Regulation of arteriolar resistance

Question 1

State Darcy’s Law and apply this to cardiac output

Answer 1

Flow = change in pressure / resistance
CO = (MAP-CVP)/TPR
CO - MAP/TPR since CVP = 0

Question 2

State Pouseille’s law

Answer 2

Resistance = (viscosity x length x 8) / (radius^4 x Pi)

Flow = (change in pressure xr^4 x Pi) / (viscosity x length x 8)

Question 3

If flow = change in pressure/resistance, then what does pressure equal?
Apply this to cardiac output
What does this mean in terms of varying resistance of arterioles?

Answer 3

Pressure = flow x resistance
(MAP-CVP) = CO x TPR
MAP = CO xTPR
Varying arteriolar resistance regulates both flow and MAP

Question 4

How do arterioles control blood flow to tissues?

Answer 4

By varying their radius (reduced radius = increased resistance and vice versa). This also affects MAP.

Question 5

What is the effect of reducing the resistance in arterioles on the vascular bed?

Answer 5

Reducing resistance by increasing radius in arterioles increases supply to the vascular bed while also reducing MAP.

Question 6

Name the two type of mechanisms that keep MAP and blood blow consistent.
State their individual functions.

Answer 6

Extrinsic: ensures TPR remains in right ballpark
Intrinsic: selfish needs of individual tissue

Question 7

State the two types of extrinsic regulation

Answer 7

Neural/hormonal

Hormonal

Question 8

Describe neural/hormonal extrinsic regulation

Answer 8

Sympathetic
Noradrenaline is released and adrenaline is released from the adrenal medulla.
They bind to alpha 1 receptors causing reduced flow through tissue and increased TPR

Question 9

What is the effect of the parasympathetic system on arteriolar resistance

Answer 9

No effect

Question 10

As well as alpha 1 receptors, what other kind of receptors does adrenaline act on in arterioles in certain tissues?
What is the effect

Answer 10

Activates Beta 2 receptors in skeletal and cardiac muscle, causes arteriolar dilatation, increased flow and reduced TPR

Question 11

List the three types of hormonal extrinsic regulation

Answer 11

Angiotensin II
Vasopressin (ADH)
Atrial Natriuretic Peptide (ANP) and Brain Natriuretic Peptide (BNP)

Question 12

Describe the effect of angiotensin II on arterioles

Answer 12

Responds to low blood volume

Causes arteriolar constriction and thus increased TPR

Question 13

Describe the effect of vasopressin (ADH) on arterioles

Answer 13

Responds to low blood volume

Causes arteriolar constriction and thus increased TPR

Question 14

Describe the effect of ANP and BNP on arterioles

Answer 14

Respond to high blood volume

Cause arteriolar dilatation and thus reduced TPR

Question 15

List the local or intrinsic mechanisms that regulate arterioles

Answer 15

Active (metabolic) hyperaemia
Pressure (flow) autoregulation
Reactive hyperaemia
Injury response

Question 16

Describe the effect of active (metabolic) hyperaemia

Answer 16

Matches blood supply to tissue’s metabolic needs
Increased metabolic activity -> increased metabolites -> paracrine EDRF release->
smooth muscle relaxation ->
arteriolar dilatation ->
increased flow to wash out metabolites (CO2, K+, H+)

Question 17

Describe the effect of pressure (flow) autoregulation

Answer 17

Reduced MAP ->
reduced flow -> 
metabolite accumulation ->
EDRF release -> 
arteriolar dilatation -> 
restore of normal flow (but could be myogenic)

Question 18

Describe the effect of reactive hyperaemia

Answer 18

Extreme version of pressure autoregulation
Occlusion of blood supply ->
increased blood flow

Question 19

Describe the effect of injury response

Answer 19

Injury response involves C fibres activating substance P which activates mast cells leading to histamine release ->
arteriolar dilatation ->
increased blood flow and permeability
(also increased leucocytes)

Question 20

What are the special areas of circulation where local mechanisms are either heavily employed or different?

Answer 20

Coronary circulation
Cerebral circulation
Pulmonary circulation
Renal circulation

Question 21

Describe the local mechanisms employed by coronary circulation

Answer 21

Blood supply interrupted by systole - ventricles contract, squashing arterioles. Still has to cope with increased demand during exercise ->
excellent ACTIVE HYPERAEMIA: adenosine triggers EDRF release
Many Beta 2 receptors swamp any sympathetic arteriolar constriction

Question 22

Describe the local mechanisms employed by cerebral circulation

Answer 22

Needs to be kept stable whatever the consequences are - excellent PRESSURE AUTOREGUULATION

Question 23

Describe the local mechanisms employed by pulmonary circulation

Answer 23

When O2 is decreased -> arteriolar constriction (opposite response of most tissues - blood redirected to better ventilated areas)

Question 24

Describe the local mechanisms employed by renal circulation

Answer 24

Filtration dependent on pressure so MAP changes could have big effects on volume - excellent PRESSURE AUTOREGULATION