CV Physiology | Frank-Starling Mechanism
Venous return (VR) is the flow of blood back to the heart. Under steady-state conditions, venous return must equal cardiac output (CO) when averaged over time. B. The cardiac output was measured using the Fick method. Oxygen uptake Mixed venous oxygen content ml O2 per liter of blood. Using this %. j— Solutions to Quiz 1 The following relationship was derived to relate cardiac output to the various model parameters (in enous Return. (L/ min.). Click the Quiz button on the left side of the screen. • After answering Increased Venous. Return. Slow Heart. Rate. Extremely Fast. Heart Rate. Exercise . What's the relationship between venous return and stroke volume? (Page 8.) .
In this way, an increase in venous return to the heart leads to an equivalent increase in cardiac output to the systemic circulation. Hemodynamically, venous return VR to the heart from the venous vascular beds is determined by a pressure gradient venous pressure, PV, minus right atrial pressure, PRA divided by the venous vascular resistance RV as shown to the figure.
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.
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.
Although the above relationship is true for the hemodynamic factors that determine the flow of blood from peripheral veins back to the heart, it is important not to lose sight of the fact that blood flow through the entire systemic circulation can be represented by either the cardiac output or the venous return, because these are equal in the steady-state owing to the circulatory system being closed. 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.
Venous return is influenced by several factors. Rhythmical contraction of limb muscles as occurs during normal locomotory activity walking, running, swimming promotes venous return by the muscle pump mechanism. Sympathetic activation of veins decreases venous complianceincreases central venous pressure and promotes venous return indirectly by augmenting cardiac output through the Frank-Starling mechanismwhich increases the total blood flow through the circulatory system.
During respiratory inspirationthe venous return increases because of a decrease in right atrial pressure. It is not as straightforward as you might think. Thus, if the end-diastolic volume increases, the muscle fibers are lengthened and the ventricle contracts more forcefully, ejecting a greater stroke volume.
The figure to the right shows this Frank-Starling effect. What factor alters the filling during diastole? For the right ventricle, this is the pressure in the right atrium, because this is the pressure that is experienced by the right ventricle as it fills.
CV Physiology | Venous Return - Hemodynamics
Since there is no valve at the entrance to the right atrium, the pressure in the right atrium is necessarily the same as the pressure in the veins at the entrance to the right atrium. This pressure in the large veins at the entrance to the right atrium is called the central venous pressure. In other words, the central venous pressure is the same at the right atrial pressure, and this is the pressure that determines the filling of the right ventricle and thus its end-diastolic volume.
The central venous pressure always is only a few mm Hg, but nonetheless it does change enough to significantly affect the stroke volume.
In particular, posture changes this pressure and that is the factor with which we are here most concerned. The Effect of Posture on Stroke Volume Recall how voluminous and thin-walled the superior and inferior vena cava are.
You probably were able to put two fingers into the superior vena cava of the pig heart. When a person is lying down, the large veins in the chest are plump with blood. And because these veins are stretched, the pressure in them is higher than when they contain less blood.
CV Physiology: Venous Return - Hemodynamics
Consequently, when lying down, the central venous pressure is relatively high, the end-diastolic volume is relatively high and thus the stroke volume is comparatively high. But this changes when we stand. The pressure in the large veins in the legs increases greatly. For example, one meter below the heart, the effect of gravity adds about 74 mm Hg of pressure.
This causes the distensible, voluminous veins to expand, and blood pools in the leg veins.
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This reduces the blood in the central veins, and the central venous pressure drops. Because these central veins are very compliant structures, pressure cannot increase again in them until blood flows back into the thorax. The Effect of Muscle Contraction on Stroke Volume Lying down, of course, is one factor that would increase the amount of blood in the veins in the thorax and thus the central venous pressure. However, another important factor is muscle contraction.
If the standing person begins walking, the contractions of the leg muscles squeeze on the leg veins, thereby forcing blood from those veins up into the thorax.
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- Venous Return - Hemodynamics
- Regulation of Cardiac Output
This is called muscle pumping. Thus, as a standing person begins walking, the end-diastolic volume and thus the stroke volume increase. Muscle pumping works on the veins, but not the arteries, because veins are large, highly compliant and the larger ones have valves.
In other words, contracting skeletal muscles serve as auxillary pumps, squeezing blood back into the central veins. Why have Such Distensible Veins?
Given that the veins are so large and compliant, you might wonder why we are put together this way. It allows blood to move around substantially in the veins, altering central venous pressure and thus stroke volume. The reason is that it allows us to function despite significant changes in extracellular fluid volume and thus blood volume.
A person can lose a liter or more of blood or extracellular fluid through hemorrhage or diarrhea and still easily maintain arterial blood pressure. As volume is lost, the distensible veins simply contrict, compensating for the lost volume.
Likewise, an increase in extracellular fluid volume is accommodated by distension of the veins. The central venous pressure is hardly affected. This would lead to a disaster fairly quickly if not corrected. If this continued for 20 minutes, one liter of blood would be transferred from your systemic circulation into you pulmonary circulation.