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CVS > Mechanics 1 > Flashcards

Mechanics 1 Flashcards

1
Q

What do cardiac myocytes require to beat?

A

external Ca

skeletal muscle can beat without this

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2
Q

Describe a single cardiac myocyte

A

rod shape
striated
100 micrometres long, 15 micrometres wide
T tubules form invaginations along the myocyte at Z line to carry surface depolarisation into the cell = 200 micrometres deep, 2 apart

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3
Q

What pumps/channels are found on the T tubule surface facing the RyR of the sarcoplasmic reticulum?

A

VGCC (L type)
Na/Ca exchanger
Na + channel (Na/K ATPase)

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4
Q

Describe excitation contraction coupling

A
  1. Depolarisation detected by L type Ca channels to cause external Ca to enter cell
  2. Some Ca directly causes contraction or binds to RyR to cause Ca release from SR
  3. Ca taken back up into SR by Ca ATPase channels
  4. Same amount of Ca that entered is effluxed by NCX in a passive process using Na downhill energy gradient
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5
Q

What is used to reuptake Ca into SR?

A

SERCA
sarcoendoplasmic reticulum CaATPase channels
active process
against [] gradient

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6
Q

How does efflux of Ca work?

A

passive process

use downhill energy gradient of Na (high Na outside cell)

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7
Q

What are the 4 events including the action potential initiation?

A 
AP 
Ca influx
crossbridge cycle
fibre shortening
contraction
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8
Q

What is the function of the T tubules?

A

facilitate the transmission of the AP to the centre of the cell

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9
Q

What forms the communication interface?

A

SR and RyR

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10
Q

What is the SR?

A

huge intracellular store of Ca

Ca is critical for myofilament contraction

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11
Q

What is the relationship between force and Ca?

A

sigmoidal curve

around 10micromol of intracellular Ca needed to produce maximal force

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12
Q

What reduces the [] of Ca needed to generate a given contraction?

A

Ca [] needed is reduced by stretch because Ca sensitivity increases with stretch

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13
Q

What is the [] of Ca needed to generate a 50% of maximal contraction?

A

[Ca] 50%

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14
Q

What causes an increase in potential free energy?

A

increase muscle length

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15
Q

Why does active force curve decrease after increase?

A

point where stretch does not generate more force because there is not enough overlap between the filaments

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16
Q

What is the passive force based on?

A

muscles’ resistance to stretch

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17
Q

What does stretch do to cardiac myocytes?

A

increase Ca sensitivity

unlike in skeletal muscle

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18
Q

Why does skeletal muscle produce less force whilst cardiac produces more?

A

cardiac muscle is more resistant to stretch due to ECM, cytoskeleton leading to more passive force generated

cardiac muscle is also less compliant

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19
Q

Which limb of length tension relationship is important for cardiac muscle only?

A

ascending limb

the pericardium resists overstretching that is characteristic of the descending limb

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20
Q

What is isometric contraction of the heart?

A

no change in length, increase tension
valves are closed
during diastole, ventricles fill and pressure increases in ventricles which exerts a preload on cardiac cells

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21
Q

What is the effect of preload on isometric contraction?

A

increase preload –> increase IC

22
Q

What is isotonic contraction of the heart?

A

change in length, same tension
AV closed, SL open
fibres shorten
during ejection phase (late systole)

23
Q

What is the effect of afterload on isotonic shortening?

A

increase afterload decreases shortening

24
Q

What is concentric?

A

shortening

most common form of isotonic contraction in heart

25
Q

What is eccentric?

A

lengthening

26
Q

What is the isometric length tension relation?

A

preload enhances contractile force
(increase in contractile energy with stretch = Starling’s Law)
stretch is set by the diastolic ventricular filling pressure (diastole determines the preload)
filling of ventricles with blood causes stretching and generates isometric contraction

27
Q

What is the isotonic afterload shortening relation?

A

afterload impairs shortening

an increase in afterload causes the rate and degree of shortening to decrease

28
Q

What happens if there is the same afterload with a large preload?

A

shortens the muscle more as the cardiac cells compensate for the greater volume of blood

29
Q

What governs the amount of force a muscle can produce?

A

preload

30
Q

What is ventricular filling equivalent to?

A

preload

31
Q

Describe preload in the heart?

A

Diastole the walls stretch as blood fills the ventricles and pressure increases –> stretch determines preload –> preload dependent on VENOUS RETURN TO THE HEART

32
Q

What are 3 measures of preload?

A

end diastolic volume
end diastolic pressure
right atrial pressure

33
Q

Describe afterload in the heart?

A

Afterload is the force the heart must over come to eject blood from the ventricles

34
Q

What is the measure of afterload?

A

diastolic arterial BP must be overcome by pressure in ventricles for blood to be ejected

35
Q

What is the change in afterload in hypertensive patients:

A

they have an increased afterload
increase BP
increased risk of high pressure in heart

36
Q

What is Frank Starling relationship?

A

increased diastolic fibre length increases ventricular contraction
increased stretching/preload increases speed of shortening

37
Q

Why does stretch actually increase strength of contraction?

A

it reduces interference by myosin heads that are pulling in the wrong direction beyond the sarcomere midpoint

38
Q

Why is Starling’s Law important?

A

larger preload means ventricles need to increase SV to ensure CO balances venous return
The law means that the heart can adapt to changes in preload (venous return)

39
Q

Factor 1 that contributes to Starling’s Law?

A

Changes in number of myofilament crossbridges that interact

  • at shorter lengths actin filaments overlap
  • reduced number of myosin crossbridges that can be made because some are pulling in the opposite direction to majority which reduces net tension generated

But this is also the case in skeletal muscle so which is cardiac muscle more sensitive than skeletal? FACTOR 2

40
Q

Factor 2 that contributes to Starling’s Law?

A

Increased Ca sensitivity with increased length
At physiological [Ca] lower active tension as only fraction of crossbridges are activated - force increases more steeply with stretch

41
Q

Why does increased length increase Ca sensitivity?

A
  1. Troponin C (TnC) is a thin filament binding to Ca and regulate cross bridge formation
    - longer sarcomere increases TnC affinity for Ca
    - conformational protein change
    - less Ca needed for same force
  2. Stretch causes lattice spacing (space between A and M filaments ) to decrease
    - increase chance of forming strong cross bridges
    - and so more force is produced for same amount of Ca
42
Q

What is the Anrep effect?

A

a sudden increase in afterload on the heart causes an increase in ventricular inotropy

if stretch is maintained there is a further slow increase in force over 5 minutes due to increased Ca transients (slow force response)

43
Q

Stroke work?

A

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

44
Q

Formula for stroke work?

A

SV x Pressure

preload and afterload influence SV
cardiac structure influences the pressure at which blood is ejected

45
Q

What is the Law of Laplace?

A

when pressure in a cylinder is constant, tension in walls increases with radius
T (wall tension) = P (pressure in vessel) x R (radius) /h
tension is constant
h takes into account wall thickness

46
Q

What is Law of Laplace used for?

A

ventricles and blood vessels

47
Q

What is kept the same in the heart? How?

A

wall tension kept same between L and R ventricle despite higher pressure in LV

  • radius of curvature of LC less than RV so higher pressures generated with similar wall tension
  • heart does not rip despite high pressures generated
48
Q

Wall tension in giraffe?

A

wall stress kept low by long narrow thin walled ventricle and a small radius to allow for a high pressure

49
Q

Wall tension in frogs?

A

pressures are lower so ventricles are spherical to give a larger radius

50
Q

What happens to wall tension in a failing heart?

A

dilated cardiomyopathy
enlarged radius
increase wall stress
bad –> aneurysm

CVS (16 decks)

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