Chapter 9 - Microvascular Permeability and the Exchange of Water and Solutes Across Microvascular Walls

Abstract

Exchange through microvascular walls is both the initial and the final step of transport of materials by the circulation. In most tissues, microvascular exchange is a passive process, driven by differences in hydrostatic pressure and solute concentration between the circulating plasma and the interstitial fluid that flank microvessel walls. Lipophilic molecules and small water-soluble molecules and ions can exchange rapidly in most vascular beds, but microvascular walls are a barrier to macromolecules, severely impeding their exchange. The consequent differences in macromolecular concentration across microvascular walls are responsible for differences in osmotic pressure, which were identified over a century ago to play an essential role in the balance of fluid between the circulating blood and the tissues.