Blood Flow Through the Lungs and Its Distribution
The blood flow through the lungs is essentially equal to the cardiac output. Therefore, the factors that control cardiac output—mainly peripheral factors- also control pulmonary blood flow. Under most conditions, the pulmonary vessels act as passive, distensible tubes that enlarge with increasing pressure and narrow with decreasing pressure. For adequate aeration of the blood to occur, it is important for the blood to be distributed to those segments of the lungs where the alveoli are best oxygenated. This is achieved by the following mechanism.
Effect of Diminished Alveolar Oxygen on Local Alveolar Blood Flow—Automatic Control of Pulmonary Blood Flow Distribution. When the concentration of oxygen in the air of the alveoli decreases below normal—especially when it falls below 70 per cent of normal (below 73 mm Hg PO2)—the adjacent blood vessels constrict, with the vascular resistance increasing more than fivefold at extremely low oxygen levels. This is opposite to theeffect observed in systemic vessels, which dilate ratherthan constrict in response to low oxygen. It is believed that the low oxygen concentration causes some yet undiscovered vasoconstrictor substance to be released from the lung tissue; this substance promotes con-striction of the small arteries and arterioles. It has been suggested that this vasoconstrictor might be secreted by the alveolar epithelial cells when they become hypoxic.
This effect of low oxygen on pulmonary vascular resistance has an important function: to distribute blood flow where it is most effective. That is, if some alveoli are poorly ventilated so that their oxygen con-centration becomes low, the local vessels constrict.This causes the blood to flow through other areas of the lungs that are better aerated, thus providing an auto-matic control system for distributing blood flow to the pulmonary areas in proportion to their alveolar oxygen pressures.
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