Figure 3: The renin-angiotensin aldosterone system (RAAS).

In response to a decrease in blood volume, BP decreases, which elicits a response from the kidney to increase BP. This renal response is mediated by the production and secretion of renin by the juxtaglomerular cells in response to decreased arterial pressure sensed by renal baroreceptors and decreased luminal NaCl concentration sensed by the macula densa. The enzyme renin cleaves angiotensinogen, also known as renin substrate to produce angiotensin I, which is further processed to angiotensin II. Angiotensin II increases blood volume by two mechanisms: directly constricting systemic and renal arteries and arterioles, and by stimulating the production of aldosterone from the adrenal gland. The subsequent increase in vascular resistance and the NaCl and water reabsorption restores blood volume and BP toward normal and reduces RAAS activity.