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.