File:Revised Starling Principle and fluid homeostasis.jpg

English: The Starling Principle states that fluid movement between blood and the interstitium is broadly determined by differences in hydrostatic pressure and colloid osmotic pressure between the blood plasma inside the blood capillary and the interstitial fluid in the surrounding tissues. (A) The traditional interpretation of the Starling Principle states that when blood capillary hydrostatic pressure (Pc) was larger than the plasma colloid osmotic pressure (πc) fluid would filter out of the capillaries (left yellow arrow). As the hydrostatic pressure begins to decline towards the venous end of the capillary, plasma colloid osmotic pressure was considered greater than the capillary hydrostatic fluid pressure. This would drive reabsorption and fluid would re-enter the venules by osmotic attraction (right yellow arrow). If true, then 90% of all interstitial fluid would re-enter the venous circulation while the remaining 10% would drain via the lymphatic system. This interpretation is incorrect. (B) With the direct measurement of interstitial hydrostatic fluid and colloid osmotic pressures, together with the discovery of the glycocalyx (or the “small pore pathway”), the old theory (A) has been disproven. Revision of the Starling forces has shown that, in the steady state, the forces driving filtration (green line) exceed the forces opposing filtration (blue line). Hence net, but dwindling, filtration will occur along the entire length of the capillary (grey area between the blue and green lines) with only periods of transient venous reabsorption as explained in C (when Starling forces temporarily change). This means that the bulk of interstitial fluid is drained in the lymph (yellow arrows). (C) Details of the blood vessel wall including the glycocalyx (yellow) are shown. The glycocalyx controls the local protein concentration within the intercellular cleft of the capillary wall, and hence the osmotic absorption gradient. If filtration declines or ceases, due to a drop in capillary pressure, venous absorption can occur. Due to reversed fluid movement, proteins in the interstitial fluid are drawn into the cleft where they accumulate in the sub-glycocalyx (as they cannot cross the glycocalyx). This raises local osmotic pressure of the sub-glycocalyx space to discourage reabsorption and ensures a return to a state of filtration. Therefore, according to the revised Starling model, venous absorption will only be transient. Pc = capillary hydrostatic pressure; Pi = interstitial hydrostatic pressure; πc = capillary plasma colloid osmotic pressure (COP); πg = sub-glycocalyx COP; πi = interstitial COP.