Heart as a Pump / Ion Channels Flashcards
To define cardiac output (CO) and to know that CO = heart rate x stroke volume.
Stroke volume = volume of blood pumped per beat.
Cardiac Output = volume of blood pumped per minute by left ventricle (HR x SV)
4-6L/min, but increases w/ exercise (25-40L)
Describe changes in pressure and volume thorough the cardiac cycle as a function of time, and to identify the four phases of the cardiac cycle.
Diastole –> Isovolumetric contraction –> Ejection –> Isovolumentric relaxation
- Diastole:
At the end of diastole, the left atrium has filled with oxygenated blood from the pulmonary vein. Contraction is triggered by an electrical signal that originates in the sinoatrial node. As the atrium begins to contract (atrial systole), the atrial pressure increases. This is seen as the “a wave” (the hump) in both the atrial pressure (black dashed line) and the ventricular pressure (red line) because at this stage, the mitral valve between the left atrium and left ventricle is open, so blood flows freely into the ventricle as the atrium contracts. - Isovolumetric contraction phase
As the wave of depolarization reaches the ventricle, it begins to contract and ventricular pressure increases. The initial increase in pressure immediately pushes the mitral valve closed because the ventricular pressure quickly exceeds that in the atrium, which is now relaxing. However, the aortic pressure (~80 mmHg in the example) is initially greater than the ventricular pressure, so the aortic valve is also closed during the initial stage of ventricular contraction. Thus, the ventricular pressure increases dramatically because the ventricle is contracting but the blood has no place to go (both valves are closed). This is the isovolumetric contraction phase of the cardiac cycle (AKA isovolumic contraction phase). - Ejection Phase
As the ventricle continues to contract, the ventricular pressure exceeds that in the aorta, thus the aortic valve is pushed open and blood begins to flow. This is the ejection phase of the cardiac cycle.
As the ventricle begins to relax, the ventricular pressure falls. Pressure decreases slowly at first, and ejection continues. However, when the ventricular pressure drops below the aortic pressure, the aortic valve closes. - Isovolumetric relaxation phase
The ventricle continues to relax with both valves closed, so the pressure falls rapidly. This is the isovolumetric relaxation phase of the cardiac cycle.
As the ventricle continues to relax, the pressure eventually falls below that in the atrium, allowing the mitral valve to open and blood to flow into the ventricle, beginning a new cycle.
Define stroke volume, ejection fraction, stroke work, and pulse pressure, and to identify them graphically on a pressure-volume loop diagram.
SV = EDV - ESV, how much blood LV pumps out with each beat
EF = SV/EDV, another way to say SV but as a percentage of EDV
Stroke work = energy per beat (in Joules), corresponds to the area inside the PV loop
Pulse Pressure: systolic - diastolic
To define preload, afterload and contractility, and to describe how altering these variables changes ventricular function.
Preload: how much the heart fills. In PV loop, front vertical line moves forward more = fatter loop
Afterload: reistance to emptying. In PV loop, front vertical line (and loop) is taller, which means it comes back down sooner = skinner loop overall)
Inotropy/contractility: strength of contraction. In PV loop, new Starling curve (diag line), slope is steeper, loop goes back farther and higher
Sketch typical “fast” and “slow” cardiac action potentials, labeling both the voltage and time axes, and describe the cells in which each type of action potential is found.
Fast: Shoot straight up, come down slowly, e.g.: _|_ occurs in muscle and purkinje fibers
Slow: slower rise and fall, e.g.: _/_, occurs in SA & AV nodes
Describe the properties of the ion channels that underlie “fast” and “slow” cardiac action potentials
Fast: Na in, little K out, Kout/Cain balance, Kout. Easy.
Pacemaker cells:
- have reduced INa and little IK1
- express If and ICa-T (which are absent in myocardial cells)
Slow: The upstroke (phase 0) is attributable to activation of ICa-T and ICa-L, NOT Ina
In slow cardiac APs, the balance between ICa and delayed rectifier current (IKr and IKs) is such that repolarization (phase 3) occurs shortly after the peak of the action potential.
Describe ionic mechanisms that are likely to account for the ability of pacemaker cells to generate rhythmic firing without neural input.
that internal calcium release and the resultant movements of sodium and calcium via the NCX sodium/calcium exchanger play an important role in generating the pacemaker potential
Describe the significance of the IK1 channels in myocardial cells that have “fast” action potentials and the If [or Ih] currents in cells having “slow” action potentials.
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Discuss the mechanism and significance of “overdrive suppression.”
Cells in other regions of the heart (incl AV node) are also capable of spontaneous activity, BUT…
The frequency at which they would fire action potentials is lower than the frequency of discharge of cells in the SA node.
Consequently, (under normal circumstances) these cells are driven by action potentials originating in the SA node - that is, an action potential will spread to them from the SA node before they reach threshold on their own (overdrive suppression).
Define absolute refractory period, relative refractory period.
A second action potential cannot be initiated (absolute refractory period) until most of the inactivation of INa is removed (during the repolarizing phase), and the threshold for a second action potential remains elevated (relative refractory period) until after repolarization is complete (complete removal of inactivation of INa and deactivation of IKr and IKs has occurred).
