12 Before diseases of the pulmonary vasculature are considered in Chapters 13 and 14, this chapter discusses a few of the general anatomic and physiologic aspects of the pulmonary vessels. Included in the discussion on physiology are several topics relating to hemodynamics of the pulmonary circulation, as well as a brief consideration of some nonrespiratory metabolic functions of the pulmonary circulation. In contrast to the systemic arteries, which carry blood from the left ventricle to the rest of the body, the pulmonary arteries, which carry blood from the right ventricle into the lungs, are relatively low-pressure, thin-walled vessels. Under normal circumstances, the mean pressure within the main pulmonary arteries is roughly one sixth the pressure in the aorta. The pulmonary trunk, which carries the outflow from the right ventricle, divides almost immediately into the right and left main pulmonary arteries, which subsequently divide into smaller branches. Throughout these progressive divisions, the pulmonary arteries and their branches travel with companion airways, following closely the course of the progressively dividing bronchial tree. By the time the vessels are considered arterioles, the outer diameter is less than approximately 0.1 mm. An important feature of the smaller pulmonary arteries is the presence of smooth muscle within the walls that is responsible for the vasoconstrictive response to various stimuli, particularly hypoxia, which allows for matching of perfusion to well-ventilated lung units. (See Chapter 1 for discussion of mismatch.) The pulmonary capillaries form an extensive network of communicating channels coursing through alveolar walls. Rather than being described as a series of separate vessels, the capillary system has been described as a continuous meshwork or sheet bounded by alveolar walls on each side and interrupted by “posts” of connective tissue, akin to the appearance of an underground parking garage. The capillaries are in close proximity to alveolar gas, separated only by alveolar epithelial cells and a small amount of interstitium present in some regions of the alveolar wall (see Figs. 8-1 and 8-2). Overall, the capillary surface area is approximately 125 m2 and represents approximately 85% of the available alveolar surface area. The design of this capillary system is extraordinarily well suited to the requirements of gas exchange, inasmuch as it contains an enormous effective surface area of contact between pulmonary capillaries and alveolar gas. Left atrial pressure is difficult to measure directly. However, a special catheter designed for this purpose, called a pulmonary artery balloon occlusion catheter or Swan-Ganz catheter, has been used widely in clinical application for such pressure measurements (Fig. 12-1). This catheter is inserted into a large vein (usually in the neck or groin) and passed through the right heart into a pulmonary artery. The catheter tip is equipped with a tiny soft balloon that lodges in a segmental pulmonary artery and temporarily blocks flow to the segment. After a short period of equilibration, because there is no blood flow passing the catheter tip, the pressure measured at the tip of the catheter reflects the pressure “downstream” in the pulmonary veins and left atrium.
Anatomic and Physiologic Aspects of the Pulmonary Vasculature
Anatomy
Physiology
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