The pulmonary circulation is a division of the circulatory system in all vertebrates. The circuit begins with deoxygenated blood returned from the body to the right atrium of the heart where it is pumped out from the right ventricle to the lungs. In the lungs the blood is oxygenated and returned to the left atrium to complete the circuit.

The other division of the circulatory system is the systemic circulation that begins upon the oxygenated blood reaching the left atrium from the pulmonary circulation. From the atrium the oxygenated blood enters the left ventricle where it is pumped out to the rest of the body, then returning as deoxygenated blood back to the pulmonary circulation.

A separate circulatory circuit known as the bronchial circulation supplies oxygenated blood to the tissues of the lung that do not directly participate in gas exchange.

Anatomy

thumb|400px|[[3D rendering of a high resolution computed tomography of the thorax. The anterior thoracic wall, the airways and the pulmonary vessels anterior to the root of the lung have been digitally removed in order to visualize the different levels of the pulmonary circulation.]]

thumb|Image showing main pulmonary artery coursing ventrally to the [[aortic root and trachea. The right pulmonary artery passes dorsally to the ascending aorta, while the left pulmonary artery passes ventrally to the descending aorta.]]

The pulmonary arteries have both an internal and external elastic membrane, whereas pulmonary veins have a single (outer) elastic layer.

Arteries

From the right ventricle, blood is pumped through the semilunar pulmonary valve into the left and right main pulmonary artery (one for each lung), which branch into smaller pulmonary arteries that spread throughout the lungs.

Physiology

Two pulmonary circulations

The lung actually possesses a high-flow, low-pressure circulation which passes deoxygenated blood from the right heart through the capillaries surrounding the alveoli to be oxygenated, and a low-flow, high-pressure (just slightly lower than systemic arterial pressure) circulation which supplies oxygenated blood to other structures of the lung (airways, supporting tissues, and the vasa vasorum) via the bronchial arteries. This oxygenated blood supplied by the bronchial arteries amounts to 1–2% of left heart output, and is drained into the pulmonary venous system and returned to the left atrium.

Pulmonary arterial pressure normally measures about 25 mmHg during systole, about 8 mmHg during diastole, for a mean arterial pressure of 15 mmHg. The pulmonary arteries and veins are short vessels. To accommodate the right ventricular stroke volume, the pulmonary arterial system has very high compliance; this is achieved by all pulmonary arteries possessing much larger diameters compared to systemic counterparts, as well as thin and distensible walls. Pulmonary blood flow is essentially equal to cardiac output. Pulmonary vessels typically function as distensible conduits that distend at higher intraluminal pressures and narrow with lower pressures. Pulmonary capillary pressure is estimated to normally stand at about 7mmHg (compared to about 17 mmHg in capillaries of the systemic circulation), though it has not been measured directly. The fetal lungs are collapsed, and blood passes from the right atrium directly into the left atrium through the foramen ovale (an open conduit between the paired atria) or through the ductus arteriosus (a shunt between the pulmonary artery and the aorta).

Clinical significance

A number of medical conditions may affect the pulmonary circulation:

  • Pulmonary hypertension describes an increase in resistance in the pulmonary arteries.
  • Pulmonary embolism is occlusion or partial occlusion of the pulmonary artery or its branches by an embolus, usually from the embolization of a blood clot from deep vein thrombosis. It can cause difficulty breathing or chest pain, is usually diagnosed through a CT pulmonary angiography or V/Q scan, and is often treated with anticoagulants such as heparin and warfarin.
  • Cardiac shunt is an unnatural connection between parts of the heart that leads to blood flow that bypasses the lungs.
  • Vascular resistance
  • Pulmonary shunt

History

thumb|The opening page of one of Ibn al-Nafis's medical works

The pulmonary circulation is archaically known as the "lesser circulation" which is still used in non-English literature.

The discovery of the pulmonary circulation has been attributed to many scientists with credit distributed in varying ratios by varying sources. In much of modern medical literature, the discovery is credited to English physician William Harvey (1578 – 1657 CE) based on the comprehensive completeness and correctness of his model, despite its relative recency. Other sources credit one or more of Greek philosopher Hippocrates (460 – 370 BCE), Arab physician Ibn al-Nafis (1213 – 1288 CE), Syrian physician Qusta ibn Luqa or Spanish physician Michael Servetus (c. 1509 – 1553 CE). Several figures such as Hippocrates and al-Nafis receive credit for accurately predicting or developing specific elements of the modern model of pulmonary circulation: Hippocrates for being the first to describe pulmonary circulation as a discrete system separable from systemic circulation as a whole and al-Nafis The Edwin Smith Papyrus (1700 BCE), named for American Egyptologist Edwin Smith (1822 – 1906 CE) who purchased the scroll in 1862, provided evidence that Egyptians believed that the heartbeat created a pulse that transported the above substances throughout the body. A second scroll, the Ebers Papyrus (c. 1550 BCE), also emphasized the importance of the heart and its connection to vessels throughout the body and described methods to detect cardiac disease through pulse abnormalities. Although they had knowledge of the heartbeat, vessels, and pulse, the Egyptians attributed the movement of substances through the vessels to air that resided in these channels, rather than to the heart's exertion of pressure. The Egyptians knew that air played an important role in circulation but did not yet have a conception of the role of the lungs.

The next addition to the historical understanding of pulmonary circulation arrived with the Ancient Greeks. Physician Alcmaeon (520 – 450 BCE) proposed that the brain, not the heart, was the connection point for all of the vessels in the body. He believed that the function of these vessels was to bring the "spirit" ("pneuma") and air to the brain. Empedocles (492 – 432 BCE), a philosopher, proposed a series of pipes, impermeable to blood but continuous with blood vessels, that carried the pneuma throughout the body. He proposed that this spirit was internalized by pulmonary respiration. He wrote that the right ventricle played a different role to the left: it transported blood to the lungs where the impurities were vented out so that clean blood could be distributed throughout the body. Though Galen's description of the anatomy of the heart was more complete than those of his predecessors, it included several mistakes. Most notably, Galen believed that blood flowed between the two ventricles of the heart through small, invisible pores in the interventricular septum. However, Avicenna's description of pulmonary circulation reflected the incorrect views of Galen.