Peripheral Circulation: Capillaries and Veins

Question 1

What are the major types of vessels within systemic circulation?

Answer 1

Aorta, Larger artery, small artery, arteriole, capillaries, venule, vein and vena cava.

Question 2

Capillary structure and function

Answer 2

Single endothelial cell wall (0.5um thick), no elastin, no smooth muscle (so cannot dilate or constrict), bring blood to within 30um of virtually every cell, slow velocity (good for diffusion)
Width 5-10um, slightly wider at the venous end than the arteriole end

Question 3

What are the mechanisms of exchange in capillaries?

Answer 3

1. Diffusion
2. Vesicular Transport
3. Bulk Flow

Question 4

Diffusion mechanism of exchange in capillaries

Answer 4

O2, CO2 and lipid soluble substances (e.g. hormones) can diffuse across through the endothelial cells

Question 5

What is Ficks Law of Diffusion

Answer 5

Rate = permeability coefficient x concentration gradient x area.
Rate of diffusion: size, charge, area, permeability, concentration gradient (CO2 binds 20x more readily than O2 = greater permeability coefficient.

Question 6

Why is low flow rate in capillaries important?

Answer 6

Ensures enough time for exchange. The larger the area the lower the velocity flow = good for diffusion.

Question 7

Why must there be flow for diffusion to occur in the capillaries?

Answer 7

The is no innervation of nerves = no constriction or dilation (constant resistance)
Only 25% open at any time
Increased tissue VO2 opens more capillaries
Flow determined by arterioles.

Question 8

Flow equation

Answer 8

Delta P (pressure difference mmHg) / Resistance to flow.

Question 9

The effects of capillary pressure when arterioles dilates

Answer 9

Pressure increases.

Question 10

The effects of capillary pressure when arterioles constrict

Answer 10

Pressure decreases.

Question 11

Explain vesicular transport in capillaries

Answer 11

Transport of large charged molecules (e.g. proteins, antibodies) through the endothelial cells using vesicles. Not a commonly used mechanism

Question 12

Explain bulk flow in capillaries

Answer 12

Transport of H20, electrolytes, small molecules (Me <70,000). They move through the fenestrations of fenestrated capillaries.

Question 13

When arterioles contract what happens

Answer 13

Contraction = constriction = increase resistance = decrease flow

Question 14

When arterioles dilate what happens

Answer 14

Relaxation = dilation = decrease resistance = increase flow

Question 15

What is the interstitial?

Answer 15

Interstitial fluid is a solution that bathes and surrounds the tissue cells of multicellular animals.

Question 16

What pressure does blood enter a capillary?

Answer 16

35mmHg. This forces fluid into the interstitium via fenestrations. (Pc)

Question 17

Why aren’t large volumes lost from the system due to filtration from the capillaries

Answer 17

Filtration = Reabsorption + Lymph flow.

Forces cause reabsorption back into the capillary and lymph. We filter slightly more than we reabsorb.

Question 18

What does starlings force/ equation tell us?

Answer 18

Illustrates the role of hydrostatic and oncotic force in the movement of fluid across the capillary membrane.

Question 19

What is the hydrostatic pressure in the intersitium?

Answer 19

0mmHg (Pif)

Question 20

What is the oncotic pressure in interstitium?

Answer 20

Very Low. pi if

Question 21

What is the hydrostatic pressure when the blood leaves the venule?

Answer 21

15 mmHg. Pressure drop inside the vessel.

Question 22

What is the capillary oncotic pressure

Answer 22

25 mmHg (pi C) osmotic pressure is due to proteins

Question 23

Any change in starlings forces will cause what?

Answer 23

A change in blood flow

Question 24

At the arteriole end of a capillary why is there filtration out of the capillary?

Answer 24

Because the hydrostatic pressure is greater than the oncotic pressure

Question 25

At the venous end of the capillary why is there reabsorption?

Answer 25

Because the hydrostatic pressure is less than the oncotic pressure.

Question 26

Any excess fluid is removed by what?

Answer 26

The lymphatic system

Question 27

What are the features of the lymphatic system?

Answer 27

Thin walled vessels, one way to prevent backflow returns to venous circulation via subclavian veins in the chest. Takes excess fluid and puts it back into the subclavian veins.

Question 28

A change in starlings forces causes what?

Answer 28

Leads to an oedema- swelling water in the extracellular space

Question 29

What does capacitance mean?

Answer 29

The ability of a hollow organ (vessel) to distend or constrict.

Question 30

Vein structure

Answer 30

Less smooth muscle in the tunica media (middle layer), capable of dilating and constricting. Floppy rather than rigid. ‘Capacity’ to hold large volumes of blood. Valves prevent backflow.
Veins are fighting gravity to get blood back into the heart. Blood enters the heart at 1mmHg, therefore valves help to increase the pressure back to the heart.

Question 31

What are the capacitance vessels?

Answer 31

Veins

Question 32

How much blood is found in the vessels at any one time?

Answer 32

60-80% of blood is found in the veins.

Question 33

How many litres of blood is found in the vessels at any one time?

Answer 33

3-3.5L

Question 34

What is the significant of veins capacitance?

Answer 34

The veins can mobilise large volumes of blood when required (e.g. exercise)

Question 35

Cardiac filling (CVP) is determined by venous return. What is venous return determined by?

Answer 35

Venous compliance

Question 36

Compliance equation?

Answer 36

Change in volume / Change in pressure

Question 37

What is the effect on arterial and venous volume when there is a 1 mmHg increase in pressure?

Answer 37

Arterial pressure will increase by 1 mL

Venous pressure will increase by 6-10mL (this is because veins at very compliant and can act as a reservoir).