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
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.
Veins
Oxygenated blood leaves the lungs through pulmonary veins, which return it to the left part of the heart, completing the pulmonary cycle.
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.
Capacity and compliance Compared to the systemic circulation, the pulmonary circulation has less than 1/8th the capacitance and contains ~1/10th the blood volume. 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.
Blood flow, blood pressure and vascular resistance The (alveolar) pulmonary circulation operates as a very low pressure and resistance system. Indeed, blood pressure in the circulation is normally just sufficient to maintain blood flow in all parts of the lungs. Nevertheless, the pulmonary circulation can accommodate significantly increased flow during periods of increased demand. Abnormally high blood pressures in the pulmonary circulation (e.g. in left-sided heart failure) cause excess fluid to transude from the blood vessels into the alveoli and accumulate here, causing pulmonary edema and impairing gas exchange. 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.
During very heavy demand (e.g. during strenuous exercise), pulmonary blood flow may be increased to 4-fold to 7-fold of normal. The additional blood flow is accommodated by increasing the number of open capillaries (up to 3-fold greater), distending capillaries (up to 2-fold greater flow), and increasing pulmonary blood pressure; the former two mechanisms accommodate the additional blood flow by reducing vascular resistance and can normally accommodate all the additional required blood flow even at peak demand with little additional increase of pulmonary blood pressure.
Blood normally passes through a pulmonary capillary in about 0.8 s, but the time spent traversing the capillaries is as little as 0.3 s during maximal blood flow.
Autoregulation of alveolar blood flow Lung tissue is capable of reducing perfusion of poorly ventilated alveoli to redirect blood flow to better ventilated ones. Reduced O2 concentration in an alveolus causes adjacent blood vessels to constrict; vascular resistance may increase more than 5-fold with very low alveolar O2 levels. Hypoxic vasoconstriction of alveolar blood vessels is thought to be mediated by increased action of vasoconstrictors (e.g. endothelin, and reactive oxygen species), decreased release of vasodilators (e.g. nitric oxide), and closing of oxygen-sensitive K+ channels in vascular smooth muscle (directly causing depolarisation and consequent constriction of muscle).
Hydrostatic effects and pulmonary blood flow The hydrostatic pressure of blood within blood vessels exhibits a gradient across the lung (as do blood vessels across any axis of the body). In an upright person, the lung normally measures 30 cm top-to-bottom for a hydrostatic pressure gradient of 23 mmHg, of which 15 mmHg is superior to the level of the heart. Consequently, in a standing person at rest, there is 5 times more blood flow at the bottom of the lung than at the top.
The hydrostatic pressure gradient can lead to three different blood flow scenarios in different parts of the lung:
Zone 1: upper-most zone where alveolar pressure always exceeds capillary blood pressure and no blood flow occurs. Zone 2: intermediate zone where peak capillary blood pressure exceeds alveolar pressure during systole, but alveolar pressure always exceeds diastolic blood pressure, so that some blood flows through the capillaries but only during systole. Zone 3: lower zone where capillary blood pressure during diastole exceeds alveolar pressure so that blood flow is continuous.
Normally, only zone 2 and zone 3 blood flow occurs in the lungs: as normal systolic pulmonary blood pressure is ~25 mmHg at heart level and the hydrostatic pressure of the blood is 15 mmHg at the top of the lung, capillary pressure at the top of the lung will amount to ~10 mmHg during systole, but because normal diastolic pulmonary blood pressure is only 8 mmHg, no blood flow will occur in parts of the lung where hydrostatic pressure of the blood exceeds 7 mmHg (this limit is situated about 10 cm about the level of the heart in vivo).
Zone 1 blood flow occurs in the context of either abnormally low systolic pulmonary blood pressure, or abnormally high alveolar pressure. When alveolar pressure exceeds capillary blood pressure, capillaries collapse and blood flow ceases. Increased blood flow through the pulmonary circulation (e.g. during physical exertion) causes zone 2 parts of the lung to progressively transition to zone 3 patterns of blood flow.
Pulmonary blood volume Pulmonary blood volume is normally about 450 mL (about 9% of total blood volume); about 70mL of this amount is contained within pulmonary capillaries, and the remainder is roughly equally distributed between the pulmonary arterial and venous systems.
The pulmonary blood pool may range from half of the normal amount to twice the normal amount as result of various physiological or pathological conditions. Vigorous expiration that creates high pressures within the lungs can cause as much as 250 mL of blood to be ejected from the pulmonary circulation. The pulmonary blood pool can act as a blood reserve that can be mobilised into the systemic circulation (e.g. after blood loss). Dysfunction of the left heart can cause significant amounts of blood to dam up in the pulmonary circulation.
Development The pulmonary circulation loop is virtually bypassed in fetal circulation. 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).
When the lungs expand at birth, the pulmonary pressure drops and blood is drawn from the right atrium into the right ventricle and through the pulmonary circuit. Over the course of several months, the foramen ovale closes, leaving a shallow depression known as the fossa ovalis.
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
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 for making great strides over the understanding of those before him and towards a rigorous model. There is a great deal of subjectivity involved in deciding at which point a complex system is "discovered", as it is typically elucidated in piecemeal form so that the very first description, most complete or accurate description, and the most significant forward leaps in understanding are all considered acts of discovery of varying significance.