Why is angiotensin converting enzyme localized in the lungs I understand that it's also found elsewhere, such as in renal capillaries, but I can't see logic behind it being located in the lungs. Isn't ACE's function, through making more angiotensin II, causing increased fluid and sodium retention and increasing peripheral resistance? What benefit does that purpose get from ACE being mainly in the lungs?

ACE is present on the luminal surface of vascular endothelia throughout the body and is abundantly present in the **endothelium-rich lungs**.

ACE in the kidney—particularly in the endothelial cells of the afferent and efferent arterioles— can produce enough ANG II to exert local vascular effects.

Thus, the kidney _receives_ ANG II from two sources:

Systemic ANG II comes from the general circulation and originates largely from the pulmonary region, and

Local ANG II forms from the renal conversion of systemic ANG I.

• In addition, the proximal tubule **secretes** ANG II into its lumen and thus achieves intraluminal concentrations in excess of those in the general circulation. ANG II in the circulation has a short half-life (~2 min).