Lecture 13: Vascular function part 2

Question 1

How does vascular function differ in skeletal muscle circulation?

Answer 1

• Strong metabolic autoregulation
• Extensive SNS innervation, ‘shut down at rest’
• During exercise SNS is ineffective compared to the vasodilators present.
• B2 receptors also cause vasodilation anyways and increased Adrenaline from exercise will promote this.

Question 2

What are the effects of metabolites on skeletal muscle vascular smooth muscle?

Answer 2

• Relax vascular SM directly or via endothelium
• Inhibit sympathetic adrenergic stimulation

Question 3

How does skeletal muscle enhance venous return?

Answer 3

• Metabolite accumulation has little direct effect on post cap vascular tone
• Myogenic responses, increased SNS, circuitlating catecholamines and histamine promote venoconstriction == enhanced venous return

Question 4

How is the cutaneous circulation characterised?

Answer 4

Characterised by:
- Low cap density
- Large number of superficial AV shunts = high SA = heat exchangers

Question 5

What regulates the flow of blood to the cutaneous AV shunts for heat exchange:

Answer 5

• Only alpha receptors are present
• Powerfully influenced by SNS
• Local factors cause the affinity of alpha receptors for NA to decrease when temperature rises.

i.e local warming causes vasodilation

Question 6

How does sweating influence cutaneous blood flow?

Answer 6

• Cholinergic innervation of sweat glands activates kinin system and bradykinin vasodilates pre cap vessels.

This together with exercise induced vasodilation facilitate heat exchange

Question 7

How is cerebral circulation autoregulated?

Answer 7

• Huge metabolism influences
• Even if total brain is constant, theres big regional variations
  Local mechanisms for this include:
  -> pH, (most sensitive to pCO2), K, PaO2
  -> Little ANS effect
  -> Increased circulatory volume may reduce flow.

Question 8

Whats happening in compartment syndrome and how does this relate to circulation?

Answer 8

• Heamorrhage of oedema rises Intersitital pressure and this in turn can:
• 20mmHg rise = post cap. collapse, this can be restored.
• 40mmHg rise = pre-cap collapse, under perfusion, or no perfusion.

This depends on compartment compliance
- Superficial - skin = super compliant
- Deep - Less comliant

Question 9

Whats happening in raynauds disease?

Answer 9

• Vascular spasm because inappropriate responsiveness of alpha adrenergic receptors following exposure to the cold.

Question 10

Whats happening in circulatory shock?

Answer 10

Sustained ischeamia can change microcirculation in a way that is not easily reversed i.e inadequate vascular tone

Question 11

How can ischemia cause inadequate vascular tone?

Answer 11

• Inadequate perfusion of blood vessels
• depletion of of NE in nerve terminals
• Perivascular accumulation of vasodilator metabolites and local release of histamine, bradykinin, serotonin or PG

= Can lead to microvascular thrombosis.