Module 6: Section 6

Question 1

Venules

Answer 1

• Directly attached to the capillaries
• Have smooth muscle in their walls
• Display little tone and generate minimal resistance to blood flow
• Respond to arteriolar chemical signals to ensure matched inflow and outflow of blood through capillaries

Question 2

Structure of the venous side

Answer 2

Capillaries to venules to small veins to larger veins

Question 3

Veins

Answer 3

• Have a large diameter and offer little resistance to flow
• Considered capacitance vessels
• The smooth muscle in their walls has little myogenic tone so they can easily distend to accommodate more blood and serve as a blood reservoir

Question 4

Venous return

Answer 4

• If venous capacity increases, more blood is being stored, it reduces the amount of avalible circulating blood
• Decreasing venous capacity results in more blood circulating

Question 5

How is venous return expressed?

Answer 5

As the volume of blood returning to the atria per minute

Question 6

Steady conditions - venous return

Answer 6

Under steady conditions, venous return must be equal to cardiac output

Question 7

6 factors affecting venous return

Answer 7

1. Sympathetic activity
2. The skeletal muscle pump
3. Gravity
4. Venous valves
5. Respiratory activity
6. Cardiac suction

Question 8

Sympathetic activity - factor affecting venous return

Answer 8

• Increasing sympathetic activity decreases venous capacity
• This increases venous return and cardiac output

Question 9

The skeletal muscle pump - factor affecting venous return

Answer 9

• Many veins lie between skeletal muscles
• When muscles contract, the veins are compressed which reduces venous capacity and increases venous pressure
• This decreases venous return

Question 10

Gravity - factor affecting venous return

Answer 10

• When standing, gravity increases hydrostatic pressure in veins below the heart, causing them to expand and reduce venous return
• Higher pressure at the venous end of capillaries decreases reabsorption and can lead to localized edema
• In healthy people, sympathetic reflexes and the muscle pump counteract this by squeezing veins and reducing hydrostatic pressure

Question 11

Venous valves - factor affecting venous return

Answer 11

• Venous vasoconstriction and the muscle pump both squeeze the veins to move the blood towards the heart
• The veins have regulatory spaced one-way valves that prevent backward flow of blood
• Also helps reduce the dramatic increase in hydrostatic pressure when standing

Question 12

Respiratory activity - factor affecting venous return

Answer 12

• Respiratory activity makes chest cavity pressure ~5 mmHg below atmospheric pressure
• Since the veins pass through the chest cavity, this helps increase the pressure gradient between the venules and heart
• This “respiratory pump” increases venous return

Question 13

Cardiac suction - factor affecting venous return

Answer 13

• During ventricular contraction, the valves are pulled downward, which increases the size of the atrial cavity
• This lowers atrial pressure below 0 mmHg, which increases venous return
• During ventricular relaxation, rapid expansion of the ventricles creates a suction effect that pulls blood into the atria, this also increases venous retirm