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Learning Objectives

Learning Objectives

By the end of this chapter the student will be able to:

  • Explain the main factors that regulate cardiac output, heart rate, and stroke volume including the autonomic nervous system.

  • Describe the impact of preload, afterload, and contractility on cardiac output.

  • Explain the various methods used for measuring cardiac output.

  • Integrate the myocardial structure with the contraction of the heart.

  • Explain the coupling of electrical excitation with the contraction of the heart.

  • Describe the regulation of [Ca2+]i and its impact on contractility.

  • Describe the effects of the autonomic nervous system on contractility.

  • Identify the importance of the Frank-Starling relationship on cardiac output.

  • Describe the relationship between cardiac and vascular curves and their coupling.

  • Explain pressure and volume in the heart during the cardiac cycle through a ventricular pressure-volume loop.

  • Describe how preload, afterload, and contractility affect the pressure-volume loop.

Introduction

Cardiac output (CO) is the total volume of blood ejected by the ventricles per minute. It is approximately 5 L/min at rest in an average healthy individual (70 kg man) and it can increase up to 20 to 25 L/min during maximum exercise. CO is critical to maintain a sufficient blood supply to all body tissues, especially to the brain and other vital organs during various stressful conditions. At all times, the CO of the right ventricle going to the pulmonary circulation is equal to the CO of the left ventricle going to the systemic circulation. Furthermore, the venous return to both sides of the heart equals their CO. This precise control of CO depends on the regulation of heart rate and stroke volume. In this chapter, the various mechanisms that regulate CO, heart rate, and stroke volume as well as the pressure-volume relationship will be discussed.

Regulation of Cardiac Output

Cardiac output varies based on the body’s requirements and depends on the heart rate (beats per minute) and the stroke volume (blood volume pumped per heartbeat) as follows:

Cardiac output = Heart rate × Stroke volume

 

On average, the heart rate is equal to 70 beats per minute and the stroke volume is approximately 70 mL/beat. Thus, the CO is 4900 mL/min or approximately 5 L/min. This means that each minute the right ventricle pumps 5 L of blood to the lungs and the left ventricle pumps 5 L of blood to the systemic circulation, which is approximately the total volume of blood in the body (~5-5.5 L). During exercise of an average untrained individual, CO can reach 20 to 25 L/min; however, it can further increase to almost 40 L/min in a trained athlete during endurance exercise. The difference between CO at rest and during maximum exercise is referred to as cardiac reserve. This variability of CO is based on the body’s demand and is regulated via several mechanisms as discussed ...

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