Because Normal Cardiovascular Function Is Essential for Life and Health, a Practical Understanding of Cardiovascular Function and Dysfunction IsVitaI to the Veterinary Clinician
Cardiovascular physiology is the study of the function of the heart, the blood vessels, and the blood. The primary function of the cardiovascular system can be summarized in one word: transport.
The bloodstream transports numerous substances that are essential for life and health, including the oxygen and nutrients required by every cell in the body. Blood also carries carbon dioxide and other metabolic waste products away from Fnetabolically active cells and delivers them to the lungs, kidneys, or liver, where they are excreted.To appreciate the importance of cardiovascular transport, the reader need only consider what happens if the heart stops contracting and circulation ceases: unconsciousness results within about 30 seconds, and irreversible damage to the brain and other sensitive body tissues occurs within a few minutes. However, circulation does not have to stop completely for significant dysfunction to occur. For example, the loss of as little as 10% of the normal blood volume can impair exercise performance. In each tissue of the body, normal function depends on the delivery of blood flow. The higher the rate of metabolism in a tissue, the greater is the requirement for blood flow. The condition of inadequate blood flow to any tissue is called ischemia. Even transient ischemia leads to dysfunction. Persistent ischemia leads to permanent tissue damage (infarction) or cell death (necrosis).
Impairment in the transport functions of the cardiovascular system is encountered frequently in veterinary medicine. Because any cardiovascular impairment inevitably leads to significant dysfunction and loss of health, a practical understanding of cardiovascular function and dysfunction is vital to the veterinary clinician.
Many veterinary students have difficulty understanding cardiovascular physiology. I hey tend to agree with William Harvey, the father of cardiovascular physiology, whose initial impression was that the motions of the heart and the blood were so complicated that they could be comprehended only by God! Harvey persisted, however, in a careful, deliberate study of cardiovascular function and in 1628 set forth the first proof that the heart propels blood through the blood vessels in a circulatory pattern.
Before Harvey’s time, it was thought that blood flowed out of the heart into the blood vessels and then returned to the heart by backward flow in the same vessels. In other words, blood was thought to flow in a tidal manner, in much the same way that air flows through a single set of airways: first into the lungs and then back out.We now take for granted that the cardiovascular system is a circulatory system, not a tidal system. However, the circularity of the cardiovascular system is precisely what makes it difficult to understand. It has no clear beginning or ending, and disturbances in one part of the cardiovascular system end up affecting all other parts as well. In recognition of this complexity, Chapters 18 to 26 are written with the goal of identifying the most basic and important concepts of normal cardiovascular function and explaining them in a way that best prepares the reader to understand, diagnose, and treat cardiovascular dysfunction (cardiovascular disease).