Capillary Fluid Exchange

Capillaries are where fluids, gases (carbon dioxide and oxygen), nutrients, and wastes are exchanged between the blood and body tissues by diffusion. Recall that diffusion is the movement of ions (Na+, K+, Ca2+) and small molecules (H2O, glucose/sugars) from an area of high concentration to low concentration. However, many larger sized particles such as proteins cannot pass easily between the endothelial cells or fenestrations of capillaries walls.
Fluid exchange is controlled by blood pressure within the capillary vessel (hydrostatic pressure) and osmotic pressure of the blood within the vessel. Fluid decreases as it moves away from the heart. Osmotic pressure (referred to as oncotic pressure or colloid oncotic pressure in clinical terms) is created by the movement of water from an area of low solute concentration, to an area of high solute concentration. The osmotic pressure is produced by high concentrations of salts and plasma proteins in the blood. Blood plasma proteins such as albumin are too large to leave blood vessels, and remain suspended in blood plasma (the liquid portion of blood) in order to maintain oncotic pressure.

As blood enters the capillary bed on the arteriole end, the blood pressure (i.e. hydrostatic pressure) in the capillary vessel is greater than the osmotic pressure of the blood in the vessel. The net result is that fluid leaves the capillary, and moves to the body tissue (filtration). At the middle of the capillary bed,