Capillary fluid filtration. Starling forces and lymph flow

Abstract

This review has focused on how the tissue Starling forces and lymph flow oppose the formation of interstitial edema, i.e., edema safety factors. When capllary pressure is increased, tissue pressure increases in most tissues, the colloid osmotic pressure gradient acting across the capillary wall increases in all tissues with the exception of the liver, and lymph flow is an important component of the edema safety factor in most capillary beds. Some tissues, such as the liver, have overflow systems other than the lymphatic system, which can become operative when filtration is increased to prevent excessive tissue swelling. All capillary and tissue Starling forces are near equilibrium in intestinal and subcutaneous tissues, but a Starling force analysis of lung indicates that the forces measured in several experimental models are far from equilibrium. To explain the differences between the data obtained in different experimental models of lung, a mathematical approach was used in which the lung was represented as a two-compartment system. When fluid accumulation is considered to occur in a septal compartment which is connected by a high resistance pathway to the perivascular spaces, the fluid balance characteristics can be evaluated. Finally, the review indicates that Starling force analyses in whole organs are very useful experimental models with which to study fluid balance in a wide variety of tissues in both functional and pathological states. However, most published studies concerning fluid balance either measured too few Starling forces or no steady state was defined during the course of the experimental procedures. To describe fluid movement adequately, all Starling forces should be measured at steady state conditions, i.e., when lymph flow, lymphatic proteins, and tissue volume are nonchanging with time. Indications are that some tissues may require several hours to attain a true steady state with respect to tissue colloids. Therefore, each expeimental model must attain steady state conditions which are clearly defined before any inference can be made concerning the behavior of Starling forces as edema develops. Future research using microscopic techniques should allow more complete models to be developed concerning the mechanisms responsible for fluid movement at the microcirculatory level. Starling force analyses still provide the major means of assessing an organ's ability to regulate its interstitial volume, and several organ systems have not been studied. It is hoped that this review will stimulate workers to investigate capillary and tissue Starling forces in other capillary beds, e.g., heart, stomach, muscle, etc., and relate these important determinants of fluid balance not only to edema formation, but also to the functional state of the particular organ. The extension of Starling force analyses to chronic edema states, e.g., myxedema, diabetes, left-sided heart failure, etc., promises to provide physiologists and clinicians with the basic information necessary to explain how interstitial fluid volume is regulated in both healthy and diseased states. This basic approach certainly will provide new diagnostic procedures and treatment regimens in the clinical setting. At the present time, investigators can measure tissue pressure, lymph flow, K(f,c), and tissue volume in human leg and forearm. Although a complete Starling force analysis has not been conducted in human tissues, the techniques are available to estimate P(t), π(t), lymph flow, π(p), tissue volume, and P(c). Perhaps studies will soon be conducted in the human model similar to those presented in this review, using simple maneuvers such as cuffs to elevate venous pressure.