Oncotic pressures opposing filtration across non‐fenestrated rat microvessels

Abstract

We hypothesized that ultrafiltrate crossing the luminal endothelial glycocalyx through infrequent discontinuities (gaps) in the tight junction (TJ) strand of endothelial clefts reduces albumin diffusive flux from tissue into the 'protected region' of the cleft on the luminal side of the TJ. Thus, the effective oncotic pressure difference (σ∆π) opposing filtration is greater than that measured between lumen and interstitial fluid. To test this we measured σ∆π across rat mesenteric microvessels perfused with albumin (50 mg ml −1 ) with and without interstitial albumin at the same concentration within a few micrometres of the endothelium as demonstrated by confocal microscopy. We found σ∆π was near 70% of luminal oncotic pressure when the tissue concentration equalled that in the lumen. We determined size and frequency of TJ strand gaps in endothelial clefts using serial section electron microscopy. We found nine gaps in the reconstructed clefts having mean spacing of 3.59 µm and mean length of 315 nm. The mean depth of the TJ strand near gaps was 67 nm and the mean cleft path length from lumen to interstitium was 411 nm. With these parameters our three-dimensional hydrodynamic model confirmed that fluid velocity was high at gaps in the TJ strand so that even at relatively low hydraulic pressures the albumin concentration on the tissue side of the glycocalyx was significantly lower than in the interstitium. The results conform to the hypothesis that colloid osmotic forces opposing filtration across non-fenestrated continuous capillaries are developed across the endothelial glycocalyx and that the oncotic pressure of interstitial fluid does not directly determine fluid balance across microvascular endothelium. The classic Starling equation describing the balance of hydrostatic and colloid osmotic (oncotic) forces which determine filtration and reabsorption across the capillary wall includes four forces:where P c and P t are the hydrostatic pressures in the capillary lumen and tissue, respectively, and π c and π t are the corresponding lumen and tissue oncotic pressures. J v /A is the filtration rate per unit area, σ is the reflection coefficient to the plasma proteins and L p is the hydraulic conductivity of the vessel wall. This relation has been tested by changing P c and π c (only rarely by changing P t or π t ) and confirmed many times in both whole organ and isolated microvessels,

The endothelial glycocalyx: a review of the vascular barrier

“…They found that when the concentration of albumin on the outside of the vessel equalled that on the inside, the effective oncotic gradient from the lumen of the vessel to the interstitium was 70% of that when there was no surrounding albumin: the classic Starling equation predicts that there should be no pressure difference. This finding was termed the 'colloid osmotic pressure (COP) paradox' [2,15,16].…”

“…Adamson et al helped to answer this question [16] by infusing albumin into non-fenestrated microvessels. They found that when the concentration of albumin on the outside of the vessel equalled that on the inside, the effective oncotic gradient from the lumen of the vessel to the interstitium was 70% of that when there was no surrounding albumin: the classic Starling equation predicts that there should be no pressure difference.…”

“…2). The oncotic pressure differences across the endothelial surface layer are tightly regulated and reach equilibrium very quickly, thereby leading to less fluid filtration [11,15,16].…”

Volume Management and Resuscitation in Thoracic Surgery

“…Together with circulating substances, it forms a barrier that prevents circulating cells and macromolecules from entering the interstitium. In contrast to the original Starling model, which explained regulation of fluid balance occurring across the entire endothelial cell, a revised model has been proposed, whereby the hydrostatic and osmotic forces act only across the EGL surface layer on the luminal aspect of the endothelium [31]. These forces reach equilibrium very quickly, resulting in a much lower fluid flux than predicted by the traditional Starling equation (Fig.…”

“…However, in vitro and in vivo deviations from the classic Starling principle have been noted [28], such as absence of the venous reabsorption [29] and lymphatic flow [30] required to prevent interstitial edema, and lack of importance of the interstitial colloid osmotic pressure in determining transendothelial fluid balance [31]. This led to further investigation into non-Starling mechanisms of barrier regulation involving the endothelial glycocalyx layer (EGL) [32].…”