Kidney as a Hormone Producer

I have concentrated upon the obvious functions of the kidney: regulation and excretion. Now we have to finish by considering a third function: the secretion of hormones. The first of these hormones which the kidney produces is called, appropriately, renin. This is part of the regulatory mechanism just out lined, and it produced by cells just beside each glomerolus usually called the "juxta-glomerular" apparatus. 

Renin of itself has no action, but it acts on a protein in the bloodstream to produce a small molecule with very powerful effects, angiotensin.

First, it constricts blood vessels and raises the blood pressure. Secondly, it causes the kidney to retain sodium. And thirdly, it stimulates the adrenal glands to produce the same hormone mentioned above which is triggered by volume receptors within the circulation. All these are powerful effects in regulating blood volume and blood pressure. It may come as a surprise to learn that the second hormone which the kidney produces is the active form of Vitamin D. This vitamin is esential for the formation and maintenance of helathy bone, and promotes the absorption of calcium from the gut. 

Being the vitamin it is a compound that we have to have in our diet because we cannot make it ourselves. Without it, children get the bone disease rickets, adults the similar disease of bones which have finished growing, called osteomalacia (soft bones). However, before vitamin D can act on bone or on calcium absorption it must be transformed twice. First in the liver, and then in the kidney to its active form. One could predict from this that patients with kidney disease might suffer from bone problems. Thirdly, the kidney produces a hormone which promotes the formation of red blood cells by the bone marrow. Red blood cells are called erythrocytes in Greek, and the hormone is called erythropoietin. It is released from the kidney if the oxygen in the blood falls low. For example, on going to high altitudes. Again one could predict that patients without kidneys or with kidney disease would became anaemic. Finally, the kidney produces several of an important group of compounds, the prostaglandins. These have only just achieved prominence, but one group is clearly very important in regulating blood flow and blood pressure and it produced in the kidney. Probably, they are part of kidney's regulation of its internal blood flow but like renin may have effects beyond the kidney.