Peripheral circulation

Question 1

What is the function of

a) arteries and arterioles
b) capillaries
c) veins and venuoles

Answer 1

a) distribution and resistance
b) exchange
c) capacitance

Question 2

What determines exchange in capillaries?

Answer 2

Concentration gradient, permeability and SA
Fick’s law:
Rate = permeability coefficient x conc gradient x area

Question 3

Describe the structure of a capillary.

Answer 3

One endothelial cell thick, no smooth muscle.
Lumen 5-10 microm, widest at venule end.
Slow velocity to 1mml.s for efficient diffusion and bring blood to within 30microm of every cell

Question 4

What exchange mechanisms does a capillary use?

Answer 4

Diffusion
Vesicular transport
Bulk flow

Question 5

What is diffusion dependent on? What molecules use this mechanism?

Answer 5

Concentration gradient
O2, CO2, lipid soluble (small uncharged).
Must be a flow present for diffusion to occur.

Question 6

Why is a large PO2 gradient needed?

Answer 6

To meet metabolic demand

Question 7

What is the resulting effect of capillary branching?

Answer 7

Increase CSA which reduces flow rate to allow more time for exchange. Velocity increases again in veins.

Question 8

What percentage of capillaries are open under normal conditions? When does this change?

Answer 8

25% open but when more tissue VO2 is required, more will open

Question 9

What determines capillary flow?

Answer 9

Arteriole resistance. Usually 35mmHg
If arterioles constrict, flow and pressure falls.
If arterioles dilate there is more energy and pressure.

Question 10

What is vesicular transport?

Answer 10

Evagination of the membrane to create vesicles for LARGE CHARGED molecules e.g. proteins, Ig

Question 11

What is bulk flow?

Answer 11

Occurs between fenestrations or junctions of endothelial cells. Used for water, electrolytes and small molecules

Question 12

Why are capillaries leaky?

Answer 12

The blood enters under high pressure so lymphatic system is require to remove the excess fluid and prevent oedema.

Question 13

How does the pressure in a capillary change over its course? Where in the body is this not true for?

Answer 13

Pressure falls at venous end.

Not true in renal capillaries, the pressure stays the same as both the afferent and efferent are arterioles.

Question 14

What determines filtration and reabsorption in a capillary?

Answer 14

Filtration occurs at beginning due to hydrostatic pressure.
As oncotic pressure increase towards venous end reabsorption occurs.
At ateriole end Pc > Onc = filtration
At venous end Pc < Onc = reabsorption
More filtration occurs than absorption overall.

Question 15

At what pressure does the blood enter and leave a capillary?

Answer 15

Enters at 35mmHg but leaves at 15mmHg as flow and velocity fall.
Oncotic pressure is 25mmHg due to the plasma proteins.

Question 16

Why doesn’t interstitial pressure play a role?

Answer 16

They are very low

Question 17

Describe the structure of a vein.

Answer 17

Limited smooth muscle in tunica media for a large degree of constriction and dilation. Valves to prevent backflow.
Hold 60-80% of total blood volume due to their capacitance.

Question 18

Why is a veins capacitance important?

Answer 18

Allows blood to be moved to the arteries during increased metabolic demand.

Question 19

What effect does a change in pressure in a vein have on volume?

Answer 19

A change in pressure brings about an even large change in volume due to compliance.
In arteries the change is proportional.

Question 20

When does a vein have its highest compliance?

Answer 20

Veins alter their shape with pressure so at low pressure and volumes the compliance is highest. At high pressures and volume, compliance is lower as it is reaching its capacity and requires a stretch rather than a change in shape.

Question 21

What effect does sympathetic innervation have on compliance and VR?

Answer 21

Increase sympathetic = increased tone, reduced compliance = increased VR. The vessels cannot hold as must volume so it returns to the heart.

Question 22

What is CVP?

Answer 22

Pressure at which the vena cava drains into the right atrium.
Affected by preloads.