Venous Return

Volume of blood flowing from veins into the right atrium each minute.

• Venous return is the sum of all the local blood flows from

individual segments of the peripheral circulation.

• Venous return at rest = 5 L/min

• Venous return (& its factors affecting) is the primary

controller of the cardiac output.

Factors affecting Venous Return:

1. Muscle pump & Unidirectional Venous valves.

2. Valve competence

3. Vascular tone — Venous tone

— Arteriolar tone

4. Respiration — Intrapleural pressure

5. Heart beat — Ejection Phase (apex to base Vent. Shortening)

— Vent. Diastole

6. Blood Volume

7. Rt. Atrial Pressure ( Rt. Atrial Pr.   Venous Return)

8. Gravity, Posture

9. Intrapericardial Pressure/Effusion/Cardiac Temponade

10. Venous Obst.

11. Degree of filling of systemic circulation. (MSFP)

Regulation of Venous Return

Normally Atrial pressure is near zero and venous return

is promoted by the driving pressure gradient in low

resistance veins.

Venous Return is Facilitated by:

I. Driving pressure by the heart & Lungs:

I. Cardiac suction effects

II. Low Atrial pressure

III. Pressure imparted to blood by cardiac

contraction

IV. Respiratory/Thoracic Pump

II. Pressure Gradient between Veins and Heart

• Mean Systemic Filling Pressure(MSFP)

• Blood Volume

• Sympathetic

Vasoconstrictor Activity

• SK. Muscle Pump

• Venous Valves preventing backflow of

blood.

Driving Pressure Promoting Venous Return

1) Effect of Cardiac suction on Venous Return:

a. During Ventricular contraction:

Apex to base shortening, AV valves drawn downward so Atrial Pressure drops below zero which increases

 Vein-to-Atrial Pr. gradient.

b. During Ventricular Relaxation:

Rapid expansion of Ventr. Chambers so

-ve Pr. in Ventricles ‘to suck the blood in’ from

atria & Veins which increases Vein-to-ventricle Pressure gradient.

“Thus Heart Functions as Suction Pump”.

2) Effect of Respiratory Activity on Venous Return:

–ve Intrapleural (intrathoracic) Pressure = -5 mmHg

(subatmospheric)

Facilitates Venous return by suction due to respiratory

pump.

More active during exercise (due to inceased Respiratory Activity)

Pressure Gradient between Veins and Heart:

Mean systemic filling Pressure: (MSFP)

 Pressure measured everywhere in systemic circulation

when all flow of blood flow is stopped.

 MSFP is the degree of filling of systemic circulation

 MSFP forces the systemic blood towards the heart.

Normal MSFP = 7 mmHg.

As Rt. Atrial Pr. is subatmospheric (eg: -2 mmHg)

& MSFP is 7 mmHg; the MSFP provides the gradient for

venous return.

 If, Rt. Atrial Pr. Rises (Heart failure, A-V valves disease);

the venous return will stop when Rt. atrial Pr. approaches

MSFP i-e 7 mmHg.

Blood Volume:

increased Blood Volume = Increased Venous Pressure & increased MSFP which leads to increased Pressure gradient.

Effect of Sympathetic Vasoconstrictor Activity:

Increased Venous tone leads to increased Venous Pressure (increased MSFP) approx. 17 mmHg

Leading to increased Pressure gradient.

Very Important during Exercise.

Effect of SK. Muscle Pump:

SK. Muscle Pump increases Venous Pressure

which squeezes blood towards heart

so decreases venous capacity

and counters the effect of gravity on veins

is very active during exercise.

Effect of veins valves:

In Large Veins one-way valves spaced at 2-4 cm intervals so permit blood to move forward towards the heart & prevent the backflow.