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Integrated Physiology and Pharmacology > The heart > Flashcards

The heart Flashcards

(90 cards)

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1
Q

What are the conducting cells of the heart?

A

Myocardium (muscle cells)

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2
Q

How is electrical activity spread between muscle cells of the heart?

A

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

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3
Q

Describe the electrical activity of the heart (4 steps)

A

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

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4
Q

How big is the SAN?

A

15mmx5mmx2mm

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5
Q

Where is the SAN?

A

Posterior aspect- junction at the superior vena cava and the right atrium

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6
Q

What is the conduction speed of the SAN?

A

0.05 m/s

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7
Q

What is the conduction velocity of the atrial myocardium and the Bachmann’s bundle?

A

1.0 m/s

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8
Q

What is the Bachmann’s bundle?

A

A branch of the anterior internodal tract that resides on the inner wall of the left atrium

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9
Q

Why is the conduction of the atrial myocardium and the Bachmann’s bundle the same?

A

So the atria contract simultaneously

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10
Q

How big is the AVN?

A

2mmx10mmx3mm

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11
Q

Where is the AVN?

A

Posterior aspect- right side interatrial septum

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12
Q

What are the AVN subzones?

A

AN (1st part, atrial to nodal)
N (2nd part, pure nodal)
NV (3rd part, nodal to ventricular cells)

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13
Q

Why is conduction slowed to 0.05m/s between the atria and the node?

A

To allow time for atrial contraction to completely finish

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14
Q

Describe AV refractoriness and how it changes

A

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

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15
Q

What is the conduction velocity at the bundle of His?

A

1 m/s

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16
Q

What is the conduction velocity of the purkinje fibres?

A

4 m/s

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17
Q

In the ventricles where does contraction and repolarisation start?

A

At the bottom of the ventricles

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18
Q

Describe spiral muscle contraction?

A

Evokes a torsion which is more efficient to get more blood out

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19
Q

What are the two types of cardiac action potential?

A

Nodal and contractile

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20
Q

Which node (SAN or AVN) is dominant?

A

The SAN is dominant but the AVN can take over but this means the atria will not contract

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21
Q

Why is skeletal muscle not suitable in the heart in relation the their action potentials?

A

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

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22
Q

Describe properties of a pacemaker cell

A

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

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23
Q

Describe the pacemaker action potential

A

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

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24
Q

How can pacemaker cells be controlled?

A

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

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25
Describe the properties of a cardiac muscle potential
No automaticity Long plateau phase Propagated and prolonged action potential Fast depolarisation and overshoot
26
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
27
What is the cardiac cycle?
The mechanical and electrical events that occur everytime your heart beats
28
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
29
What does isovolumetric mean?
There is no change in volume as the pressure is not yet high enough
30
Why is Bachmann's branch important?
It has a good conduction velocity so both atria contract simultaneously
31
How much blood flows from the atria to the ventricles passively?
80%
32
How much does the atria contribute to blood flow into the ventricles?
10%
33
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
34
What side of the heart is the tricuspid valve?
The right side | Left side = mitral/ bicuspid valve
35
What is EDV?
End diastolic volume | Where ventricular volumes are maximal
36
What is the typical EDV of the left ventricle (LVEDV)?
120ml
37
What are the typical end diastolic pressures of both the right and left ventricle?
Right = 3-6mmHg | Left - 8-12mmHg
38
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
39
What does phase 2 start with?
QRS complex ie ventricular depolarisation
40
When do the AV valves close?
When intraventricular pressure exceeds atrial pressure | This prevents backflow of blood
41
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
42
The first heart sound, S1, is caused by what?
Closure of the AV valves
43
Why is S1 usually split?
Split by 0.04s because the mitral valve closure slightly precedes the tricuspid closure (Left before Right -alphabet)
44
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
45
What might the c wave be due to?
The bulging of the mitral valve leaflets back into the atrium
46
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
47
Why are there no heart sounds heard normally during ejection?
The opening of a healthy valve is silent
48
What is S2 caused b?
When the intraventricular pressures fall at the end of phase 4 the aortic and pulmonar valves abruptly close
49
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)
50
What is the rate of pressure decline in the ventricles determined by?
The rate of relaxation in the muscle fibres - this is called lusitropy
51
What is the rate of relaxation regulated b?
Largel the sarcoplasmic reticulum that are responsible for rapidly re-sequestering calcium following contraction
52
What is the end systolic volume in the left ventricle?
50ml
53
What is the stroke volume?
Approx 70ml | EDV-ESV = stroke volume
54
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
55
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
56
What do the jugular veins drain?
The face and the cranial vault
57
What causes S3?
The tensing of the chordae tendinae and the AV ring during vetricular relaxation and filling
58
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
59
What is the heart rate of a newborn?
70-190 beats per minute
60
What is the heart rate of an infant under 1?
80-120 beats per minute
61
What is the heart rate of children between 1 and 10?
70-130 beats per minute
62
What is the heart rate of children over 10/adults?
60 - 100 beats per minute | Well trained athletes can be as low as 40
63
What is excitation-contraction coupling?
Electrical excitation causes contraction in muscle cells
64
What are the roles of t-tubules and intercalated discs in myocytes?
They help transmit action potentials rapidly in the myocardium
65
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
66
What is the sarcoplasmic reticulum?
Fluid filled membrane sac surrounding each myofibril Acts a calcium store Cisterns and triads
67
What is the calcium concentration of the sarcoplasma if the muscle is relaxed?
0.1um
68
What is the calcium concentration of the sarcoplasmic reticulum in a relaxed muscle?
10mM
69
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
70
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
71
Muscle tension is directly proportional to what?
The number of cross-bridges
72
The number of cross bridges is proportional to what?
The sarcomere length
73
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
74
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
75
What stops the heart from expanding away from the goldilocks zone?
The pericardium/mediastinum
76
What is titin?
A stiff spring like protein in heart muscle
77
What is the maxiumum tension of heart muscle?
2.2um (Maximum cross bridges form)
78
A 2.2um sarcomere is produced by what pressure?
10-12mmHg filling pressure in the intact heart eg pre-systole
79
What is isometric contraction?
No change in length
80
What is isotonic contraction?
The length changes but the tension does not
81
What is the pre-load?
Initial stretching, sarcomere length, indicated by ventricular EDV
82
What is the after-load?
Force against which the ventricles act to eject blood - essentially arterial blood pressure and vascular tone
83
What does Vmax increase with?
Contractility
84
In isotonic contraction the heavier the load the slower the what?
Contraction | There is an inverse relationship between shortening velocity and afterload
85
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
86
When does contractility increase?
When more crossbridges form per stimulus | It may also reflect the qualitative state of the actin/myosin cross bridges
87
What does digoxin do?
Increases the contractility of the heart
88
What does noradrenaline increase?
``` Both Po (maximal force) and Vmax ie has both positive inotropic and chronotropic effects ```
89
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
90
What is the Bowditch Staircase?
An autoregulation method by which myocardial tension increases with an increase in heart rate

Integrated Physiology and Pharmacology (17 decks)

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