Non-gravitational causes of uneven blood flow Random variations in the resistance of blood vessels Some evidence that proximal regions of an acinus receive more blood flow than distal regions In some animals some regions of the lung have an intrinsically higher vascular resistance
Evolutionary pressure for hypoxic pulmonary vasoconstriction Pulmonary blood flow in the fetus is only about 15% of the cardiac output Most of the output of the right ventricle bypasses the lung through the ductus arteriosus The pulmonary vascular resistance is high because of hypoxic vasoconstriction in the very muscular pulmonary arteries Immediately after birth, and pulmonary blood flow must increase dramatically The great fall in pulmonary vascular resistance is due mainly to the release of hypoxic vasoconstriction In addition the ductus arteriosus gradually closes
Substances metabolized by the lung Biological activation: Angiotensin I is converted to the vasoconstrictor, angiotensin II via ACE Biological inactivation:. Examples include bradykinin, serotonin, prostaglandins E 1, E 2, and F 2 alpha. Norepinephrine is also partially inactivated Not affected: Examples include epinephrine, prostaglandins A 1 and A2, angiotensin II and vasopressin. Metabolized and released: Examples include the arachidonic acid metabolites - the leukotrienes, and prostaglandins. Secreted: Immunoglobulins, particularly IgA, in bronchial mucus.