Lesson 5: Venous Return and Startlings Law

Question 1

Describe the structure of veins and their functions.

Answer 1

• Carry blood towards the heart.
• Mostly carry deoxygenated blood.
• No stretch - very thin layer of smooth muscle.
• Thin walled
• Large lumen
• Have valves - to prevent backflow of bood

Question 2

Define venous return and explain what Starling’s Law is.

Answer 2

• The return of blood to the right side of the heart via the vena cava.
• Up to 70% of the total voolume of blood is contained in the veins at rest.
• This means that a large amount of blood can be returned to the heart when needed.
• During exercise the amount of blood returning to the heart increases.
• If more blood is being pumped back to the heart, then more blood needs to be pumped out, so SV will increase - this is Starling’s Law

Question 3

What are the 6 venous return echanisms?

Answer 3

Main ones:
- The skeletal muscle pump
- The respiratory pump
- Pocket valves

Other ones:
- Smooth muscle in the walls of veins
- Gravity
- The suction pump action of the heart.

Question 4

What is the skeletal muscle pump?

Answer 4

When msucles 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 5

What is the respiratory pump?

Answer 5

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

Question 6

What are pocket valves?

Answer 6

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

Question 7

Define blood pressure.

Answer 7

The force exerted by the blood against the bloof vessel wall.

Question 8

Define systolic pressure.

Answer 8

The pressure in the arteries when the ventricles are contracting.

Question 9

Define diastolic pressure.

Answer 9

The pressure in the arteries when the ventricles are relaxing.

Question 10

What is a typical BP reading?

Answer 10

120mmHg/80

Question 11

What is the impact of BP on venous return?

Answer 11

When systolic blood pressure increases, there is also an increase in venour return, and when systolic pressure decreases, there is a decrease in venous return.

Question 12

What is the impact of a pressure gradient between the right atrium and the vena cava on venous return?

Answer 12

• Venous return is the same as SV.
• If venous return increases, the heart contracts with more force, which will increase the ejection fraction and SV (starling’s law).
• Venous return is determined by a pressure gradient.
• The pressure gradient is the mean systemic pressure minus the right atrial pressure, and resistance is the total peripheral vascular resistance

Question 13

What is the formula for venous return?

Answer 13

venous pressure (Pv) - right atrial pressure (Pra) / venous vascular resistance (Rv)

Question 14

What increases/decreases venous return?

Answer 14

Increases venous return:
- An increase in venous pressure (PV)
- A decrease in right atrial pressure (PRA)
- A decrease in venous resistance (RV)

Decreases venous return:
- An increase in right atrial pressure.