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SECTION V: CARDIOVASCULAR PHYSIOLOGY: INTRODUCTION
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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.
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In this section, the components of the cardiovascular system that permit it to serve the body’s needs for substance transfer will be considered. First, the electrical activity that allows the chambers of the heart to contract in an ordered fashion, to move the circulation unidirectionally, will be discussed. Then, the properties of blood and its components that suit them to transport dissolved solutes to and from the interstitial fluid will be considered. The properties of the circulatory “plumbing,” or blood vessels, will be addressed, along with the mechanisms that regulate them. Finally, the specialized properties of the circulation in areas of the body with unique needs will be discussed.