There are four tissue types in the body: epithelial tissue, connective tissue, muscle tissue, and nervous tissue. There are 12 major organ systems in the body. Organs and organ systems work together to maintain homeostasis.
Review Questions
What is homeostasis?
What are the four levels of organization in an organism?
What is the difference between a tissue and an organ?
List the four types of tissues that make up the human body.
A classmate says that all four tissue types are never found together in an organ.
Why do you think an organ is able to do many more jobs than a single tissue can?
Identify the organ system to which the following organs belong: skin, stomach, brain, lungs, and heart.
Give an example of how two organ systems work together to maintain homeostasis.
Further Reading / Supplemental Links
http://en.wikipedia.org/wiki/Tissue_%28biology%29
Vocabulary
cardiovascular system
The body system that include the heart, blood, and blood vessels.
connective tissue
Tissue that is made up of different types of cells that are involved in structure and support of the body; includes blood, bone, tendons, ligaments, and cartilage.
epithelial tissue
A tissue that is composed of layers of tightly packed cells that line the surfaces of the body; examples of epithelial tissue include the skin, the lining of the mouth and nose, and the lining of the digestive system.
homeostasis
The ability of the body to maintain a stable internal environment in the response to external changes.
muscular tissue
Tissue that is composed of cells that have filaments that move past each other and change the size of the cell. There are three types of muscle tissue: smooth muscle, skeletal muscle, and cardiac muscle.
nervous tissue
Composed of nerve cells and related cells.
organ
A structure made of two or more tissues that work together.
organ system
A group of organs that work together.
tissue
SA group of cells that work together for a common purpose.
Points to Consider
What are the levels of organization of the integumentary system?
What other body systems does the integumentary system work with to maintain homeostasis?
Lesson 16.2: Integumentary System
Lesson 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.
Check Your Understanding
What is homeostasis?
What is epithelial tissue?
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. 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.
Figure 16.5
Skin acts as a barrier that stops water and other things, like soap and dirt, from getting into your body.
Your Skin and Homeostasis
Your integumentary system has many roles in homeostasis, including protection, the sense of touch, and regulating body temperature. Keeping water out of the body is an important role for your integumentary system. If this were not so, the man in Figure 1 would not be able to bathe. All of your body systems work together to maintain stable internal conditions. Each of the parts that make up your integumantary system has a special role in maintaining homeostasis which we will explore a little later.
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 2 mm thick. It has many important functions, some of these include:
It acts as 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.
It helps regulate body temperature. It does this by making sweat, a watery substance which cools the body when it evaporates.
It helps you to gather information about your environment. Special nerve endings in your skin sense heat, pressure, cold and pain.
It helps the body get rid of some types of waste, which are removed in sweat.
It acts as a sun block. A chemical called melanin is made by certain skin cells when they are exposed to sunlight. Melanin blocks sun light from getting to deeper layers of skin cells, which are easily damaged by sun light.
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. The layer of skin that 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 are 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. These layers are the epidermis and the dermis. A fatty layer, called subcutaneous tissue, lies under the dermis, but it is not part of your skin. The layers that make up your skin are shown in Figure below.
Figure 16.6
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; thin and hairy, which is the most common type on the body, and thick and hairless, which is found on parts of the body that experience a lot of friction, such as the palms of the hands or the soles of the feet.
Epidermis
Epidermis is the outermost layer of the skin. It forms the waterproof, protective wrap over the body's surface and is made up of many layers of epithelial cells (discussed in lesson 1). The epidermis is divided into several layers where epithelial cells are formed by mitosis in the lowest layer. The epithelial cells move up through the layers of the epidermis, changing shape and composition as they divide and become filled with keratin. The skin cells at the surface of the epidermis form a thin layer of flattened, dead cells. 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 is fed by diffusion from the blood vessels of the dermis.
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 membrane made of collagen fibers. As you can see in Figure below, the dermis contains the 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. Tiny muscles in the dermis pull on hair follicles which cause hair to stand up. This can happen when you are cold or afraid. The resulting little “bumps” in the skin are commonly called goosebumps, shown in Figure below.
Figure 16.7
Goose bumps 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 s
tart in the dermis. Oil glands secrete an oily substance, called sebum, into the hair follicle. An oil gland is shown in Figure below. Sebum “waterproofs” hair and the skin surface to prevent them from drying out. It can also stop the growth of bacteria on the skin. Sebum is the cause of the oily appearance of skin and hair. 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, also called acne.
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 the skin temperature, which in turn helps to control body temperature. The skin also releases excess water, salts, and other wastes in sweat. A sweat gland is shown in Figure below.
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. Both hair and nails are important parts of your integumentary system.
Fingernails and toenails both grow from nail beds. A nailbed 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 you 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.
Figure 16.8
The structure of fingernails is similar to toenails. The is the part of the nail that extends past the finger, beyond the nail plate. The is what we think of when we say nail, the hard portion made of the tough protein keratin. The is the crescent shaped whitish area of the nail bed. The is the fold of skin at the end of the nail.
Hair sticks out from the epidermis, although it grows from hair follicles deep in the dermis, as shown in Figure below. Hair is 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 the result of 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.
Figure 16.9
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 radiation (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 do. 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 than children and teens who have not.
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.
Figure 16.10
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.
Because some types of skin cancer are easy to cure, the dangers of too much sunlight are not always taken seriously by people. It is important to remember that a more serious form of skin cancer, called melanoma, is also associated with long-term sun exposure. Melanomas are difficult to treat, and potentially deadly tumors. The best way to avoid skin cancer is to cover up when outside in the sun, and to wear sunscreen.
Bathing and Skin Hygiene
During the day, your skin can collect many different things. Sweat, oil, dirt, dust, and dead skin cells can build up on the skin surface. If not washed away, the mix of sweat, oil, dirt, and dead skin cells can encourage the excess growth of bacteria. These bacteria feed on these substances and cause a smell that is commonly called body odor. Dirty skin is also more prone to infection. Bathing every day helps to remove dirt, sweat and extra skin cells, and helps to keep your skin clean and healthy.
Acne
Hormones can affect your skin. Certain hormones cause oil glands in the skin to make an oil called sebum. When too much sebum is made by oil glands, it can cause the hair follicles to get blocked with dead skin cells. Within these blocked pores bacteria and yeast begin to multiply. In response to the growth of the bacteria and yeast, the skin inflames. This skin inflammation produces the red bumps that are called acne. Up to 85% of teenagers get acne. Acne usually goes away by adulthood. Frequent washing can help reduce the amount of sebum and dead skin cells on the skin. But washing cannot prevent the excessive sebum production that leads to acne.
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. These new cells will eventually replace the damaged tissues.
When an injury is deep enough through the epidermis into the dermis, bleeding occurs. A blood clot and scab soon forms. After the scab is formed, cells in the base 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.
Figure 16.11
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.
Lesson 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. It also helps you to sense your surroundings.
Skin is made up of two layers, the epidermis and the dermis. Hair and nails are made of the same type of protein as skin.
Nails grow from nail beds and hairs grow from hair follicles in the skin.
Acne is a skin problem that occurs when the skin produces too much sebum.
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 sunblock and a hat when outdoors can help prevent skin cancer.
Review Questions
Identify two functions of skin.
How does the integumentary system help maintain homeostasis?
Describe the structure of skin.
Identify the layer of skin from which hair grows.
In what way are hairs and nails similar to skin?
Name two functions of nails.
Name two functions of hair.
What type of skin problem happens when the skin makes too much sebum?
The World Health Organization recommends that no person younger than 18 years old use a tanning bed. Why do you think using a tanning bed is not recommended?
How does washing your skin help to keep you healthy?
Why are stitches sometimes needed if a person gets a deep or long cut in their skin?
Further Reading / Supplemental Links
http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5540a9.htm
http://www.cdc.gov/Features/SkinCancer
http://en.wikipedia.org/wiki
Vocabulary
CK-12 Life Science Page 39