Cardiac Cycle, Regulation of Cardiac Output Flashcards
What are the 3 waves seen during a cardiac cycle on and ecg?
-P wave- results from atrial depolarization
-QRS wave- results from ventricular depolarisation
-T wave- results from ventricular repolarisation
Systole vs Diastole
Systole - phase of contraction
Diastole - phase of relaxation
What happens in atrial/ventricular diastole?
-Both atria and both ventricles are relaxed
-AV valves and semilunar valves are closed
-Venous return of blood fills both atria (vena cavae into right atrium and pulmonary veins into left)
-Pressure build in atriia - AV valves open and blood flows into ventricles - ventricles 80% filled
What happens in Atrial systole/ Ventricular diastole?
-Spontaneous depolarization in SA node passes into surround atria (P wave)
-Depolarization and contraction of atrial myocardinal cells
-Cardiac muscle in both atria contract - ventricles still relaxed and final 20% blood pumped in
-Ventricles are full of blood - end-diastolic volume
What happens in Atrial diastole/ Ventricular systole?
-Atrial myocytes repolarize - ventricular myocytes depolarize (QRS wave)
-As atria relax- venous blood fills atria again, full ventricles begin contraction
-Ventricular pressure greater than atrial pressure so AV valves close
-When pressure in right ventricle rises above pressure in pulmonary artery, pulmonary semilunar valve opens
-When pressure in left ventricle rises above pressure in aorta, aortic semilunar valve opens
-Both ventricles eject 2/3s of blood they contain (stroke volume) and third of initial volume is left (end-systolic volume)
What is the isovolumetric contraction?
When the pressure in the ventricles is not strong enough to push open the semilunar valves
What happens in atrial/ventricular diastole?
-Ventricle myocardial cells repolarize and begin to relax (T wave)
-For a moment both AV and semilunar valves are closed - isovolumetric relaxation
-atria filling with venous blood, atrial pressure rises again- cycle again
What are the lub-dub sounds caused by?
The lub sound is caused by blood turbulence created by closure of the AV valves as systole begins
The dub sound results from blood turbulence created by closing of the semilunar valves in the beginning of diastole
What is the EDV, ESV and SV?
-End-diastolic volume is 135ml - volume of blood in ventricle at end of diastole
-End-systoliuc volume is 60ml - volume of blood in ventricle at end of systole
-Stroke volume - 135-60= 75ml - vol of blood ejected by each ventricle during systole
How is cardiac output measured?
-Vol of blood pumped per min by each ventricle
-cardiac output = stroke vol x cardiac rate
-At rest cardiac output is 5.5L/min
What is cardiac rate and stroke volume modulated by?
The autonomic nervous system (involuntary whereas somatic NS is voluntary e.g skeletal muscles)
Autonomic NS - Sympathetic division vs Parasympathetic division
-Provides mechanism to allow involuntary neural control of most tissues/organs in body
Sympathetic division - nerves release the neurotransmitter, noradrenaline/ norepinephrine onto cells in target organs which binds to adrenergic receptors
Parasympathetic division - nerves release the neurotransmitter, acetylcholine onto cells in target organs which binds to muscarinic acetlycholine receptors
Effects of autonomic NS
-Sympathetic and parasympathetic nerves frequently innervate the same tissue/organ
-Noradrenaline and acetylcholine generally have opposing effects on the activity of the tissue/organs
How does noradrenaline affect cells?
Noradrenaline, released by sympathetic nerve endings, binds to ß adrenergic receptors and stimulates opening of Na+ and Ca2+ (HCN) channels in pacemaker cells of the SA node.
What is responsible for triggering spontaneous depolarizations?
HCN channels
.increased opeing increases rate of AP
-increases heart rate
How do paraysmpathetic (vagus) nerve affect cells?
Release acetylcholine which binds to muscarinic acetylcholine receptors on SA node cells
-Promotes opening of K+ channels on plasma membrane of pacemaker cells
-Directly inhibits opening of HCN channels
-Overall effect is to slow the rate of depolarisation of SA node autorhythmic cells - reduces heart rate
What causes increased heart rate during exercise?
-Reduction in vagus nerve activity
-Increased sympathetic nerve stimulation
Regulation of stroke volume
-Increase in EDV stretches tge ventricular muscle cells causing increase force of contraction of these cells
-Increase contractility increases stroke volume
-relationship knows as Frank Starling law of the heart
-Intrinsic ability of vcardiac muscle to increase its strength of contraction can be modulated by the autonomic NS
-Noradrenaline releasted from sympathetic nerves binds to ß adrenergic receptors and increases intracellulat Ca2+ by opening Ca2+ channels - increases contration strength and stroke vol increases
