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.
In principle, this sounds like a simple system. However in practice, exquisite minute-to-minute regulation is needed to ensure that organs receive the substances that they need when they need them, particularly in the face of ever-changing demands. For example, when an individual begins to exercise, there is a prompt demand for additional oxygen and glucose in the contracting muscles to sustain muscle activity. In the brain, there is no capacity to store glucose and blood flow must be maintained to ensure consciousness, even in the face of hydrostatic challenges (eg, moving from a recumbent to a standing position). The cardiovascular system must therefore be able to adjust the rate at which the plasma is circulated around the body as a whole and to redirect plasma flow to the places where it is most needed. Further, the body is an “open” system, meaning that some bodily constituents (eg, water) are constantly being lost to the environment. The circulation, and the organs that condition it, must respond promptly to these threats to homeostasis to ensure the proper functioning of vital body systems, which typically operate within a narrow range of osmolarity, pH, oxygen saturation, etc.
In this section, we will consider the components of the cardiovascular system that permit it to serve the body’s needs for substance transfer. We will first consider the electrical activity that allows the chambers of the heart to contract in an ordered fashion, to move the circulation unidirectionally. We will then consider the properties of blood and its components that suit them to transport dissolved solutes to and from the interstitial fluid. The properties of the circulatory “plumbing,” or blood vessels, will next be addressed, along with the mechanisms that regulate them. Finally, we will consider the specialized properties of the circulation in areas of the body with unique needs.
Obviously, a functional cardiovascular system is vital for life, and irreversible damage quickly ensues in a number of organs if the heart stops beating. Less drastic disorders of the cardiovascular system also represent a considerable burden of disease. In fact, cardiovascular diseases, collectively, are the leading cause of death and responsible for significant disability worldwide. In the United States, heart disease and stroke are the first and third most common causes of death, and an estimated one in three American adults has some form of cardiovascular disorder. Cardiovascular diseases are also leading causes of hospitalization, and account for the highest economic burden of any disease category. Finally, while there have been impressive gains in treatment and prevention of some cardiovascular diseases, the growing epidemic of obesity, and the rise in the proportion of the population with at least one cardiovascular risk factor, has been cause for considerable alarm among public health officials. All of these facts underscore the relevance of a thorough understanding of cardiovascular physiology for the aspiring health professional.