Peripheral Circulation: Capillaries and Veins

Question 1

Which vessels are specialised for distribution and resistance?
Which vessels are specialised for exchange?
Which vessels are special for capacitance?

Answer 1

large/small arteries and arterioles
capillaries
veins

Question 2

Describer how a capillary’s structure is related to its function

Answer 2

no elastin
no smooth muscle - cannot dilate/constrict
bring blood to within 30micrometers of virtually every cell
slow velocity of flow (1mm/s)
single endothelial cell wall (0.5microns)

Question 3

What are the 3 mechanisms of exchange in the capillaries?

Answer 3

1. Diffusion
2. Vesicular transport
3. Bulk flow

Question 4

What is Fick’s law of diffusion?

Answer 4

Rate = permeability coefficient x conc grad x area

Question 5

For diffusion to occur there must be …… in the capillaries

Answer 5

flow

Question 6

Is there innervation to the capillaries?

Answer 6

No
cannot constrict or dilate
constant resistance

Question 7

increased tissue ……… opens more capillaries

Answer 7

VO2 (metabolic rate)

Question 8

Which vessels determine flow rate?

Answer 8

arterioles

Question 9

How is flow related to pressure and resistance?

Answer 9

Flow = Pressure/Resistance

Question 10

What is vesicular transport used for?

Answer 10

large charged molecules e.g. proteins and antibodies

Question 11

Describe bulk flow

Answer 11

fenestration’s in endothelial cells and junctions between two endothelial cells allows exchange between blood and interstitium
H2O, electrolytes, small molecules (<70,000)

Question 12

Capillaries are leaky. Blood enters a capillary at high pressure, forcing fluid into the interstitium. Large volumes are not lost from the system -Why?

Answer 12

Starling Forces

• hydrostatic forces
• fluid is reabsorbed using oncotic pressure form plasma proteins

Question 13

How are filtration, reabsorption and lymph flow related in an equation?

Answer 13

Filtration = reabsorbed + lymph flow

Question 14

Give the symbols and approximate pressures in the following situations:

1. Blood enters the capillary with a hydrostatic pressure
2. Hydrostatic pressure in the interstitium
3. Capillary oncotic pressure
4. Blood leaves the capillary with a hydrostatic pressure
5. Interstital oncotic pressure

Answer 14

1. Pc = 35 mmHg
2. Pif = 0 mmHg
3. πc = 25 mmHg
4. Pc = 15 mmHg
5. πif = 0 mmHg

Question 15

When is Pc > πc ? What does this result in?

Answer 15

at the arteriolar end of the capillary

causes filtration of fluid out of the capillary

Question 16

When is Pc < πc? What does this result in?

Answer 16

at venous end of capillary

causes reabsorption of fluid into capillary

Question 17

Any excess fluid is removed by the …………. …………

Answer 17

lymphatic system

Question 18

Describe the structure and function of the lymph vessels

Answer 18

thin walled vessels
one way valves prevent back flow
resturn to venous circulation via subclavian veins in chest

Question 19

Changes in starling forces can lead to ………..

Give three examples

Answer 19

oedema
heart failure - peripheral/pulmonary oedema
malnutrition e.g. Kwashiorkor disease
allergic reaction

Question 20

Describe the structure and function of veins

Answer 20

less smooth muscle in tunica media
capable of dilating and constricting
floppy rather than rigid 
capacity to hold large volumes
valves to prevent backflow

Question 21

What proportion of blood vessels is found in veins?

Answer 21

60-80%

Question 22

Large volumes of blood can be mobilised from the veins in certain situations. Give an example

Answer 22

exercise

Question 23

Cardiac filling (CVP) is determined by ………. ……..

Answer 23

venous return

Question 24

Venous return determines …….. ……….

Answer 24

venous compliance

Question 25

• 1mmHg increase in arterial pressure might lead to increase in arterial volume of ………ml
• 1mmHg increase in venous pressure would lead to increase in venous volume of ……….ml

Answer 25

1

6-10

Question 26

Veins are very ……… and can therefore act as a ……..

Answer 26

complaint

reservoir

Question 27

Draw a compliance curve for a vein and the shape of the vein at each point on the curve.

Answer 27

see lecture notes

• lower compliance at higher pressures and volumes
• highest compliance at low pressures and volumes

Question 28

Veins (and therefore venous compliance) are under the control of the ………. ……. ………
How?

Answer 28

sympathetic nervous system

• Stimulation of sympathetic nerves, catecholamine or Angiotensin II
• increase tone
• decreased compliance
• fall in capacitance
• increases venous return

Question 29

Blood reenters heart at minimal pressure.

This is called ……….. ……….. …………

Answer 29

central venous pressure

Question 30

What 5 factors affect Central Venous Pressure

Answer 30

venous capacitance  (venous tone)
gravity
respiratory pump
skeletal muscle pump
increase blood volume