The heart Flashcards
What are the conducting cells of the heart?
Myocardium (muscle cells)
How is electrical activity spread between muscle cells of the heart?
Gap junctions- cell- cell connections that form a physical link at the intercolated discs and they cause propagation of electrical signals from one cell to another.
They are faster than chemical synapses
Describe the electrical activity of the heart (4 steps)
1) Excitability is initiated at the SAN
2) Conduction to the atria then AVN then AV ring
3) Excitability passes through the bundle of His
4) The Purkinje system causes ventricles to contract so blood is ejected
How big is the SAN?
15mmx5mmx2mm
Where is the SAN?
Posterior aspect- junction at the superior vena cava and the right atrium
What is the conduction speed of the SAN?
0.05 m/s
What is the conduction velocity of the atrial myocardium and the Bachmann’s bundle?
1.0 m/s
What is the Bachmann’s bundle?
A branch of the anterior internodal tract that resides on the inner wall of the left atrium
Why is the conduction of the atrial myocardium and the Bachmann’s bundle the same?
So the atria contract simultaneously
How big is the AVN?
2mmx10mmx3mm
Where is the AVN?
Posterior aspect- right side interatrial septum
What are the AVN subzones?
AN (1st part, atrial to nodal)
N (2nd part, pure nodal)
NV (3rd part, nodal to ventricular cells)
Why is conduction slowed to 0.05m/s between the atria and the node?
To allow time for atrial contraction to completely finish
Describe AV refractoriness and how it changes
AV refractoriness prevents excess contraction in ventricles and it increases at a high heart rate. This longer delay allows the max amount of blood from atria to the ventricle to maximise the delivery of blood/oxygen
What is the conduction velocity at the bundle of His?
1 m/s
What is the conduction velocity of the purkinje fibres?
4 m/s
In the ventricles where does contraction and repolarisation start?
At the bottom of the ventricles
Describe spiral muscle contraction?
Evokes a torsion which is more efficient to get more blood out
What are the two types of cardiac action potential?
Nodal and contractile
Which node (SAN or AVN) is dominant?
The SAN is dominant but the AVN can take over but this means the atria will not contract
Why is skeletal muscle not suitable in the heart in relation the their action potentials?
Skeletal muscle has a short duration and fast firing rate which means summation of the action potential can occur. You don’t want summation in cardiac cells as the ventricles would continue to contract too far as there is still calcium in the cell
Describe properties of a pacemaker cell
They show automaticity and rhythmicity
Examples are AV nodes and purkinje fibres
Show a gradual depolarisation and repolarisation - they shows a pre-potential which is a slow slope with gradual depolarisation
Describe the pacemaker action potential
The pre-potential is caused by a decrease in potassium influx and an increase in cation influx
The threshold is between -40mV and -50mV and is reacehed by the pre-potential
After threshold there is an increase in calcium influx followed by a quick potassium efflux which causes repolaristion
It is regulated by innervation, temperature and other pacemakers
How can pacemaker cells be controlled?
By both sympathetic and parasympathetic innervation
Vagal fibres (Ach) - causes hyoerpolarisation and a decreased pre-potential slope (parasympathetic stimulation)
Noradrenaline (sympathetic) causes increased prepotential slope, increases firing rate
Describe the properties of a cardiac muscle potential
No automaticity
Long plateau phase
Propagated and prolonged action potential
Fast depolarisation and overshoot
Describe the stages of a cardiac muscle potential
1) Voltage gated Na channels open
2) Sodium inflow depolarises the membrane and triggers the opening of more Na channels = positive feedback cycle and rising membrane voltage
3) Na channels close at +30mV
4) Calcium entering through slow calcium channels prolongs depolarisation of membrane creating a plateau. This plateau falls slightly because of some potassium leakage, however most remain closed
5) Calcium channels close and calcium is transported out the cell. Potassium channels open causing rapid efflux so its membrane returns to resting potential
What is the cardiac cycle?
The mechanical and electrical events that occur everytime your heart beats
What are the four major stages of the cardiac cycle and are they part of systole or diastole?
1) Inflow of blood phase - diastole
2) Isovulumetric contraction - systole
3) Outflow of blood phase - systole
4) Isovolumetric relaxation - diastole
What does isovolumetric mean?
There is no change in volume as the pressure is not yet high enough
Why is Bachmann’s branch important?
It has a good conduction velocity so both atria contract simultaneously
How much blood flows from the atria to the ventricles passively?
80%
How much does the atria contribute to blood flow into the ventricles?
10%
What is atrial fibrilation and when could it be an issue?
There is no p wave in the ECG so there is a reduced atrial kick
At rest this may not be a big issue but could be when exercising
What side of the heart is the tricuspid valve?
The right side
Left side = mitral/ bicuspid valve
What is EDV?
End diastolic volume
Where ventricular volumes are maximal
What is the typical EDV of the left ventricle (LVEDV)?
120ml
What are the typical end diastolic pressures of both the right and left ventricle?
Right = 3-6mmHg
Left - 8-12mmHg
Why does the pulmonary system have a low pressure?
Capillaries are one cell thick
If they were to fill up too much due to higher blood pressures they would cause oedema of the lungs
This may prevent effective gas exchange
What does phase 2 start with?
QRS complex ie ventricular depolarisation
When do the AV valves close?
When intraventricular pressure exceeds atrial pressure
This prevents backflow of blood
Ventricular contraction triggers the contraction of which other type of muscles and what is the purpose of this?
Papillary muscles
They are attached to chordae tendinae which attach to leaflets of the valves
Tension in the AV leaflets prevent them from bulging back too far in to the atria and becoming leaky
The first heart sound, S1, is caused by what?
Closure of the AV valves
Why is S1 usually split?
Split by 0.04s because the mitral valve closure slightly precedes the tricuspid closure (Left before Right -alphabet)
What happens between the time when the AV valves close and the aortic/pulmonary valves open?
Pressure rapidly rises but there is no change in volume
No ejection occurs
Isovolumetric contraction
What might the c wave be due to?
The bulging of the mitral valve leaflets back into the atrium
When does ejection begin?
When intraventricular pressure exceeds the pressures within the aorta and the pulmonary artery
Causes the aortic and pulmonary valve to open
Why are there no heart sounds heard normally during ejection?
The opening of a healthy valve is silent
What is S2 caused b?
When the intraventricular pressures fall at the end of phase 4 the aortic and pulmonar valves abruptly close
What is the dictrotic notch?
Valve closure is associated with a small backflow of blood into the ventricles
This gives a characteristic notch on tracings (a fall in pressure in the atria)
What is the rate of pressure decline in the ventricles determined by?
The rate of relaxation in the muscle fibres - this is called lusitropy
What is the rate of relaxation regulated b?
Largel the sarcoplasmic reticulum that are responsible for rapidly re-sequestering calcium following contraction
What is the end systolic volume in the left ventricle?
50ml
What is the stroke volume?
Approx 70ml
EDV-ESV = stroke volume
What is the advantage of having some blood left in the ventricles after contraction?
It gives ‘headroom’ when you start exercising - more blood can be ejected straight away
What is the v wave?
Left atrial pressure continues to rise due to venous return from the lungs
The peak of the LAP is at the end of this phase is the v wave
What do the jugular veins drain?
The face and the cranial vault
What causes S3?
The tensing of the chordae tendinae and the AV ring during vetricular relaxation and filling
How is the heart asynchronous?
The right atria contracts before the left atria
The left ventricle contracts before the right ventricle
The right ventricle contracts before the left ventricle
What is the heart rate of a newborn?
70-190 beats per minute
What is the heart rate of an infant under 1?
80-120 beats per minute
What is the heart rate of children between 1 and 10?
70-130 beats per minute
What is the heart rate of children over 10/adults?
60 - 100 beats per minute
Well trained athletes can be as low as 40
What is excitation-contraction coupling?
Electrical excitation causes contraction in muscle cells
What are the roles of t-tubules and intercalated discs in myocytes?
They help transmit action potentials rapidly in the myocardium
What is the sarcolema?
Thousands of invaginations form t-tubules
Allow action potentials to stimulate all parts, deep into the myocyte simultaneously = faster rate of contraction
What is the sarcoplasmic reticulum?
Fluid filled membrane sac surrounding each myofibril
Acts a calcium store
Cisterns and triads
What is the calcium concentration of the sarcoplasma if the muscle is relaxed?
0.1um
What is the calcium concentration of the sarcoplasmic reticulum in a relaxed muscle?
10mM
What is the role of calcequestrin?
In the SR it binds to free calcium so the calcium concentration decreases
This means the pumps can work more efficiently so more calcium can be stored
What happens to the myosin/actin binding site when sarcoplasmic calcium is low?
The tropomyosin band obscures myosin/actin binding site, preventing the head from sticking to the actin molecule
Muscle tension is directly proportional to what?
The number of cross-bridges
The number of cross bridges is proportional to what?
The sarcomere length
How can the length tension relationship be measured?
1) Record length of muscle
2) Electrically twitch and record force
3) Lengthen the muscle
4) Repeat steps 1-3 over a range of muscle lengths
What is the relationship between length and tension in cardiac myocytes?
Short means that actin interferes with binding sites so there is no force generation
Longer = myosin heads can stick so a force is genereated
If it is long enough it can reach the ‘goldilock’s zone’
Too long however and there is no opportunity for binding
What stops the heart from expanding away from the goldilocks zone?
The pericardium/mediastinum
What is titin?
A stiff spring like protein in heart muscle
What is the maxiumum tension of heart muscle?
2.2um (Maximum cross bridges form)
A 2.2um sarcomere is produced by what pressure?
10-12mmHg filling pressure in the intact heart eg pre-systole
What is isometric contraction?
No change in length
What is isotonic contraction?
The length changes but the tension does not
What is the pre-load?
Initial stretching, sarcomere length, indicated by ventricular EDV
What is the after-load?
Force against which the ventricles act to eject blood - essentially arterial blood pressure and vascular tone
What does Vmax increase with?
Contractility
In isotonic contraction the heavier the load the slower the what?
Contraction
There is an inverse relationship between shortening velocity and afterload
What are the key results when you measure the veolicity of shortening and afterload?
An increase in pre load gives a maximal force (Po)
For any given afterload an increase in preload increases velocityc
Vmax is constant - it indicates the cardiac muscle contractility
When does contractility increase?
When more crossbridges form per stimulus
It may also reflect the qualitative state of the actin/myosin cross bridges
What does digoxin do?
Increases the contractility of the heart
What does noradrenaline increase?
Both Po (maximal force) and Vmax ie has both positive inotropic and chronotropic effects
What is the frequency force relationship?
Inter-beat duration influences the force of contraction
Increase in frequency reduces time between each beat so contractility increases
This is due to changes in calcium availability - calcium accumulates with each beat as there is less time for removal
What is the Bowditch Staircase?
An autoregulation method by which myocardial tension increases with an increase in heart rate
