The heart Flashcards
Semester 1 year 1
What is the heart surrounded by?
A protective fluid-filled sac - pericardium
What do valves do?
-prevent backflow during cardiac cycle
-control unidirectional blood flow in cardiac cycle
What are the types of valves?
-AV valves (mitral + tricuspid) connect to cardiac wall via chordae tendinae + papillary muscles
-semilunar valves (aortic + pulmonary) are small fibrous nodules come together
What cells are found in the heart wall?
-epicardium (maintain integrity of heart)
-myocardium
-endocardium (inner lining)
-all cells are electrically active
Describe conducting and contractile cells?
-conducting cells rapidly spread action potentials
-contractile cells have action potentials that lead to contraction, generating a force
Describe the electrophysiology of cardiac cells
-myocytes are involved in excitation-contraction coupling
-myocardium are extensively branched muscle fibre cells that are connected by intercalated discs (ID)
-action potentials propagate from cell to cell
Describe the myocardial cell structure
-intercalated discs are part of sarcolemma
-gap junctions allow depolarising current to flow from cell to cell
-desmosomes anchor fibres together
What phases does the depolarisation sequence consist of?
-systole (contraction phase)
-diastole (relaxation phase)
Describe the depolarisation phase
-electrical signal originates in SAN (primary pacemaker)
-action potential propagation
-reaches fibrous atrioventricular ring
-AVN acts as secondary pacemaker
-action potential carried to ventricular muscle via left + right branches of its specialised conducting cells
-action potential propagation flows from cell to cell
What does the fibrous atrioventricular ring do?
Prevents the spread of depolarisation straight to ventricle, allowing for optimal atrial emptying
Is ventricle contraction synchronised or in series?
Synchronised
What is the order of the cardiac cycle coordinated sequence?
-atrial systole
-isovolumetric ventricular contraction
-rapid ventricular ejection
-reduced ventricular ejection
-isovolumetric ventricular relaxation
-rapid ventricular filling
-reduced ventricular filling
Describe atrial systole
-atrial depolarisation following stimulation in SAN
-pressure increases due to contraction
-ventricles relax + mitral/tricuspid valves open so ventricles further fill
Describe isovolumetric ventricular contraction
-ventricles start to contract but volume stays the same
-pressure increases due to contraction
-ventricular pressure > atrial pressure, so mitral/tricuspid valves close
Describe rapid ventricular ejection
-ventricular pressure exceeds aortic pressure
-semilunar valves (SLV) open so blood ejected rapidly due to pressure grad.
-stroke volume = vol. ejected from left ventricle
-ventricle volume decreases + arterial pressure increases
Describe reduced ventricular ejection
-ventricular pressure reduces due to repolarisation
-SLV open + blood still ejected, so vol. decreases
-arterial vol. decreases as elastic walls recoil, forcing blood through to ‘arterial’ tree
-blood returns to heart, so atrial pressure increases
Describe isovolumetric ventricular relaxation
-begins after ventricles fully repolarised
-ventricles relaxed, so pressure decreases
-SLV close as ventricular pressure is below arterial pressure
-all valves closed + ventricular volume remains constant
Describe rapid ventricular filling
-ventricular pressure below atrial pressure, so mitral/tricuspid valves open
-ventricles begin to fill from atria so there’s a rapid vol. increase
Describe reduced ventricular filling
-longest phase of cardiac cycle
-includes last portion of ventricular filling
What do myocytes vary in?
Anatomy and channels
What type of currents do myocytes have during the cardiac cycle?
-time dependent
-voltage gated (VG)
Describe the process that an ECG measures
-cell A depolarises beyond threshold
-sodium + calcium channels open, so cations enter, so depolarisation
-depolarisation causes flow of +ive charge to cell B
What is the instantaneous vector?
EC current
What is detected in an ECG trace?
Sum of the vectors
What do the electrodes in an ECG measure in and what can the direction of the electrical activity cause?
-electrodes measure in planes around the heart
-direction of electrical activity can cause a +ive or -ive deflection
In an ECG, what does the P wave represent?
Depolarisation of the atria
In an ECG, what is the PR interval?
The time between atrial depolarisation and ventricular depolarisation
In an ECG, what does the QRS complex represent?
Depolarisation of the ventricles
In an ECG, what does the ST segment represent?
Region between the end of ventricular depolarisation and beginning of ventricular repolarisation
In an ECG, what does the T wave represent?
Repolarisation of ventricles
