Under steady-state conditions, venous return must equal cardiac output (CO) when Although the above relationship is true for the hemodynamic factors that . Discuss how venous return regulates cardiac output Frank Starling law of the from Describe the relationship between blood flow, mean arterial pressure and . Strong sympathetic stimulation of the large vessels, particularly the veins, can an average of three fold, which greatly increases venous return and cardiac output. As an example he quotes experimental work where heavy exercise in the overall (whole body) proportional relationship of cardiac output to metabolic rate.
However, there are often temporary differences between cardiac output and venous return.
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Whenever such differences exist, the volume of the central venous compartment must be changing. Because the central venous compartment is enclosed by elastic tissues, any change in central venous volume produces a corresponding change in central venous pressure. On the other hand central venous pressure has an equally important negative effect on venous return.
Thus, central venous pressure is always automatically driven to a value that makes cardiac output equal to venous return.
CV Physiology | Venous Return - Hemodynamics
The Venous Function Curve Anatomically the peripheral venous compartment is scatered throughout the systemic organs, but functionally it can be viewed as a single vascular space that has a particular pressure PPV at any instant of time. The normal operating pressure in the peripheral venous compartment is usually very close to mean circulatory filling pressure. Moreover, the same factors that influence mean circulatory filling pressure have essentially equal influences on peripheral venous pressure.
Thus, "Peripheral venous pressure" can be viewed as essentially equivalent to "mean circulatory filling pressure. When the peripheral venous pressure is assumed to be 7 mm Hg, there will be no venous return when the central venous pressure PCV is also 7 mm Hg. This relationship is summarized by the venous function curve, which shows how venous return increases as central venous pressure drops.
If central venous pressure reaches very low values and falls below the intrathoracic pressure, the veins in the thorax are compressed, which therefore tends to limit venous return. In the example in Figureintrathoracic pressure is taken to be 0 mm Hg and the flat portion of the venous function curve indicates that lowering central venous pressure below 0 mm Hg produces no additional increase in venous return. The two ways in which peripheral venous pressure can change were: Second, peripheral venous pressure can be altered through changes in venous tone produced by increasing or decreasing the activity of sympathetic vasoconstrictor nerves supplying the venous smooth muscle.
In addition, an increase in any force compressing veins from the outside has the same effect on the pressure inside veins as an increase in venous tone. Thus, such things as muscle exercise and wearing elastic stockings tend to elevate peripheral venous pressure.
Venous Return - Hemodynamics
Shift of the venous function curve Whenever peripheral venous pressure is altered, the relationship between central venous pressure and venous return is also altered. For example, whenever peripheral venous pressure is elevated by increase in blood volume or by sympathetic stimulation, the venous function curve shifts upward and to the right. By similar logic, decreased peripheral venous pressure caused by blood loss or decreased sympathetic vasoconstriction of peripheral veins shifts the venous function curve downward and to the left.
Determination of Cardiac Output and Venous Return by Central Venous Pressure The significance of the fact that central venous pressure simultaneously affects both cardiac output and venous return can be best seen by plotting the cardiac function curve and the venous function curve on the same graph Figure Central venous pressure, as defined earlier, is the filling pressure of the right heart.
Strictly speaking, this pressure directly affects only the stroke volume and output of the right heart pump. In most contexts, however, "cardiac output" implies the output of the left heart pump.
How is it then, as we have previously implied, that central venous pressure profoundly affects the output of the left side of the heart? The proper answer is that changes in central venous pressure automatically cause essentially parallel changes in the filling pressure of the left side of the heart i. Consider, for example, the following sequence of consequences that a small step increase in central venous pressure has on a heart that previously was in a steady state: The circulatory system is made up of two circulations pulmonary and systemic situated in series between the right ventricle RV and left ventricle LV as depicted in the figure.
Balance is achieved, in large part, by the Frank-Starling mechanism. For example, if systemic venous return is suddenly increased e. Increased pulmonary venous return to the left atrium leads to increased filling preload of the left ventricle, which in turn increases left ventricular stroke volume by the Frank-Starling mechanism. In this way, an increase in venous return to the heart leads to an equivalent increase in cardiac output to the systemic circulation.
Therefore, increased venous pressure or decreased right atrial pressure, or decreased venous resistance leads to an increase in venous return. PRA is normally very low fluctuating a few mmHg around a mean of 0 mmHg and PV in peripheral veins when the body is supine is only a few mmHg higher. Because of this, small changes of only a few mmHg pressure in either PV or PRA can cause a large percent change in the pressure gradient, and therefore significantly alter the return of blood to the right atrium.Biology Help: Venous Return, Venous Pressure, Compliance
For example, during lung expansion inspirationPRA can transiently fall by several mmHg, whereas the PV in the abdominal compartment may increase by a few mmHg.
These changes result in a large increase in the pressure gradient driving venous return from the peripheral circulation to the right atrium. Therefore, one could just as well say that venous return is determined by the mean aortic pressure minus the mean right atrial pressure, divided by the resistance of the entire systemic circulation i. There is much confusion about the pressure gradient that determines venous return largely because of different conceptual models that are used to describe venous return.