The Respiratory System
Respiration. You have learned that cell respiration takes place when the cells take in oxygen and give off carbon dioxide. This process maintains the life of the cell by providing oxygen for the burning of food in the cell.
Body respiration, or breathing, takes place in the lungs. When you breathe, you draw air into your lungs by moving a large sheet of tissue called the diaphragm, and by the action of the rib muscles. Look at the illustration to see how the diaphragm causes the lungs to expand and contract.
When air is drawn into your lungs, it passes through several parts of the respiratory system. It is first drawn through the nose and mouth. There it is cooled or warmed by the moist tissues that line the passages. Foreign bodies are filtered out by tiny hairs in the nose.
The air passes through the nasal passages and the throat and is drawn into the trachea, or wind-pipe, and the bronchial tubes. These tubes lead to air sacs called alveoli. In the alveoli, the transfer of gases into the blood takes place.
Oxygen in blood. The alveoli are surrounded by a dense concentration of capillaries that carry the blood from the veins. This blood has a high concentration of carbon dioxide and a low concentration of oxygen. Because the walls between the capillaries and the air sacs are so thin, both carbon dioxide and oxygen can pass through them.
Carbon dioxide passes out of the blood, and oxygen passes into it. The oxygen is picked up by a compound called hemoglobin. Hemoglobin gives the blood its red color.
After the blood obtains oxygen in the alveoli, it is pumped to the left side of the heart, then through the arteries to the capillaries, and then to the cells of the body, where cell respiration takes place, and the process begins again.
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