By Billie Rubin
Guess who regulates how many red blood cells (RBCs) we need at any given time. Bone marrow? Liver? Spleen? Your lungs? Give up? It’s your kidneys. Yup, they don’t just make urine. It all starts when those little kidneys sense the level of oxygen in our blood. When the oxygen level is low, the kidneys put out a hormone called erythropoietin.
The bone marrow has special receptors for the erythropoietin and when it gets some, it starts to crank out RBCs. As more RBCs mature and start picking up oxygen in the blood and taking it to various tissues, the kidney senses that the oxygen level is now okay and stops producing erythropoietin and the bone marrow slows down for the time being. Interesting feedback loop, eh?