kidneys
Kidneys The kidneys participate in several complex endocrine pathways and produce certain hormones. A decline in blood flow to the kidneys stimulates them to release the enzyme renin, triggering the renin-angiotensin-aldosterone RAAS system, and stimulating the reabsorption of sodium and water. The reabsorption increases blood flow and blood pressure. The kidneys also play a role in regulating blood calcium levels through the production of calcitriol from vitamin D3, which is released in response to the secretion of parathyroid hormone PTH . In addition, the kidneys produce the hormone erythropoietin EPO in response to low oxygen levels. EPO stimulates the production of red blood cells erythrocytes in the bone marrow, thereby increasing oxygen delivery to tissues. You may have heard of EPO as a performance-enhancing drug in a synthetic form .