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