Transport Pathways and Processes

Abstract

The goal of this chapter is to chart procedures for identifying endothelial transport pathways and mechanisms for water and solutes. An excellent description of the discovery of the microcirculation, of the identification of the capillaries as the main sites of interchange of fluid and solutes, and of the development of the concept of capillary permeability has been written by E. M. Landis.²⁵ In the first half of this century, it was recognized that capillaries in most organs of the body allowed relatively free exchange of water and small solutes (crystalloids), but restricted the escape of plasma proteins and other macromolecules (colloids). In 1896, Ernest Starling⁵³ explained the retention of vascular volume as the result of a balance of the osmotic pressure of plasma colloids and the hydrostatic pressure of capillary blood. Imbalance of hydrostatic and colloid osmotic forces results in fluid movement into or out of the bloodstream. This process is called ultrafiltration. The fluid moved (ultrafiltrate) contains water and small solutes in equilibrium with plasma, but only traces of plasma proteins. The direction and rate of fluid movement are proportional to sign and magnitude of the difference between hydrostatic and effective osmotic forces (positive = outward).^{24, 35} Exchange of water and solutes smaller than plasma proteins was attributed to passive diffusion.^{22, 23} Differences in capillary permeability from organ to organ were recognized: the capillaries of the brain were known to be nearly impermeable to ions and small molecules, while the sinusoids of the liver and spleen were known to be permeable to plasma proteins. It was also recognized that capillaries in most organs allowed small (“trace”) amounts of plasma proteins to escape into the interstitial fluid, and that the lost protein was returned to the bloodstream by the lymphatics.^{16, 29}