Venous return - gravity

    Cardiovascular Physiology Concepts
                                    Richard E. Klabunde, Ph.D.

HOME

Search Contents

Keywords Tutorials

Topics:

Arrhythmias

Cardiac Valve Disease

Coronary Artery Disease

Edema

Heart Failure

Hypertension

Peripheral Artery Disease

Click here for information on Cardiovascular Physiology Concepts, published by Lippincott Williams & Wilkins (200 5)

Effects of Gravity on Venous Return

Gravitational forces significantly affect venous return and therefore cardiac output, and arterial and venous pressures. To illustrate this, consider a person who is lying down and then suddenly stands up. As the person stands, gravity acts on the vascular volume so that blood accumulates in the lower extremities. (Compare the size of veins in the top of your feet while lying down and standing.) Because venous compliance is high and the veins readily expand with blood, most of the blood volume shift occurs in the veins. Therefore, venous volume and pressure becomes very high in the feet and lower limbs when standing. This shift in blood volume decreases thoracic venous blood volume and therefore central venous pressure decreases. This decreases right ventricular filling pressure (preload), leading to a decline in stroke volume by the Frank-Starling mechanism. Left ventricular stroke volume also falls because of reduced pulmonary venous return (decreased left ventricular preload). This causes cardiac output and arterial blood pressure to fall. If arterial pressure falls appreciably upon standing, this is termed orthostatic or postural hypotension. Normally, baroreceptor reflexes are activated to restore arterial pressure by causing peripheral vasoconstriction and cardiac stimulation.

Without the operation of important compensatory mechanisms, standing upright would lead to significant edema in the feet and lower legs in addition to orthostatic hypotension. Venous pooling and reduced venous return are rapidly compensated in a normal individual by myogenic and neurogenic vasoconstriction of veins, the functioning of venous valves, by muscle pump activity, and by the abdominothoracic pump. When these mechanisms are operating, capillary and venous pressures in the feet will only be elevated by 10-20 mmHg, mean aortic pressure will be maintained, and central venous pressure will be only slightly reduced.

Despite the operation of compensatory mechanisms, changes in posture still affect cardiovascular balance. Therefore, a person who is standing upright has increased systemic vascular resistance (sympathetic mediated), decreased venous compliance (due to sympathetic activation of veins), decreased stroke volume and cardiac output (due to decreased preload), and increased heart rate (baroreceptor-mediated tachycardia). These changes help to maintain normal mean aortic pressure when a person stands.

RK Revised 04/04/07

DISCLAIMER: These materials are for educational purposes only, and are not a source of medical decision-making advice.