Approximately 370 years ago, it was established that the heart's pumping action is essential to maintain the continuous circulation of blood throughout the body. Our current understanding of the detailed function of this amazing pump, its regulation, and modern treatments for heart disease is, in comparison, very recent. The heart is actually two pumps in one. The right side of the heart receivesblood from the body and pumpsblood through the pulmonary (pu ? l _ mo¯-na¯ r-e¯) circulation, which carriesblood to the lungs and returns it to the left side of the heart. In the lungs, carbon dioxide diffuses from the blood into the lungs, and oxygen diffuses from the lungs into the blood. The left side of the heart pumps blood through the systemic circulation, which delivers oxygen and nutrients to all remaining tissues of the body. From those tissues carbon dioxide and other waste products are carried back to the right side of the heart (figure 20.1). The heart of a healthy 70 kg person pumps approximately 7200 L (approximately 1900 gallons) of blood each day at a rate of 5 L/min. For most people, the heart continues to pump for more than 75 years. During periods of vigorous exercise, the amount of blood pumped per minute increases severalfold. The life of the individual is in danger if the heart loses its ability to pump blood for even a few minutes. Cardiology (kar-de¯-ol _ o¯-je¯) is a medical specialty concerned with the diagnosis and treatment of heart disease.

This chapter describes the functions of the heart (p. 680), size, shape, andlocation of the heart (p. 680), the anatomy of the heart (p. 682), the route of bloodflow through the heart (p. 688), and its histology (p. 690) and electrical properties (p. 693). The cardiac cycle (p. 698), mean arterial blood pressure (p. 704), regulation of the heart (p. 706), and the heart and homeostasis (p. 708) are described. The chapter ends with the effects of aging on the heart (p. 711).