Cardiovascular Physiology 1- Generation of the heart beat Flashcards
What is the purpose of the cardiovascular system?
Transport system for materials on which the cells of the body absolutely depend on.
Example of materials:
O2, CO2, Hormones, Heat
By these processes, homeostasis can be maintained and therefore able to survive- Mechanism of dissipating heat or getting warmth
How is the cardiovascular system organised?
- 4 chambers, right and left side
- Blood flows in one direction
- Systemic circulation= The blood vessels that carry blood from the left side of the heart to the tissues and back to the right side of the heart
How is the heart beat able to generate its own action potential?
- Action potential generated in the SAN
- Signal for myocardial contraction comes from specialised myocardial cells known as autorhythmic cells (pacemaker cells)= Set the rate of the heartbeat
Able to generate AP:
-Unstable membrane potential, starting at -60mV and slowly drifts upwards towards threshold= Pacemaker potential NOT resting potential as it never ‘rests’ at a constant value
What is the ionic basis of SAN generated Action potential?
1) Pacemaker potential= -60mV, If channels (allow current (I) to flow) opens to allow Na+ IN and K+ leaves
If channels open when membrane is hyper polarised at the end of reploarisation from the previous AP
2) Results in increasingly positive charge which slowly depolarises SAN
3) As membrane potential becomes positive, the If channels gradually close and one set of Ca2+ channels opens
4) Influx of Ca2+ ions continue depolarisation and membrane potential slowly moves towards threshold
5) Membrane potential reaches threshold= Different set of voltage-gated Ca2+ channels open= Calcium rushes in= Steep depolarisation
6) Peak of action potential= Calcium channels close and slow K+ channels start to open
7) Repolarisation= K+ efflux Membrane potential reduces as it becomes less positive= Goes back to -60mV
What are the differences between cardiac AP generation and nerves and muscles generation?
1) Depolarisation is due to opening of Ca2+ channels NOT Na+
2) Cardiac pacemaker cell K+ permeability does NOT remain constant between AP while nerve and skeletal muscle does
3) Single muscle fibre can have graded contractions= the fibre varies the amount of force it generates while in nerve and skeletal= All-or-none
What is the heart rate determined by?
The speed at which pacemaker cells depolarise
How does action potential of SAN spread throughout the heart to coordinate contraction?
1) Depolarisation begins in SAN (right atrium)
2) Spread of wave of depolarisation spreads to the atrioventricular node
3) Delay of passage of electrical signal from the atria to the ventricle to allow the atria to contract
4) Depolarisation moves into ventricles as it travels down the bundle of His
5) Reaches apex of heart and depolarisation moves to left and right bundle branches through the punkinje fibres
5) Depolarisation then spreads throughout the ventricular myocardium
What are the different pacemaker cells which could potentially drive the heart rate?
SAN: Highest frequency of AP generation= Drives the heart rate
However, if SAN becomes dysfunctional, then AVN determines HR (around 50/min)
BUT: If this fails, Purkinje fibres can also determine heart rate at around 30/min=Cannot survive
What is complete heart block?
When conduction tissue between atria and ventricles is damaged and becomes non functional
What is contractile cells of the heart and how are they different from pacemaker cells?
Contractile cells make up the atria and the ventricles= Cells must be able to shorten and lengthen their fibres and the fibres must be flexible enough to stretch
Cardiac pacemaker cells (autorhythmic cells)= Carry the impulses that are responsible for the beating of the heart= Occur throughout the heart
Difference: Contractile cells: Striated muscle, with fibres organised into sarcomeres and are bigger
How are the individual cardiac muscle cells organised?
Branch and join neighbouring cells end-to-end to create a complex network
Cell junctions= Intercalated disks have interdigitated membranes + Made up of desmosomes and gap junctions
What is the ionic basis of cardiac muscle cell action potential (contractile myocardium)?
DIFFERENT TO IONIC MECHANISM AND SHAPE FROM SAN
1) Resting membrane potential: Stable resting potential of about -90mV
2) Rising phase= Depolarisation
When a wave of depolarisation moves into a contractile cell through gap junctions= Membrane potential rises= Na+ Channels open and membrane potential reaches between +20 to +30mV and the channels close
3) Peak potential + Initial repolarisation= K+ Channels open to allow fast limited efflux to give a very small repolarisation
4) Plateau: AP then flattens due to: Fast K+ channels close + Slow Voltage gated Ca2+ channels start to open = Starts to enter the membrane
5) Rapid repolarisation: Plateau ends when Ca2+ channels close and K+ permeability increases= Ca2+ channels close, slow K+ channels open= K+ exists the cells rapidly and cell returns to resting potential
How does an action potential start cardiac muscle contraction?
L-type Ca2+ channels in the transverse tubules initiate a much larger Ca2+ release from the sarcoplasmic reticulum
What is the refractory period? Why is it much longer in cardiac muscle AP?
Refractory period: Time following an action potential during which a normal stimulus cannot trigger a second action potential
Much longer: Prevent tetanus as cardiac muscles must relax between contractions so ventricles can fill with blood
By the time a second AP is triggered, membrane will be completely relaxed
This contrasts skeletal muscle contraction= A second potential is fired immediately after the refractory period= summation
What is pulmonary circulation?
The blood vessels that go from the right ventricle to the lungs, back to the left atrium
