Christina Hunt, age 14, of Gulfport, Miss., for her question:
WHAT REGULATES BREATHING?
The breathing of human beings and of many animals is called "external respiration." With breathing there is a taking in of oxygen and the giving off of carbon dioxide. The exchange of gases between the blood and other tissues in the body is another form of respiration, called "internal respiration" or "tissue respiration."
Breathing is regulated by the brain. A specialized group of cells in the brain forms what is called the respiratory center. This center 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.
When the amount of carbon dioxide returns to normal, breathing also becomes normal.
In human beings and also in many animals, breathing continues constantly without conscious effort. You can try to hold your breath for a while, but soon you will reach a point where you can no longer do so. Then automatic, rhythmic breathing starts again.
Automatic breathing is made up of two separate acts: "inspiration," or breathing in, and "expiration," or breathing out.
The chest muscles act to expand and contract the chest cavity, causing the lungs to either fill or empty. A pause occurs between inspiration and expiration. The pause is shorter during fast breathing because at this time there is a greater need for additional oxygen.
Your lungs are your chief organs of breathing. They are two almost pyramid shaped structures that lie in the chest cavity. The spongy lung tissue is divided into many small air sacs called "alveoli."
The thin walls of the alveoli are filled with a network of tiny blood vessels called capillaries.
Also important in breathing is the diaphragm, a muscle which forms the chest cavity floor.
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 "vital capacity."
The bloodstream acts as the transportation system, carrying oxygen to all of the cells and carbon dioxide away from them. The exchange of these gases between the blood cells and the tissue cells is just as automatic as external respiration.