Cardiac Cycle Flashcards
Cardiac Cycle Def
Start of 1 beat to the start of the next. Complete cycle of heart activity
Systole Def.
Ventricular contraction. 2/5 of cycle at rest (increases during activity)
Diastole Def.
Ventricular Relaxation. 3/5 of cycle at rest
Cardiac Muscle (cardiomyocytes) Features
myogenically (involuntarily) active, irregular fibre shapes, mostly signle nucleate (some binucleated), straited, contain gap junctions (intercalated disks). Sponatneously active (operate seperate to extrinsic signalling)
Intercalated disks function
Separate the heart into 2 syncytia. The 2 Atria and the 2 Ventricles. Current flows through interlacted disks after depolaristion to stimulate adjacent cell
SA node outline
Cation leaky membrane keeps RMP at ~ -55mV. Gets depolarised by slow Na+ influx. Depolarises ar ~ -40mV opening Ca channels. 200 msecs later (time dependent) Ca channels close and K+ eflux occurs. Generates action potential sent onto rest of cells
Extrinsic control of SA node
Autonomic nervous system
SA node receptors
Beta1-ARs (sympathetic, increases cAMP) and M2 Receptors (parasympathetic, decreases cAMP)
Relationship between cAMP and Na influx
Increased cAMP = Icreased Na influx
Heart Conduction Pathway
Depolarisation starts at SA node in right atrium, depolaristaion flows through left and right atria, depolarisation reaches AV node and slows (no direct connectrion between atria and ventricles), depolarisation occurs in ventricles from AV nose down His bundles and Purkinje fibres, spreads slowly from endocardium to epicardium
Reason why action potential travels through AV nose before
Delay allows for complete ventricular filling and limits transmission of abnormal rhythms
Function of Ca channels opening during repolarisation
Creates plateau phase. Allows full contraction and relaxation of heart before next action potential (prevents tetanic contraction). This and period causes rhythmic contractions
Cardiac Vessels Outline
Account for pressure changes in vessels and compartments of heart
Diastole Ventricular Pressure Outline
Drops to 0mmHg, chamber enlarges (relaxes) counteracting pressure from atrium filling. As ventricle fills with blood from atrial systole, ventricular pressure rises
Systole Ventricular Pressure Outline
Pressure rises as Ventricle contracts (AV valves close). Ventricle pressure exceeds atrial pressure and blood is ejected out of heart (higher volume out of left). Pressure drops until AV valves open beginning next cycle
Diastole Ventricular Volume
Fills passively due to low ventricular pressure. Atria contraction occurs at end of diastole delivering final 25% of blood ventrical volume
Systole Ventricular Volume
No ejection at systole start (hasn’t yet exceeded aortic/pulmonary pressure). Ejection begins rapid, slowing at systoles end. As pressure drops, filling begins again
Electro-cardiogram (ECG) Outline
Shows electrical changes in heat over cycle
ECG P-wave outline
Atria depolarisation. Starts at SA node moves through atria. Sinus rhythm
ECG QRS complex
Ventricular depolarisation. Depolarisation spreads from epi to endocardium
ECG T Wave
Ventricular repolarisation. Same direction as depolarisation (epi to endo cardium). Both charge and direction of current is reversed
ECG PR Interval
Conduction through AV node. Conduction slows. Falls in range 0.2>x>0.12 secs
ECG QT Interval
Ventricular Systole. Time taken for ventricular depolarisation and repolarisation. Heart rate dependent. Shorter at faster rates
ECG ST Segment Outline
Interval between ventricular depolarisation and repolarisation
