2-4
2-4. PHYSIOLOGY OF RESPIRATION
a. The walls of the alveoli are composed of a thin, permeable membrane. It is here that oxygen passes from the alveoli into the tiny capillaries that surround each alveoli. Carbon dioxide in the blood passes from the capillaries into the alveoli to be exhaled. This exchange of oxygen and carbon dioxide in the lungs is called external respiration.
b. The oxygen that enters the capillaries is carried by the red blood cells in a chemical combination with hemoglobin. This blood, now oxygenated, returns to the heart to be pumped out to the body through the arteries. The oxygen passes part of the blood into the cells of the body, while carbon dioxide waste from the cells is passed into the blood that returns to the heart. This exchange of gases between the capillary blood and the cells of the body is called internal respiration.
