The pulmonary circulation, which makes up approximately 10% of the circulating blood volume, has a total blood volume of about 500 mL. In healthy adults, there is usually around 75 mL of blood in the alveolar-capillary network at any given time. The pulmonary capillary bed, the largest vascular bed in the body, covers an area of 70 to 80 m2, which is almost equal to the alveolar surface area. During exercise, the pulmonary capillary blood volume can increase from 75 mL to as high as 200 mL due to the recruitment of new capillaries caused by increased pressure and flow. This recruitment of new capillaries is a unique characteristic of the lungs, allowing for compensation during periods of stress such as exercise.

Oxygenated blood exits the alveolus through a network of small pulmonary venules (15 to 500 µm in diameter) and veins. These small vessels merge together to form larger pulmonary veins (>500 µm in diameter), through which the oxygenated blood returns to the left atrium of the heart. Unlike arteries, arterioles, and capillaries, which follow the branching patterns of the airways closely, venules and veins are located at a considerable distance from the airways.

The structure of the pulmonary circulation involves arteries with thin walls and minimal smooth muscle. These pulmonary arteries are seven times more compliant than systemic vessels, making them easily distensible. This high compliance state requires lower pressure for blood flow compared to the noncompliant arterial walls of the systemic circulation. Under normal conditions, the vessels in the pulmonary circulation are dilated and have larger diameters compared to similar arteries in the systemic system. All these factors contribute to a very compliant, low-resistance circulatory system, facilitating blood flow through the pulmonary circulation with the relatively weak pumping action of the right ventricle. The right ventricle is less muscular than the left ventricle, which is attributed to this low-resistance, low-work system. The pressure gradient differential for the pulmonary circulation, from the pulmonary artery to the left atrium, is only 6 mm Hg (14 mm Hg in the pulmonary artery minus 8 mm Hg in the left atrium). This pressure gradient differential is less than 7% of the pressure gradient differential of 87 mm Hg present in the systemic circulation (90 mm Hg in the aorta minus 3 mm Hg in the right atrium).

The vessels in the pulmonary circulation can be categorized into three groups (extra-alveolar, alveolar, and microcirculation) based on their physiological properties, although their anatomical definition is not well-established. The extra-alveolar vessels, including arteries, arterioles, veins, and venules, are larger than their systemic counterparts. They are not influenced by changes in alveolar pressure but are affected by changes in intrapleural and interstitial pressure. Therefore, the caliber of these extra-alveolar vessels is influenced by lung volume and lung elastin. At high lung volumes, the decrease in pleural pressure increases vessel caliber, whereas at low lung volumes, an increase in pleural pressure decreases vessel caliber. On the other hand, alveolar capillaries reside within the interalveolar septa and are highly sensitive to changes in alveolar pressure but not to changes in pleural or interstitial pressure. Positive-pressure ventilation increases alveolar pressure, compressing these capillaries and obstructing blood flow. The pulmonary microcirculation consists of small vessels involved in the exchange of fluids and solutes, contributing to fluid balance in the lung.

The structure of the alveolar-capillary network is as follows.
The total blood volume of the pulmonary circulation is approximately 500 mL, which is approximately 10% of the circulating blood volume. Approximately 75 mL of blood is present in the alveolar-capillary network of healthy adults at any one time. The pulmonary capillary bed is the largest vascular bed in the body. It covers a surface area of 70 to 80 m2, which is nearly as large as the alveolar surface area. During exercise, the pulmonary capillary blood volume increases from 75 mL to as high as 200 mL because of the recruitment of new capillaries as the result of an increase in pressure and flow. This recruitment of new capillaries is a unique feature of the lungs, and it allows for compensation in periods of stress, as in the case of exercise.
The oxygenated blood leaves the alveolus through a network of small pulmonary venules (15 to 500 µm in diameter) and veins. These small vessels quickly coalesce to form larger pulmonary veins (>500 µm in diameter), through which the oxygenated blood returns to the left atrium of the heart. In contrast to arteries, arterioles, and capillaries, which closely follow the branching patterns of the airways, venules and veins run quite distant from the airways.
Structure of the Pulmonary Circulation
The arteries of the pulmonary circulation have thin walls, with minimal smooth muscle. They are seven times more compliant than systemic vessels, and they are easily distensible. This highly compliant state of the pulmonary arterial vessels requires lower pressure for blood flow through the pulmonary circulation than do the more muscular, noncompliant arterial walls of the systemic circulation. The vessels in the pulmonary circulation, under normal circumstances, are in a dilated state and have larger diameters than do similar arteries in the systemic system. All of these factors contribute to a very compliant, low-resistance circulatory system, which aids in the flow of blood through the pulmonary circulation via the relatively weak pumping action of the right ventricle. This low-resistance, low-work system also explains why the right ventricle is less muscular than the left ventricle. The pressure gradient differential for the pulmonary circulation from the pulmonary artery to the left atrium is only 6 mm Hg (14 mm Hg in the pulmonary artery minus 8 mm Hg in the left atrium). This pressure gradient differential is less than 7% of the pressure gradient differential of 87 mm Hg present in the systemic circulation (90 mm Hg in the aorta minus 3 mm Hg in the right atrium).
Structures of the Extra-Alveolar and Alveolar Vessels and the Pulmonary Microcirculation
Although not well defined anatomically, vessels in the pulmonary circulation can be divided into three categories (extra-alveolar, alveolar, and microcirculation) on the basis of differences in their physiological properties. The extra-alveolar vessels (arteries, arterioles, veins, and venules) are larger than their systemic counterparts. They are not influenced by alveolar pressure changes, but they are affected by intrapleural and interstitial pressure changes. Thus the caliber of extra-alveolar vessels is affected by lung volume and by lung elastin. At high lung volumes, the decrease in pleural pressure increases the caliber of extra-alveolar vessels, whereas at low lung volumes, an increase in pleural pressure decreases vessel caliber. In contrast, alveolar capillaries reside within the interalveolar septa, and they are very sensitive to changes in alveolar pressure but not to changes in pleural or interstitial pressure. Positive-pressure ventilation increases alveolar pressure and compresses these capillaries and thus blocks blood flow. The pulmonary microcirculation comprises the small vessels that participate in liquid and solute exchange in maintenance of fluid balance in the lung.
Structure of the Alveolar-Capillary Network