Regulation of Arteriolar resistance Flashcards

1
Q

What is Darcy’s law?

Flow = ?

A
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2
Q

What does poiselluille’s law tell us about resistance?

A

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

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3
Q

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

A

Flow x resistance

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4
Q

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

A

Flow = difference in pressure /resistance

CO = Mean arterial pressure / total peripheral resistance : therefore

Mean arterial pressure = CO x total peripheral resistance

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5
Q

What does arteriolar radius affect?

A

Flow through individular vascular beds

AND

Mean arterial pressure

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6
Q

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

A

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
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7
Q

What controls extrinsic control?

A

Nerves and hormones

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8
Q

What is the neural extrinsic control?

A
  • Sympathetic nerves
  • release noradrenaline
  • binds to a1-receptors
  • causes arteriolar constriction
  • therefore decreased flow through that tissue, and tends to increase TPR
  • Parasympathetic nerves
  • usually no effect
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9
Q

What is the hormonal extrinsic control?

A

•noradrenaline
–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
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10
Q

Name the other hormonal extrinsic control mechanisms

A

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.

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11
Q

How do ACE inhibitors work?

A

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

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12
Q

What are the intrinsic methods of control of arteriolar resistance?

A

Active (metabolic hyperaemia)

Pressure (flow) autoregulation

Reactive hyperaemia

The injury response

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13
Q

What is meant by hyperaemia?

A

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

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14
Q

How does active hyperaemia result in arteriole dilation?

A

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

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15
Q

How does pressure autoregulation result in arterile dilation?

A

Decrease mean arterial pressure - reduced flow

Metabolites accumulate

EDFR

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16
Q

What is meant by reactive hyperaemia?

A

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.

17
Q

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

A

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

18
Q

What is the purpose of histamine?

A

Aids delivery of blood born leucocytes etc to injured area

19
Q

Which areas show special arteriolar control?

A

Coronary circulation

Cerebral circulation

Pulmonary circulation

Renal circulation

20
Q

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

A

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.

21
Q

When is coronary circulation switched off?

A

Systole

–but still has to cope with increased demand during exercise

–shows excellent active hyperaemia

22
Q

Describe cerebral circulation

A

Needs to be kept stable, whatever

Shows excellent pressure autoregulation

23
Q

Describe pulmonary circulation

A
  • 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

24
Q

Describe renal circulation

A

–Main function is filtration which depends on pressure

–Changes in MAP would have big effects on blood volume

–Shows excellent pressure autoregulation

25
Q
A