The role of the kidney in blood volume regulation: The kidney as a regulator of the hematocrit

The kidney plays a pivotal role in the regulation of blood volume by controlling the plasma volume and red blood cell (RBC) mass. Further, it is proposed that the kidney coordinates the relative volumes of these 2 blood components and in so doing regulates the hematocrit. This novel function as proposed is a functional concept whereby the kidney does not simply produce erythropoietin, but that the kidney regulates the hematocrit is termed the critmeter function. The kidney is unique in that it can indirectly report on blood volume as a tissue oxygen signal. It is proposed that the kidneys detect small changes in tissue oxygen tension for erythropoietin production at the critmeter, a functional unit of marginal oxygen tension within the kidneys. As the production of erythropoietin is modulated by angiotensin 11, the renin-angiotensin system entrains the production of erythropoietin as part of the effector signals of the feedback loop of blood volume regulation.