Exchange in, and control of, the Peripheral Circulation

Question 1

What is the difference between clefts and pores?

Answer 1

Clefts are the gaps between epithelial cells, pores are within the cells, allowing for exchange of gasses/substances etc.

Question 2

Continuous vws Fenestrated vs Discontinuous capillaries

Answer 2

Continuous:
No clefts or pores (eg brain - hence blood brain barrier)
No pores, clefts only eg most capillaries/muscles

Fenestrated:
Clefts and pores eg kidneys/intestine - specialised for fluid exchange)

Discontinuous:
Clefts and MASSIVE pores - eg liver, big pores are for the proteins

Question 3

How does exchange happen?

Answer 3

Either by diffusion or Carrier-mediated transport (mainly diffusion)

Diffusion is good because: 
self-regulating
non-saturable
non-polar substances across the phospholipid membrane
polar substances through clefts/pores

carrier mediated transport eg glucose into brain

Question 4

What are starlings forces and bulk flow? WHere does the extra fluid go and how much of it is there?

Answer 4

As the blood reaches the capilaries has approx 40mmHg pressure. The hydrostatic Pressure forces a bulk flow out of water, ions etc into interstitial space. The pressure decreases to about 20mmHg in the venules and veins heading back to the heart and so the hydrostatic force decreases. The main component left is the proteins and this therefore draws back in via oncotic pressure an increasing amount of interstitial fluid.

Overall ~20l is lost and ~17l is regained each day

Remaining 3l is returned via lymphatic system.

In other words going on in the background of all that diffusion is a mass exchange of fluid – called bulk-flow.
Hydrostatic pressure pushes fluid out through the leaky capillaries. That builds up an osmotic (oncotic) pressure which draws fluid back in.
The balance of these hydrostatic and osmotic pressures is known as Starling’s forces.

Question 5

When can Oedema occur?

Answer 5

If there is a problem with the lymphatic drainage system back to the heart, the extra 3L interstitial fluid isn’t picked up and will lead to oedema, eg filariasis/surgery

Raised CVP (central venous pressure), causing excess fluid to be released eg due to ventricular failure

Hypoproteinemia eg due to nephrosis, liver failure, nutrition \9less proteins to draw back in the oncotic pressure )

Increased capillary permeability eg in inflammation, eg rheumatism

Question 6

What is Darcy’s law?

Answer 6

Flow = change in pressure/resistance

Question 7

What is Poiseuille’s law?

Answer 7

Resistance = (viscosity x length x 8) / (radius cubed x pi)

Basically increasing viscosity and/or length will increase resistance, and increasing the radius will decrease the resistance 4 fold.

Question 8

Darcy and Poiseuille’s law combined =?

Answer 8

Flow = (Change in pressure x radius cubed x pi) / viscosity x length x 8

Basically if you increase pressure/radius, flow will be increased (radius has 4 fold effect)

If you decrease viscosity and length, flow will be increased

Question 9

How can we relate: change in pressure = flow x resistance to our body?

Answer 9

change in pressure = flow x resistance

Therefore
MAP - CVP = CO x TPR

As CVP is v low

MAP =CO x TPR

MAP = Mean Arterial Pressure
CVP = Central Venous Pressure
CO = Cardiac Output
TPR = Total Perhipheral Resistance

Question 10

What is the most fundamental equation in relation to pressure and blood flow?

Answer 10

MAP = CO x TPR

Question 11

In order to keep the blood flow to each vascular bed sufficient, and keep mean arterial pressure in the right range, you have to engage in some resistance juggling. How is it managed?

Answer 11

Via:
Local (intrinsic) mechanisms - concerned with meeting the selfish needs of each individual tissue
Central (extrinsic) mechanisms – concerned with ensuring that the total peripheral resistance (and therefore MAP) of the whole body stays in the right ball park

Question 12

What does Hyperaemia mean?

Answer 12

High Blood Flow

Hyper = high
Aemia = blood flow

Question 13

What are 4 mechanisms for intrinsic control of blood flow?

Answer 13

Active (metabolic) herperaemia
Pressure (flow) autoregulation
Reactive Hyperaemia
The injury response

Question 14

What happens in Active (metabolic) hyperaemia?

Answer 14

Tissue is producing more waste products (metabolites) as a result of being more active - higher metabolic rate (shock horror!). The endothelial cells pick this up and release a paracrine signal - either NO (Nitric oxide) or EDRF (Endothelium derived relaxation factor), which results in arteriolar dilation. This washes out the extra metabolites and this adaptation matches blood supply to the metabolic needs of that tissue.

Question 15

What happens in Pressure/Flow autoregulation?

Answer 15

Trigger is a decrease in perfusion pressure:
Low MAP causes Low flow
metabolites accumulate
triggers release of paracrine signal (eg EDRF/NO)
arterioles dilate and flow is restored to normal
an adaptation to ensure that a tissue maintains its blood supply despite changes in MAP

Question 16

WHat happens during reactive hyperaemia?

Answer 16

Trigger is occlusion of blood supply:

• this causes a subsequent increase in blood flow
• an extreme version of pressure autoregulation

eg red arm post tight bp monitor (metabolite accululation due to occlusion, mass vasodilation post area and then blood flows like crazy once released) - looks like a reaction - reaactive

Question 17

WHat happens during an injury response?

Answer 17

Eg graze skin but not cut, injury causes redness (vasodilation) because oh histamine release from mast cells which causes arteriolar dilation (increased blood flow and increased permeability). Histamine is released as a result the C-fibre which sent a signal to the dorsal root ganglia tahat it hurt also branching off and releasing peptide P which stimulates the mast cells.

Question 18

What are the central controls of blood flow and pressure?

Answer 18

Neural - Sympathetic and hormonal

Question 19

How does the sympathetic system regulate MAP?

Answer 19

Sympathetic nerves
release noradrenaline
binds to alpha 1-receptors
causes arteriolar constriction
therefore reduces flow through that tissue, and tends to increase TPR

Question 20

What affect does the parasympathetic system have of MAP/TPR?

Answer 20

usually no effect

genitalia and salivary glands are the exception

Question 21

What is the key hormonal influence on MAP/TPR and how does it affect?

Answer 21

Adrenaline!

released from adrenal medulla
binds to Alpha 1-receptors
causes arteriolar constriction
therefore decrease flow through that tissue, and tends to Increase TPR

but in some tissues, eg skeletal and cardiac muscle, it also activates Beta 2-receptors
causes arteriolar dilation
therefore increasing flow through that tissue, and tends to decreased TPR
Significance re exercise

Question 22

What does Angiotensin II do?

Answer 22

produced in response to low blood volume
causes arteriolar constriction
therefore increases TPR

Question 23

What does Vasopressin (= antidiuretic hormone) do?

Answer 23

released in response to low blood volume
causes arteriolar constriction
therefore decreases TPR

Question 24

What does Atrial natriuretic factor do?

Answer 24

released in response to high blood volume
causes arteriolar dilation
therefore decreases TPR

Question 25

How is the coronary artery system different?

Answer 25

For some reason (either the aortic valve closing off the supply or the pressure upon heart contraction squashing arteries) the coronary arteries have their lowest pressure during systole. As the heart is v important the cells esp during exercise are v good at regulating MAP - expresses many B2 receptors. .

Question 26

How is cerebral circulation different?

Answer 26

needs to be kept stable, whatever

shows excellent pressure autoregulation

Question 27

How is pulmonary circulation different?

Answer 27

Opposite in that low oxygen leads to arterial constriction, ensures that blood is directed to the best ventilated parts of the lung

Question 28

How is renal circulation different?

Answer 28

Imp to maintain the pressure so that too much fluid isn’t filtered out and you end up like a prune, but also that the MAP isn’t too low so you end up like a plump plum.

main function is filtration which depends on pressure
changes in MAP would have big effects on blood volume
shows excellent pressure autoregulation