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Without muscle, humans could not live. The primary job of muscle is to move the bones of the skeleton, but muscle also makes the heart beat and constitutes the walls of other important hollow organs.

There are three types of muscle tissue:

Skeletal Muscle

This type of muscle creates movement in the body. There are more than 600 skeletal muscles, and they makes up about 40 percent of a person’s weight. When the nervous system signals the muscle to contract, groups of muscles work together to move the skeleton. These signals and movements are nearly involuntary, yet they do require conscious effort. However, humans do not need to concentrate on individual muscles when moving.

Cardiac Muscle

Cardiac muscle is involuntary muscle. This type makes up the walls of the heart and creates the steady, rhythmic pulsing that pumps blood through the body from signals from the brain. This muscle type also creates the electrical impulses that produce the heart’s contractions, but hormones and stimuli from the nervous system can also affect these impulses, such as when your heart rate increases when you’re scared.

Smooth Muscle

Another involuntary muscle type, smooth muscle makes up the walls of hollow organs, respiratory passageways, and blood vessels. It’s wavelike movements pass things through bodily system, such as food through your stomach or urine through your bladder. Like cardiac muscle, smooth muscle also contracts in response to stimuli and nerve impulses.