Editing Anatomy and Physiology (section)

=== Vessels ===

Arteries carry blood away from the heart and are thick and elastic. Arterioles are the main site of resistance. Capillaries are one cell thick and are the only site of exchange. Veins are thin walled, hold about two thirds of blood volume, and rely on valves and skeletal muscle pumping for return.

'''Starling forces''' at the capillary determine edema: hydrostatic pressure pushes fluid out and oncotic pressure from plasma proteins pulls it back. Low albumin (liver failure, malnutrition, nephrotic syndrome) lowers oncotic pressure and causes edema despite a normal or low blood volume. Normal serum albumin is about 3.5 to 5.0 g/dL.