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Factors Affecting Venous Return




Venous Pressure

  • Venous pressure on average is about 15 mmHg.

  • Pressure in right atrium is near 0 mmHg.

  • The pressure gradient from veins to right atrium drives movement of blood from veins to the right atrium.


Sympathetic Activity

Sympathetic stimulation

Constriction of veins

Blood flows to heart

Increased venous return.


Venous Valves

  • In standing position, gravity tends to pull blood down ⟶ venous return tends to decrease.

  • Venous valves allow flow of blood only towards the heart counters the effect of gravity helps in venous return.


Muscle Pump

  • Many veins have valves that allow blood flow only towards the heart.

  • Many such veins lie in between skeletal muscles.

  • Activity in these nearby muscles increases blood flow through veins in the following way:

Contraction of muscles

Squeezes the vein

Local increase in pressure inside the vein

1. Valve proximal to heart opens and blood flows up.

2. Valve distal to heart closes and prevents the backflow.

Muscles relax

1. Valve proximal to heart closes and prevents backflow of blood that was pushed up.

2. Valve distal to heart opens and pulls blood up.

The cycle is repeated with contraction and relaxation of adjacent muscles

If the muscle activity stops

Venous pumping by this mechanism stops.


Respiratory Pump

Inspiration

Diaphragm moves downwards

1. Decreased intrathoracic pressure

2. Increased intraabdominal pressure

Increased pressure gradient in veins from abdomen to thorax

More blood flows from lower region of body to the thorax

More blood enters the heart i.e. increased venous return.


Suction by Heart

Ventricular relaxation

Decreased pressure in ventricles

Ventricles pull blood from atria

Atria in turn pulls blood from veins

Increased venous return.


Ventricular systole

Atrio-ventricular valves are pulled into the ventricles

Atrial volume increases

Pressure in atria decreases

Atria pull blood from the veins

Increased venous return.

 

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