What is the physiology behind a captopril renogram?

With untreated renal artery stenosis, the renin-angiotensin mechanism responds to a decreased GFR by producing renin, which in turn converts angiotensinogen to angiotensin I. Angiotensin I is converted to angiotensin II, which constricts the efferent arterioles of the renal glomeruli to maintain the GFR. ACE inhibitors, such as captopril, disrupt this compensatory mechanism by dilating the efferent arterioles and decreasing the GFR.