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Pulmonary circulation

Synonyms: small circulation
German: Lungenkreislauf

1 Definition

The pulmonary circuit is the part of the circulatory system which describes the movement of the blood departing from the heart into the lungs and back again into the heart.

2 Physiology

The deoxygenated blood coming from the right heart's ventricle drains via the truncus pulmonalis and the pulmonary arteries into the capillaries of the lung. In here, the exchange of gas takes place - while carbondioxide (CO2) is released, the blood gets charged with oxygen (O2). The oxygenated blood continues its path via various pulmonary veines (venae pulmonales) and returns to the left atrium of the heart. Futher, it gets pumped into the left ventricle via the mitral, bicuspid valve and departs into the systemic circulation.

Regarding the physiology of the fetus, some differences can be observed. The truncus pulmonalis is directly connected with the aorta through the ductus arteriosus (ductus arteriosus botalli). Furthermore, an open conduit between the right and left atrium of the heart, called foramen ovale can be found. This two structures are short circuits which enable the blood to byspass the lungs and the pulmonary circuit. The reason for the existence of this shunting-mechanisms is the fetal exchange of gas inside of the placenta instead of the lungs.

Deoxygenated blood
right ventricle
pulmonary valve
truncus pulmonalis
arteries of the lung
capillaries of the lung
Oxygenated blood
veines of the lung
left atrium
mitral, bicuspid valve
left ventricle

The resistance of the pulmonary vessels is described as pulmonal-vascular resistance. It amounts estimated 1/10 of the perpiphere resistance of the systemic circulation. Accordingly, the blood pressure inside of the pulmonary circulation is notably lesser (20/8) in comparison to the systemic circulation (120/80 mmHg). Hypoxia in the lungs' alveoles causes vasoconstriction in the lung's vessels of the corresponding parts, which in turn causes an increase of the pulmonal-vascular resistance. This regulation is called hypoxic pulmonary vasoconstriction or Euler-Liljestrand mechanism and leads to a limited perfusion of insufficiently ventilated parts of the lungs. Due to this mechanism the blood of the pulmonary circulation is conducted into parts which are ventilated better.

3 Clinical significance

An increase of the resistance of the pulmonary circuit's vessels is described as pulmonary hypertension.

Specialties: Anatomy

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