Anatomic and Physiologic Aspects of the Pulmonary Vasculature

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Anatomic and Physiologic Aspects of the Pulmonary Vasculature

ANATOMY

PHYSIOLOGY

Pulmonary Vascular Resistance

Distribution of Pulmonary Blood Flow

Pulmonary Vascular Response to Hypoxia

Other Aspects of Pulmonary Vascular Physiology

The pulmonary vasculature is responsible for transporting deoxygenated blood to the alveoli and then carrying freshly oxygenated blood back to the left atrium and ventricle for pumping to the peripheral tissues. Although the pulmonary circulation is often called the “lesser circulation,” the lungs are the only organ system that receives the entire cardiac output. This extensive system of pulmonary vessels is susceptible to a variety of disease processes, ranging from those that primarily affect the vasculature to those that are either secondary to airway or pulmonary parenchymal disease or due to transport of material that is foreign to the pulmonary vessels, including blood clots.

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.

Anatomy

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.

The pulmonary veins, which are responsible for transporting oxygenated blood from the pulmonary capillaries to the left atrium, progressively combine into larger vessels until four major pulmonary veins enter the left atrium. Unlike the pulmonary arteries and their branches, the pulmonary venous system does not follow the course of the corresponding bronchial structures until the level of the hilum.

The bronchial arteries, which are part of the systemic circulation, provide nutrient blood flow to a variety of nonalveolar structures, such as the bronchi and the visceral pleural surface. There is significant variability in the anatomy of the bronchial circulation. Generally, a single bronchial artery of variable origin (upper right intercostal, right subclavian, or internal mammary artery) supplies the right lung. Two bronchial arteries, usually arising from the thoracic aorta, supply the left lung. Venous blood from the large extrapulmonary airways drains via bronchial veins into the azygos vein and eventually into the right atrium. In contrast, venous blood from intrapulmonary airways drains into the pulmonary venous system emptying into the left atrium. This blood leaving the intrapulmonary airways and draining back to the left atrium never entered the pulmonary capillary bed, thus providing a small amount of anatomic shunting of deoxygenated blood to the systemic arterial circulation.

An extensive network of lymphatic channels is also located primarily within the connective tissue sheaths around small vessels and airways. Although these channels do not generally course through the interstitial tissue of the alveolar walls, they are in sufficiently close proximity to be effective at removing liquid and some solutes that constantly pass into the interstitium of the alveolar wall.

Physiology

Pulmonary Vascular Resistance

Although the pulmonary circulation handles the same cardiac output from the right ventricle as the systemic circulation handles from the left ventricle, the former operates under much lower pressures and has substantially less resistance to flow than the latter. The systolic and diastolic pressures in the pulmonary artery normally are approximately 25 and 10 mm Hg, respectively, in contrast to 120 and 80 mm Hg in the systemic arteries. The pulmonary vascular resistance (PVR) can be calculated according to Equation 12-1:

The change or drop in pressure across the pulmonary circuit is the mean pulmonary artery pressure (mPA) minus the mean left atrial pressure (mLA), and the flow is the cardiac output (Equation 12-2). Thus:

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.

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