READ: Introduction to Animals
3. Cells, Tissues, Organs
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.
The bodies of these swimmers are working hard to maintain homeostasis while they are in the icy pool water. Otherwise, their life processes would stop working as soon as they got into the water.
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.
Your body has four main types of tissue: nervous tissue, epithelial tissue, connective tissue, and muscle tissue. They are found throughout your body.
- 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.
- 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.
- 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.
- 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.
The four different tissue types work together in the heart as they do in the other organs.