Crucial for understanding the cardiovascular system is that it is not exactly a closed system. Structurally, the system 'closed' in that formed elements do not leave the system. This distinguishes it from 'open' systems such as with insects. However, the movement of substances in the capillary beds between the blood, extracellular fluid, and intracellular fluid is a favorite MCAT topic. Movement across membrane barriers depends on fluid pressures and concentration gradients in particular locations, with reference to the permeability of cell membranes for particular substances. The capillary beds and the kidneys are two locations where these concerns are paramount. Many MCAT passages over the years have dealt with this material.