Pulmonary Blood Flow
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Introduction
The bronchial circulation nourishes the airways, and comes off the aorta.
Blood pumped from the right ventricle crosses the pulmonary valve and enters the pulmonary trunk, which divides into the two pulmonary arteries. As blood enters pulmonary capillaries, it flows past air-filled alveoli and exchanges carbon dioxide for fresh oxygen. This new blood flows along pulmonary veins and enters the left atrium.
The pulmonary vascular bed receives the entire output of the right ventricle. Because it is a low pressure system, it responds to gravity, with greatest blood flow going to the base of the lungs. Given constant alveolar pressures throughout the lung, there are three zones of pressure difference.
Zone I, at the top of the lung, has more alveolar pressure than arterial pressure. This poorly perfused area increases with physiologic dead space. The alveolus acts as a Starling resistor to prevent blood flow through poor alveoli. In zone II, in the middle, pressures are equal, and there will be moderate flow. Zone III, at the bottom of the lung flow has the greatest blood flow as arterial pressure exceeds that of the alveoli.
Blood flows well through alveoli with higher O2, and vascular resistance increases with hypoxia. In situations of generalized hypoxia, such as altitude, global vasoconstriction can result in pulmonary hypertension. Giving oxygen to patients with hypoxia decreases resistance. Acidosis and increased sympathetic tone can also cause lesser degrees of vasoconstriction.
Increasing vascular pressure causes distension of existing vessels and recruitment of new vessels, both which decrease resistance.
LV failure causes blood to back up in the lungs, causing arterial pressures to rise above alevolar pressures. This opens up apical vessels, resulting in vascular redistribution, or cephalization of blood flow.
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