Physiology: Intrinsic and Extrinsic Control of Circulation

Question 1

What is the function of the aorta and large arteries?

Answer 1

High pressure reservoir/conduit

Provides the pressure head that drives blood through the systemic circuit

Question 2

What is the function of arteries and arterioles?

Answer 2

Resistance vessels

Control of these vessels determines the volume flow through the circulation

Question 3

Describe how vascular tone influences flow of blood.

Answer 3

Increase in vascular tone - vasoconstriction - reduced local blood flow
Decrease in vascular tone - vasodilation - increased local blood flow

Question 4

What does vascular tone regulate?

Answer 4

Regional blood flow (flow to organs)

Arterial blood pressure

Question 5

What regulates vascular tone?

Answer 5

Intrinsic and extrinsic mechanisms (from brain)

Question 6

Describe the hierarchy of control processes.

Answer 6

1 - myogenic regulation
2 - endothelial secretions, vasoactive metabolites, autacoids
3 - extrinsic factors

Question 7

What is autoregulation?

Answer 7

When arterial pressure is altered, blood flow in many vascular beds remains constant.

Question 8

What is the myogenic response?

Answer 8

Flow autoregulation

Resistance vessels respond directly to a change in pressure by vasoconstriction or vasodilation

Question 9

What happens when a vessel narrows due to a myogenic response?

Answer 9

Increases endothelial shear stress which stimulates the endothelium to produce NO
This prevents excessive myogenic constriction

Question 10

What happens with metabolic regulation (active hyperemia)?

Answer 10

Operates by releasing local dilator substances.

Helps to maintain blood flow to organs (brain, heart muscle, skeletal muscle) to match metabolic activity

Question 11

Name examples of local dilator substances.

Answer 11

Carbon dioxide
Lactic acid
Hydrogen ions etc.

Question 12

What is reactive hyperemia?

Answer 12

Occurs in response to complete obstruction to blood flow.

E.g. MI, peripheral artery disease

Question 13

What does the endothelium produce that alters blood flow?

Answer 13

Vasoconstrictors - endothelin, angiotensin II

Vasodilators - NO, prostacyclin, endothelial derived hyperpolarizing factor, adenosine

Question 14

When is NO mainly produced?

Answer 14

Mainly from shear stress

Circulating insulin and oestrogen can activate NO production to a lesser extent

Question 15

List the factors affecting NO production.

Answer 15

Shear stress - atherosclerosis and exercise
Pregnancy - high oestrogen levels = vasodilation
Drugs - GTN (vasodilators)

Question 16

Why do we have extrinsic controls?

Answer 16

To regulate the overall TPR, and hence control BP

To allow the brain to control blood flow to individual organs

Question 17

Name the extrinsic controls.

Answer 17

Vasomotor nerves (vasoconstrictor, vasodilator) 
Hormones (adrenaline, vasopressin, angiotensin II)

Question 18

What is normally the most important extrinsic control?

Answer 18

Sympathetic vasoconstrictor fibres (noradrenergic)

Question 19

What are key facts about the sympathetic vasoconstrictor nerves?

Answer 19

Controlled by brainstem
Innervate most arterioles and veins of body
Terminate at the edge of the tunica media in strings of synaptic varicosities
Varicosities release vesicles (containing NA and ATP)

Question 20

What does the NA released by the sympathetic vasoconstrictor nerves do?

Answer 20

Activates alpha adrenoreceptors on vascular myocyte

Alpha adrenoreceptors cause vasoconstriction

Question 21

Describe how reduced sympathetic activity leads to vasodilation?

Answer 21

Increase in BP –> activates baroreflex –> inhibits sympathetic activity –> reduced blood pressure
Exercise (increase in body temp) –> reduced sympathetic nerve activity to the skin –> vasodilation –> increased blood flow –> heat loss

Question 22

Name the hormonal factors controlling circulation.

Answer 22

Adrenaline (vasoconstriction)
Angiotensin II - control hypovolemia and cardiac failure
Vasopressin - hypovolemia (vasoconstriction)
Atrial natriuretic peptide - vasodilator + diuretic effects