The Lung Removes Many Hormones and Toxins from the Blood and Inactivates Many Others
Because it receives the total cardiac output, the pulmonary capillary bed with its vast endothelial surface is ideally placed to cleanse the blood of substances produced in other parts of the body.
The endothelial cell surface, which is enlarged by projections and by depressions known as caveolae, is the site of many enzymes involved in the uptake and metabolism of vasoactive substances. Serotonin is almost totally removed by uptake into endothelial cells, where it is degraded by monoamine oxidase. Norepmephriiie is also removed to some degree, but acetylcholine, epinephrine, and histamine are not removed. The peptides bradykinin and angiotensin are metabolized by angiotensin-converting enzyme (ACE) located on the endothelial surface. Bradykinin is inactivated, whereas angiotensin I is converted into angiotensin II. The lung degrades the majority of prostaglandin E2 and prostaglandin F2, but prostacyclin (prostaglandin I2) is unaffected. Leukotrienes are broken down by neutrophils, which are numerous in the pulmonary circulation.Many exogenous toxic substances are also removed from the blood by the pulmonary endothelium. This process at times can cause severe lung injury. For example, the toxins from Crotalaria species of plants can cause smooth muscle hypertrophy in the pulmonary arterioles, which leads to pulmonary hypertension.
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