Renin is a protein produced by the kidneys in response to a drop in blood pressure in the renal artery. Renin promotes the production of angiotensin II, which in turn stimulates the secretion of aldosterone by the adrenal glands. Aldosterone plays a crucial role in maintaining blood volume and blood pressure. Any drop in blood pressure in the kidneys is, therefore, quickly corrected by the activation of the renin-angiotensin-aldosterone system. This test reports the amount (mass) of renin in the blood and must be interpreted with aldosterone levels based on the posture (lying or standing) at the time of sample collection. Results can be affected by many medications and dietary salt intake. The aldosterone/renin ratio can be measured to eliminate the effect of posture and other interferences.
High renin levels in the blood are often due to a drop in blood pressure in the renal artery (renal artery stenosis, heart failure, etc.). Low renin levels are most often due to hypertension of undetermined cause (essential hypertension) or caused by a benign tumour in the adrenal glands secreting aldosterone or another mineralocorticoid.