Skeletal muscle blood flow ranges from 2.7 ml/100g/min at rest to 100 ml/100g/min during exercise. Sympathetic nervous activity maintains resting vascular tone but vasodilator metabolites override sympathetic activity during exercise. The muscle pump enhances perfusion by lowering venous pressure during contraction. Splanchnic blood flow is 25% of cardiac output at rest and increases after eating as blood is coupled to nutrient absorption. The liver acts as a large compliant reservoir that stores 20% of blood volume. Renal blood flow is 23.3% of cardiac output with autoregulation maintaining flow over a range of pressures.

1. Skeletal muscle circulation
 Enormous range of blood flow in skeletal
muscle: 2.7 ml/100g/min at rest (15.6% of CO)
 During exercise: 100 ml/100g/min (80-85% of
CO)
 Resistance vessels have high resting tone
(myogenic)

2.  Neural
 neural control dominates at rest
 tonic sympathetic nervous system vasoconstrictor activity (1
Hz) alpha 1 adrenergic receptor mediated
 an increase in sympathetic nervous system activity (4-5 Hz)
can decrease flow by 70%
 – vasodilatation at rest is passive due to withdrawal of
sympathetic nervous system activity
 sympathetic-cholinergic fibers are anatomically present -
physiological role is uncertain

3.  with increased activity there is an increase in
 the production of vasodilator metabolites
 n vasodilator metabolites are dominant during
 exercise although sympathetic nervous system
 activity to the working muscle is also enhanced

4.  Mediators of Vasodilation
 – increased interstitial [K+] stimulates
 Na+/K+ATPase hyperpolarizes membrane
 – interstitial acidosis/hypoxia hyperpolarizes
 membrane
 – interstitial hyperosmolarity
 – adenosine?

5. Physical factors
 Cyclical contraction and
relaxation of active skeletal
muscle vessels
 vessels are compressed
during the contraction phase
blood flow becomes
intermittent
 muscle perfusion is
enhanced by the muscle
pump
 during activity muscle pump
lowers the venous pressure
which increases the
pressure gradient driving
flow

6.  Autoregulation
 blood flow is relatively constant from 60 to 120
mmHg (mainly myogenic)
 Reactive Hyperemia
 brief occlusion of blood flow is followed by a
transient increase in flow

7. Role of Skeletal Muscle Circulation in
Blood Pressure Control
 large mass of tissue: 40 - 45% of body weight
 major site of resistance vessels
 Peripheral resistance regulated by controlling muscle
resistance
 resistance influenced by
 tonic vasoconstrictor activity
 metabolic vasodilators
 regulation by reflex mechanisms (baroreceptors,
cardiopulmonary receptors, etc.)

8. Splanchnic circulation
 Blood supply of
 Intestines
 Pancreas
 Spleen
 Liver
Mesenteric arteries ->
intestines -> portal vein ->
liver -> hepatic vein
Liver is supplied by hepatic
artery

9.  Blood flow 25% of resting CO - can increase by
30 -100% after a meal
 blood flow is closely coupled to absorption of
water, electrolytes and nutrients
 Series/parallel configuration: the venous
drainage from the capillary bed of the
gastrointestinal tract, spleen and pancreas
flows into the portal vein, which provides most
of the blood flow to the hepatic circulation

10.  Hepatic artery provides the remainder of the
blood flow into the liver
 High compliance venous system (25
ml/mmHg/kg) acts as a reservoir (especially
the liver)
 Contains 20% of the blood volume at rest

11.  Sympathetic nervous system
 innervation of arterioles, precapillary sphincters and
venous capacitance vessels
 little or no basal sympathetic nervous system tone
 sympathetic nervous system activity strong vaso-
and venoconstriction
 redistributes BF, and increases functional
circulating blood volume (“mobilization”)

12. Parasympathetic
 no innervation of blood vessels
 Increased activity, increased motility, increased
metabolism
 functional hyperemia due to local vasodilator
metabolites (NO?)

13. Hormones
 Gastrin, cholecystokinin functional hyperemia
 Angiotensin II, vasopressin vasoconstriction

14.  Autoregulation
 – poorly developed metabolic mechanism
dominates
 Autoregulatory escape
 increased sympathetic nervous system activity
causes a transient decrease in BF
 after 2 -4 minutes blood flow returns towards
normal due to accumulation of metabolites
(adenosine) and vasodilation of arterioles
 veins remain constricted

15.  Hypotension
 – vasoconstriction due to sympathetic nervous
system, angiotensin II and vasopressin -> increased
TPR
 – venoconstriction displaces blood centrally
increased central venous pressure

17. Reservoir function
 Spleen pumps blood & ability to plasma into
lymphatics
 Sympathetic stimulation causes spleen to contract
strongly and discharges blood into circulation
 Liver is a large expandable organ
 Act as a reservoir when there is excess of blood
 Releases extra blood into the circulation

18. Renal circulation
 At rest 420.0 ml100g/min (1260 ml/min) 23.3 % CO
 Pressure drop across the glomerulus is only 1-3 mmHg
 Further drop at the efferent arteriole
 Regulation
 Norepinephrine, Angiotensin II – vasoconstriction
 Dopamine – vasodilatation
 – Sympathetic activity (alpha receptor) – vasoconstriction
 – Stimulation of renal nerves - increases renin secretion
 Autoregulation is present
 – Myogenic effect, NO may be involved
 Renal cortex high blood flow poor O2 extraction but in medulla
low blood flow but high O2 extraction

19. Points to remember
 Blood flow ml/min or ml/10g/min
 % cardiac output
 Autoregulation
 Metabolic hyperaemia
 Reactive hyperaemia
 Local factors eg. Nitric oxide
 Neural and hormonal factors
 Other factors
 Effects of ischemia