Venous Return

Question 1

Venous return

Answer 1

The return of blood to the right side of the heart via the vena cava.
Up to 70% of the total volume of blood is contained in the veins at rest. This means a large amount of blood can be returned to the heart when needed. During exercise the amount of blood returning to the heart increases (venous return increases). Therefore more blood needs to be pumped out which increases stroke volume, this is starlings law.

Question 2

Venous return mechanisms

Answer 2

The pressure of the blood in the large veins is very low and this makes it difficult to return blood to the heart. The large lumen of the veins offers little resistance to blood flow. This means that active mechanisms are needed to help venous return.

1) The skeletal muscle pump
2) The respiratory pump
3) Pocket valves

Question 3

The skeletal muscle pump

Answer 3

When muscles contract and relax they change shape
This change in shape means that the muscles press on the nearby veins and cause a pumping effect and squeeze the blood towards the heart.

Question 4

The respiratory pump

Answer 4

When muscles contract and relax during breathing in and breathing out, pressure changes occur in the thoracic and abdominal cavities.
These changes in pressure compress the nearby veins and assist blood return to the heart.

Question 5

Pocket valves

Answer 5

It is important that the blood in the veins only flows in one direction
The presence of valves ensures that this happens
This is because once the blood has passed through the valves, they close to prevent the blood from flowing back.

Question 6

Other factors that aid venous return

Answer 6

A very thin layer of smooth muscle in the walls of the veins. This helps squeeze the blood back to the heart.
Gravity helps the blood return to the heart from the upper body.
The suction pump action of the heart.