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The heart pumps a volume of blood into the lungs which after getting oxygenated is transported back to heart. This oxygenated blood is then pumped out of the heart and into the aorta and subsequently into the systemic circulation. As a consequence, at any given time the lungs (pulmonary vasculature) contain around 500ml of blood, thereby allowing them to function as a reservoir of the blood. This reservoir volume is increased by 500 ml when the person is in the supine or lying down position. This is because in supine position there’s an increased venous return (due to the effect of gravity) to the right heart from the peripheries and therefore more blood accumulates in the central parts of body. Upon standing up, this extra 500ml of blood gets redistributed under the effect of gravity, to the now more dependent parts of the body which includes the peripheral tissues and the lower extremities.
The division of vasculature in the pulmonary circuit is somewhat different from that in the systemic circulation. The arteries in the pulmonary circuit divide in a binary fashion, thereby following the pattern of division of the airways. The veins too exhibit a pattern similar to the arterioles and the bronchioles, thereby finally converging into forming one large pulmonary vein. These pulmonary veins transport the oxygenated blood back to the heart.
Apart from the primary function of lungs to deliver oxygenated blood to the heart, it has several secondary functions as well:
Filtration: Pulmonary vascular bed acts as a filter for the blood received by the right side of the heart and before this blood leaves the left side of the heart to enter the systemic circulation. Blood arriving from the right side of the heart to the lungs may contain an embolus or a thrombus or any foreign particle that may result in obstruction of the vessels. The pulmonary artery divides into numerous small capillaries with diameters too small to allow passage of any dislodged particles into the pulmonary artery and subsequently in to the left side of the heart. Therefore, the pulmonary vascular bed acts as a filtration barrier that prevents introduction of any particles into the systemic circulation.
Metabolic Functions: The pulmonary vasculature also has some metabolic functions:
- The endothelium of the pulmonary vessels is lined by fibrinolytic enzymes that cause lysis of the fibrin clots which get stuck in the pulmonary vasculature.
- The pulmonary capillary endothelium is lined by another enzyme as well. This enzyme is called the Angiotensin Converting Enzyme (ACE). ACE functions to covert Angiotensin Ito Angiotensin II. ACE is also present in the endothelium of other vascular beds and the plasma, however, the amount present in the pulmonary vascular bed is the highest. ACE within the pulmonary vascular bed is responsible for more than 70% of the total conversion of Angiotensin I to Angiotensin II in the body.