Lecture 12- Venous blood flow and the heart

Question 1

Where is the extra blood we have stored?

Answer 1

Systemic veins

Question 2

How much of the blood at any given time is not in the systemic veins?

Answer 2

1/3

Question 3

In regards to volume arteries are…

Answer 3

High pressure but low volume

Question 4

In regards to volume veins are…

Answer 4

Low pressure but high volume

Question 5

Why is it that veins can be capacitance vessels and arteries cannot?

Answer 5

• Arteries have rigid walls with thick tunica media, they therefore can’t expand to store more blood
• Veins have thin walls however and so easily expand +change shape to hold more blood

Question 6

What is compliance?

Answer 6

The extent to which a vessel allows deformation in response to an applied force (how easy it is to stretch the walls)

Question 7

What is the equation for compliance?

Answer 7

Change in V/change in P

The more pressure it takes to change the volume of a vessel the less compliant it is.

Question 8

What do we see with the volume of veins as pressure increases?

Answer 8

• Small change in pressure leads to a large change in volume (will just keep expanding)
• Therefore, high compliance

Question 9

What do we see with the volume of arteries as pressure increases?

Answer 9

• Change in pressure big for not much change in volume

- Therefore, low compliance

Question 10

What is meant by the veins have survival value?

Answer 10

• When an arterial puncture causes a loss of arterial blood below a critical value a neural signal is sent to the veins.
• The veins then undergo venoconstriction pushing the ‘extra’ blood back to the heart in order to replace what has been lost
• Its like a blood transfusion

Question 11

What is a potential consequence of veins having high vascular compliance?

Answer 11

-While supine (laying down), venous volume is uniform
from head to toe.
-In the upright position, venous volume below the heart increases; whereas venous volume above the heart decreases due to gravity
-This is known as venous pooling in the legs and feet
-It’s dangerous as it prevents good blood flow back to the heart (could result in fainting)

Question 12

Why does venous pooling occur but the volume of arteries remains constant?

Answer 12

• Veins due to high compliance don’t have enough strength in their walls to resistance blood falling down due to gravity.
• Arteries have thick walls so hold their shape better preventing pooling

Question 13

How do valves counteract venous pooling?

Answer 13

Create individual pockets of blood. The blood inside these pockets can not fall backwards as a result of the valves ensuring uni-directional flow. This creates a smooth column of blood the whole way up.

Question 14

What’s a second method to counteract venous pooling?

Answer 14

• ‘Tone’ of surrounding tissue also counteracts venous pooling
• For example, muscle as it is stiff will prevent the veins from expanding and taking more blood

Question 15

Why are elderly people sometimes prone to fainting?

Answer 15

Have lost muscle mass over time, meaning there is not much to help counteract venous pooling (can’t stiffen veins) so harder for blood to counteract gravity and may end up at feet rather than heart.

Question 16

What is a second way muscle tone surrounding veins counteracts pooling?

Answer 16

Tissue actively squeezes on veins to push blood forward up towards the heart.

Question 17

What also happens when muscle squeeze blood in the veins, how is this counteracted?

Answer 17

Some of the blood will go up as a result of contraction but some will also be squeezed in the wrong direction (down). This is however, where the valves come in to prevent it falling down further and pooling in feet/legs.

Question 18

How does this squeezing action of muscles on veins work when we are breathing?

Answer 18

• When we breath we pump the muscles (diaphragm +intercostal) onto the veins and squeeze blood up to the heart.
• This timing makes sense as points in which we are breathing harder such as exercising is when there is a high demand for oxygenated blood.
• The harder you work the more return of blood you get to be oxygenated and then sent around the body.

Question 19

What is Starling’s law of the heart?

Answer 19

The more stretched muscle fibers are before a
contraction, the stronger the contraction will be.

In other words the more blood there is in the ventricles as a result of higher venous return the larger the stroke volume.

Question 20

How does Starling’s law of the heart relate to contraction at the cellular level?

Answer 20

The more stretched out the sarcomeres are as a result of increased blood volume the more those actin and myosin filaments are able to be pulled and thus the more force generated.