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Cardiology > Cardiovascular mechanics > Flashcards

Cardiovascular mechanics Flashcards

1
Q

What is excitation-contraction coupling?

A

Process whereby the excitatory event turns into the contractile event via a series of processes

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2
Q

Describe a single ventricular event

A

Electrical event
Ca2+ influx
Ca2+ release
Contractile event

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3
Q

How was the importance of calcium for the heart beat discovered?

A

Sydney Ringer

Saline solution with pipe water allowed the heart to contract

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4
Q

How big are ventricular cells?

A

100 μm long and 15 μm wide

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5
Q

What are t-tubules?

A

Finger-like invaginations from the cell surface

Carry surface level depolarisation into the heart of the cell

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6
Q

How big are t-tubules?

A

T-tubule openings up to 200 nm in diameter

Spaced (approx. 2 μm apart) so that a T-tubule lies alongside each Z-line of every myofibril

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7
Q

What percentage of cell volume do myofibrils occupy?

A

46%

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8
Q

Why are there so many mitochondria in muscle cells?

A

Provide ATP for muscle contraction

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9
Q

What key calcium handling proteins are in the t-tubules?

A

L-type calcium channel

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10
Q

What causes the L-type calcium channels to open?

A

Response to the action potential arriving

Calcium moves into cell down concentration gradient

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11
Q

Where does calcium go after entering the cell?

A

Binds to SR Ca2+ (ryanodine receptor) release channel

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12
Q

What does Ca2+ binding result in?

A

Conformational change

Allows calcium to flow from the sarcoplasmic reticulum into the cytosol

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13
Q

How is relaxation induced?

A

ATP is used to restore calcium into the sarcoplasmic reticulum

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14
Q

What is the sodium/calcium exchange system?

A

Doesn’t use ATP

Uses the downhill energy gradient of Sodium to expel calcium out of the cell

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15
Q

What happens as more and more calcium enters the cell?

A

More force produced
Activation of more myofilaments
Sigmoidalish relationship

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16
Q

What happens when you lengthen a muscle?

A

More force produced with increasing length
Baseline force also increases
‘Active force production line”

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17
Q

What happens to elastic component when lengthened?

A

Elastic components stretch

Passive force produced

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18
Q

What is isometric contraction?

A

Non-shortening force production

Muscle fibres do not change length but pressures increase in both ventricles

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19
Q

How is cardiac and skeletal muscle different?

A

Cardiac muscle more resistant to stretch and less compliant than skeletal muscle
Less passive force in skeletal muscle

20
Q

What causes this difference in skeletal and cardiac properties?

A

Due to properties of the extracellular matrix and cytoskeleton

21
Q

What is unique about cardiac muscle?

A

It cannot be overstretched as the heart is contained in the pericardial sac that does not allow it to overstretch

Therefore Only ascending limb of the relation is important for cardiac muscle

22
Q

What is isotonic contraction?

A

Shortening of fibres and blood is ejected from ventricles

23
Q

What contraction is involved in the cardiac cycle?

A

Under normal circumstances: both isometric and isotonic

24
Q

Define preload

A

Weight that stretches muscle before it is stimulated to contract

25
Q

Define after load

A

Weight not apparent to muscle in resting state; only encountered when muscle has started to contract

26
Q

What is the relationship between shortening and load in isotonic contraction?

A

If load is great the shortening the muscle can do is very small

27
Q

What is the relationship between muscle length (preloaded) and stretch?

A

More interdigitation of the fibres occur, more stretch with length

28
Q

What is the in vivo correlate of preload?

A

As blood fills the heart during diastole, it stretches the resting ventricular walls

This stretch (filling) determines the preload on the ventricles before ejection

Preload is dependent on venous return

29
Q

What are measures of preload?

A

End-diastolic volume
End-diastolic pressure
Right atrial pressure

30
Q

What are the in vivo correlates of after load?

A

Afterload is the load against which the left ventricle ejects blood after opening of the aortic valve

Any increase in afterload decreases the amount of isotonic shortening that occurs and decreases the velocity of shortening.

31
Q

What is the measure of after load?

A

Diastolic blood pressure

32
Q

Why is it important to keep diastolic pressure low?

A

That is the pressure against the heart to work to eject blood
If diastolic blood pressure is high then the heart has to do more work

33
Q

What happens as filling of the heart increases?

A

Force of contraction increases

34
Q

Define the frank-starling relationship?

A

Increased diastolic fibre length increases ventricular contraction

35
Q

What is the consequence of the F-S relationship?

A

Ventricles pump greater stroke volume so that, at equilibrium, cardiac output exactly balances the augmented venous return

36
Q

What are the factors causing the F-S relationship?

A

Changes in the number of myofilament cross bridges that interact

Changes in the Ca2+ sensitivity of the myofilaments

37
Q

How does the number of cross bridges interacting affect force?

A

As the muscle stretches
Allow for more cross bridges to form
Causes greater force

38
Q

What happens at shorter lengths?

A

At shorter lengths than optimal the actin filaments overlap on themselves so reducing the number of myosin cross bridges that can be made

39
Q

How does Ca2+ sensitivity affect force?

Hypothesis 1

A

Ca2+ required for myofilament activation
Troponin C (TnC) is thin filament protein that binds Ca2+
TnC regulates formation of cross-bridges between actin and myosin
At longer sarcomere lengths the affinity of TnC for Ca2+ is increased due to conformational change in protein
Less Ca2+ required for same amount of force

40
Q

How does Ca2+ sensitivity affect force?

Hypothesis 2

A

With stretch the spacing between myosin and actin filaments (so-called “lattice spacing”) decreases

With decreasing myofilament lattice spacing, the probability of forming strong binding cross-bridges increases

This produces more force for the same amount of activating calcium

41
Q

Define stroke work

A

Work done by heart to eject blood under pressure into aorta and pulmonary artery

42
Q

What is stroke work equal to?

A

volume of blood ejected during each stroke (SV) multiplied by the pressure at which the blood is ejected (P)

SV x P

43
Q

What affects stroke volume?

A

Preload and afterload

44
Q

What affects pressure?

A

Cardiac structure

45
Q

What is the law of LaPlace?

A

When the pressure within a cylinder is held constant, the tension on its walls increases with increasing radius

46
Q

What is law tension equal to?

A

Pressure in vessel x Radius in vessel

T= P x R

Incorporating wall thickness (h), this can be amended to:
T= (PxR)/h

47
Q

What is the significance of radius of curvature?

A

Radius of curvature of walls of LV less than that of RV allowing LV to generate higher pressures with similar wall stress

Cardiology (16 decks)

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