Why does arteriolar consctriction decrease blood pressure downstream of arterioles?

Hey guys - just wondering considering that arterial vasoconstriction increases blood pressure how come arteriolar constriction decreases blood pressure considering that resistance still increases and flow decreases when arterioles constrict just like with arteries - thanks :smile:

Reply 1

macpatgh-Sheldon

20

Hi Tom (btw, hockey is out atm shamefully! :frown:

),

To look at your Q logically, actually the medium-sized arterioles are the main resistance vessels i.e. their constriction is the main determinant of the INCREASE in blood pressure noted in several contexts e.g. administration of alpha-adrenergic receptor agonist drugs will tend to increase blood pressure by this very mechanism i.e. the receptors on the [circular] smooth (unstriated) muscles in the tunica media of these arterioles are mainly alpha receptors e.g. phentolamine or ephedrine will result in a rise in systemic b.p. Another situation where this applies is in the rare phaeochromocytoma (tumour of the adrenal medulla) which secretes large amounts of adrenaline and noradrenaline [the preferred terms lately are the US ones, epinephrine and norepinephrine, respectively] leading to paroxysmal bouts of dangerous upstrokes in blood pressure e.g. on pressure on the abdomen induced by change in posture compressing the adrenals [here the situation is very complex because NA has a mainly alpha receptor agonist action while adrenaline has both alpha and beta agonist activity [beta-1 effect will increase rate and force of contraction of the heart [positive chronotropic and inotropic effects, respectively, tending to increase b.p., while stimulation of the beta-2 receptors in muscular arterioles will lead to relative vasodilatation, tending to offset this rise in b.p.].

The same situation occurs [more like used to occur - now history!] with the outdated anti-depressants, the MAOIs (monoamine oxidase inhibitors) when the patient inadvertently consumed amine-containing foods like cheese and Chianti wine against medical advice; the breakdown of catecholamines was inhibited by the blocking of action of MAO, so that large amounts of these amines [mainly centrally] increased sympathetic outflow, leading to a very high b.p. and consequent high risk of haemorrhagic stroke.

The title of your post is more correct than the body in that "the b.p. downstream of the constriction drops"; simply because the narrowed part of the vessel[s] dissipates the force generated by the heartbeat, so that the post-constriction part of the arteriole has a deflated pressure. An analogy might help to grasp this principle: if you have a narrow pipe/hose into which water is being pumped from a large tap, the pressure within the pipe will be high, but at its open end where the water is released, pressure will drop (bottleneck effect).

One final example [slightly different] to clarify this point:-
In coarctation of the aorta (a congenital cause of secondary hypertension), the blood pressure is usually [depending on the precise location of the stenotic part] higher in the right arm than in the left arm because proximal to the narrowing is the branching from the aorta of the brachiocephalic artery, whose branch to the right arm will carry a higher b.p. than the left subclavian artery supplying the left arm, which branches out distal to the coarctation.

Make sense?

I hope I have explained clearly enough to illustrate the point.

Be safe!
M.

Reply 2

username4233074

OP

13

[quote="macpatgh-Sheldon;88044578"]Hi Tom (btw, hockey is out atm shamefully! :frown:

),

To look at your Q logically, actually the medium-sized arterioles are the main resistance vessels i.e. their constriction is the main determinant of the INCREASE in blood pressure noted in several contexts e.g. administration of alpha-adrenergic receptor agonist drugs will tend to increase blood pressure by this very mechanism i.e. the receptors on the [circular] smooth (unstriated) muscles in the tunica media of these arterioles are mainly alpha receptors e.g. phentolamine or ephedrine will result in a rise in systemic b.p. Another situation where this applies is in the rare phaeochromocytoma (tumour of the adrenal medulla) which secretes large amounts of adrenaline and noradrenaline [the preferred terms lately are the US ones, epinephrine and norepinephrine, respectively] leading to paroxysmal bouts of dangerous upstrokes in blood pressure e.g. on pressure on the abdomen induced by change in posture compressing the adrenals [here the situation is very complex because NA has a mainly alpha receptor agonist action while adrenaline has both alpha and beta agonist activity [beta-1 effect will increase rate and force of contraction of the heart [positive chronotropic and inotropic effects, respectively, tending to increase b.p., while stimulation of the beta-2 receptors in muscular arterioles will lead to relative vasodilatation, tending to offset this rise in b.p.].

The same situation occurs [more like used to occur - now history!] with the outdated anti-depressants, the MAOIs (monoamine oxidase inhibitors) when the patient inadvertently consumed amine-containing foods like cheese and Chianti wine against medical advice; the breakdown of catecholamines was inhibited by the blocking of action of MAO, so that large amounts of these amines [mainly centrally] increased sympathetic outflow, leading to a very high b.p. and consequent high risk of haemorrhagic stroke.

The title of your post is more correct than the body in that "the b.p. downstream of the constriction drops"; simply because the narrowed part of the vessel : :