Capillary Exchange and Venous Return

Question 1

Which of arterioles and venules have more smooth muscle wrapped around them?

Answer 1

Arterioles

Question 2

What is the function of pre-capillary sphincters?

Answer 2

Control blood flow into the capillary bed

Question 3

What vessels to meta-arterioles empty into? What does the next vessel empty into?

Answer 3

Meta-arterioles empty into thoroughfare channels which empty into venules.

Question 4

Why is it advantageous that blood flow through capillaries is slow due to their highly branched nature and small diameter?

Answer 4

Allows time for nutrient exchange.

Question 5

List the blood vessels in order of decreased pressure.

Answer 5

Aorta, arteries, arterioles, capillaries, venules, veins, Venae Cavae.

Question 6

How much of the body’s total water in extracellular fluid?

Answer 6

1/3

Question 7

How much of the body’s extracellular fluid is part of plasma?

Answer 7

20%

Question 8

Do substances pass straight from capillaries to muscle cells with nothing in between?

Answer 8

No, they first enter interstitial fluid.

Question 9

Are proteins able to pass between muscle cells and plasma?

Answer 9

No

Question 10

What causes osmotic pressure (OP)?

Answer 10

Proteins that are not filtered through.

Question 11

Does the hydrostatic pressure of capillaries need to be greater or lesser than the osmotic pressure of capillaries to drive filtration?

Answer 11

Greater.

Question 12

How is the net filtration pressure calculated?

Answer 12

NFP = [HPc - HPif] - [OPc - OPif]

Question 13

What happens to filtered fluid?

Answer 13

Passes into the lymphatic system to become lymph. It eventually drains into the subclavian vein to enter the systemic circuit.

Question 14

What is extracellular oedema?

Answer 14

Failure to maintain the correct distribution of body water, which leads to excess accumulation of interstitial fluid.

Question 15

Does vasodilation increase or decrease capillary hydrostatic pressure?

Answer 15

Increase.

Question 16

Does vasoconstriction increase or decrease hydrostatic pressure?

Answer 16

Decrease.

Question 17

At what capillary pressure in (mm Hg) does the transition point between filtration and absorption occur?

Answer 17

26 mmHg

Question 18

Does arteriole constriction decrease or increase the transition point?

Answer 18

Decrease- less filtration.

Question 19

Does arteriole dilation increase or decrease the transition point?

Answer 19

Increase- more filtration.

Question 20

Compare veins and arteries in terms of pressure, resistance, wall thickness and valves.

Answer 20

Veins have lower pressure, thinner walls, low resistance, compliant walls and one way valves. Arteries do not have valves.

Question 21

Why must veins have a large diameter and low resistance?

Answer 21

To account for low pressures in the veins.

Question 22

What are the three mechanisms that allow venous return?

Answer 22

Pressure gradient, skeletal muscle ‘pump’ and the respiratory ‘pump’.

Question 23

Explain how pressure gradient is used to increase venous return.

Answer 23

Sympathetic activation causes slight constriction of the veins. As the veins decrease in diameter, pressure in them increases, increasing the pressure gradient between the veins and the RA. This increases venous return and SV.

Question 24

How does the skeletal muscle pump contribute to venous return?

Answer 24

As muscles contract, they decrease the diameter of veins, increasing pressure. The pressure gradient is increased.

Question 25

How does the respiratory pump contribute to venous return?

Answer 25

As the diaphragm constricts, thoracic pressure decreases and the abdominal pressure increases. The veins in the abdomen are squeezed, increasing pressure in them. The pressure gradient is increased and so is venous return.