Cells exist within a body fluid compartment known as the interstitial fluid, and the cardiovascular system has evolved to ensure that the composition of the interstitial fluid is maintained within a narrow range. Homeostasis is accomplished by pumping a separate fluid compartment—plasma—around the body, where it can be “conditioned” as it passes through specific organs that add nutrients, oxygen, hormones and needed metabolites, and/or remove waste products. The plasma then delivers needed substances to other organs and tissues. Efficient transfer of substances between the cells and the plasma is accomplished by dense networks of capillaries, which offer little resistance to the transfer of substances across their walls, and provide for short diffusion distances between the capillaries and the sites at which products will be utilized. The pumping function in this system is provided by the heart, a four-chambered organ that drives blood around two circuits in series, one that perfuses the lungs and one that serves the remainder of the body.