Cardiac centre Flashcards
Cardiac centre
events associated with one heartbeat
cycle
In a normal cycle the atria contract while ventricles relax followed by atrial relaxation and ventricular contraction
Systole
contraction
Diastole
relaxation
Relaxation period
end of heartbeat chambers in diastole
isocolumetric relaxation
mitral and tricuspid valves closed no change in blood volume in ventricles
ventricular filling
ventricle diastyole
intraventicular pressure drops below atrial pressure
AV valve opens
passive ventricle filling
rapid ventricular filling
diastasis- only small colume enters
atrial systole pumps 20-25mls of blood
End-diastole volume
each ventricle contain 130mls
atrial systole 20-30% of EDV
Aortic and Pulmonary valves remain shut
Ventricular Stystole
near end of atrial systole ventricles depolarise and ventricular contraction begins
A-V valves are pushed closed by blood
after 0.05 seconds all valves are shut
isovolumetric contraction occurs
ventricular ejection occurs when pressure in ventricle opens Ao and pulmonary valve
t wave
ventricular polarasation
Cardiac Output
CO = SV x HR
ml per min
the total amount of blood ejected from the left ventricle in one minute
Cardiac reserve
Cardiac reserve refers to the difference between the rate at which the heart pumps blood and its maximum capacity for pumping blood at any given time
cardiac reserve is 4-5x than at rest
Age related changes
decreased maximal CO
changes in nodal and conducting cells
decreased elasticity of fibrous skeleton
Stroke volume factor
preload
contractility
afterload
Preload
greater preload on cardiac muscles fibers prior to contraction increases their force of contraction
if HR > 160 bpm SV falls due to reduced filling time
low heart rate have longer filling times raising preload giving larger stroke volumes
Starlings law of the heart
The more the heart is filled in diastole, the greater the stretch and the greater the force of systole
Contractility
the strength of contraction at a given preload
positive inotropic substances
calcium
adrenaine
negative inotrophic substances
beat blockers
calcium channel blockers
Afterload
the pressure in the aorta and pulmonary artery that needs to be exceeded
increased after load result in reduced SV
more blood remainsin ventricle at end of systole
preload
volume of blood im ventricles at end of diastole
increased
-hypervolemia
regurgataion of cardiac valves
heart failure
Afterloas
resistance left ventricle must overcome to circulate blood
- increase
- hypertension
- vasoconstriction
higher after load = higher cardiac workload
regulation of HR
autonomic control
sympathetic and parasympathetic
sympathetic
sympathetic nerve increases the HR and strength of contraction
parasympathetic
parasympathetic nerve slows HR
Input to cardiovascular centre
cerebral cortex
limbic system
hypothalamus
proprio-receptors
chemo-receptor
baro-receptors
output to heart (sympatheitc)
cardiac accelerator nerve
increased rate of spontaneous depolarization of SA node and AV node increasing HR
Increased contractility of atria and ventricles increasing stroke volume
output to heart (parasympatheitc)
Vagus nerves (cranial nerve X)
decreased rate of spontaneous depolarization in SA node and AV node decreasing HR
Chemical regulaiton hormones
adrenal medulla produces adrenaline and noradrenalince
thyroid hormones
increases HR and contractility
chemical regulation IONS
increased potassium and sodium decrease HR and contractility
increased calcium increases HR and contractility
Phases
Atrial contraction isovolumetric contraction rapid ejection reduced ejection isovolumetric relaxation rapid filling reduced filling
atrial systole
P wave of the EGC
electrical depolarization of the atria
atria contract
atrial pressure increases
blood flows through AV valve
Aortic + Pulmonary valves remain shut so no blood leaves ventricles
Delay before qrs complex because allow ventricles to fill before contracting
rapid blood flow into ventricles
(1/3 of filling time), pumps 20-25 mls of blood, each ventricle contains about 130ml (EDV) pumps 20-30% of EDV.
End diastole volume
after atrial contraction atrial pressure falls
pressure gradient across AV valve
AV valve closes
ventricular volumes are maximal-EDV
Preload
left ventricle end diastole volume
120ml
Isovolumetic contraction
QRS complex of the the ECG
ventricular depolirisation
excition-contraction coupling myocyte contraction
rapid increase in intraventricular pressure
AV valve closes when intraventricular pressure exceeds atrial pressure
contraction of ventricles increases pressure with no change in volume as semilunar valves are closed
Rapid ejection
interventricular pressie exceed the pressures within the aorta and pulmonary artery
aortic valves and pumonic valve open
blood ejected due to pressure gradient between ventricles and arteries
Reduced ejection
T- wave of ecg
ventricular repolarization
ventricle emptying falls
isovolumetric relaxation
all valves closed
intraventricular pressures fall aortic and pulmonic valve close
rapid filling
ventricles relax intraventricular pressure falls below atrial pressure
AV valves rapidly open
passive ventricular filling
intraventricular pressure continues to fall as ventricles relaxation occurs
reduced filling
ventricle fill with blood
increase intraventricular pressure
decreased pressure gradient
90% of ventricle filling is passive
Cardiac cycle
- all chambers in diastole at end of heartbeat,. The pressure in the ventricles falls and therefore Aortic and Pulmonary valves close
- Isovolumetric relaxation - Mitral & tricuspid closed, no change in blood volume in ventricles. The ventricles continue to relax & pressure falls.
- rapid ventricular filling (1/3 of filling time) AV (Mitral & tricuspid) valves open. Blood pours from atria into the ventricles passively.
- Atrial Diastasis only small volume enters
- Atrial systole resembles P wave (final 1/3 of time), pumps 20-25 mls of blood, each ventricle contains about 130ml (EDV) pumps 20-30% of EDV. Aortic + Pulmonary valves remain shut so no blood leaves ventricles Delay before qrs complex because allow ventricles to fill before contracting
- Ventricular contraction QRS wave
- Ventricular systole - near end of atrial systole ventricles depolarise and ventricular contraction begins, A-V valves are pushed closed by blood All valves are now closed (0.05 secs)
- Isovolumetric contraction occurs, contraction of ventricles increases pressure with s semilunar valves are closed therefore pressure in ventricles rises
- Ventricular ejection interventricular pressure exceed the pressures within the aorta and pulmonary artery opening Aortic and Pulmonary valves , lasts 0.25 secs valves close, relaxation begins again
- T wave resemble ventricular repolarisation, QRS complex cover atrial repolarisation
3 factors that govern cardiac output
Preload- (end diastolic volume) i that is amount of blood returning to left ventricle i.e. venous return. Greater venous return greater CO. Starlings law of heart s the amount of strectch of the left ventricle at the end of diatole prior to contraction related to muscle sarcomere length.
Afterload- pressure in the ascending aorta that has to overcome to open aortic valve and eject blood.
Contracility- the force of contraction at a given preload
