Right atrial pressure (mm Hg)
Analysis of the effect on cardiac output of (1) moderate sympathetic stimulation (from point A to point C), (2) maximal sympathetic stimulation (point D), and (3) sympathetic inhibition caused by total spinal anesthesia (point B). (Redrawn from Guyton AC, Jones CE, Coleman TB: Circulatory Physiology: Cardiac Output and Its Regulation. 2nd ed. Philadelphia: WB Saunders Co, 1973.)
each other at point A, which represents a normal venous return and cardiac output of 5 L/min and a right atrial pressure of 0 min Hg. Note in the figure that maximal sympathetic stimulation (green curves) increases the mean systemic filling pressure to 17 mm Hg (depicted by the point at which the venous return curve reaches the zero venous return level). And the sympathetic stimulation also increases pumping effectiveness of the heart by nearly 100 per cent. As a result, the cardiac output rises from the normal value at equilibrium point A to about double normal at equilibrium point D—and yet the right atrial pressure hardly changes. Thus, different degrees of sympathetic stimulation can increase the cardiac output progressively to about twice normal for short periods of time, until other compensatory effects occur within seconds or minutes.
Effect of Sympathetic Inhibition on Cardiac Output. The sympathetic nervous system can be blocked by inducing total spinal anesthesia or by using some drug, such as hexamethonium, that blocks transmission of nerve signals through the autonomic ganglia. The lowermost curves in Figure 20-15 show the effect of sympathetic inhibition caused by total spinal anesthesia, demonstrating that (1) the mean systemic filling pressure falls to about 4 mm Hg and (2) the effectiveness of the heart as a pump decreases to about 80 per cent of normal. The cardiac output falls from point A to point B, which is a decrease to about 60 per cent of normal.
Effect of Opening a Large Arteriovenous Fistula. Figure 20-16 shows various stages of circulatory changes that occur after opening a large arteriovenous fistula, that is, after making an opening directly between a large artery and a large vein.
1. The two red curves crossing at point A show the normal condition.
Analysis of successive changes in cardiac output and right atrial pressure in a human being after a large arteriovenous (AV) fistula is suddenly opened. The stages of the analysis, as shown by the equilibrium points, are A, normal conditions; B, immediately after opening the AV fistula; C, 1 minute or so after the sympathetic reflexes have become active; and D, several weeks after the blood volume has increased and the heart has begun to hypertrophy. (Redrawn from Guyton AC, Jones CE, Coleman TB: Circulatory Physiology: Cardiac Output and Its Regulation. 2nd ed. Philadelphia: WB Saunders Co, 1973.)
2. The curves crossing at point B show the circulatory condition immediately after opening the large fistula. The principal effects are (1) a sudden and precipitous rotation of the venous return curve upward caused by the large decrease in resistance to venous return when blood is allowed to flow with almost no impediment directly from the large arteries into the venous system, bypassing most of the resistance elements of the peripheral circulation, and (2) a slight increase in the level of the cardiac output curve because opening the fistula decreases the peripheral resistance and allows an acute fall in arterial pressure against which the heart can pump more easily. The net result, depicted by point B, is an increase in cardiac output from 5 L/min up to 13 L/min and an increase in right atrial pressure to about +3 mm Hg.
3. Point C represents the effects about 1 minute later, after the sympathetic nerve reflexes have restored the arterial pressure almost to normal and caused two other effects: (1) an increase in the mean systemic filling pressure (because of constriction of all veins and arteries) from 7 to 9 mm Hg, thus shifting the venous return curve 2 mm Hg to the right, and (2) further elevation of the cardiac output curve because of sympathetic nervous excitation of the heart. The cardiac output now rises to almost 16 L/min, and the right atrial pressure to about 4 mm Hg.
Pulsatile blood flow in the root of the aorta recorded using an electromagnetic flowmeter.
4. Point D shows the effect after several more weeks. By this time, the blood volume has increased because the slight reduction in arterial pressure and the sympathetic stimulation have both reduced kidney output of urine. The mean systemic filling pressure has now risen to +12 mm Hg, shifting the venous return curve another 3 mm Hg to the right. Also, the prolonged increased workload on the heart has caused the heart muscle to hypertrophy slightly, raising the level of the cardiac output curve still further. Therefore, point D shows a cardiac output now of almost 20 L/min and a right atrial pressure of about 6 mm Hg.
Other Analyses of Cardiac Output Regulation. In Chapter 21, analysis of cardiac output regulation during exercise is presented, and in Chapter 22, analyses of cardiac output regulation at various stages of congestive heart failure are shown.
In animal experiments, one can cannulate the aorta, pulmonary artery, or great veins entering the heart and measure the cardiac output using any type of flowme-ter. An electromagnetic or ultrasonic flowmeter can also be placed on the aorta or pulmonary artery to measure cardiac output.
In the human, except in rare instances, cardiac output is measured by indirect methods that do not require surgery. Two of the methods commonly used are the oxygen Fick method and the indicator dilution method.
Pulsatile Output of the Heart as Measured by an Electromagnetic or Ultrasonic Flowmeter
Figure 20-17 shows a recording in a dog of blood flow in the root of the aorta made using an electromagnetic flowmeter. It demonstrates that the blood flow rises rapidly to a peak during systole, and then at the end of systole reverses for a fraction of a second. This reverse flow causes the aortic valve to close and the flow to return to zero.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.