Pulmonary circulation includes a vast network of arteries, veins, and lymphatics that function to exchange blood and other tissue fluids between the heart, the lungs, and back. They are designed to perform certain specific functions that are unique to the pulmonary circulation, such as ventilation and gas exchange. The pulmonary circulation receives the entirety of the cardiac output from the right heart and is a low pressure, low resistance system due to its parallel capillary circulation. The system can be divided into the following components:
The arterial circuit arises from the main pulmonary artery arising from the right ventricle and runs a course of only 5 cm before dividing into right and left main branches and many subsequent branches to form an extensive network of small arteries, arterioles, and capillaries. The pulmonary arteries are thinner (one-third the thickness of their counterpart systemic vessels) and have a larger diameter. The combined effect makes them much more distensible and compliant (approximately 7mL/mmHg).
The venous circuit begins with the venules that drain the capillaries. They join to form smaller veins and eventually merge to form the main pulmonary veins draining into the left atrium. Like the arteries, the pulmonary veins are thinner and more distensible than the counterpart systemic veins and accommodate more blood because of their larger compliance.
Lymphatics play a crucial role in maintaining a dry alveolar membrane and preventing accumulation of tissue fluid around the pulmonary circulation. They can be found close to the terminal bronchioles and drain the mediastinal lymphatics before emptying into the right lymphatic duct.
It is appropriate to mention that a similar system of lymphatics and vessels exists between the parietal and visceral pleurae, draining the pleural fluid which plays an important role in providing a viscous medium for expansion of lungs during their respiratory excursion. The large negative pleural pressure (approximately -4 to -7 mmHg) exists because of an efficient efferent venous and lymphatic system that keeps the alveoli closely tethered to the visceral pleura and prevents them from collapsing inwards.
In addition to the pulmonary circulation, the lung parenchyma receives oxygenated blood via the bronchial circulation (accounting for about ~1% of the cardiac output) which arises from the aorta, and thus left ventricle. The bronchial circulation has superifical and deep systems. The superficial system drains into the hemiazygos and azygos veins, which ultimately drain into the right heart with the systemic venous return. However, the deep circulation drains into the pulmonary vein and thus left ventricle. As a result, the deep bronchial system effectively functions as an arteriovenous shunt. However, its venous return to the left heart is minimal (0-0.5% of cardiac output) and does not affect cardiac output to any significant degree as volumes between right and left ventricles are nearly identicle.
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