FIGURE 11 Blood flow in pulmonary capillaries (C) surrounds the alveoli (A) like a "sheet" of blood flow. PA, pulmonary arteriole; marker = 50 um. (From Weibel, Chap. 82 in Crystal et al, eds., The lung: Scientific foundations. Philadelphia: Lippincott-Raven, 1997.)

Therefore, increasing pulmonary arterial pressure will increase flow by two mechanisms: (1) the pressure gradient for Ohm's law is increased and (2) the transmural pressure is increased, which increases vessel size and decreases PVR. Figure 12 shows how PVR becomes even smaller when pulmonary arterial pressure is increased. Increasing pulmonary venous pressure also decreases PVR, because some of this pressure increase is transmitted to the capillaries. As discussed earlier, capillary dimensions significantly affect PVR. Hence, pressure affects resistance and vice versa in the pulmonary circulation, in contrast to the systemic circulation in which resistance primarily affects pressure.

Alveolar pressure is the outside pressure for calculating transmural pressure in most pulmonary vessels. Alveolar pressure varies with the ventilatory cycle, but it is generally near zero (i.e., atmospheric pressure; see Chapter 19). Therefore, vascular pressure is the primary determinant of transmural pressure in pulmonary vessels. However, large positive alveolar pressures can occur with some forms of artificial ventilation, and this will tend to collapse pulmonary capillaries.

Increasing transmural pressure can affect capillary dimensions by two mechanisms: recruitment and disten-tion. At very low pressures, some capillaries may be closed, and increasing pressure will open them by recruitment. At higher pressures, capillaries are already open, but they may be distended or stretched by increased transmural pressure. Together, recruitment and distention increase the effective size of the pulmonary capillaries and reduce PVR.

Another important determinant of pulmonary vessel size is lung volume, but this effect differs for different types of vessels. Extra-alveolar vessels are surrounded by lung parenchyma, which acts as a tether or support structure to hold the vessels open. Therefore, lung volume is more important than alveolar pressure for determining the dimensions of extra-alveolar vessels. At high lung volumes, the extra-alveolar vessels are pulled open by tissues outside the vessels. At low lung volumes, this tethering effect is reduced and the extra-alveolar vessels narrow. Also, extra-alveolar vessels have smooth muscle and elastic tissue that tend to collapse the vessels at low lung volumes. The effects of lung volume on

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