Nervous and Hormonal Control

Question 1

What is intrinsic control?

Answer 1

- Regulate local blood flow to organs/tissues
Important – regional hyperaemia 
(increase in blood flow).
- Vasodilators eg inflammation, local 
metabolites such as Nitric oxide, 
Prostaglandins, Endothelin, K+, H+

Question 2

What is extrinsic control?

Answer 2

Regulate TPR to control blood pressure.
Brain function selectivity alters blood flow to
organs according to need eg. during exercise,
temperature regulation etc.

Question 3

What nerves does extrinsic control affect ?

Answer 3

vasoconstrictors - eg. noradrenaline

vasodilators - eg. acetylcholine, nitric oxide

Question 4

What hormones does extrinsic control affect ?

Answer 4

vasoconstrictor – eg. adrenaline, angiotensin II

vasodilators – eg. anti-natriuretic peptide (ANP)

Question 5

Describe the process of the sympathetic vasoconstriction system

Answer 5

• An action potential moves down the axon and
  arrive at a varicosity.
• Depolarisation at the varicosity activating
  voltage gated Ca2+ channels.
• Ingress of calcium causes release of
  neurotransmitters - mainly noradrenaline.
• NA diffuses to the vascular smooth muscle
  cells where it binds mainly α1 – contraction;
  some α2 – contraction and β2 – relaxation.
• The noradrenaline is then taken up again and
  recycled or broken down.

*Adrenaline from the adrenals and released into
the circulation can also act at α1 or β2 receptors.

Question 6

Describe in detail what occurs in the varicosities of the sympathetic vasoconstriction nerves

Answer 6

• Release of NA (Noradrenaline) can be modulated by Angiotensin II acting on AT1 receptor increasing NA release.
• NA release causes vasoconstriction
• Metabolites prevent vasoconstriction to maintain blood flow; K+, adenosine, histamine & serotonin etc feed back and inhibit NA release.
• NA can also negatively feed back itself via α2 receptors to limit its own release.
• Lots of modulation occurring at the
  neurotransmitter level at the varicosity.
  It produces vasoconstriction and
  vasodilation as required.

Question 7

What is one role of the sympathetic vasoconstriction nerves ?

Answer 7

Distinct sympathetic pathways innervate
different tissues
- Switching on vasoconstriction in some vessels
and off in other vessels (producing
vasodilation).
- eg. exercise, increased sympathetic nerve
stimulation to GI (less blood flow), reduce
sympathetic nerve stimulation to skin (more
blood flow, cool down).

Question 8

What is another role of the sympathetic vasoconstriction nerves ?

Answer 8

Pre-capillary vasoconstriction
- Leads to downstream capillary pressure drop so
increased absorption of interstitial fluid into blood
plasma to maintain blood volume (important in
hypovolemia).

Question 9

What is another role of the sympathetic vasoconstriction nerves ?

Answer 9

Control resistance arterioles
- Produces vascular tone allows vasodilatation /increased blood flow to occur, controls TPR.
- Maintains arterial blood pressure and blood flow to
brain myocardium & kidney etc

Question 10

What is another role of the sympathetic vasoconstriction nerves ?

Answer 10

Control venous blood volume
- Venoconstriction leads to decreased venous blood volume increasing venous return, this increases stroke volume via Starling’s law.

Question 11

Describe sympathetic vasoconstriction nerves

Answer 11

Controlled by brainstem
- Provides central control of blood flow &
blood pressure.

Innervate most arterioles & veins
- NA activates a1-adrenoceptors on vascular smooth
muscle cells causing vasoconstriction.
- Sympathetic nerve activity is tonic (1 action potential per second). Tonic sympathetic activity sets vascular tone.
- Decrease in sympathetic activity producing
vasodilatation is an important principle in
pharmacological treatment of cardiovascular disease,
eg. Hypertension.