Cardiac Output

Cardiac Output

What is cardiac output? Mention factors effecting cardiac output? Add a note on venous return. (or) Enumerate any three factors influencing venous return to the heart. Explain any one of them.
Answer:

Cardiac output:

• Definition:
• The amount of blood pumped out by each ventricle into the circulation per minute is called cardiac output.

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Factors effecting cardiac output:

1. Venous return:

Venous return Definition:

• It is the amount of blood, which is returned to the heart from different parts of the body.

Effect on cardiac output:

• When venous return increases, cardiac output also increases.
• But when it decreases and heart rate also decreases then the cardiac output does not decrease.
• Venous return is directly proportional to cardiac output if other factors remains constant.

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Factors effecting venous return:

1. Thoracic pump or respiratory pump.
   • During inspiration the following changes occur.
     1. Expansion of thoracic cavity.
     2. Decrease in intra-throacic pressure.
     3. Increase in diameter of inferior vena cava.
     4. This is turn increases venous return.
2. Cardiac pump.
   1. Vis A tergo-A force from behind.
      • It is imparted by the contraction of the heart.
   2. Vis A fronte-force acting from front.
      • It attracts blood in the veins towards the heart.
3. Muscle pump:
   1. On contraction of muscle:
      • Veins in between muscles are squeezed.
      • The valves present proximal to the muscle is opened.
      • This propels the blood towards heart.
   2. On relaxation of muscle.
      • The valves proximal to the muscle is closed.
      • This prevents back flow of blood.
      • When muscular activity increases the venous return also increases.
4. Total blood volume.
   • Increase in blood volume increases venous return.
5. Sympathetic tone.
   • Increase in sympathetic activity causes constriction of venules.
   • This, the turn, increases venous return.
6. Gravity.
   • Gravitational force reduces the venous return.
7. Venous pressure.
   • Pressure in venules is 12-18 mm Hg.
   • Pressure in inferior and superior venacava – 5.5 mmHG
   • At the junction of vena cava and right atrium pressure is 4.6 mm Hg.
   • Pressure in atrium falls to zero during atrial diastoles.
   • This pressure gradient helps in venous return.

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2. Force of Contraction:

• The cardiac output is directly proportional to the force of contraction only if other factors remains constant.
• This in turn depends on diastolic period and ventricular filling.

3. Heart rate:

• It is directly proportional to cardiac output if other factors remains constant.
• Marked increase in heart rate, increases cardiac output.
• Marked decrease in heart rate, decreases cardiac output.

4. Peripheral resistance:

• It is the resistance against which the heart has to pump the blood.
• Thus, cardiac output is inversely proportional to peripheral resistance.

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