Renal artery stenosis (RAS) is narrowing of one or both renal arteries that reduces blood flow to the kidneys. The two main causes are atherosclerosis (90% of cases, typically in older adults with cardiovascular risk factors, affecting the proximal renal artery) and fibromuscular dysplasia (FMD, 10%, typically in young women, affecting the mid-to-distal renal artery). When blood flow to the kidney decreases, the juxtaglomerular cells sense reduced perfusion and release renin, activating the RAAS cascade: renin → angiotensin I → angiotensin II → aldosterone. This causes vasoconstriction and sodium/water retention, raising blood pressure. This is a secondary cause of hypertension — the elevated BP is a response to the kidney's perception of low blood flow.