SUMMARY Afferent arteriolar diameters, relative flow distribution, and flow conductance factors are estimated by nonlinear regression analysis of the sieving effect on microspheres in different vascular structures of the dog renal cortex. The data presented are from experiments in which microspheres of 10-30 fim were injected into the abdominal aorta during normotension and after lowering the blood pressure to the lower limit of autoregulation. Microscopic examination of the spheres trapped in the glomeruli and the renal arteries showed an increasing exclusion of microspheres greater than 15 fim from the afferent arterioles during normotension. This effect was most pronounced for the deeper cortical layers and can be explained mainly as geometrical exclusion of spheres from afferent arterioles. During hypotension, progressively larger microspheres entered glomeruli and afferent arterioles, presumably due to vasodilation of the vessels. There was a significant redistribution of microspheres larger than 15fim from the outer to the inner cortex during hypotension without a corresponding redistribution of smaller spheres or the estimated blood flow. Approximately the same degree of dilation of afferent arterioles was observed during autoregulatory hypotension in three cortical layers.IT IS commonly believed that the variation of the preglomerular vascular resistance during autoregulation takes place in the afferent arteriole. There is no direct evidence supporting this; neither hydrostatic pressure in this vessel nor its diameter has been measured during autoregulation. The autoregulatory changes in preglomerular vascular resistance may take place exclusively in the interlobular artery or in addition to the afferent arteriole.It also is not clear whether the variation in the preglomerular vascular resistance is identical in all of the so-called cortical layers. Lack of autoregulation in the inner medulla (dog)' and outer cortex (rat), 2 as well as identical autoregulation in all layers'' (dog), have been reported by measuring the local blood flow with various indirect methods.The intension of this study was to measure the variation in diameter of the afferent arteriole in the autoregulating dog kidney. For this purpose we used an original version of the microsphere method, the principle of which is to record the diameter population of the spheres which become trapped in (or pass through) the afferent arterioles and the population of sphere diameters presented to these vessels. 4 ' 5 Methods Mongrel dogs of either sex (body weight, 17-25 kg) were anesthetized with sodium pentobarbital (25 mg/kg given intravenously as an initial dose), and N 2 O was supplied by conventional anesthetic apparatus connected to an endotracheal tube. The arterial oxygen saturation, pH, and Pco 2 were within normal limits during the experFrom Medical Department A, University School of Medicine, Bergen, Norway.Address for reprints: Professor J. Ofstad, Medical Department A, University School of Medicine, 5016 Haukeland, Bergen, Norway.Receiv...

Hypertensive disease caused by stenosis of the renal artery in man seems to correspond fairly well with the experimental counterpart in different laboratory animals. Increased secretion of renin into the blood stream occurs early in the pathogenesis. Comparison of the renin concentration in the renal venous blood from both kidneys (renin ratio) is at present considered to be the most important diagnostic and prognostic method in this condition. Renin concentration ratios varying from 1.5 to 2.5 have been reported as highly indicative of curable renal hypertensive disease in man. A considerable number of patients cured by operation have ratios less than these values.Some questions should be considered before renin concentration ratios are applied in clinical practice : 1) What is the error of method in renin ratio measurments ?2) What is the relation between the ratio of renin concentrations and that of renin secretion rates ?3) Does the ratio express an all -or-nothing kind of mechanism or a graded biological phenomenon? (What does the popuation of the ratios look like?) Error of method in renin ratio measurementsOn the right side sampling from the short main stem carries the risk of sampling only from one section of the kidney if the catheter is pushed too far into the vessel, or the possi-bility of aspirating blood from the vena cava if the catheter has not been introduced far enough into the renal vein. On the left side the renal vein is so long that the risk of introducing the catheter into a branch is minimal. The greatest errors in sampling on the Ieft side are caused by aspirating blood from the spermatic or ovarian vein emptying in the inferior vein wall or from the left suprarenal vein emptying in the superior vein wall.One main cause of sampling errors on both sides is the laminar character of the blood stream in the renal veins. Although the evidence of laminar flow in the renal vein is indirect, it is quite conclusive. The Reynold's number of the blood stream is much lower than that thought critical for the onset of a turbulent flow regime. When roentgen contrast medium is injected into the renal vein, the findings are well compatible with a laminar type of flow, and the difficulties in obtaining flow measurements using indicator dilution techniques (dye dilution or thermodilution) is probably also partly due to the presence of this type of flow. We have ourselves in experiments with dye dilution measurements of the renal blood flow in man observed that dye injected into a branch feeding only the lower pole of the kidney was not recovered from the venous flow when the sampling catheter was placed in the upper part of the renal vein. The presence of a laminar blood flow has the obvious consequence that measuring renin concentrations in renal venous blood may be of little help in conditions where only a part of the kidney