Regulation of Arteriolar resistance

Question 1

What is Darcy’s law?

Answer 1

Question 2

What does poiselluille’s law tell us about resistance?

Answer 2

Resistance of a vessel is mainly determined by radius of vessel

Question 3

How can you denote darcy’s law in terms of systemic circulation?

Answer 3

Flow = difference in pressure /resistance

CO = Mean arterial pressure / total peripheral resistance : therefore

Mean arterial pressure = CO x total peripheral resistance

Question 4

What does arteriolar radius affect?

Answer 4

Flow through individual vascular beds

Mean arterial pressure

Question 5

What is done to keep blood flow to each vascular bed sufficient as well as keeping mean arterial pressure in the right range?

Answer 5

Resistance juggling

Two levels of control over smooth muscle surrounding arterioles:

1. Intrinsic mechanisms - meets selfish needs of each individual tissue
2. Extrinsic mechanisms - ensuring total peripheral resistance of whole body stays in right range

Question 6

What controls extrinsic control?

Answer 6

Nerves and hormones

Question 7

What is the sympathetic neural extrinsic control?

Answer 7

Sympathetic nerves:

Noradrenaline binds to α₁ receptors and causes arteriolar constriction
Decreased flow through that tissue and increased TPR and MAP

Question 8

What is the parasympathetic neural control?

Answer 8

Parasympathetic nerves:

• Usually no effect
• Genitalia and salivary glands are exceptions (↑ flow)

Question 9

What is the hormonal extrinsic control?

Answer 9

Adrenaline released from adrenal medulla binds to α₁ receptors and causes arteriolar constriction

↓ flow through that tissue and ↑ TPR and ↑ MAP

Question 10

What can sometimes happen in hormonal extrinsic control?

Answer 10

In some tissues, (skeletal and cardiac muscle) activates β₂ receptors

Causes arteriolar dilation

Therefore ↑ flow through that tissue and tends to ↓ TPR

Significant during exercise

Question 11

Name the other hormonal extrinsic control mechanisms

Answer 11

Angiotensin - low blood blood volume causes arteriolar constriction

Vasopressin - (antidiuretic hormone) - low blood volume causes arteriolar constriction

Atrial & Brain natriuretic peptide - high blood volume causes arteriolar dilation

Question 12

How do ACE inhibitors work?

Answer 12

decrease bp
(Angiotensin converting enzyme)

Converts angiotensin 1 to angiotensin 2

Reduces amount of angiotensin 2 so decreases TPR

Question 13

What are the intrinsic methods of control of arteriolar resistance?

Answer 13

Active (metabolic hyperaemia)

Pressure (flow) autoregulation

Reactive hyperaemia

The injury response

Question 14

What is meant by hyperaemia?

Answer 14

Hyperaemia is an increase in blood flow due to an increase in metabolism

Question 15

How does active hyperaemia result in arteriole dilation?

Answer 15

Increase in metabolic activity - increase concentraion of metabolites

Triggers the release of EDFR (Endothelium derived relaxing factor- paracrin/local effect)

Increase flow to wash out metabolites

Blood supply matches metabolic needs

Question 16

How does pressure autoregulation result in arterial dilation?

Answer 16

Decrease MAP - reduced flow

Metabolites accumulate

EDFR

Question 17

What is meant by reactive hyperaemia?

Answer 17

Brief increase in organ blood flow that occurs following a brief period of ischaemia. Following this ischaemia there will be a shortage of oxygen and a build up of metabolic waste.

Question 18

Which areas show special arteriolar control?

Answer 18

Coronary circulation

Cerebral circulation

Pulmonary circulation

Renal circulation

Question 19

What type of receptors are present in the coronary arteries? And what are there significance?

Answer 19

Beta - 2 receptors causing arteriolar dilation

Sympathetic arteriolar constriction around the body causes reduced blood flow however the coronary arteries rich supply in beta receptors overcomes this.

Question 20

When is coronary circulation switched off?

Answer 20

Systole

–but still has to cope with increased demand during exercise

–shows excellent active hyperaemia

Question 21

Describe cerebral circulation

Answer 21

Needs to be kept stable, whatever

Shows excellent pressure autoregulation

Question 22

Describe pulmonary circulation

Answer 22

• Decreased O2 causes arteriolar constriction

–ie the opposite response to most tissues

–Ensures that blood is directed to the best ventilated parts of the lung

Question 23

Describe renal circulation

Answer 23

–Main function is filtration which depends on pressure

–Changes in MAP would have big effects on blood volume

–Shows excellent pressure autoregulation