Mechanics Flashcards
Law of Laplace; Cardiac myocytes; Starling's law of the heart; Pressure volume loops; Phases of the cardiac cycle; Pressures and volumes; Ejection fraction
What is the structure and function of a cardiomyocyte?
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
What are the steps of E-C coupling in cardiac muscle?
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)
Why is the L type Ca2+ channel needed in cardiomyocytes but not in skeletal muscle?
DHPR mechanical link does not exist in cardiomyocytes
Define isometric contraction
No change in length put pressure increases in ventricles when valves closed
Define isotonic contraction
Shortening of fibres leads to the ventricular blood ejection when valves open
When ventricles contract
Define preload, and how does it influence pressure volume loops?
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
Define afterload, and how does it influence pressure volume loops?
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
What is Starling’s law of the heart (Frank-Starling mechanism)?
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)
What is stroke work?
Work done by heart to eject blood under pressure to arteries
Stroke volume x pressure
What factors affect heart contraction?
Preload - amount of blood filling in ventricle
Afterload - pressure in aorta
In a pressure-volume loop, what do the letters A-D represent?
A= mitral valve closes B= aortic valve opens C= aortic valve closes D= mitral valve opens
What is the two factor cause of the Frank-Starling mechanism?
Increased stretch=reduced actin-myosin overlap=more crossbridge formations
Changes in Ca2+ sensitivity - Troponin C has higher Ca2+ affinity when stretched
What is stroke work?
Work done by heart to eject blood under pressure to the arteries
Stroke work = stroke volume x pressure
What is the Law of LaPlace?
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
What is the relationship between the Law of LaPlace and mechanics?
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
What is contractility?
Force of contraction from the myocardium
Measured by ejection fraction
Increased by sympathetic stimulation
What are the 7 stages of the cardiac cycle?
1) Atrial systole
2) Isovolumetric contraction
3) Rapid ejection
4) Reduced ejection
5) Isovolumetric relaxation
6) Rapid passive filling
7) Reduced passive filling
What are the electrical and mechanical events, valve movements and point on the PV loop associated with atrial systole?
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
What are the electrical and mechanical events, valve movements and point on the PV loop associated with isovolumetric contraction?
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
What are the electrical and mechanical events, valve movements and point on the PV loop associated with rapid ejection?
Ventricles depolarised and contracting (ST segment)
Pressure gradient outwards, isotonic contraction forces blood into arteries
AV closed
SL open
No noise
B→C
What are the electrical and mechanical events, valve movements and point on the PV loop associated with reduced ejection?
Ventricles repolarise (T wave) Decrease in ventricle pressure reduces gradient ∴ valves close AV closed SL closing No noise B→C
What are the electrical and mechanical events, valve movements and point on the PV loop associated with isovolumetric relaxation?
Diastolic ∴ no electrical events Ventricles relax and fibre length is constant AV closed SL closed S2 from SL closing C→D
What are the electrical and mechanical events, valve movements and point on the PV loop associated with rapid passive filling?
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
What are the electrical and mechanical events, valve movements and point on the PV loop associated with reduced passive filling?
Isoelectric ECG period Most blood has entered ventricles ∴ gradient reduced and slower filling AV open SL closed No noise D→A
Define end diastolic volume
Volume of blood in ventricle just before contraction
110ml normal
Define end systolic volume
Volume of blood in ventricle just before end of blood expulsion - residual vol of blood in ventricles
40ml normal
How is stroke volume calculated?
EDV-ESV
What are the normal pressure values of systolic, diastolic, pulse, mean arterial and atrial?
systolic - 120mmHg diastolic - 80mmHg pulse - 40mmHg mean atrial - 93 mmHg atrial - <30mmHg
Define ejection fraction
SV/EDV
Clinical sign of how well the heart is working (normally 60-70%)
What is the length tension relation in cardiac muscle?
Elastic band analogy longer stretch (preload) increased force of contraction
What are important properties of cardiac muscle?
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
Define cardiac output
stroke volume x HR
volume of blood pumped by heart per minute
approx 5L
Define stroke volume
end diastolic volume - end systolic volume
volume of blood pumped by heart per beat
approx 70ml
Define pulse pressure
systolic BP - diastolic BP
approx 40mmHg
Define mean arterial pressure
DBP +( 1/3 x pulse pressure)
approx 93 mmHg
