Figure 4.02. Changes in cross-sectional area of airways through successive generations of branchings.
Like the systemic and pulmonary circulations, the supply tubes branch and branch and branch, becoming much more numerous. While they become smaller in diameter and shorter in length at each of the 23 generational branchings, the total cross-sectional area becomes larger. Again like the blood system, flow velocity through this air conducting system decreases as we reach the very smallest components.
The alveoli, the end-organs of most of the gas exchange, are about 1/2 mm in diameter. While they very adequately carry out the function assigned, they are far larger than blood capillaries. Consequently there are only several hundred million alveoli while there are billions of capillaries.
By the way, the alveolar ducts are also sites of some gas exchange.
The Pulmonary Tree with some blood vessels as seen in a "corrosion" preparation: 1 = trachea; 2 = left principal bronchus; 3 = right principal bronchus; 4 = aorta, 5 = thoracic aorta; 6,7,8 = bronchial arteries.
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