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BRS CVR > Cardiovascular Mechanics > Flashcards

Cardiovascular Mechanics Flashcards

1
Q

What does a ventricular cell require for contraction?

A

External Ca2+

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2
Q

Outline the process of contraction in a single ventricular cell.

A

Electrical event (AP)

Calcium transient (amount of calcium in sarcoplasm has ^ for short period of time).

Contractile

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3
Q

Will the heart beat without external Ca2+?

A

Heart WON’T beat without external Ca2+, different to skeletal muscle which can contract.

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4
Q

What type of indicator is activated upon Ca2+ binding?

A

Fluorescent indicator activated upon Ca2+ binding, transient rises in Ca2+ concordant with contraction.

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5
Q

Can the hearts contractility be sustained by saline solution with bicarbonate of soda and KCL?

A

No

The addition of lime or a calcium salt will restore good contractility.

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6
Q

What are T-tubules?

A

Finger-like invaginations of the cell surface.

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7
Q

What is the length and width of ventricular cells?

A

Length: 100 micrometers; Width: 15 micrometers.

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8
Q

Dimensions of T-tubules?

A

200nm, T-tubules are separated by 2micrometers, intermediate between each Z-line of myofibril, transmitting surface depolarisation deep into the cell.

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9
Q

Proportion of myofibrils, MC and SR in single ventricular cell?

A

Myofibrils - 46%
MC - 36% - High proportion of MC to provide ATP for contractility (ATP for sliding filament theory).
SR - 4%

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10
Q

Outline the process of excitation-contraction coupling in the heart.

A
  1. L-type Ca2+ channel, upon excitation, the depolarisation is sensed by the ion channel in the cardiomyocyte, consequently this opens the channel in response to AP.
  2. Extracellular Ca2+ moves across concentration gradient intracellularly by diffusion.
  3. Minor proportion of Ca2+ activates actin filaments and directly causes contraction.
  4. Majority of Ca2+ binds to RyR on SR (SR Ca2+ release channel); receptor undergoes conformational change (Ligand gated); opening RyR → Efflux from SR.
  5. Ca2+ binds to TnC on actin filaments to stimulate shortening of sarcomere (sliding filament theory)
  6. Relaxation period: Ca2+ actively pumped into a stored position by Ca2+ ATPase channels of SR. Same amount of Ca2+ that came into the cell is effluxed by a Na-Ca exchange.
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11
Q

Does excitation-contraction coupling in heart require energy?

A

Doesn’t require energy; energy is transferred via the concentration gradient of Na+ to expel Ca2+ from cell.

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12
Q

Explain the relationship between the force production and intracellular Ca2+.

A

Force-Ca2+ relationship sigmoidal. Intracellular cytoplasmic Ca2+ ^ will subsequently result in a greater force exerted by the muscle.

10 micrometers intracellular concentration sufficient to produce maximum force.

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13
Q

How can we add Ca2+ in SR?

A

Sympathetic stimulation

Increased phosphorylation of some proteins and increase Ca2+ influx into cell.

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14
Q

What is the Length-tension relationship?

A

Increase in muscle length causes an increase in force.

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15
Q

Active force production line.

A

Cardiac preparation increase, muscular force increase (Tension system).

Elastic components (Elastin) stretch, passive tension is produced; cytoskeletal components of cells stretching, EPE is stored, this occurs during no shortening of the muscle → Isometric contraction.

Muscle length increases, passive force increases.

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16
Q

What is an isometric contraction?

A

Tension provided does’t cause muscular shortening; exerts pulling force on transducer.

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17
Q

LTR cardiac vs. skeletal muscle

A

Overstretch the muscle beyond the actin and myosin filament overlap will result in a decrease in force; behaviour is exhibited in skeletal muscles.

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18
Q

What is active and passive force?

A

Passive - Based on the resistance to stretch of the muscle.

Active - Dependent on sarcomere shortening, forces act in the direction of point of muscular attachment towards centre.

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19
Q

What is the total force?

A

Passive + Active

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20
Q

What type of muscle is less compliant?

A

Cardiac muscle → less compliant, and resilient to stretch in comparison to skeletal muscle, thus exerts more passive force.

21
Q

What are the 2 forms of contraction the heart uses?

A

Isometric

Isotonic

22
Q

What is isometric contraction?

A

Muscle fibres don’t change length but pressure increases in both ventricles.

23
Q

What is isotonic contraction?

A

Shortening of fibre and blood is ejected from ventricles.

24
Q

Examples of isometric and isotonic contractions?

A

Isometric - Planque,

Isotonic - Bicep curls, bench press.

25
Q

What is preload?

A

Degree to which cardiac muscle cells are stretched from filling of the ventricles prior to contraction. Therefore, preload is a way of expressing end-diastolic volume. Increasing ventricular filling → increases EDV and cardiac muscle is stretched to a greater degree.

26
Q

What is the difference between force produced with a small preload and a large preload?

A

Small preload - shorter muscle lengths > less force can be produced.

Large preload - longer muscle lengths > more force can be produced.

27
Q

What is EDV?

A

End-diastolic volume → Volume of blood present in the ventricles before ventricular systole.

28
Q

What factors effect EDV?

A

Filling time - Duration of ventricular diastole during filling.

HR - Greater the contraction rate, the shorter the filling time, thus lowers EDV and preload.

29
Q

How does preload affect contractility?

A

Increased preload → Increases contractility

30
Q

What is cardiac contractility?

A

Tension developed and velocity of shortening (Strength of contraction) of myocardial fibres at a given preload, and afterload.

31
Q

How does sympathetic stimulation affect preload?

A

Increases venous return to the heart, contributes ventricular filling, EDV + preload. Atria and ventricles in diastole, majority of ventricular filling and atrial kick.

32
Q

In vivo correlates of preload.

A 
As blood fills the heart during diastole, it stretches the resting ventricular cells. 
This stretch (filling) determines the preload on the ventricles before ejection. 
Preload is dependent on venous return.
33
Q

What do preload measure include?

A

EDV, end-diastolic pressure and right atrial pressure.

34
Q

What is afterload?

A

Load against which left ventricle ejects blood after opening aortic valve.

35
Q

What does any increase in afterload do to the amount of isotonic shortening?

A

Decreases the amount of isotonic shortening that occurs and decreases the velocity of shortening.

36
Q

What decreases afterload?

A

Vasodilators and factors decreasing resistance.

37
Q

What does more afterload do to the velocity of shortening?

A

More afterload = Less sarcomere shortening

More afterload = Decreases velocity of shortening.

38
Q

What do measures of afterload include?

A

Diastolic arterial BP.

39
Q

What does increased afterload do?

A

Decrease shortening + decrease velocity of shortening.

40
Q

Definition of F-S relationship.

A

Increased diastolic fibre length increases ventricular contraction.

41
Q

What is the consequence of the F-S relationship?

A

Ventricles pump greater SV so that, at equilibrium CO exactly balances that augmented venous return.

42
Q

Name the 2 factors that are thought to have caused the Frank-Starling Relationship.

A

Change in the number of myofilament cross-bridges that interact.
Changes in the Ca2+ sensitivity of the myofilaments.

43
Q

Explain how changes in the number of myofilament cross-bridges that interact causes the F-S relationship.

A

Ventricular stretching ^ contact between the myosin heads with the myosin binding sites, lattice-spacing decreases.
Decreasing myofilament lattice spacing, increasing the probability of forming strong-binding-cross-bridges; providing more force for the same amount of activating calcium.

44
Q

Explain how changes in Ca2+ sensitivity of myofilaments causes the F-S relationship.

A

Ca2+ required for myofilament activation, troponin C (TnC), is thin filament protein that binds Ca2+, subsequently causing tropomyosin to expose myosin binding sites, regulating 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; thereby less Ca2+ is required for equivalent amount of force.

45
Q

Define SW + equation.

A

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

SW = SV x P

46
Q

What factors affect SW?

A

SV affected by PRELOAD and AFTERLOAD

Pressure affected by CONTRACTILITY

47
Q

Define the Law of Laplace + equation.

A

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

Wall tension = Pressure in vessel x Radius of vessel
T=PR
T=PR/h

48
Q

Which ventricle lower radius?

A

LV allowing it to generate higher pressures than the RV with similar wall stress/

BRS CVR (15 decks)

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