Human beings, like all animals, must have a steady supply of oxygen to maintain life. Oxygen plays an important role in the process of turning food into energy. However, it also combines with carbon from food to produce carbon dioxide, which is a gas. As a waste product of the process, it must be removed from the body. The respiratory system is the body's way of taking in oxygen and getting rid of carbon dioxide through the process of breathing.

Parts of the respiratory system

In human beings, the respiratory system is made up of the nasal cavity, throat (pharynx), voice box (larynx), windpipe (trachea), bronchi, and lungs. Air enters this system through the nose and then moves into the nasal cavity. It also enters through the mouth. These two airways meet at the upper part of the throat, called the pharynx. From there, two passageways open downward, one for food and one for air. The air passage contains the voice box, or larynx, and the vocal cords in its upper part. In the lower part is the windpipe, or trachea.

The trachea separates inside the lungs into two main branches, or bronchi—one for each lung. Within the lungs, the bronchi divide into smaller and smaller branches. They end with the alveoli, the tiniest structures of all. They are so tiny that the lungs of an average person contain more than 600 million alveoli. The alveoli are elastic air sacs containing tiny blood vessels called capillaries. Through the capillaries carbon dioxide is exchanged for oxygen. Then the oxygen is circulated by the blood through the circulation system to all the cells of the body.

The breathing cycle

A cluster of nerve cells located at the base of the brain controls breathing in humans. This cluster of nerve cells is called the respiratory center. It sends messages to the diaphragm, a muscle at the base of the lungs, telling the diaphragm to move. This allows the lungs to expand and contract.

The lungs are protected by the ribs. In the process of breathing in, the rib muscles raise the ribs and the diaphragm moves downward, enlarging the chest cavity. Air then flows into the lungs, making them expand. When the rib muscles and the diaphragm relax and the chest cavity grows smaller, the lungs then decrease in size with the chest cavity. Air is forced out of the lungs.

The lungs of an average adult never empty completely, even after exhaling fully. The rate of breathing varies in the average adult from about six to eight breaths per minute when asleep to about 16 per minute when awake. It can increase to as many as 100 per minute under stress.