Comparison of chloride concentration and osmolality in proximal tubular fluid, peritubular capillary plasma and systemic plasma in the rat.

1. Chloride concentration and osmolalities were compared in consecutively collected samples of proximal tubular fluid, peritubular capillary plasma and systemic plasma. 2. Mean chloride concentrations (m-mole/l) were 141.3+/-2.6 in tubular fluid, 114.8+/-1.7 in peritubular capillary plasma and 119.4+/-1.8 in systemic plasma. 3. Mean osmolalities (m-osmole/kg H2O) were 297+/-2.2 in tubular fluid, 293+/-2.4 in peritubular capillary plasma and 299+/-1.8 in systemic plasma. 4. These differences are discussed in relation to the anatomical and functional organization of the peritubular capillaries and renal tubules.