Physiology: Intrinsic and Extrinsic Control of Circulation Flashcards
What is the function of the aorta and large arteries?
High pressure reservoir/conduit
Provides the pressure head that drives blood through the systemic circuit
What is the function of arteries and arterioles?
Resistance vessels
Control of these vessels determines the volume flow through the circulation
Describe how vascular tone influences flow of blood.
Increase in vascular tone - vasoconstriction - reduced local blood flow
Decrease in vascular tone - vasodilation - increased local blood flow
What does vascular tone regulate?
Regional blood flow (flow to organs)
Arterial blood pressure
What regulates vascular tone?
Intrinsic and extrinsic mechanisms (from brain)
Describe the hierarchy of control processes.
1 - myogenic regulation
2 - endothelial secretions, vasoactive metabolites, autacoids
3 - extrinsic factors
What is autoregulation?
When arterial pressure is altered, blood flow in many vascular beds remains constant.
What is the myogenic response?
Flow autoregulation
Resistance vessels respond directly to a change in pressure by vasoconstriction or vasodilation
What happens when a vessel narrows due to a myogenic response?
Increases endothelial shear stress which stimulates the endothelium to produce NO
This prevents excessive myogenic constriction
What happens with metabolic regulation (active hyperemia)?
Operates by releasing local dilator substances.
Helps to maintain blood flow to organs (brain, heart muscle, skeletal muscle) to match metabolic activity
Name examples of local dilator substances.
Carbon dioxide
Lactic acid
Hydrogen ions etc.
What is reactive hyperemia?
Occurs in response to complete obstruction to blood flow.
E.g. MI, peripheral artery disease
What does the endothelium produce that alters blood flow?
Vasoconstrictors - endothelin, angiotensin II
Vasodilators - NO, prostacyclin, endothelial derived hyperpolarizing factor, adenosine
When is NO mainly produced?
Mainly from shear stress
Circulating insulin and oestrogen can activate NO production to a lesser extent
List the factors affecting NO production.
Shear stress - atherosclerosis and exercise
Pregnancy - high oestrogen levels = vasodilation
Drugs - GTN (vasodilators)
Why do we have extrinsic controls?
To regulate the overall TPR, and hence control BP
To allow the brain to control blood flow to individual organs
Name the extrinsic controls.
Vasomotor nerves (vasoconstrictor, vasodilator) Hormones (adrenaline, vasopressin, angiotensin II)
What is normally the most important extrinsic control?
Sympathetic vasoconstrictor fibres (noradrenergic)
What are key facts about the sympathetic vasoconstrictor nerves?
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)
What does the NA released by the sympathetic vasoconstrictor nerves do?
Activates alpha adrenoreceptors on vascular myocyte
Alpha adrenoreceptors cause vasoconstriction
Describe how reduced sympathetic activity leads to vasodilation?
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
Name the hormonal factors controlling circulation.
Adrenaline (vasoconstriction)
Angiotensin II - control hypovolemia and cardiac failure
Vasopressin - hypovolemia (vasoconstriction)
Atrial natriuretic peptide - vasodilator + diuretic effects
