What are the kidneys?
These endocrine organs are located behind the abdominal cavity, on both sides of the spine, just below the diaphragm. As organs this pair is responsible for mainly three things; A fundamental part of the urinary system, performs functions to help maintain homeostasis, and is a part of the endocrine system releasing important hormones into the blood. As an endocrine gland the kidneys affect blood pressure, encourages red blood cell production, increases calcium levels in blood, and stimulates the production of platelets.
Hormones of the Kidneys
Renin: Produced in the juxtaglomerular cells and is released by instructions from the broreceptors (pressure sensitive cells) and from low sodium chloride levels in the kidneys filtrate, this hormone activates the renin-angiotensin system and instructs the liver to release angiotesin I which in turn causes the vascular system to constrict, produces the thirst reflex via the hypothalamus, and the kidneys to increase sodium and water retention which all cause the increase of blood pressure.
Erythropoietin (EPO): Produced in the extraglomerular mesangial cells this hormone instucts bone marrow to produce red blood cells (erythrocytes).
Calctriol (dihydroxyvitamin d3): This hormone is produced in the proximal tubule of the nephron in the kidneys, as an active form of D3 it will help increase calcium levels in the body and blood by instructing the kidneys to limit the transfer of Ca2 in the blood to urine, increasing release of calcium from the bones to blood, and increases the absorption of calcium and phosphate in the gastrointestinal tract. As well it restricts PTH (parathyroid hormone) from being released.
Thrombopoietin (TPO): This hormone is made in both the liver (parenchymal  and sinusoidal endothelial cells) and kidneys (proximal convoluted tubule cells). It is also created in the stiated muscle and stromal cells in the bone marrow. When secreted it is responsible for stimulating the production of red blood cells or platelets in the megakaryocytes (Bone marrow cells). The negative feedback systems of this hormone are unique in the sense that TPO will bond directly to platelets and are destroyed, apposed to most hormonal negative feedback system restricts the hormone receptors.
