Transcapillary Fluid Exchange in the Renal Cortex

• In the microcirculation of the renal cortex, fluid exchange proceeds at very high rates in two anatomically and functionally distinct capillary systems, the glomerular and the peritubular capil-laries. Net ultrafiltration is confined to the glomeru-lus, which consists of a tuft of capillaries surrounded by the ultrafiltrate enclosed within Bowman's capsule at the proximal end of the nephron. Normally, this ultrafiltrate is almost entirely reab-sorbed during passage along the renal tubule and returned to the circulation via uptake into the surrounding network of peritubular capillaries. Thus, the processes of ultrafiltration and absorption, which in the peripheral microcirculation typically occur across the arterial and venous ends of the capillaries, respectively, proceed in the kidney across two distinct sets of capillaries connected in series by the efferent arteriole. The rate of fluid exchange through capillary walls is governed by the difference between the opposing hydrostatic and colloid osmotic pressures, as originally conceived by Ludwig (1) and refined by Starling (2). At any point along a capillary this relationship may be expressed as = fc[(Pc-P/)-(irc-ir,)], (1) where / " is the net transcapillary fluid flux averaged over several cardiac cycles (positive for ultrafiltra-tion and negative for absorption), AP and An-are the transcapillary hydrostatic and colloid osmotic pressure differences, respectively, P c and ir G are the hydrostatic and colloid osmotic pressures in the capillary, P/ and 717 are the corresponding pressures in the surrounding interstitial fluid, and k is the effective hydraulic permeability of the capillary wall measured in the presence of an osmotically active solute; k generally differs from the hydraulic permeability measured using pure water (L p). To define k, the radially averaged protein concentration is used to compute osmotic pressure, but the concentration at the capillary wall and thus the true osmotic pressure are likely to be somewhat different. Under these conditions, a radial concentration gradient will be manifested as a resistance to transcapillary fluid movement (R) in series with the resistance presented by the capillary wall. Evidence consistent with the Ludwig-Starling hypothesis has been inferred largely from studies of single capillaries in omentum, mesentery, and skeletal muscle of amphibia and mammals (3—5) and from studies using more indirect, isogravimetric or isovolumetric methods for estimating transcapillary fluid exchange in whole organs (6, 7). In general, this evidence shows that AP is slightly greater than A77 at the arterial end of a capillary (thereby favoring net ultrafiltration) and is slightly less …