The effects of changes in haematocrit on the intrarenal distribution of blood flow in the dog's kidney.

1. The effect of changes in the haematocrit of blood perfusing the kidney on its intrarenal distribution was studied in dogs.2. Two types of preparations were employed. (i) In the isolated perfused kidney evidence is presented that flow in the autoregulating preparation represents predominantly cortical flow while flow in the ;low flow non-autoregulating' kidney reflects medullary flow. (ii) In the intact kidney renal blood flow rate and its intrarenal distribution was studied by the injection of (133)Xe into the renal artery and measuring its clearance from the kidney by an external counter.3. In both types of preparation cortical flow was found to be independent of changes in P.C.V. but medullary flow varied inversely with haematocrit.4. A change in the haematocrit of the perfusing blood leads to alteration of its viscosity. It was argued that an increase in viscosity must lead to a reduction in the resistance of the cortical afferent arterioles but that medullary afferent arterioles were not able to respond in this manner.5. These findings demonstrate that changes in total body haematocrit cause a redistribution of blood flow between renal cortex and medulla.