from Guyton and Hall textbook of medical physiology, 4th South Asia ed

from Guyton and Hall textbook of medical physiology, 4th South Asia edition write a 5 mark short note on Venous Return

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Short Note: Venous Return

(Guyton and Hall Textbook of Medical Physiology, 4th South Asia Edition)

Definition

Venous return is the quantity of blood flowing from the veins into the right atrium each minute. It is the counterpart of cardiac output - the two must be equal (except transiently when blood is stored in or removed from the heart and lungs). This equality is maintained because the heart pumps only what it receives, a concept central to cardiovascular regulation.

Factors Determining Venous Return

Three principal factors govern venous return from the systemic circulation to the heart:

Right atrial pressure (RAP) - Impedes flow of blood from the veins into the right atrium. A rise in RAP reduces venous return; a fall increases it.
Mean Systemic Filling Pressure (MSFP) - The pressure that "pushes" blood toward the heart. It represents the degree of filling of the systemic circulation and is measured when all blood flow is stopped. Normal MSFP is approximately 7 mmHg. It rises with increased blood volume or venous tone, and falls with the reverse.
Resistance to venous return - The total resistance the blood must overcome between the peripheral vessels and the right atrium, primarily at the level of the arterioles, small arteries, and venules.

Venous Return Curve

The venous return curve plots venous return against right atrial pressure:

When the heart fails completely (RAP rises toward MSFP of ~7 mmHg), venous return falls to zero - because there is no longer a pressure gradient from the systemic circulation to the right atrium.
As RAP falls below MSFP, venous return progressively increases.
When RAP falls below -4 to -8 mmHg, the large veins in the chest collapse (due to negative intrathoracic pressure), and venous return reaches a plateau (maximum ~5-6 L/min at rest) - it cannot increase further regardless of how negative RAP becomes.

The driving pressure for venous return is:

Venous Return = (MSFP - RAP) / Resistance to Venous Return

Changes in the Venous Return Curve

Factor	Effect on Curve
Increased blood volume	Shifts curve right; raises MSFP
Sympathetic stimulation	Raises MSFP (venous constriction); shifts curve right
Decreased blood volume	Lowers MSFP; shifts curve left
Increased arteriolar resistance	Rotates curve to a lower slope (reduces VR for any given RAP)
Decreased arteriolar resistance	Rotates curve to a higher slope (increases VR)

Analysis of Cardiac Output Using the Venous Return Curve

The venous return curve and the cardiac output curve are plotted together on the same axes. Their intersection point (the "equilibrium point" or "operating point") determines the actual cardiac output and right atrial pressure simultaneously achieved by the cardiovascular system. For a normal heart at rest, this equilibrium occurs at a cardiac output of ~5 L/min with a RAP of about 0 mmHg.

When the heart is stimulated (e.g., by sympathetic activation), the cardiac output curve shifts upward-left and the new equilibrium shifts to a higher cardiac output. Conversely, cardiac failure shifts the cardiac output curve downward and the equilibrium shifts to a lower cardiac output with elevated RAP.

Control of Cardiac Output by Venous Return - Frank-Starling Mechanism

Within physiological limits, the heart pumps whatever blood is delivered to it. When venous return increases, more blood fills the ventricles, stretching the myocardial fibers. By the Frank-Starling mechanism, this stretch increases the force of contraction, raising stroke volume to match the increased venous return. Thus, under normal conditions, venous return is the primary determinant of cardiac output, not the intrinsic pumping capacity of the heart.

Source: Guyton and Hall Textbook of Medical Physiology, Chapter 20 - Cardiac Output, Venous Return, and Their Regulation.

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