Cardiac output curves at different levels of intrapleural pressure and at different degrees of cardiac tamponade. (Redrawn from Guyton AC, Jones CE, Coleman TB: Circulatory Physiology: Cardiac Output and Its Regulation. 2nd ed. Philadelphia: WB Saunders Co, 1973.)
2. Breathing against a negative pressure, which shifts the curve to a more negative right atrial pressure (to the left).
3. Positive pressure breathing, which shifts the curve to the right.
4. Opening the thoracic cage, which increases the intrapleural pressure to 0 mm Hg and shifts the cardiac output curve to the right 4 mm Hg.
5. Cardiac tamponade, which means accumulation of a large quantity of fluid in the pericardial cavity around the heart with resultant increase in external cardiac pressure and shifting of the curve to the right. Note in Figure 20-7 that cardiac tamponade shifts the upper parts of the curves farther to the right than the lower parts because the external "tamponade" pressure rises to higher values as the chambers of the heart fill to increased volumes during high cardiac output.
Combinations of Different Patterns of Cardiac Output Curves.
Figure 20-8 shows that the final cardiac output curve can change as a result of simultaneous changes in (a) external cardiac pressure and (b) effectiveness of the heart as a pump. Thus, by knowing what is happening to the external pressure as well as to the capability of the heart as a pump, one can express the momentary ability of the heart to pump blood by a single cardiac output curve.
There remains the entire systemic circulation that must be considered before total analysis of cardiac regulation can be achieved. To analyze the function of the systemic circulation, we first remove the heart and lungs from the circulation of an animal and replace them with a pump and artificial oxygenator system. Then, different factors, such blood volume, vascular resistances, and central venous pressure in the right atrium, are altered to determine how the systemic circulation operates in different circulatory states. In these studies, one finds
Combinations of two major patterns of cardiac output curves showing the effect of alterations in both extracardiac pressure and effectiveness of the heart as a pump. (Redrawn from Guyton AC, Jones CE, Coleman TB: Circulatory Physiology: Cardiac Output and Its Regulation. 2nd ed. Philadelphia: WB Saunders Co, 1973.)
three principal factors that affect venous return to the heart from the systemic circulation. They are as follows:
1. Right atrial pressure, which exerts a backward force on the veins to impede flow of blood from the veins into the right atrium.
2. Degree of filling of the systemic circulation (measured by the mean systemic filling pressure), which forces the systemic blood toward the heart (this is the pressure measured everywhere in the systemic circulation when all flow of blood is stopped—we discuss this in detail later).
3. Resistance to blood flow between the peripheral vessels and the right atrium.
These factors can all be expressed quantitatively by the venous return curve, as we explain in the next sections.
In the same way that the cardiac output curve relates pumping of blood by the heart to right atrial pressure, the venous return curve relates venous return also to right atrial pressure—that is, the venous flow of blood into the heart from the systemic circulation at different levels of right atrial pressure.
The curve in Figure 20-9 is the normal venous return curve. This curve shows that when heart pumping capability becomes diminished and causes the right atrial pressure to rise, the backward force of the rising atrial pressure on the veins of the systemic circulation decreases venous return of blood to the heart. If all nervous circulatory reflexes are prevented from acting, venous return decreases to zero when the right atrial pressure rises to about +7 mm Hg. Such a slight rise in right atrial pressure causes a drastic decrease in venous return because the systemic circulation is a distensible bag, so that any increase in back pressure causes blood to dam up in this bag instead of returning to the heart.
At the same time that the right atrial pressure is rising and causing venous stasis, pumping by the heart also
Normal venous return curve. The plateau Is caused by collapse of the large veins entering the chest when the right atrial pressure falls below atmospheric pressure. Note also that venous return becomes zero when the right atrial pressure rises to equal the mean systemic filling pressure.
approaches zero because of decreasing venous return. Both the arterial and the venous pressures come to equilibrium when all flow in the systemic circulation ceases at a pressure of 7 mm Hg, which, by definition, is the mean systemic filling pressure (Psf).
Plateau in the Venous Return Curve at Negative Atrial Pressures—Caused by Collapse of the Large Veins. When the right atrial pressure falls below zero—that is, below atmospheric pressure—further increase in venous return almost ceases. And by the time the right atrial pressure has fallen to about -2 mm Hg, the venous return will have reached a plateau. It remains at this plateau level even though the right atrial pressure falls to -20 mm Hg, -50 mm Hg, or even further. This plateau is caused by collapse of the veins entering the chest. Negative pressure in the right atrium sucks the walls of the veins together where they enter the chest, which prevents any additional flow of blood from the peripheral veins. Consequently, even very negative pressures in the right atrium cannot increase venous return significantly above that which exists at a normal atrial pressure of 0 mm Hg.
Mean Circulatory Filling Pressure and Mean Systemic Filling Pressure, and Their Effect on Venous Return
When heart pumping is stopped by shocking the heart with electricity to cause ventricular fibrillation or is stopped in any other way, flow of blood everywhere in the circulation ceases a few seconds later. Without blood flow, the pressures everywhere in the circulation become equal. This equilibrated pressure level is called the mean circulatory filling pressure.
Effect of Blood Volume on Mean Circulatory Filling Pressure.
The greater the volume of blood in the circulation, the greater is the mean circulatory filling pressure because extra blood volume stretches the walls of the vascula-ture. The red curve in Figure 20-10 shows the approximate normal effect of different levels of blood volume on the mean circulatory filling pressure. Note that at a blood volume of about 4000 milliliters, the mean circulatory filling pressure is close to zero because this is the "unstressed volume" of the circulation, but at a volume of 5000 milliliters, the filling pressure is the normal value of 7 mm Hg. Similarly, at still higher volumes, the mean circulatory filling pressure increases almost linearly.
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