circulation through special regions 3
TRANSCRIPT
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)
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
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
Mediators of Vasodilation – increased interstitial [K+] stimulates Na+/K+ATPase hyperpolarizes membrane – interstitial acidosis/hypoxia hyperpolarizes membrane – interstitial hyperosmolarity – adenosine?
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
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
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.)
Splanchnic circulation Blood supply of
Intestines Pancreas Spleen Liver
Mesenteric arteries -> intestines -> portal vein -> liver -> hepatic vein
Liver is supplied by hepatic artery
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
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
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”)
Parasympathetic no innervation of blood vessels Increased activity, increased motility, increased
metabolism functional hyperemia due to local vasodilator
metabolites (NO?)
Hormones Gastrin, cholecystokinin functional hyperemia
Angiotensin II, vasopressin vasoconstriction
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
Hypotension – vasoconstriction due to sympathetic nervous
system, angiotensin II and vasopressin -> increased TPR
– venoconstriction displaces blood centrally increased central venous pressure
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
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
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