<P> The' single effect' describes the fact that the ascending thick limb of the loop of Henle is not permeable to water but is permeable to sodium chloride . This allows for a countercurrent exchange system whereby the medulla becomes increasingly concentrated, but at the same time setting up an osmotic gradient for water to follow should the aquaporins of the collecting duct be opened by ADH . </P> <P> Although the kidney cannot directly sense blood, long - term regulation of blood pressure predominantly depends upon the kidney . This primarily occurs through maintenance of the extracellular fluid compartment, the size of which depends on the plasma sodium concentration . Renin is the first in a series of important chemical messengers that make up the renin--angiotensin system . Changes in renin ultimately alter the output of this system, principally the hormones angiotensin II and aldosterone . Each hormone acts via multiple mechanisms, but both increase the kidney's absorption of sodium chloride, thereby expanding the extracellular fluid compartment and raising blood pressure . When renin levels are elevated, the concentrations of angiotensin II and aldosterone increase, leading to increased sodium chloride reabsorption, expansion of the extracellular fluid compartment, and an increase in blood pressure . Conversely, when renin levels are low, angiotensin II and aldosterone levels decrease, contracting the extracellular fluid compartment, and decreasing blood pressure . </P> <P> The kidneys secrete a variety of hormones, including erythropoietin, and the enzyme renin . Erythropoietin is released in response to hypoxia (low levels of oxygen at tissue level) in the renal circulation . It stimulates erythropoiesis (production of red blood cells) in the bone marrow . Calcitriol, the activated form of vitamin D, promotes intestinal absorption of calcium and the renal reabsorption of phosphate . Part of the renin--angiotensin--aldosterone system, renin is an enzyme involved in the regulation of aldosterone levels . </P> <P> Calculations of kidney performance are an important part of physiology and can be estimated using the calculations below . </P>

What are the hormones produced by the kidney