SHORT TERM REGULATION OF MEAN ARTERIAL BLOOD PRESSURE


RAPIDLY ACTING NERVOUS MECHANISMS

1) BARORECEPTOR REFLEXES

Anatomy
• Baroreceptors are especially abundant in the:
  a) carotid sinuses [located in wall of ICA just above carotid bifurcation]
  b) walls of the aortic arch
• Impulses are transmitted from:
  a) carotid sinus via the glossopharangeal nerve (CN-IX) to the medulla
  b) aortic arch via the vagal nerve (CN-X) to the medulla



response of baroreceptors to pressure

 < 60 mmHg see no stimulation of baroreceptors

  • 60 - 160 mmHg see maximum stimulation
  • see maximum   at normal pressures [I = impulses]
  • the baroreceptors respond much more to a rapidly changing pressure than to a stationary pressure
  • they adapt in 1 — 2 days to whatever pressure they are exposed to; have no long term effect in BP regulation


baroreceptor reflex
• stimulated baroreceptors inhibit vasoconstrictor centre of medulla —>
 i) vasodilation of peripheral vasculature
 ii) decreased HR & contractility
 —> reduced BP
 [low BP has an opposite effect]
• baroreceptors play a major role in maintaining BP during postural changes


 2) CHEMORECEPTOR REFLEXES

Anatomy
 Chemoreceptors are located in the:
  a) carotid bodies [located in the carotid bifurcation]
  b) aortic bodies in walls of the aortic arch
Impulses are transmitted via the vagus [along with nerve fibres from baroreceptors] into the vasomotor centre
Each body has its own blood supply —> each body is in close contact with arterial blood

chemoreceptor reflex
1° reduced arterial BP —> reduced O2; increased CO2 & H+ —> stimulate chemoreceptors —> excite vasomotor centre —> increase BP
 [& increased resp stim]
1°reduced O2; increased CO2 & H+ —> stimulate chemoreceptors —> excite vasomotor centre —> increase BP
Only works strongly with BP < 80 mm Hg



3) ATRIAL & PULMONARY ARTERY REFLEXES

Anatomy
• Both the atria & pulmonary arteries have stretch receptors in their walls—low pressure receptors
• pulmonary artery receptors are similar to baroreceptors in operation, atrial receptors operate as follows:


atrial reflexes
stretched atria —>
 1) slight reflex vasodilation of peripheral arterioles —>
  i) reduced peripheral resistance —> reduced BP back down to normal
  ii) increased blood flow into capillaries —> increased capillary pressure —>    third space shifting —> reduced blood volume
 2) reflex dilatation of afferent arterioles of kidney —> increased urine production
 3) stimulate hypothalamus —> decreased ADH —> reduced resorption of H2O in    kidney —> increased urine secretion
 4) increased HR [Bainbridge reflex] —> offload fluid from heart


4) CNS ISCHEMIC RESPONSE

• reduced blood flow to vasomotor centre in brain stem —> ischaemia of medulla —> increased local[CO2] —> excite vasomotor centre —> increased BP
• has a tremendous magnitude in increasing BP: is one of the most powerful activators of the sympathetic vasoconstrictor system
• Only becomes active at arterial BP < 50 mmHg — ‘last ditch stand’
• Cushing reaction: increased Intracranial pressure —> compression of arteries in brain —> CNS ischaemic response —> increased BP

note that in all the above reflexes, the increased sympathetic output not only stimulates the arteries & arterioles but also constricts the veins —> increased mean systemic pressure —> increased cardiac output —> increased BP



RAPIDLY ACTING HORMONAL MECHANISMS

1) NORADRENALIN—ADRENALIN VASOCONSTRICTOR MECHANISM

• Sympathetic stimulation —> stimulate adrenal medulla —> release of Ad & NAd —> excite heart; vasoconstrict most blood vessels
• May act on metarterioles which are not innervated


2) VASOPRESSIN VASOCONSTRICTOR MECHANISM


  • Reduced BP —> hypothalamus secretes vasopressin via post pituitary —> direct vasoconstriction —> increased peripheral resistance/MSFP —> increased BP
  • Very potent; plays an important role in correcting BP when is acutely dangerously low —> important short term role
  • Important long term role as ADH (same substance)



3) RENIN—ANGIOTENSIN VASOCONSTRICTOR MECHANISM


  • at least 20 minutes are required before this system can become fully active
  • it has a relatively long duration of action