READ: Introduction to Animals

What are human skeletons made out of? Bone. What is connected to the bone? Muscle. What layer rests on top of muscle to protect your body? Skin. Bones, muscle, and skin form the foundation of your body.

The person in the above figure is moving. How do his bones help him to move? How are his muscles working with his bones? What would happen if the he did not have skin? What would happen if his bones were removed? Human bodies cannot work without a collection of different tissues and organs. If you remove one, the others will not operate in the same way.

While reading this chapter, think about the individual functions of bones, muscles, and skin. But also remember to consider how they work together to form a whole human.

CK-12 Foundation, Biology. http://creativecommons.org/licenses/by-nc-sa/3.0/

Lesson Objectives

• List the levels of organization in the human body.
• Identify the four types of tissues that make up the body.
• Identify 12 organ systems.
• Describe how organs and organ systems work together to maintain homeostasis.

Vocabulary

CK-12 Foundation, Biology. http://creativecommons.org/licenses/by-nc-sa/3.0/

Cells, Tissues, Organs

Homeostasis

The men in Figure below just jumped into freezing icy water. They are having fun, but imagine how cold they must feel! What happens to their bodies when one moment they are warm and the next they are freezing? If their bodies are working right, they will begin to shiver. Shivering helps the body return to a stable temperature.

The ability of the body to maintain a stable internal environment in response to change is called homeostasis. Homeostasis allows your body to adapt to change. Change might be from jumping into cold water or running in hot weather. Or it might be from not getting enough food when you are hungry. Homeostasis is a very important characteristic of living things.

Homeostasis and Cells

Cells are the most basic units of life in your body. They must do many jobs to maintain homeostasis, but each cell does not have to do every job. Cells have specific jobs to maintain homeostasis. For example, nerve cells move electrical messages around the body, and white blood cells patrol the body and attack invading bacteria.

There are many additional different types of cells. Other cells include red blood cells, skin cells, cells that line the inside of your stomach, and muscle cells.

Groups of Cells Form Tissues

Cells are grouped together to carry out specific functions. A group of cells that work together is called a tissue. Your body has four main types of tissues, as do the bodies of other animals. These tissues make up all structures and contents of your body. An example of each tissue type is shown in Figure below.

1. Epithelial tissue is made up 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.
2. Connective tissue is made up of many different types of cells that are all involved in structure and support of the body. Examples include tendon, cartilage, and bone. Blood is also classified as a specialized connective tissue.
3. Muscle tissue is made up of bands of cells that contract and allow bodies to move. There are three types of muscle tissue: smooth muscle, skeletal muscle, and cardiac muscle.
4. Nervous tissue is made up of the nerve cells that together form the nervous system. Nervous tissue is found in nerves, the spinal cord, and the brain.

Groups of Tissues Form Organs

A single tissue alone cannot do all the jobs that are needed to keep you alive and healthy. Two or more tissues working together can do a lot more. An organ is a structure made of two or more tissues that work together. The heart, shown in Figure below, is made up of the four types of tissues.

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Groups of Organs Form Organ Systems

Your heart pumps blood around your body. But how does your heart get blood to and from every cell in your body? Your heart is connected to blood vessels such as veins and arteries. Organs that work together form an organ system. Together, your heart, blood, and blood vessels form your cardiovascular system.

What other organ systems can you think of?

Organ Systems Work Together

Your body’s 12 organ systems are shown in Table below. Your organ systems do not work alone in your body. They must all be able to work together to maintain homeostasis.

For example, when the men in Figure above jumped into the cold water, their integumentary system (skin), cardiovascular system, muscular system, and nervous system worked quickly together to ensure the icy water did not cause harm to their bodies.

For example, the nervous system sent nerve messages from the skin to tell the cardiovascular system to reduce the blood flow to the skin. Blood flow is then increased to the internal organs and large muscles to help keep them warm and supply them with oxygen. The nervous system also sent messages to the respiratory system to breathe faster. This allows for more oxygen to be delivered by the blood to the muscular system.

One of the most important functions of organ systems is to provide cells with oxygen and nutrients and to remove toxic waste products such as carbon dioxide. A number of organ systems, including the cardiovascular and respiratory systems, all work together to do this.

CK-12 Foundation, Biology. http://creativecommons.org/licenses/by-nc-sa/3.0/