Mechanics

Question 1

What is the structure and function of a cardiomyocyte?

Answer 1

Rod-shaped
100μm long
T-tubule invaginations that are 200nm in diameter
Carry action potentials to sarcoplasmic reticulum and lining up the Z lines
Many mitochondria close to myofibrils for rapid ATP synthesis

Question 2

What are the steps of E-C coupling in cardiac muscle?

Answer 2

1) AP opens L-type Ca2+ channels leading to Ca2+ influx
2) Influx causes conformational change in RyR, causing efflux of Ca2+ from sarcoplasmic reticulum
3) Ca2+ binds to troponin, moves tropomyosin
4) Ca2+ ATPase uses ATP to pump Ca2+ against conc gradient back to SR (all released by SR is reabsorbed)
5) Na+/Ca2+ exchanger uses Na+ gradient to efflux Ca2+ from cell (all enters via L type Ca2+ channel exits)

Question 3

Why is the L type Ca2+ channel needed in cardiomyocytes but not in skeletal muscle?

Answer 3

DHPR mechanical link does not exist in cardiomyocytes

Question 4

Define isometric contraction

Answer 4

No change in length put pressure increases in ventricles when valves closed

Question 5

Define isotonic contraction

Answer 5

Shortening of fibres leads to the ventricular blood ejection when valves open
When ventricles contract

Question 6

Define preload, and how does it influence pressure volume loops?

Answer 6

Wall stress at end of diastole, measure using End diastolic volume/pressure
Force stretching a muscle before contraction - blood filling in diastole
Determined by amount of venous return
More blood = more force = greater contraction

Question 7

Define afterload, and how does it influence pressure volume loops?

Answer 7

Wall stress during systole, measure using DBP
Force exerted on a muscle during contraction that was apparent when not contracting - pressure of blood in arteries
Pressure in aorta that left ventricle needs to overcome
Larger afterload = reduced shortening of muscles

Question 8

What is Starling’s law of the heart (Frank-Starling mechanism)?

Answer 8

Increase in diastolic fibre length (preload) causes an increase in ventricular contraction causing an increase in stroke volume
So that CO balances venous return (preload)

Question 9

What is stroke work?

Answer 9

Work done by heart to eject blood under pressure to arteries

Stroke volume x pressure

Question 10

What factors affect heart contraction?

Answer 10

Preload - amount of blood filling in ventricle

Afterload - pressure in aorta

Question 11

In a pressure-volume loop, what do the letters A-D represent?

Answer 11

A= mitral valve closes
B= aortic valve opens
C= aortic valve closes
D= mitral valve opens

Question 12

What is the two factor cause of the Frank-Starling mechanism?

Answer 12

Increased stretch=reduced actin-myosin overlap=more crossbridge formations
Changes in Ca2+ sensitivity - Troponin C has higher Ca2+ affinity when stretched

Question 13

What is stroke work?

Answer 13

Work done by heart to eject blood under pressure to the arteries
Stroke work = stroke volume x pressure

Question 14

What is the Law of LaPlace?

Answer 14

When pressure within the cylinder is constant, tension on walls increases with increasing radius
Wall Tension = Vessel pressure x vessel radius
Incorporate wall thickness T=(PxR)/h

Question 15

What is the relationship between the Law of LaPlace and mechanics?

Answer 15

To achieve = wall tension in each ventricle, pressure in right must be reduced (it has a larger radius)
Left ventricle has a smaller radius ∴ can generate greater pressure for the same wall tension
Heart failure leads to dilation ∴ larger radius ∴ greater wall stress

Question 16

What is contractility?

Answer 16

Force of contraction from the myocardium
Measured by ejection fraction
Increased by sympathetic stimulation

Question 17

What are the 7 stages of the cardiac cycle?

Answer 17

1) Atrial systole
2) Isovolumetric contraction
3) Rapid ejection
4) Reduced ejection
5) Isovolumetric relaxation
6) Rapid passive filling
7) Reduced passive filling

Question 18

What are the electrical and mechanical events, valve movements and point on the PV loop associated with atrial systole?

Answer 18

Pacemaker potential from SAN, AP flows over atria causing depolarisation and contraction (P wave)
Atrial contraction
AV open, SL closed
S4 abnormal - indicates valve incompetence
D→A

Question 19

What are the electrical and mechanical events, valve movements and point on the PV loop associated with isovolumetric contraction?

Answer 19

Wave of depolarisation spreads over ventricles causing contractions (QRS complex)
Myocardium contracts without fibre shortening, increasing pressure with no change in volume
AV closed
SL closed
S1 due to AV closure
A→B

Question 20

What are the electrical and mechanical events, valve movements and point on the PV loop associated with rapid ejection?

Answer 20

Ventricles depolarised and contracting (ST segment)
Pressure gradient outwards, isotonic contraction forces blood into arteries
AV closed
SL open
No noise
B→C

Question 21

What are the electrical and mechanical events, valve movements and point on the PV loop associated with reduced ejection?

Answer 21

Ventricles repolarise (T wave)
Decrease in ventricle pressure reduces gradient ∴ valves close
AV closed
SL closing 
No noise
B→C

Question 22

What are the electrical and mechanical events, valve movements and point on the PV loop associated with isovolumetric relaxation?

Answer 22

Diastolic ∴ no electrical events
Ventricles relax and fibre length is constant
AV closed
SL closed
S2 from SL closing
C→D

Question 23

What are the electrical and mechanical events, valve movements and point on the PV loop associated with rapid passive filling?

Answer 23

Isoelectric ECG period
AV opens to allow rapid filling of ventricles down gradient
AV open
SL closed
S3 abnormal sound due to valve abnormalities
D→A

Question 24

What are the electrical and mechanical events, valve movements and point on the PV loop associated with reduced passive filling?

Answer 24

Isoelectric ECG period
Most blood has entered ventricles ∴ gradient reduced and slower filling
AV open
SL closed
No noise
D→A

Question 25

Define end diastolic volume

Answer 25

Volume of blood in ventricle just before contraction

110ml normal

Question 26

Define end systolic volume

Answer 26

Volume of blood in ventricle just before end of blood expulsion - residual vol of blood in ventricles
40ml normal

Question 27

How is stroke volume calculated?

Answer 27

EDV-ESV

Question 28

What are the normal pressure values of systolic, diastolic, pulse, mean arterial and atrial?

Answer 28

systolic - 120mmHg
diastolic - 80mmHg
pulse - 40mmHg
mean atrial - 93 mmHg
atrial - <30mmHg

Question 29

Define ejection fraction

Answer 29

SV/EDV

Clinical sign of how well the heart is working (normally 60-70%)

Question 30

What is the length tension relation in cardiac muscle?

Answer 30

Elastic band analogy
longer stretch (preload) increased force of contraction

Question 31

What are important properties of cardiac muscle?

Answer 31

Does not overstretch as encased in the pericardium
More resistant to stretch than skeletal muscle so generates a greater passive force as length increases
More compliant than skeletal muscle
Uses both isotonic and isometric contraction

Question 32

Define cardiac output

Answer 32

stroke volume x HR
volume of blood pumped by heart per minute
approx 5L

Question 33

Define stroke volume

Answer 33

end diastolic volume - end systolic volume
volume of blood pumped by heart per beat
approx 70ml

Question 34

Define pulse pressure

Answer 34

systolic BP - diastolic BP

approx 40mmHg

Question 35

Define mean arterial pressure

Answer 35

DBP +( 1/3 x pulse pressure)

approx 93 mmHg