Angiotensin-converting enzyme (ACE), a vital component of the renin-angiotensin-aldosterone system, is abundant in lung endothelial cells. ACE converts the inactive decapeptide, angiotensin I, into the active octapeptide, angiotensin II. This potent vasoconstrictor narrows blood vessels, increasing resistance to blood flow and elevating blood pressure. Angiotensin II also stimulates aldosterone production, encouraging kidney cells to reabsorb more sodium and water from urine, thereby increasing blood volume and pressure. In addition, ACE interacts with the endogenous vasodilator peptide bradykinin and degrades it, promoting nitric oxide generation and relaxing smooth muscle cells in blood vessels. However, ACE degrades bradykinin into inactive peptides, reducing its vasodilatory effect. ACE inhibitors halt ACE's action, preventing the conversion of angiotensin I to angiotensin II, allowing blood vessels to dilate, and reducing blood pressure. Lower levels of angiotensin II yield less aldosterone, prompting the kidneys to excrete more sodium and water, further decreasing blood volume and pressure. ACE inhibitors also slow the degradation of bradykinin, contributing to decreased blood pressure and vasodilation. Structurally, ACE inhibitors are categorized into sulfhydryl-containing, dicarboxylate-containing, and phosphorus-containing groups. Sulfhydryl-containing ACE inhibitors, such as captopril, have a sulfhydryl group. Dicarboxylate-containing ones, like enalapril and lisinopril, contain two carboxylate groups, while phosphorus-containing ACE inhibitors, like fosinopril, feature a phosphonate group.