The blood flow through the lungs is essentially equal to the cardiac output. Therefore, the factors that control cardiac output—mainly peripheral factors, as discussed in Chapter 20—also control pulmonary blood flow. Under most conditions, the pulmonary vessels act as dis tensible tubes that enlarge with increasing pressure and narrow with decreasing pressure. For adequate aeration of the blood to occur, the blood must be distributed to the segments of the lungs where the alveoli are best oxygenated. This distribution is achieved by the following mechanism.
Decreased Alveolar Oxygen Reduces Local Alveolar Blood Flow and Regulates Pulmonary Blood Flow Distribution. When the concentration of O2 in the air of the alveoli decreases below normal, especially when it falls below 70 percent of normal (i.e., below 73 mm HgPo2), the adjacent blood vessels constrict, with vascular resistance increasing more than fivefold at extremely low O2 levels. This effect is opposite to the effect observed in systemic vessels, which dilate rather than constrict in response to low O2 levels. Although the mechanisms that promote pulmonary vasoconstriction during hypoxia are not completely understood, low O2 concentration may stimulate release of vasoconstrictor substances or decrease release of a vasodilator, such as nitric oxide, from the lung tissue.
Some studies suggest that hypoxia may directly induce vasoconstriction by inhibition of oxygen-sensitive potassium ion channels in pulmonary vascular smooth muscle cell membranes. With low partial pressures of oxygen, these channels are blocked, leading to depolarization of the cell membrane and activation of calcium channels, causing influx of calcium ions. The rise of calcium concentration then causes constriction of small arteries and arterioles.
The increase in pulmonary vascular resistance as a result of low O2 concentration has the important function of distributing blood flow where it is most effective. That is, if some alveoli are poorly ventilated and have a low O2 concentration, the local vessels constrict. This constriction causes the blood to flow through other areas of the lungs that are better aerated, thus providing an automatic control system for distributing blood flow to the pulmonary areas in proportion to their alveolar O2 pressures.
