KEY POINTS

1. Capillaries, the smallest blood vessels, are the sites for the exchange of water and solutes between the bloodstream and the interstitial fluid.

2. Lipid-soluble substances diffuse readily through capillary walls, whereas lipid-insoluble substances must pass through capillary pores.

3. Fick's law of diffusion is a simple mathematical accounting of the physical factors that affect the rate of diffusion.

4. Water moves across capillary walls both by diffusion (osmosis) and by bulk flow.

5. The Starling equation quantifies the interaction of oncotic and hydrostatic forces acting on water.

6. Several common physiological changes alter the normal balance of Starling forces and increase the filtration of water out of capillaries.

7. Edema is a clinically noticeable excess of interstitial fluid.