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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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