Nephron function in the early phase of ischemic renal failure. Significance of erythrocyte trapping. Trapping of red blood cells (RBCs) in renal medulla vasculature in postischemic acute renal failure (ARF) was found to depend upon the length of the ischemic period. Thus trapping occurred after 45 minutes but not 25 minutes of ischemia. By prior hemodilution to a hematocrit (hct) of 30%, RBC trapping after 45 minutes of ischemia could be completely prevented. Likewise hemo-concentration (hct = 60%) before 25 minutes of ischemia resulted in extensive RBC trapping. By increasing or decreasing the hct, the contribution of RBC trapping to the functional defects and decrease in renal blood flow that follows minor (25 min) and more substantial (45 min) ischemia was investigated. Renal blood flow (RBF) was measured by microspheres, and vascular and tubular pressure by the micropuncture technique. Glomerular filtration rate (GFR) was estimated from inulin clearance, and tubular function from urine osmolality and sodium and potassium excretion. It was found that postischemic RBF was not correlated to RBC trapping but depended on the length of ischemia. After both 25 and 45 minutes of ischemia tubular obstructions occurred in the proximal tubules and/or loops of Henle, causing an increase in proximal tubular pressure. These obstructions were dependent on the length of ischemia but not on RBC trapping. After hemoconcentration and 25 minutes of ischemia there was an increment in distal tubular pressure, indicating that abundant RBC trapping may contribute to an increase in tubular pressure by compression of medullary tubules and thereby reduce GFR. When the damage was more severe other factors came into play and the contribution of RBC trapping to the decrease in GFR was minimal. 