Tissues and Organs
Related cells joined together are collectively referred to as a tissue. The cells in a tissue are not identical, but they work together to accomplish specific functions. A sample of tissue removed for examination under a microscope (biopsy) contains many types of cells, even though a doctor may be interested in only one specific type.
Connective tissue is the tough, often fibrous tissue that binds the body's structures together and provides support and elasticity. It is present in almost every organ, forming a large part of skin, tendons, joints, ligaments, blood vessels, and muscles. The characteristics of connective tissue and the types of cells it contains vary, depending on where it is found in the body.
The body's functions are conducted by organs. Each organ is a recognizable structure (for example, the heart, lungs, liver, eyes, and stomach) that performs specific functions. An organ is made of several types of tissue and therefore several types of cells (see Cells). For example, the heart (see Biology of the Heart) contains muscle tissue that contracts to pump blood, fibrous tissue that makes up the heart valves, and special cells that maintain the rate and rhythm of heartbeats. The eye (see Structure and Function of the Eyes) contains muscle cells that open and close the pupil, clear cells that make up the lens and cornea, cells that produce the fluid within the eye, cells that sense light, and nerve cells that conduct impulses to the brain. Even an organ as apparently simple as the gallbladder (see Gallbladder and Biliary Tract) contains different types of cells, such as those that form a lining resistant to the irritative effects of bile, muscle cells that contract to expel bile, and cells that form the fibrous outer wall holding the sac together.