38. The pulmonary circulation. Control of lung vessels

Pulmonary circulation

The pulmonary circulation is responsible for transporting deoxygenated blood to the lungs to be oxygenated, and to transport the blood back to the heart. When blood passes through the capillaries around the alveoli oxygen enters the blood from the alveoli while CO2 leaves the blood and enters the alveoli.

The pulmonary circulation also partly provides nutrients and oxygen to the lung tissue itself, but the lungs have another circulation which delivers the majority of nutrients and oxygen to the lung; the bronchial circulation. The bronchial circulation arises from the aorta and empties into the pulmonary veins.

The pulmonary and systemic circulations are organized in series and not in parallel. This means that the same amount of blood passes through the pulmonary circulation as the systemic circulation per minute. In other words, the cardiac output of the right ventricle is the same as the cardiac output of the left ventricle, 5 L/min in rest.

At any given point in time around 9% of the blood in your body is in the pulmonary circulation. The remaining 91% is in the systemic circulation. Like the systemic circulation the pulmonary circulation is comprised of arteries, capillaries and veins.

The pulmonary circulation has much lower pressure and resistance than the systemic circulation. The pulmonary arterial pressure is 24/9 mmHg, compared to the systemic arterial pressure, which is 120/80 mmHg. The pulmonary vascular resistance is also much lower than the systemic vascular resistance (total peripheral resistance).

When a person is lying down blood flow is uniform throughout the lung. However, when standing blood is unevenly distributed because of gravity. Blood flow is lowest in the apex and highest at the base of the lung.

Regulation

Hypoxia in the pulmonary circulation causes vasoconstriction of local vessels. Note that this is opposite of that in the systemic circulation, where hypoxia causes local vasodilation.

The difference can be explained. If a part of the lung is poorly ventilated (oxygenated), there is not much to gain by having full perfusion of this part. Because hypoxia causes local vasoconstriction blood will be redirected from poorly ventilated, hypoxic regions of the lung toward well-ventilated regions of the lung.