The transport equation describing the flow of solute across a membrane has been modified on the basis of theoretical studies calculating the drag of a sphere moving in a viscous liquid undergoing Poiseuille flow inside a cylinder. It is shown that different frictional resistance terms should be introduced to calculate the contributions of diffusion and convection. New sieving equations are derived to calculate r and A,/Ax (respectively, the pore radius and the total area of the pores per unit of path length). These equations provide a better agreement than the older formulas between the calculated and the experimental glomerular sieving coefficients for [6I]polyvinylpyrrolidone (PVP) fractions with a mean equivalent radius between 19 and 37 A. From r and A,/Ax, the mean effective glomerular filtration pressure has been calculated, applying Poiseuille's law. A value of 15.4 mm Hg has been derived from the mean sieving curve obtained from 23 experiments performed on normal anesthetized dogs.In 1951, Pappenheimer et al. developed the so-called "pore theory" to account for the transcapillary transport of uncharged, lipid-insoluble solutes in mammalian muscles (24). According to this theory, convective flow and net diffusion contribute to solute flow across the membrane, in this case the capillary walls, both processes being impeded by the steric hindrance at the entrance of the "pores" (supposed to exist between the cells) and by frictional forces within the pores (20,22,23,25).The solute flow due to diffusion was calculated as D(c -c 2 )AW/Ax X A,/A, where D is the free diffusion coefficient, cl and c 2 , respectively, the solute concentrations in filtrand and filtrate and A,/Ax the pore area freely available to water per unit of length. The term A,/A, describes the restriction to the motion and can be calculated as 1/K 1 X SD where SD = [1 -(a,/r)]2 is the steric hindrance term (a, is the radius of the solute molecules

SUMMARY1. The effect of acute haemorrhage on the plasma renin concentration was studied in the dog and man.2. Plasma-renin concentration was regularly increased after the larger bleeds; after the smaller haemorrhages plasma-renin concentration remained unchanged.3. The results are discussed in relation to current hypotheses concerning the control of renin and aldosterone secretion.

Hypothetical mechanisms have been postulated to explain the presence of proteins in urine after severe exercise. Recently, it has been shown that several amino acids inhibit tubular protein reabsorption. Seven healthy men, hyperhydrated, were studied during a 2-min bicycle exercise at supramaximal load. The subjects were tested without or with lysine perfusion (0.4 g/kg body wt iv). In both testing conditions, blood lactate increased to 13.8 mmol/l. Total protein urinary excretion increased to 1.10 and 1.67 mg/min, without and with lysine perfusion, compared with 79 micrograms/min at rest. In the meantime, albumin excretion increased 48- and 74-fold, respectively, while beta 2-microglobulin excretion increased 97- and 1,043-fold compared with basal values. The renal clearance of albumin increased to 8.4 microliters/min without lysine and to 12.0 microliters/min with lysine perfusion (rest 0.18). beta 2-Microglobulin clearance increased to 10.0 and 39.3 ml/min, respectively (rest 0.05). These data clearly demonstrate that postexercise proteinuria is of mixed type after exhaustive short-term exertion. Both increased glomerular permeability and partial tubular reabsorption inhibition to proteins appear to be involved.

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