________________
chemical reactions in which carbon dioxide and water are formed as waste products. The entire process of respiration brings oxygen from the atmosphere into the lungs and bloodstream, and then into all the cells of the body. Thus there are two levels of respiration: the first occurs in the lungs and is called external respiration; the second occurs in the cells and is called internal respiration.
External respiration: The muscles of respiration draw air into the lungs, so that oxygen can be absorbed into the blood and transported to the cells. Oxygen moves into the cell and carbon dioxide moves out of the cell to be carried back to the lungs to be disposed of. This is exchange of gases by diffusion between the alveoli and the blood. Each alveolar wall is one-cell thick and is surrounded by a network of tiny capillaries (the walls of which are also only one-cell thick). The total area for gas exchange in the lungs is 70 to 80 square metres.
Venous blood arriving at the lungs has travelled from all the active tissues of the body, and contains high levels of carbon dioxide and low levels of oxygen. Carbon dioxide diffuses from venous blood down its concentration gradient into the alveoli until equilibrium with alveolar air is reached. By the same process, oxygen diffuses from the alveoli into the blood. The slow flow of blood through the capillaries increases the time available for diffusion to occur. When blood leaves the alveolar capillaries, the oxygen and carbon dioxide concentrations are in equilibrium with those of alveolar air.
The changeover from absorption of gases to elimination of gases takes place at the pause between inhalation and exhalation. The point of exchange is as vital as the exhalation and inhalation. In yogic terms this point is the most important part of pranayama.
Internal respiration: This is exchange of gases by diffusion between blood in the capillaries and the body cells. Gaseous exchange does not occur across the walls of the arteries carrying blood from the heart to the tissues, because
88
