Chapter 30: The Heart as a Pump

The cardiac cycle is meticulously detailed, beginning with late diastole where passive ventricular filling accounts for a majority of the volume, followed by atrial contraction to complete the process. Ventricular systole is characterized by an initial phase of isovolumetric contraction, where pressure builds sharply without volume change, eventually forcing open the semilunar valves for rapid ejection into the systemic and pulmonary systems. Important clinical measurements are introduced, such as the ejection fraction—a key metric of ventricular health—and the Frank-Starling law, which describes how the heart adjusts its contractile force based on the volume of blood it receives, a concept known as preload. The text clarifies the relationship between heart rate and the duration of cardiac phases, noting that diastole is significantly shortened during high heart rates, which can compromise coronary perfusion and ventricular filling. Physical diagnostic indicators like heart sounds and arterial pulses are explained as products of valve closures and pressure waves, while abnormal sounds or murmurs are linked to conditions like valvular stenosis or regurgitation. Methods for quantifying cardiac performance, including Fick’s principle and thermodilution, provide a basis for assessing output in both healthy and diseased states by dividing oxygen consumption by the difference in oxygen content between arterial and venous blood. Furthermore, the discussion extends to the pathophysiology of circulatory shock and heart failure, distinguishing between systolic and diastolic dysfunctions and their impact on ventricular pressure-volume loops. Myocardial oxygen demand is also analyzed, revealing that pressure-based work, such as overcoming high afterload in aortic stenosis, consumes more energy than volume-based work. The chapter concludes with an analysis of how the heart adapts to the extreme demands of physical exercise through integrated neural and hormonal controls, highlighting how athletes manage increased cardiac output through enhanced stroke volume compared to sedentary individuals.