficult to cure. For this reason you should always wear sunscreen with a high sun protection factor (SPF), a hat, and clothing when out in the sun. As people age, their skin gets wrinkled. Wrinkles are caused mainly by UV radiation and by the loosening of the connective tissue in the dermis due to age.

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Sunburn is caused by overexposure to UV rays. Getting sunburned as a child or a teen, especially sunburn that causes blistering, increases the risk of developing skin cancer later in life.

Injury

Your skin can heal itself even after a large cut. Cells that are damaged or cut away are replaced by cells that grow in the bottom layer of the epidermis and the dermis. When an injury cuts through the epidermis into the dermis, bleeding occurs. A blood clot and scab soon forms. After the scab is formed, cells at the bottom of the epidermis begin to divide by mitosis and move to the edges of the scab. A few days after the injury, the edges of the wound are pulled together.

If the cut is large enough, the production of new skin cells will not be able to heal the wound. Stitching the edges of the injured skin together can help the skin to repair itself. The person in Figure below had a large cut that needed to be stitched together. When the damaged cells and tissues have been replaced, the stitches can be removed.

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Sewing the edges of a large cut together allows the body to repair the damaged cells and tissues, and heal the tear in the skin.

Summary

• Skin acts as a barrier that keeps particles and water out of the body.
• The skin helps to cool the body in hot temperatures, and keep the body warm in cool temperatures.
• Skin is made up of two layers, the epidermis and the dermis.
• Pimples occur when the skin produces too much sebum.
• Hair and nails are made of keratin, the same protein as skin.
• Nails grow from nail beds and hairs grow from hair follicles in the skin.
• Skin cancer can be caused by excess exposure to ultraviolet light from the sun or tanning beds.
• Frequent bathing helps keep the skin clean and healthy.
• Wearing sun block and a hat when outdoors can help prevent skin cancer.

Vocabulary

dermis

The layer of skin directly under the epidermis; made of a tough connective tissue that contains the protein collagen.

epidermis

Outermost layer of the skin; forms the waterproof, protective wrap over the body's surface.

integumentary system

The outer covering of your body that is made up of skin, hair, and nails; has many roles in homeostasis, including protection, sense of touch, and controlling body temperature.

keratin

A tough, waterproof protein found in dead skin cells.

melanin

Brownish pigment that gives skin and hair their color.

oil gland

A structure that secretes, an oily substance, called sebum, into the hair follicle; sebum “waterproofs” hair and the skin surface to prevent them from drying out and can also stop the growth of bacteria on the skin.

sunburn

A burn to the skin that is caused by overexposure to UV radiation from the sun's rays or tanning beds; may lead to an increased risk of skin cancer.

sweat gland

Small structures of the skin that secrete sweat; found all over the body.