Introduction to Cardiovascular System Flashcards
What is the general role of the heart and blood?
Heart - pump blood around the body
Blood - carry metabolites around the body
What is the general role of the arteries, veins and capillaries?
Arteries - distribute oxygenated blood to cells and tissues
Capillaries - metabolite exchange
Veins - return blood to the heart
Where does the heartbeat begin and why?
The apex of the heart
This allows blood to be squeezed out of the top of the heart
Where does electrical activity begin in the heart?
Sino-atrial node
What is sinus rhythm?
The normal rhythm of the heart which is determined by the SA node
How do the valves prevent electrical impulses travelling from the atria to the ventricles?
They are insulation as they are made from non-electrically conducting material
How is the electrical activity of the heart stimulated?
How does it reach the AV node?
- action potential is initiated in SA node
2. it is propagated to the atrioventricular node via internodal tracts in the atria
How does the speed of the action potential vary in the SA node and the AV node?
Why?
Cells of the AV node transmit the action potential more slowly and delay the impulse by 100 ms
This gives time for the atria to completely empty into the ventricles before they contract
What happens to the electrical impulse as it leaves the AV node?
The impulse spreads down to the ventricles along the bundle of His
What is significant about the bundle of His?
It is the only electrical connection between the atria and the ventricles
How does the AV bundle divide?
It divides into left and right bundle branches
There are 2 bundles going to the left ventricle
There is 1 bundle going to the right ventricle
What happens to the impulse after it passes down the bundle of His and the bundle branches?
It spreads to the contractile cells of the ventricles through an extensive network of Purkinje fibres
What is diastole?
The resting period where the heart fills with blood
What is significant about the period between heartbeats?
There is a period between heartbeats where there is no electrical activity
This allows the heart to recharge ready for the next contraction
Why can electrical impulses travel so quickly through the heart?
Gap junctions in the intercalated discs allow the spread of the action potential throughout the myocardium like a functional syncytium
what is a syncytium?
A functional whole
It is one thing even though it is made up from multiple components
What is the main, secondary and backup pacemaker of the heart?
Main - SA node
Secondary - AV node
Backup - all the other electrically active myocytes
What is the SA node influenced by?
How do other heart cells respond to these stimuli?
Influenced by the blood (e.g. hormones like adrenaline) and parasympathetic and sympathetic nerves
The rest of the heart cannot respond as intelligently to the stimuli
what is an escape pacemaker?
an electronic device that will oversee the heartbeat and intervene if something goes wrong
What is the “brake” of the heartbeat?
The vagus nerve which slows down the activity of the heart
It releases acetylcholine
How does atropine work?
It is a drug that blocks the effect of ACh on the M2 receptors
ACh is released by the vagus nerve, and slows the activity of the heart
What is the “accelerator” of the heartbeat?
The “accelerator” speeds up the activity of the heart
Adrenaline is a natural way to speed up and strengthen the heartbeat
Why is adrenaline not often used as a drug?
It has more side effects than atropine
What is the resting potential of cells within the SA node?
They have no true resting potential
They generate regular, spontaneous action potentials
What is the difference in the depolarising current in non-pacemaker and pacemaker cells?
Non-pacemaker action potentials see the depolarising current carried into the cell by fast Na+ currents
In pacemaker cells, the current is carried into the cell via slow Ca2+ currents
Why are ‘slow response’ action potentials produced in SA nodal cells?
There are no fast Na+ channels in SA nodal cells
The SA nodal cells take longer to depolarise
What are the 3 phases of pacemaker action potentials, in order?
- phase 4 (If)
- phase 0 (ICa)
- Phase 3 (Ik)
what happens at the beginning of phase 4?
At the end of repolarisation, the membrane potential is very negative (-60 mV)
“funny” channels open that conduct slow inward Na+ currents
What is the “funny current”?
what does it cause?
depolarising slow inward Na+ current
It causes the membrane potential to begin to spontaneously depolarise - the pacemaker potential
What happens during phase 4 as the membrane potential reaches -50 mV?
The (transient) T-type Ca2+ channels open
The inwards directed Ca2+ current further depolarises the cell
What happens during phase 4 as the membrane potential reaches -40 mV?
The (long-lasting) L-type Ca2+ channels open
These cause more Ca2+ to enter the cell until an action potential is generated
At what magnitude is the action potential generated in the pacemaker cell?
The threshold value of -35 mV
What happens during phase 4 relating to K+?
Slow decline in the outward movement of K+
Fall in K+ conductance contributes to the depolarising pacemaker potential
What is phase 0 of the pacemaker action potential?
The depolarisation phase of the action potential
What is Phase 0 mainly caused by?
Increased Ca2+ conductance through L-type calcium channels that begin to open towards the end of phase 4
Why is the rate of depolarisation much slower in pacemaker cells than other cardiac cells?
The movement of Ca2+ through the L-type channels is not rapid
What begins to decline during phase 0?
funny currents and Ca2+ currents through T-type channels
The respective channels begin to close