A&P Chapter 9
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What is the cell membrane of the cardiac cell called?
What is the difference in resistance between the sarcolemma and the intercalated disc?
intercalated disc has a much lower resistance
What lies between intercalated disc? What is the function?
Gap junctions - highly permeable to ions so excitation spreads from cell to cell
Why are gap junctions so important?
because individual cardiac cells are not innervated - they rely on the stimulus from the SA node to be carried through the intercalated disc
What is the resting membrane potential of cardiac muscle? Where does it peak with its action potential?
-85/95, peaks at 20 = 105 mV action potential
How long is the plateau of ventricular muscle?
What is responsible for the plateau in the cardiac muscle action potential?
- 1) large amounts of Na and Ca enter the cell
- 2) after AP, permeability to K transiently decreases
What are the 3 types of cardiac muscle?
- 1) atrial muscle
- 2) ventricular muscle
- 3) Excitatory & conductive muscle fibers
What initiates contraction of cardiac muscle?
spontaneous electrical excitation of SA node
Where does the calcium for cardiac muscle contraction come from?
sarcoplasmic reticulum and extracellular calcium from the T-tubule
What happens during phase 0 of a cardiac action potential?
rapid depolarization (to +20) due to opening of fast sodium channels and slow calcium channels, K permeability is decreased. This is the absolute refractory period.
What happens during phase 1 of a cardiac action potential?
- Slight repolarization:
- Fast sodium gates close
- Chloride enters the cell
- Potassium leaves the cell
- Calcium still entering cell
What happens during phase 2 of a cardiac action potential?
- Calcium gates open more halting repolarization
- ~ 0 mV
- 0.2-0.3 seconds long
- Na gate still closed
What happens in phase 3 of a cardiac action potential?
- calcium channels close
- potassium channels open more d/t increased Ca levels
- Absolute refractory period ends/relative begins
What happens in phase 4 of the cardiac action potential?
- Resting Membrane Potential:
- Na/K pump restores balance of ions
How many phases are in the action potential of the SA node? What are their names?
What is the resting membrane potential of the SA node?
What is the threshold of the SA node?
What happens in phase 0 of SA node action potential?
- threshold of -40 is reached
- Na channels activate
- Ca influx begins
What happens in phase 3 of SA node action potential?
- membrane more permeable to K
How long is the absolute refractory period in a ventricular cardiac cell? atrial?
- 0.25-0.30 seconds in ventricular
- 0.15 in atrial
How long is the relative refractory period in a cardiac cell?
How does the T tubule of cardiac muscle differ from that of skeletal muscle?
- 5x larger
- contains 25x more calcium
What regulates the amount of calcium in T tubule?
extracellular calcium level
What binds to calcium and keeps it available for diffusion into the cardiac muscle fiber?
mucopolysccharides - negatively charged so it cancels the charge of the calcium ion
What is the effect of hyperkalemia on the cardiac muscle?
- decreased contractility
- blocked conduction from atria to ventricles
- slight depolarization of cells = weaker AP
- widening QRS
What is the effect of hypercalcemia on the cardiac muscle?
hyperpolarization - spastic contraction, shortened QTi
How does hypocalcemia (or a calcium channel blocker) effect the cardiac cycle?
Calcium is not available to open K gates so depolarization takes longer = slower HR
If the LV is pumping 6 liters/min, how much is the RV pumping?
What is EDV? What is a normal volume?
- end diastolic volume - the volume in the ventricle at the end of diastole. (the most volume that will ever be in the ventricle)
- 120 ml
What is ESV? What is a normal volume?
end systolic volume - the volume of blood in the ventricle at the end of systole (the emptiest a ventricle will ever be)
EDV-ESV = ??
What are the inlet valves?
the AV valves (mitral/tricuspid)
In the right atrial pressure wave, what do a, c, and v represent?
- a = atrial contraction
- c = closure of the AV valve
- v = volume entering the atria
What is the formula for ejection fraction?
EF = SV/EDV x 100
2 patients have an EF of 20%. One has concentric hypertrophy (hypertrophic), the other has eccentric hypertrophy (dilated). Which one will likely be more symptomatic? why?
The one with concentric hypertrophy. His ventricle can hold less volume so his 20% = a lower cardiac output than the eccentric 20%
Which valves have a higher velocity, AV or semilunar?
semilunar - active process, smaller opening
What does the middle of the pressure/volume loop represent?
stroke volume or external work
Where is afterload represented on the pressure/volume loop?
upper right corner
Where is preload represented on the pressure/volume loop?
lower right corner
Where is isovolumetric contraction represented on the pressure volume loop?
Where is isovolumetric relaxation represented on the pressure/volume loop?
Where is diastole on the pressure/volume loop?
Where is systole on the pressure/volume loop?
Where does the mitral valve close in the pressure volume loop?
lower right corner
Where does the mitral valve open in the pressure volume loop?
lower left corner
where does the aortic valve open in the pressure volume loop?
upper right corner
where does the aortic valve close in the pressure volume loop?
upper left corner
If preload increases, what does that do to the pressure volume loop?
extends it to the right
What does an elevated BP look like on the pressure volume loop?
upper right corner will be higher than previously
What are the two types of cardiac work?
external and kinetic (velocity of blood flow)
What does increased contractility look like on the pressure volume loop?
left side shifted to the left
What 3 ways can you increase contractility?
- more myocytes
- better synchrony
What 3 ways can you increase preload?
- better venous return
- better compliance
- higher blood volume
what 2 ways can you decrease after load?
- lower aortic pressure
- widen aortic valve opening
What 4 ways can you increase CO?
- increase contractility
- increase preload
- decrease after load
- optimize heart rate
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