Regulation of Arteriolar resistance

Question 1

What is Darcy’s law?

Flow = ?

Answer 1

Question 2

What does poiselluille’s law tell us about resistance?

Answer 2

Resistance of a vessel is mainly determined by the radius of the vessel

Question 3

If flow = difference in pressure / resistance what does pressure equal?

Answer 3

Flow x resistance

Question 4

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

Answer 4

Flow = difference in pressure /resistance

CO = Mean arterial pressure / total peripheral resistance : therefore

Mean arterial pressure = CO x total peripheral resistance

Question 5

What does arteriolar radius affect?

Answer 5

Flow through individular vascular beds

AND

Mean arterial pressure

Question 6

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

Answer 6

Resistance juggling

Two levels of control over the smooth muscle surrounding arterioles:

1. Intrinsic mechanisms - concerned with meeting the selfish needs of each individual tissue
2. Extrinsic mechanisms - concerned with ensuring that the total peripheral resistance of the whole body stays in the right ball park

Question 7

What controls extrinsic control?

Answer 7

Nerves and hormones

Question 8

What is the neural extrinsic control?

Answer 8

• Sympathetic nerves
• release norepinephrine
• binds to a1-receptors
• causes arteriolar constriction
• therefore decreased flow through that tissue, and tends to increase TPR
• Parasympathetic nerves
• usually no effect

Question 9

What is the hormonal extrinsic control?

Answer 9

•Epinephrine

–released from adrenal medulla

–binds to a1-receptors

–causes arteriolar constriction

–therefore decreased flow through that tissue, and tends to increase TPR

BUT in some tissues, eg skeletal and cardiac muscle, it also activates b2-receptors

• causes arteriolar dilation
• therefore increase flow through that tissue, and tends to decreaseTPR

Question 10

Name the other hormonal extrinsic control mechanisms

Answer 10

Angiotensin - When blood volume is LOW - causes arteriolar constriction

Vasopressin - (antidiuretic hormone) - When blood volume is LOW - causes arteriolar constriction

Atrial natriuretic peptide & Brain natriuretic peptide - When blood volume is HIGH - causes arteriolar dilation.

Question 11

How do ACE inhibitors work?

Answer 11

ACE inhibitors are used to decrease blood pressure

ACE is angiotensin converting enzyme (converts angiotensin 1 to angiotensin 2)

ACE inhibitor reduces the amount of angiotensin 2 and so decreases total peripheral resistance

Question 12

What are the intrinsic methods of control of arteriolar resistance?

Answer 12

Active (metabolic hyperaemia)

Pressure (flow) autoregulation

Reactive hyperaemia

The injury response

Question 13

What is meant by hyperaemia?

Answer 13

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

Question 14

How does active hyperaemia result in arteriole dilation?

Answer 14

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 15

How does pressure autoregulation result in arterile dilation?

Answer 15

Decrease mean arterial pressure - reduced flow

Metabolites accumulate

EDFR

Question 16

What is meant by reactive hyperaemia?

Answer 16

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 17

How does the injury response result in an increased blood flow?

Answer 17

C - fibres pick detect injury and send action potentials to the brain.

Mast cell degranulation is caused by a messenger sent by the brain.

Histamine is released causing arteriolar dilation

Increased blood flow and increased permeability

Question 18

What is the purpose of histamine?

Answer 18

Aids delivery of blood born leucocytes etc to injured area

Question 19

Which areas show special arteriolar control?

Answer 19

Coronary circulation

Cerebral circulation

Pulmonary circulation

Renal circulation

Question 20

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

Answer 20

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 21

When is coronary circulation switched off?

Answer 21

Systole

–but still has to cope with increased demand during exercise

–shows excellent active hyperaemia

Question 22

Describe cerebral circulation

Answer 22

Needs to be kept stable, whatever

Shows excellent pressure autoregulation

Question 23

Describe pulmonary circulation

Answer 23

• 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 24

Describe renal circulation

Answer 24

–Main function is filtration which depends on pressure

–Changes in MAP would have big effects on blood volume

–Shows excellent pressure autoregulation