The kidneys are not just about producing urine. They perform many other important functions silently of which we are not aware. In fact in advanced kidney failure, the functioning of  almost any other vital organ like the heart and the brain can be severely affected. But that is for a future post.

Kidneys maintain a normal hemoglobin level in the blood. 

Hemoglobin is the protein which carries oxygen to all the organs of the body. It is well known that the  red blood cells which carry the hemoglobin are produced by the bone marrow. What is not well known is the fact that it is the kidneys which stimulate the bone marrow to produce the RBCs. The kidneys are so sensitive, they can detect the oxygen levels in the blood. Whenever the oxygen content of the blood decreases  they release a hormone called erythropoietin (from Greek erythros 'red' and poiein 'make' : the hormone that makes red blood cells)  into the bloodstream.  This hormone reaches the bonemarrow and stimulates it to produce more RBCs. The system works so well that most people maintain a normal hemoglobin without any trouble. 

The above facts have some practical relevance. Low hemoglobin levels are very common in patients with kidney failure. It leads to weakness, fatigue, paleness and breathlessness. In fact in quite a few patients kidney failure is diagnosed when they are investigated for a persistently low hemoglobin. Erythropoietin injections are available and help to maintain good hemoglobin levels inspite of kidney failure. Before the injections became available kidney failure patients used to need blood transfusions to maintain hemoglobin levels. Patients getting hundreds of blood transfusions was quite common in those days. Now-a-days it is quite uncommon. 

So the next time you see a normal hemoglobin report, you would do well to offer a silent prayer of gratitude to your excellent pair of kidneys.