The Structure of the Small Pulmonary Arteries Varies Among Species

The main pulmonary arteries that accompany the bronchi are elastic, but the smaller arteries adjacent to the bronchioles and the alveolar ducts are muscular. The adult pig and the cow have a thick muscle layer in the smaller pulmonary arteries; the horse has less muscle; and the sheep and dog have only a thin muscle layer. The amount of smooth muscle in the wall of small pulmonary arteries determines the reactivity of the vasculature to alveolar hypoxia and other neural and humoral stimuli (see later discussion).

The small pulmonary arteries lead into pulmonary capillaries, which form an extensive branching network of vessels within the alveolar septum, almost covering the alveolar surface. Not all capillaries are perfused in the resting animal. As a result, vessels that are unperfused in the resting animal can be recruited when pulmonary blood flow increases (e.g., during exercise). Pulmonary veins with thin walls conduct blood from capillaries to the left atrium and also form a reservoir of blood for the left ventricle. The reservoir of blood in the pulmonary veins is available for sudden increases in cardiac output (e.g., at the start of a sudden burst of exercise).

Functionally, Pulmonary Blood Vessels Can Be Classified as Alveolar and Extra-Alveolar Vessels

Alveolar vessels are the thin-walled capillaries that perfuse the alveolar septum (Figure 46-1). They are exposed almost directly to the pressure changes that occur in the alveoli during breathing. Extra-alveolar vessels include the pulmonary arteries, and veins, which occur together with bronchi in a loose connective tissue sheath called the bronchovascular bundle. This bundle is bounded by a limiting membrane to which alveolar septa are attached (Figure 46-2). The behavior of extra-alveolar vessels is determined by pressure changes within the connective tissue space of the bronchovascular bundle, which approximate pleural pressure, rather than by changes in alveolar pressure. The bronchovascular bundle is also the initial site of accumulation of edema fluid when animals develop pulmonary edema.

The Pulmonary Blood Vessels Offer a Low Resistance to Flow

Pulmonary vascular pressures can be measured by advancing a catheter through the jugular vein into the right ventricle and pulmonary artery. Even though the pulmonary circulation receives the total output of the right ventricle, pulmonary arterial pressures are much less than systemic pressures. Pulmonary arterial systolic, diastolic, and mean pressures average approximately 25, 10, and 15 mm Hg, respectively, in mammals at sea level but these pressures are somewhat greater in larger than in smaller mammals. If the catheter is advanced until it becomes wedged in a pulmonary artery, the occluded vessel becomes an extension of the catheter, allowing estimation of pulmonary venous pressure, also known as pulmonary wedge pressure. Pulmonary wedge pressure (average, 5 mm Hg) is only slightly greater than left atrial pressure (average, 3 to 4 mm Hg). The small difference in pressure between the mean pulmonary artery (15 mm Hg) and left atrial (4 mm Hg) pressures indicates that the pulmonary circulation offers little vascular resistance to blood flow. Pulmonary vascular resistance (PVR) is calculated as follows:

PVR=(Ppa−Pla)/Q˙

where Ppa is mean pulmonary arterial pressure, Pla is left atrial pressure, and is cardiac output.

Although PVR is low in the normal resting animal, it decreases even further when pulmonary blood flow and pulmonary vascular pressures increase, as occurs during exercise. This is because an increase in pressure recruits previously unperfused vessels and serves to distend all vessels. More importantly, pulmonary vascular smooth muscle relaxes during exercise so that small arteries and veins dilate.

Micropuncture studies have shown that approximately half the vascular resistance in the pulmonary circulation is precapillary, and that the capillaries themselves provide a considerable portion of resistance to blood flow (Figure 46-3). Unlike the arterioles in the systemic circulation, the small arteries in the pulmonary circulation neither provide large resistance nor dampen the arterial pulsations; consequently, pulmonary capillary blood flow is pulsatile. The pulmonary veins provide little resistance to blood flow.

The Distribution of Pulmonary Blood Flow Within the Lung Is Influenced by Several Factors

The understanding of the distribution of blood flow within the lung was based for many years on experiments performed in humans or on dog lungs postured vertically to mimic the position of lungs in humans. Such experiments indicated that there is a vertical gradient of perfusion, with blood flow per unit lung volume increasing from the top to the bottom of the lung. Elegant models that considered pulmonary arterial, pulmonary venous, and alveolar pressures were proposed to explain gravity-dependent distribution of blood flow.

This description of gravitational zones provides a good theoretical basis for understanding the effects of pressures on pulmonary blood flow of bipeds. In comparison with quadrupeds, a large fraction of the human lung lies both below and above the level of the pulmonary artery so that gravity acting on the vertical height of the lung results in relative over-perfusion of the basal region and under-perfusion of the uppermost regions. In quadrupeds by contrast, the bulk of the lung is dorsal to the heart and the resting mean pulmonary perfusion pressure is sufficient to perfuse the whole lung height so gravity plays only a minor role in determining distribution of blood flow.

Blood flow is preferentially distributed to the dorsocaudal region of the lung of standing quadrupeds (Figure 46-4). This distribution is accentuated by exercise and may even persist when posture changes during anesthesia. The branching pattern of pulmonary arteries and arterioles and the relative resistances of each vessel are the major determinants of blood flow distribution.

Passive Changes in Vascular Resistance Result from Changes in Vascular Transmural Pressure

The diameter of blood vessels is a function of the pressure difference between the inside and the outside of the vessel, called the transmural pressure. Pressure within the vessels increases when the blood volume therein increases as occurs during exercise. This leads to an increase in transmural pressure, which causes the vessels to dilate. Transmural pressure can also increase if the pressure surrounding the vessel decreases. This occurs in large pulmonary arteries and veins as the lung inflates. These vessels are contained in the bronchovascular bundle, which is enlarged by the traction of the surrounding alveolar septa during lung inflation. Consequently, pressure in the perivascular connective tissue of the bronchovascular bundle decreases. This leads to an increase in transmural pressure, and the extra-alveolar arteries and veins therefore dilate.

The overall effects of lung volume on PVR reflect opposing effects on alveolar and extra-alveolar vessels (Figure 46-5). At residual volume, PVR is high because extra-alveolar vessels are narrowed. As the lung inflates to functional residual capacity, resistance decreases, primarily because of dilation of extra-alveolar vessels (arteries and veins). Further inflation above functional residual capacity increases PVR, primarily because alveolar capillaries are flattened by the high tension in the stretched alveolar septa. Capillaries become progressively more elliptical and therefore offer more resistance to flow.