Local Control of Blood Flow
1. Active or Reactive Hyperemia
Hyperaemia= increase in flow
Active or Reactive hyperaemia= relationship between disturbance in metabolites that
will cause vasodilation of arterioles.
2 diff ways to disturbance of metabolites
(i)Enhancing metabolic activity of cells surrounding blood vessels
(ii)Accumulation because we have reduced flow to the area
Key dilators is Adenosine
Concept of Vasodilation
As the Adenosine diffuses and reaches the smooth muscle causes and starts smooth
muscle cells it to relax, we see a much larger increase in diameter of blood vessels and
now u can see rich network of blood flow. A large flow occurs, vasodilation allows
opening of low resistance pathway so much more flow is distributed to downstream
capillaries that will assist in deliverance of oxygen and removal of waste products which
will once those signals are removed allow the blood vessels to come to their original
Norepinephrine is added which causes vasoconstriction and prevents blood from
moving downstream capillaries. Once its removed, it comes to its resting diameter again
Vasodilator is added, adenosine is going on to the blood vessel, and the capacity of
blood vessel to go in opposite direction and opens up a lot of capillaries.
Local Control of Blood Flow is not associated with metabolic demands.
Its associated with organ trying to maintain its blood flow aat constant level. Its to help
ensure that’s there is no ups and down in perfusion of an orgran just based on
fluctuation based on arterial pressure . Its called flow autoregulation coz its designed
to maintain steady flow.
Actual response that’s allowing auto-regulation to occur is called Myogenic response. The initiator is change in arterial pressure (nothing to do with metabolism) . The
second graph on the left side how steady arterial pressure and it increases (prob due to
infusion of fluid, or changed posture etc). When you increase pressure, that is inside the
blood vseels pushing outside, pressure is pushing on the walls of blood vessels.
Arterioles are somewhat elastic, when increase outward force, the tendency passively
arterioles wall will be stretched out. Filling ballon with water).. Increasing the pressure
will push out the walls of the arterioles, (i)PASSIVE RESPONSE just happens with the
increase in pressure pushes out walls
But smooth muscles react to this, they don’t like being stretched out, and the SECOND
PART is the (ii)ACTIVE RESPONSE. The smooth muscle cells have stretched sensitive
channels in their membranes, these channels become activated and allow Ca to enter
smooth muscle cells. Increasing intracellular calcium in the muscle cell causes
CONTRACTION to occur. Smooth muscle cells contract lowering the diameter and that
will reduce BLOOD Flow (negative feedback but helps maintain steady blood
This response does not happen in a lot of responses, . Important in kidneys, brain also likes to
maintain steady blood flow.
Response to the stimulus (negative feedback)
If blood pressure is changed ;increase /decrease, you will not see a change that goes
Blood flow is able to maintained despite changes in pressure (small graph steady
Dashed line =where low or very high pressure , smooth muscle cells are not capable of
maintain flow anymore. Line stays horizontal is the optimal range, for flow auto-
regulation, if you increase pressure / decrease beyond that, then flow-auto regulation
would not occur.
SUMMARY: LOCAL REGULATION
The two mechanisms are not exclusive of each other. They are both diff features, and
used depending on the situation.
Normal arteriolar time: has some level of tone and constriction
Vasoconstriction: Endothelin increases
All factors arein balance but any change will cause the change in arterioal Control of Blood –Systemic a.k.a intrinsic (since its not coming from inside the organ)
TWO main influences that can cause blood vessels to change diameter
1. Neural – exerted Sympathetic nervous system
Sympathetic nerves are running down the spinal cord and there is branches of them
that go to heart to regulate cardiac function. Also, branches go out and go out to other
parts as well and these nerves terminate in close proximity to arterioles. So whatever
sympathetic nerves do will cause responses in arterioles
2. Hormonal influences can have broad ranging effects, since they are circulating and
can affect a lot of organs.
(these haromnes have cross-over functions and also blood vessels)
These are the effectors that body uses to homeostaticly regulate MAP
Sympathetic neuron comes in and terminating very close to an arterial, so it doesn’t
actually synapse with it, (unlike motor neuron) . They relase norephinrine in close
proxmitity so it can diffuse and reach arteriole.
On smooth muscle cells on arterioles, is the adrenergic receptor (predominant receptors
on arterioles) .. little horizontal line with vertical line is to illustrate action potential that
are coming down the sympathetic nerve and causing release of norepinephrine. Under
resting conditions, there is some level of sympathetic nerve activity. Action potential is
being fired periodically ,so norepinephrine is present and adrenergic receptors are
being activated by it.
This resting level of norepinephrine being released from the nerve is helping to set the
normal vascular tone where the arteriole is partially constricted. This is one of the
things that leads to partial contstriction coz we always have little norpehinrpine being released and activating adregeneric receptors. But from this, basal or resting level of
nerve activity , we can have increase or decrease in nerve activity. If we get activation of
nerve to higher leve (exercise, stressed out) much more rapid of action potential. Every
little line will cause more norphephrine wil be released, more frequent action potential ,
the more norpepinephrine will be released which means you will activate greater
number of adregrencic receptors and cause more CONSTRICTION.
This constriction will last as long as the nerve is releasing additional norepinephrine.