Chapter 1 - The heart

Abstract

A human heart pumps approximately 108,000 times per day, more than 39 million times in 1year, and nearly 3 billion times during a 75-year lifespan at the normal heart rate of 75 beats per minute. This chapter describes the location, size, and external details of the heart, and the internal anatomy of the heart to include the chambers, valves, and layers within the wall. It follows the flow of blood that reaches the heart in a step-by-step manner until it ejects blood, and describes the characteristics of the cardiac muscle fiber. The electrical activity within a cardiac conductive (pacemaker) cell is compared to that of a cardiac contractile cell. The parts of the cardiac conduction system are described, and the normal pacemaker of the heart named. The details of an electrocardiogram are then provided. A normal cardiac cycle is set out, beginning with the depolarization of the pacemaker cells and ending with the next round of pacemaker depolarization. Electrical events are correlated to mechanical changes, pressure changes, and blood flow changes. The chapter explains how cardiac output is measured, and describes how each component of cardiac output is regulated. The Frank-Starling law of the heart is set out; this principle states that, within physiological limits, the force of contraction is directly proportional to the initial length of the muscle fiber. The terms “preload” and “afterload” are defined. Finally, the chapter describes the mechanisms by which the parasympathetic and sympathetic nervous systems affect cardiac output.