Organ Circulations
Cardiac Circulation
Right coronary artery usually most important
Artery-Artery anastomoses called collaterals. Density lowest at epicardium.
Extraction is 13 ml/100 ml blood, 90%, so must increase CBF for more oxygen.
A decrease in supply or an increase in consumption will decrease balance.
Myocardium has little preference for substrate. Glucose uptake is stimulated during hypoxia – sympathetics.
Autoregulation is excellent in coronary circulation. Blood pressure usually remains constant.
Flow in ventricle decreases markedly with each systole, especially in the endocardium. But these vessels are more numerous and more dilated. So average flow is equal. Reduction in transmural pressure below normal will decrease end/epi ratio below 1.
Hypoxia is most potent stimulator of dilation. Adenosine released by parenchymal cells.
Sympathetics can produce dilation and parasympathetics barely.
Skeletal Muscle Circulation
Muscle mass is 43% of body weight.
Blood flow can vary from 250-12500 ml/min.
Huge contributor of peripheral resistance, blood pressure in toto.
In exercise – skeletal muscles can increase both flow and extraction
At rest – 25% of total, during exercise – 90%
Flow regulation at arteriole level. 1.5-150 w/ exercise.
Also metabolic dilation.
Local control overrides neural control during exercise. Ignore sympathetics. Functional sympatholyasis
Cerebral Circulation
Very weak sympathetic control
As intracranial pressure rises, transmural pressure falls, and vessels tend to collapse. Collapse leads to myogenic and metabolic dilation and flow is autoregulated up to a point.
When flow falls to a critical level, a massive sympathetic discharge is triggered and the arterial pressure begins to rise in parallel with the rise intracranial pressure. Called Cushing Response.,
Regional metabolic control. Adenosine, K and H
Cutaneous Circulation
Low resting flow. Range from 50-2800.
Arteries and arterioles control perfusion of subdermal venous plexuses and capillary loops
Capillary loops run perpendicular to skin surface.
Subcutaneous venous plexus, secondary route for heat exchange, large volume, run parallel, core temperature regulation, prevent freezing, filling controlled by A-V anastomoses.
When warm, open shunts, venous plexus is major surface for heat loss to regulate core temperature.
Countercurrent: superficial arteries and veins are separated, deep vessels run parallel. When cold, superficial vessels contract. Blood in deep arteries runs next to blood in deep veins and exchange.
CNS role: preoptic region of anterior hypothalamus is major control. Temperature receptors are both in skin and hypothalamus, initiate reflex to change sympathetic tone
Kinin system: Heat > sympathetic cholinergic signal to sweat glands > kallikrein acts on kininogen to form bradykinin. Dilates and opens A-V anastomoses.
White reaction – due to vasoconstriction, not neural
Triple response – red line (vessel trauma), flare or red blush (axon reflex from mechanical stimulation via histamine), wheal (increased capillary permeability followed by fluid and protein leaks.
Intestinal
Supply to muscularis and mucosa
Capillary loops with countercurrent flow to facilitate absorption of Na and H2o and shunting of o2
Neural control is exclusively sympathetic
Local control: autoregulation is poorly developed, functional hyperemia is well developed, glucose and FA are mediators along with gastrin and chol.
Hepatic Blood Flow
Large flow – 25% of CO at rest
75% inflow from portal vein
25% O2 inflow via portal vein (Mean pressure is 10mmHg), 75% from hepatic artery (Mean pressure is 90).
Low O2 and high substrate from gut
Paired arteriole and venule pass in parallel with bile ductule into hepatic acinus. Then to hepatic sinusoids (capillaries)
Blood leaves liver via hepatic venules.
Blood enters lobule with same pressure, thus pressure drop in arterioles is large
Elevation of pressure causes filtration out of sinusoids and into peritoneal cavity.
Local control is well developed, sensitive to O2
Sympathetic innervation, most important as volume reservoir, contains 15% of volume.
