Lec. 8-b. Cardiac output and venous return (factors affecting venous return.)

Intro.

As explained before, Right atrial pressure, Mean Systemic (filling) pressure, and Resistance of vessels to blood flow are the major factors that affect venous return and thus COP as well. Each of these factors can be broken down into multiple factors that can be deduced from the relationship of the previous three main factors. $VR=\frac{P_V-P_{RA}}{R_V}$ Where we can substitute $P_V$ (venous pressure) with $M_{CP}$ (mean circulatory pressure), giving: $VR=\frac{M_{CP}-P_{RA}}{R_V}$ These secondary factors are:

1. Pressure gradient.
2. Respiration.
3. Gravity.
4. Vascular system.
5. Skeletal muscles.
6. Blood volume.

Factors affecting venous return.

Pressure gradient.

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$M_{CP}-P_{RA}$Where $M_{CP}$ is the circulatory pressure inside the entire circulation after heart has stopped, and $P_{RA}$ is the right atrial pressure. $M_{CP}$ is usually 7-8 mmHg while $P_{RA}$ is usually 2 mmHg. Venous return is 0 once $M_{CP}$ = $P_{RA}$.

(Note: do not get confused between $M_{CP}$ (mean circulatory pressure, the pressure inside the ENTIRE circulation once heart is stopped) and $M_{SP}$ (mean systemic pressure, the pressure inside the systemic circulation ONLY once heart is stopped.))

Respiratory movements.

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VR changes with respiratory movements, during inspiration VR increases due to the increased negative pressure in the thorax sucking in blood from outside the thorax. during expiration VR decreases due to decreased negative pressure in the thorax sucking in blood from outside the thorax. Exaggerated forms of these two phenomena are Muller’s experiment (forced inhalation with closed glottis) and Valsalva’s experiment (forced exhalation with closed glottis.)

Gravity.

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In the recumbent position, gravity has no effect on VR. While standing erect, gravity assists VR from the upper body and hinders it from the lower body. The effect of gravity in countered by negative thoracic pressure, pumping action of muscles, and the presence of valves.

Vascular system.

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The mechanical coupling of vessels and the diameters of different vessels has an effect on venous return.

Venous and Capillary diameter.

VD in veins or capillaries causes stagnation of blood, reducing (or totally diminishing) VR and COP.

Arteriolar diameter.

VD in arterioles causes increased BF to the capillaries and in turn, the venous circulation, increased VR and COP.

Arterial pulsations.

Arterial pulsations are mechanically coupled to veins, assisting in the movement of blood, increasing VR and COP.

Skeletal muscle contraction.

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Muscles work as peripheral hearts, their tones and contractions constantly prevent blood from pooling in the veins that are embedded in them.

Blood volume.

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• Increased BV -⇒ increased $M_{CP}$ -⇒ increased VR and COP.
• Decreased BV -⇒ decreased $M_{CP}$ -⇒ decreased VR and COP.