Heart rate

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SA node, Bradycardia and Tachycardia

SA node

inervated of sympathetic and parasympathetic fibres, exert continous influenced heart rate

Bradycardia - stimulation of parasympathetic nerves, reduces heart rate

prolonged stimulation may stop heart

Block impulses - increase in heart rate

Tachycardia - stimulation of sympathetic nerves, increase in heart rate

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Autonomic pathways

Autonomic pathways occur in the medulla oblongata

some fibres cardio inhibit

others cause vasoconstriction

Several hormones alter heart rate - adrenaline, B1 adrenoreceptors in SA node

increase NA premeability, increase heart rate

Thyroxine - increases excitability of heart muscle

enhances adrenaline

Cortisol - stimulates cardiac muscle cells - enhances action of adrenaline

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Stroke Volume

stroke volume can be increased 

increase rate of filling of heart, increase volume of blood

Ventricles contract more strongly - expel more blood during cycle

Increased end - distolic volume

rise in rate of filling of heart

results in greater volume of blood in ventricle

stretches cells in Myocardium - enables cells to contract with increased force - eject more blood - more stroke volume

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Cardiac Reflexes

change in blood pressure initiates autnomic reflexes - alter cardiac output

Cardiovascular centre - vasometer centre

depressor area

pressor area

connected to heart by sympathetic pathway - cardioinhibitory centre

connected by parasympathetic pathway

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Baroreceptor reflexes

baroreceptors - spray type nerve ending inwalls of major arteries - respond to stretch of blood vessel

caused by increase in BP 

Major baroreceptors - wall of aortic arch

carotid sinuses

nerve exhibit tonic activity

Chemoreceptors respond to lower O2 raised CO2 or H 

found in aortic and carotid bodys

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Postural Hypotension

Principal action of baroreceptor reflex

brings BP up when it falls 

On standing up after lying down

venous pooling leads to - a fall in venous return

immediate drop in stroke volume

drop in cardiac output

fall in venous return leads to 

reduction in Arterial BP

and initiates barorecpetor reflex

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