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The complexities of osmolarity and partitioning of fluids explained


This article first outlines definitions and descriptions of key terms that will be used in the discussions that follow including osmolarity, osmolality, osmotic and oncotic pressure. The physicochemical properties of water, ions and organic molecules are discussed in terms of their biological roles. Similarly, the interactions of amphipathic molecules and their three-dimensional structures in aqueous and non-aqueous environments are then explored.

The movement of solutes and solvents across semipermeable membranes is considered and an assessment is made of the contribution of such mechanisms to normal physiology. Firstly, simple diffusion is described, followed by comparisons with facilitated diffusion and energy-dependent active transport focusing on the differences in kinetics and rate-limiting factors arising as a result. The mechanisms underlying Gibbs–Donnan equilibria are discussed, with particular emphasis as to how they arise across a selectively permeable membrane by way of a worked example. The equilibrium of forces influencing fluid movements across the capillary endothelium, known as Starling forces, is then described. Comparisons are made between the classic model as first proposed by Starling in 1896 and the modified glycocalyx model, which has developed over the past 25 years. Finally, the impact of these differences on our understanding of trans-capillary fluid flux is discussed.


Waterhouse, B.R. and Farmery, A.D. (2015) Osmolarity and partitioning of fluids. Anaesthesia and Intensive care Medicine. September 24th. .

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