Fluid exchange across endothelium.

The fluid pathway between plasma and lymph comprises 3 matrices of biopolymer chains arranged in series (endothelial glycocalyx, basement membrane, interstitial matrix), each of differing area, thickness, density and biochemical composition. Fluid exchange obeys the Starling principle but the 'balance' of pressures commonly favours filtration even 'downstream', and not venular reabsorption as still widely mistaught. At tissue level the maintenance of fluid balance remains controversial. The 3-pore theory is reviewed and updated following aquaporin characterisation. The permeability of the endothelial layer can be altered by both intracellular (Ca2+i and cyclic nucleotides) and extracellular mechanisms (albumin, orosomucoid), leading to gross 'hole' formation through as well as between cells (inflammatory stimuli) or more subtle changes (e.g. atrial natriuretic peptide). This is currently a fertile zone of interaction between classical physiology and molecular studies.