Ethan Davis, age 10, of Davenport, Iowa., for his question:
HOW MUCH AIR IS IN EACH BREATH WE TAKE?
During ordinary breathing, each breath lets out and replaces about one seventh of the air in the lungs. The new air mixes with the old air in the lungs. A person breathing quietly inhales and exhales about one pint of air at each breath. This amount, called tidal air, is enough for ordinary body functioning.
A person who is `exercising can inhale about four quarts of air in each breath. This is known as the vital capacity. Breathing is regulated by the brain. A group of cells in the brain form the respiratory center, which is extremely sensitive to carbon dioxide. When there is a slight increase in carbon dioxide in the blood, the center speeds up the impulses it sends to the muscles of respiration. This causes faster breathing. For a person to breathe in, his chest must expand. The lungs fill the chest cavity completely, no matter how large it is. So when the chest expands, the lungs do also.
Expansion lowers the air pressure inside the lungs, creating a vacuum that pulls air in from the atmosphere. To do this, the diaphragm contracts, pulling its dome downward. This provides more room in the chest cavity. As the diaphragm contracts, the muscles surrounding the ribs also contract. The ribs move upward and outward, making the chest cavity expand frontward.
Expiration results when the process of inspiration is reversed. The muscles act to force out air, just as it is forced out when abellows is closed. First, the diaphragm relaxes and resumes its dome shape, reducing the space in the chest. Then the rib muscles relax, lowering the ribs to make the chest cavity smaller. These combined actions squeeze air out of the lungs.
Breathing is one form of respiration, called external respiration. The exchange of gases between the blood and other tissues in the body is called internal respiration.
Internal respiration is also sometimes called tissue respiration. A third type of respiration, called cellular respiration, happens within the cell itself. The bloodstream acts as a transportation system, carrying oxygen to all the cells and carbon dioxide away from them.
The heart pumps blood containing oxygen to even the remotest parts of the body. As the blood flows through the body's capillaries, the oxygen passes through the thin vessel walls into the tissue fluid and cells. At the same time, it picks up carbon dioxide and carries it back to the lungs to be discharged from the body.
Cellular respiration is a chemical process inside the cells. Oxygen is burned with various foods to give the cells energy. Living cells contain substances called respiratory enzymes that cause respiration to take place in the cells.