Physiology 3 - The Cardiac Cycle Flashcards
What are the events during the cardiac cycle?
- Passive Filling2. Atrial Contraction3. Isovolumetric ventricular Contraction4. Ventricular Ejection5. Isovolumetric ventricular relaxation
What happens during passive filling?
The pressure in the atria and ventricles is close to 0.AV valves open so that venous return flows into the ventricles.
What causes the flow of blood during passive filling?
A small pressure gradient.
What is the pressure in the aorta during passive filling?Is the aortic valve open?
80mmHgNo
How full, roughly, do the ventricles become during passive filling?
About 80% full.
What happens during atrial contraction?
The atrias contract completing the end diastolic volume
What does the P wave in the ECG signal?
Atrial depolarisation
What happens between the P wave and the QRS complex?
Atrial contraction
What is the end diastolic volume in a resting normal adult?
approx .130ml
When does ventricular contraction start?
After the QRS complex
When do the AV valves shut?What does this produce
When ventricular pressure exceeds atrial pressureS1
Why is phase 3 called isovolumetric ventricular contraction?
The tension rises around a closed volume
When does the aortic/ pulmonary valves open?
When the ventricular pressure exceeds the aortic/ pulmonary pressure
What is ejected during ventricular ejection?
Stroke volume
SV=? (equation + values)
SV = EDV - ESV = 135 - 65 = 70ml
T wave represents?
Ventricular repolarisation
When does the aortic/ pulmonary valves shut?What does the produce?
When the ventricular pressure falls below aortic/ pulmonary pressureS2
What signals the start of isovolumetric ventricular relaxation?
Closure of the aortic/ and pulmonary valves
Why is stage 5 called isovolumetric ventricular relaxation/
The tension falls around a closed box
When does the heart start a new cycle?
When the ventricular pressure falls below atrial pressure causing the AV valve to open
What does S1 signal?
The start of systole (closure of mitral and tricuspid valves)
What does S2 signal?
The end of systole and beginning of diastole (closure of aortic and pulmonary valves)
What is the JVP an indirect measure of?How does this work?
The pressure in the right atriumThe jugular vein becomes the SVC which connects to the right atrium meaning any changes in the RA will be reflected in the JVP
What is the SA node made of?
Specialised pacemaker cells
Sinus rhythm?
Heart is controlled by the SA node
How is an action potential generated?
Cells in the SA node exhibit spontaneous pacemaker potential meaning the membrane potential gradually drifts until it reaches threshold which causes the cell to fire
Pacemaker potential?
Gradual drift of the membrane potential towards depolarisation
What is the ionic basis of the pacemaker potential?
There is a decrease in K efflux whilst there is slow Na influx
What causes depolarisation of the SA node cells?
At threshold, activation of voltage-gated Ca channels occur resulting in Ca influx
What is the ionic basis of repolarisation?
Activation of K channels results in K efflux
Passage of excitation through the heart path?
SA node -> cell-to-cell conduction -> AV node -> bundle of his -> left and right branches -> purkinje fibres
What does cell-to-cell conduction occur via?
Gap junctions
What are gap junctions part of?
Intercalated disc which are specialised intracellular attachment between cardiac muscle cells - they are made up of gap junctions, ask adherents and desmosomes
Location of the AV node?
Base of the right atria just above the junction of the atria and ventricles
How fast is the AV bode conduction velocity?
Fast
Resting membrane potential of of contractile muscle cells?
-90mV
What causes the rising phase (depolarisation) of the action potential in contractile muscle cells?What does this reverse the membrane potential to?What phase is this?
Fast Na+ influx+30Phase 0
What causes phase 1 of ventricular muscle action potential?
Closure of Na+ channels and transient K+ efflux
What causes phase 2 of ventricular muscle action potential?what is another name for this phase?
Mainly the influx of Ca through voltage-gated Ca channelsPlateau phase
What causes phase 3 of ventricular muscle action potential?
Closure of Ca channels and K efflux
What is phase 4?
Resting membrane potential
What is the main influence of heart rate/
Autonomic nervous system
What is the parasympathetic supply to the hear?
The vagus nerve
What type of nerve supply to the SA node dominates under resting conditions
The vagal tone - slows the intrinsic heart rate from approx. 100 to 70bpm
Where in the heart does the vagus supply?
The AV and SA nodes
What does the vagal nerve stimulation do?
Increases AV nodal delayDecreases heart rate
What neurotransmitter does the vagus nerve use to slow the heart?What is a competitive inhibitor of this and can be therefore used to treat extreme bradycardia?
acetlycholine acting through M2 receptorsAtropine
How does vagal stimulation affect the pacemaker potential?What happens to the slope of the pacemaker potential?What is the name for this effect
The cell hyper polarises meaning it takes longer to reach threshold, frequency of AP decreases Slope decreasesNegative chronotropic effect
Where do cardiac sympathetic nerves supply
AV nodeSA node Myocardium
What effects does sympathetic stimulation of the heart have?
Increases heart rateDecreases AV nodal delayIncreases force of contraction
Neurotransmitter for sympathetic supply to heart
Noradrenaline acting through B1-adrenoceptors
What effect does noradrenaline have on the slope of the pacemaker potential?
Speed at which pacemaker potential reaches threshold?Frequency of action potentials?
Effect?
Increases slopeIncreasesIncreasesPositive chronotropic effect
P wave?
Atrial depolarisation
QRS complex
Ventricular depolarisation
T wave
Ventricular repolarisation
PR interval
Largely AV nodal delay
ST segment
Ventricular systole
TP interbal
Diastole
