Summary The Control of Cardiac Output PDF

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Summary The Control of Cardiac Output...

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Summary: The Control of Cardiac Output

The heart rate is influenced by the autonomic nervous system, circulating hormones, and venous return. ■ Sympathetic stimulation increases the heart rate, and parasympathetic stimulation decreases it. Under resting conditions, parasympathetic tone dominates, and the heart rate is slightly slower than the intrinsic heart rate. When the activity level rises, venous return increases and triggers the Bainbridge reflex. The result is an increase in sympathetic tone and an increase in heart rate. ■ Circulating hormones, specifically E, NE, and T3, accelerate heart rate. ■ An increase in venous return stretches the pacemaker cells and increases heart rate. ■ The stroke volume (SV) is the difference between the enddiastolic volume (EDV) and the end-systolic volume (ESV). ■ The EDV is determined by the available filling time and the rate of venous return. ■ The ESV is determined by the amount of preload (the degree of myocardial stretching), the degree of contractility (adjusted by hormones and autonomic innervation), and the afterload (the arterial resistance to blood flow out of the heart). During exercise, increasing both the stroke volume and the heart rate can increase the cardiac output by 300–500 percent, to 18–30 L/min. The difference between resting and maximal cardiac outputs is the cardiac reserve. Trained athletes exercising at maximal levels may increase cardiac output by nearly 700 percent, to 40 L/min. Cardiac output cannot increase indefinitely, primarily because the available filling time shortens as the heart rate increases. At heart rates up to 160–180 bpm, the combination of increased venous return rate and increased contractility compensates for the reduced filling time. Over this range, cardiac output and heart rate increase together. But if the heart rate continues to climb, the stroke volume begins to drop. Cardiac output first plateaus and then declines. The Heart and the Vessels of the Cardiovascular System The purpose of cardiovascular regulation is to maintain adequate blood flow to all body tissues. The heart cannot accomplish this by itself, and it does not work in isolation. For example, when blood pressure changes, the cardiac centers adjust not only the heart rate but also the diameters of peripheral blood vessels. These adjustments work together to keep the blood pressure within normal limits and to maintain circulation to vital tissues and organs. In Chapter 21 we complete this story by detailing the cardiovascular responses to changing activities and

circulatory emergencies. We then conclude our discussion of the cardiovascular system by examining the anatomy of the pulmonary and systemic circuits....