Download Local regulation of arterial blood flow Local (tissue) blood flow: why

Local regulation of arterial blood flow
Flow = pressure gradient/resistance
Remember Poiseuille’s Law: radius
is major factor affecting resistance
Resistance =
8 x length x viscosity
π x radius4
Distribution of blood flow (Cardiac Output) at rest
The % of CO each organ receives is not constant
Blood delivered to all organs at same MAP:
driving force for blood flow is identical for all organs.
Blood flow distribution to each organ is controlled by:
1)  Vascularisation (ie. the number of blood vessels)
2) Degree of vasoconstriction or vasodilation of
arterioles supplying that organ (most important in
short term)
Local (tissue) blood flow: why does it vary?
Tissue blood flow is proportional to metabolic need
Tissues need:
Oxygen
Removal of H+ ions (pH regulation)
Nutrients (eg glucose, amino acids)
Regulation of other ions (eg. Na+, K+)
Removal of carbon dioxide
Transport of hormones to tissues
Some specific tissues have special requirements eg.
Blood flow to the skin: thermoregulation
Blood flow to the kidneys: excretion of waste products in urine
Acute control of blood flow (short term – seconds to minutes)
eg in exercising muscle is achieved by vasoconstriction and vasodilation
Long-term control of blood flow (days/weeks/months)
eg. scar tissue, cancerous tissue, increased muscle mass.
Angiogenesis: growth of new blood vessels.
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Arterioles
The major resistance vessels.
Very little elastic tissue
Thick layer of smooth muscle (innervated by sympathetic nerves).
Vasoconstriction:
Contraction of smooth muscle
Decrease in radius of arteriole
Increased resistance to blood flow
Vasodilation:
Relaxation of smooth muscle
Increase in radius of arteriole
Decreased resistance to blood flow
Arteriolar smooth muscle has vascular tone.
Intrinsic factors influence contractile activity of
arteriolar smooth muscle, therefore influencing blood flow.
Local control of blood flow: Intrinsic control
Changes within tissues that alter radius of arterioles (ie. that affect the
smooth muscle of the arterioles supplying the tissue)
Important in matching blood flow with the metabolic needs of tissues
Local controls are most important in:
1) Skeletal and cardiac muscle.
Why? Their metabolic activity varies most
2) Brain
Why? Its blood supply must be kept constant
Local influences are chemical or physical
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Local chemical changes
Occur during increased metabolic activity in tissues
eg oxygen concentration decreases
Local vasodilation of arterioles via relaxation of smooth
muscle
Process is called active hyperaemia (or functional
hyperaemia)
Active hyperemia can result in up to a 50-fold increase
in muscle blood flow with maximal exercise.
Increased blood flow meets increased metabolism.
When metabolic needs decrease, chemical changes
result in vasoconstriction.
Local chemical changes
Decreased oxygen (hypoxia)
Increased CO2: More is produced as a result of increased metabolism.
Increased acid: Carbonic acid is generated from CO2; lactic acid is produced
from anaerobic metabolism of ATP production.
Increased potassium ions
Eg. in exercising muscle: Repeated action potentials (K+ out, Na+ in) may
outpace the ability of the Na+/K+ pump to restore resting concentration
gradients, causing accumulation of K+ in tissue fluid.
Adenosine
Formed from cellular AMP, especially in cardiac muscle.
Released in response to increased metabolism or decreased oxygen.
Prostaglandin release
Local chemical messengers.
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Endothelial cells
Single layer of specialised epithelial cells lining the lumen of
every blood vessel.
Release chemicals important in regulation of the radius of
arterioles.
Chemicals are released in response to chemical (eg.
decrease in oxygen) or physical (eg. stretching) changes.
Act on the underlying smooth muscle to alter its state of
contraction.
These chemicals are vasoactive ie. “act on vessels”
EDRF
Endothelial-derived relaxing factor.
Causes vasodilation by stimulating relaxation of smooth
muscle.
Identified as NO (nitric oxide).
Local physical changes
Temperature
Heat increases blood flow to an area by causing localised vasodilation.
Cold causes vasoconstriction and therefore decreased blood flow.
Myogenic responses to stretch
Nerve-independent contractile activity initiated by the muscle itself.
Arteriolar smooth muscle responds to being stretched by myogenically
increasing its tone (contracting), therefore resisting the stretch.
Conversely, a decrease in stretch results in decreased myogenic tone.
Vasoactive substances may also contribute to these responses.
Remember: Arterioles do not contain much elastic tissue – not very
stretchy, so will respond strongly to this mechanical stretch.
Myogenic and metabolic responses are important in reactive hyperaemia.
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Reactive hyperaemia
If blood supply to a region is reduced, arterioles in the region dilate due to
1) myogenic relaxation (less stretch because of reduced blood flow)
2)Changes in local chemical composition:
eg. Oxygen concentration decreases
Concentrations of carbon dioxide and other metabolites increase – they
accumulate in the tissue
When blood flow restored, much higher than normal: hyperaemia.
Arterioles are vasodilated as a result of chemical changes and myogenic
relaxation
Pressure autoregulation
When mean arterial blood pressure falls (eg.
in heart failure), blood flow decreases
because driving force decreases.
Chemical changes in tissues and reduced
stretch cause vasodilation.
This causes a further decrease in blood
pressure, worsening the problem (other
mechanisms try to compensate).
Example: Cerebral autoregulation
(ie blood flow to brain)
In increased blood pressure,
vasoconstriction occurs in order to reduce
blood flow towards normal.
Pressure autoregulation aims to keep tissue
blood flow constant despite varying mean
arterial pressure.
This can be important in hypertension and
hypotension
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Arterioles - extrinsic control
Factors outside the organ that influence blood flow
Sympathetic nerve influence on smooth muscle (constriction: via
action of noradrenaline on α1 receptors)
Hormonal influence on smooth muscle (adrenaline constricts via α1
receptors)
During exercise, extrinsic factors cause an initial vasoconstriction in
muscle
Local changes over-ride this and cause dilation - competition
between vasoactive factors
Effect of exercise on Effect of exercise on
blood flow
TPR
Note large change in muscle blood flow
Remember
that
Total
peripheral resistance (TPR)
is the sum of the resistance of
all peripheral vasculature in
the systemic circulation.
Because
of
widespread
vasodilation during exercise,
TPR decreases
This facilitates
during exercise
blood
flow
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