Specialised Circulations

Describe the basic anatomy of the coronary arteries in the human heart

Right coronary artery supplied RV. Left coronary artery divides into LCx and LAD branches (associated with MI). Branches of coronary arteries come back round to the back of the heart (supply O2/nutrients to myocardium)

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Describe features of coronary circulation (1)

Other areas of the body with constant and continuous flow. Coronary arteries decrease to very low levels (almost 0) during cardiac cycle due to contraction of heart/systole, no flow in forward direction, most flow during diastole

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Describe features of coronary circulation (2)

Heart (high requirement for O2) - important to extract O2 during diastole, variation in flow in coronary arteries. Ventricular systole, smaller coronary arteries compressed, pressure isn't high enough to get blood down (against ventricular pressure).

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Describe features of coronary circulation (3)

When ventricles relax, pressure higher, blood able to get down coronary arteries, suction effect (smaller capillaries open, blood into coronary circulation)

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Describe features of coronary circulation (4)

Most perfusion occurs during diastole, origin of coronary arteries (coronary ostium)

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How do you calculate oxygen extraction?

O2ER = VO2 (oxygen consumption) / DO2 (oxygen delivery). High oxygen extraction in relation to coronary flow (high demand from myocardium)

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What is the link between coronary blood flow and O2 consumption

High O2 extraction in relation to coronary flow (myocardium highly dependent on O2, susceptible to lower O2 conditions, damage occurs if there is interruption to O2 supply_

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What is the dominant control mechanism in the coronary artery?

Metabolic vasodilation (active hyperaemia - release of metabolites cause vasodilatation e.g. adenosine, lactic acid, endothelial NO generation to maintain flow/dilatation)

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What is the O2 extraction coefficient?

~60% during maximum exercises (~25% at rest)

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What is the density of capillaries in coronary circulation?

High - ~ 3000 mm^2 (compared to skeletal muscle of 400 mm^2, functional end arteries - arterioles down vascular tree have smooth muscle which can dilate, other parts of the body only proximal arterioles have smooth muscle, arteries contain less)

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Describe features of coronary collateral circulation (1)

Atheroscleosis, plaque rupture, thrombosis, causes area of ischaemia. Development of collaterals (over time, ischaemia, blockage not perfused, collateral growth)

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Describe features of coronary collateral circulation (2)

Occlusive coronary artery disease - blockage, stops flow in distal LAD, development of arteries which grow out from proximal artery, can grow across and supply myocardium, can also join with arteries from the other side

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Describe features of coronary collateral circulation (3)

Collateral growth varies in people. Ability to grow collaterals - chances of survival are higher

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Describe features of coronary collateral circulation (4)

Despite a blockage, there is perfusion of artery distal to blockage, no forward pressure in artery, extensive collateral growth from other branch, still blood flow via retrograde perfusion despite stenosis

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Describe features of cerebral circulation (1)

Higher requirement for circulation, high density of blood vessels in the brain. Circle of Willis (provides perfusion all around the brain, via cerebral arteries, join carotid arteries together - if there is a blockage, perfusion can still occurs

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Describe features of cerebral circulation (2)

Arrangement of brain's arteries creates redundancy. Brain receives 15% of CO. Flow to brain can be regulated. Cerebral activity causes localised changes in blood flow (metabolic changes in activity). Form of autoregulation. Flow controlled by pCO2

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Describe features of cerebral circulation (3)

CO2 builds up in metabolising tissues. Changes in cerebral blow flow in relation to CO2 is marked. Not much neurological regulation of brain blood flow (sympathetic stimulation). Mainly metabolic autoregulation

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Describe features of cerebral circulation (4)

Normal pCO2 is 40 mmHg. Steep rise in cerebral blood flow. Myogenic/metabolic. Changes in pO2 (decrease/no change in cerebral blood flow). CO2 induces NO release from endothelium cells (vasodilation)

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What is the density of capillaries in cerebral circulation?

High - ~ 3000-4000 mm^2 (sensitive to ischaemia). Tight endothelial junctions, prevents transfer of molecules across BBB (transfer of hydrophilic small molecules requires transporters - consider when developing drugs, cross/don't cross BBB)

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Describe features of pulmonary circulation (1)

Gravitational perfusion gradient (upright). Hydrostatic pressure between base/top of lungs is marked. Under normal conditions capillaries in top part of lungs collapse during diastole. Systole - perfusion pressure is high enough to open vessels

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Describe features of pulmonary circulation (2)

Vessels in the middle are continuously patent. Vessels at the bottom are distended by gravity. Lung has reserved capacity to increase flow in response to demand of O2 e.g. exercise (perfusion pressure increases, top vessels open, more gas exchange)

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Describe features of pulmonary circulation (3)

Graph - normal people at sea-level. Adaptive to living at high altitudes (low pO2) - different link between perfusion and ratio of CO: body SA, perfusion more efficient. Inverse relationship between gravitational perfusion gradient and LVEDP

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Describe features of renal circulation (1)

Fenestrated endothelium, lot of exchange of solutes, filtered in glomerulus (tangled capillaries, high SA for filtration/exchange), flow regulated by vasoconstriction/vasodilation in response to stimuli

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Describe features of renal circulation (2)

(e.g. increase flow by vasodilation of afferent arteriole, increase perfusion pressure, opposite is true when restricting afferent arteriole).

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What is the capillary pressure in renal circulation?

~50 mmHg

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What does extrinsic neurohormonal mechanisms involve?

Control flow through glomerulus e.g. NA acting on alpha 1 adrenergic receptors (vasoconstriction)

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What does intrinsic autoregulation involve?

Involves adenosine receptor (a1 and a2 receptors)

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Describe features of juxtoglomerular feedback (tubuloglomerular feedback) (1)

Afferent arteriole in contact with DCT. Macula densa cells (release adenosine in response to high Na, constricts afferent, reduce glomerular filtration to conserve Na in blood). High Na in blood (increase filtration/excrete Na)

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Describe features of juxtoglomerular feedback (tubuloglomerular feedback) (2)

Transporters in loop of Henle (e.g. Na, K, Cl co-transporters - major target of diuretic drugs)

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Describe features of juxtoglomerular feedback (tubuloglomerular feedback) (3)

Increased BP/decreases Na in filtrate causes release of prostaglandins, activates renin-angiotensin system, constricts efferents, increase perfusion pressure in glomerulus, increase GFR

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Describe features of juxtoglomerular feedback (tubuloglomerular feedback) (4)

Decreased BP/increased Na in filtrate, adenosine released from macula densa cells, activate adenosine receptors, constrict afferents, decrease GFR - want Na/H2O retention to restore blood volume

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Describe features of baroreceptors

Baroreceptors in carotid arteries response to central changes in BP, perfusion in kidney responds to stimulus via sympathetic neural control mechanism

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Describe features of skin circulation (1)

Important for temperature regulation. Blood flow to upper layers of the skin can increase/decrease to regulate heat exchange. Specialised capillary loops in epidermis. Cold conditions (vasoconstriction). Hot conditions (vasodilation)

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Describe features of skin circulation (2)

Presence of AV anastomoses provides mechanism for large changes in blood flow (between arteries and veins, venous plexus)

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How would decreased body temperature be detected?

By peripheral and hypothalamus thermoreceptors increases sympathetic outflow to skin (NA and a2 adrenergic receptor)causing cutaneous arterioles and AVAs to constrict - reduce heat loss

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How would increased body temperature be detected?

Decreased sympathetic outflow. Cholinergic sympathetic afferent also induce sweating and bradykinin release (causes further vasodilation) - increase heat loss

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Specialised Circulations

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