Capillary fluid filtration. Starling forces and lymph flow.

STARLING (1894, 1896) described the basic forces responsible for producing fluid shifts between the circulating blood and the surrounding tissue spaces. On page 324 of his classical article entitled "On the absorption of fluids from the connective tissue spaces," Starling described how capillary pressure determines transudation into the tissues and the osmotic pressure of the proteins determines absorption from the tissues. The paper also investigated how tissue pressure affects absorption, and Starling came to the correct conclusion that tissue pressure would not cause fluid to be absorbed from the tissue unless this pressure was not transmitted to the veins. Starling also recognized that a small amount of fluid was lost continually from the circulation ("frictional resistance of the capillary wall") and formed lymph, but believed that this was small and that the forces tending to move fluid out of and into the circulation were almost balanced. Finally, Starling postulated that tissue colloidal osmotic pressure decreased following elevation of capillary pressure and provided the opposing forces to limit transudation of fluid into interstitial spaces. Thus, Starling defined the basic forces responsible for maintaining fluid balance between the microcirculation and interstitium, and physiologists have termed tissue pressure (Pt), capillary pressure (Pc), plasma colloidal osmotic (TTP), and tissue colloidal osmotic (wt) pressure as Starling forces. However, a mathematical relationship between those forces was not formulated by Starling in his original papers. Staub (1978) presents a delightful discourse concerning the historical development of the mathematical description of the Starling hypothesis: