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Model 1: Opening and Closing of Heart Valves
Heart valves open and close, somewhat like a door, depending on the pressures on the two
sides of the valve.
Blood Flow
When the pressure in chamber A is greater than the pressure in chamber B (PA >
PB), valve 1 is open. (When pressure on the “upstream side” of the valve is greater,
the valve is open.)
When the pressure in chamber B is greater than the pressure in chamber A (PB >
PA), valve 1 is closed. (When pressure on the “downstream side” of the valve is
greater, the valve is closed.)
1. INDICATE when PB < PC:
a. is valve 2 open or closed? Open
b. is valve 1 open or closed? Closed
2. INDICATE when PC < PB:
a. is valve 2 open or closed? Closed
b. is valve 1 open or closed?
3. STATE the relationship between pressures in chambers A, B, and C that results in both
valves being closed.
This is due to downstream flow.
4. If chamber A represents the left atrium and chamber B represents the left ventricle,
a. valve 1
Bicuspid valve (mitral valve)
b. chamber C
c. valve 2
Aortic Valve
5. Using your answer to question 4, PREDICT what would happen in a person’s circulatory
system if valve 2 in Model 1 did not close completely.
Model 2: Electrical Activity in the Heart
6. LABEL this heart diagram using each bold
term described below:
The SA node (sinoatrial node) is located in the
right atrium near the entrance of the superior
vena cava. The SA node contains cells that
spontaneously generate action potentials at a
rate of 80-100 beats/minute.
Action potentials propagate throughout the
atrial myocardium from cell to cell via
intercalated discs and through specialized internodal pathways. (no label)
The AV node (atrioventricular node) is located at the junction between the atria and ventricles.
Action potentials are propagated through the AV node very slowly. The AV node contains cells that
spontaneously generate action potentials at a rate of 40-60 beats/minute.
The AV bundle in the interventricular septum receives electrical activity from the AV node. This is the
only pathway for electrical activity to move from the atria to the ventricles.
Action potentials propagate from the AV bundle through the bundle branches to Purkinje fibers,
which are large-diameter cells that propagate action potentials very rapidly to myocardial cells
throughout the ventricles. Purkinje fibers spontaneously generate action potentials at a rate of 20-40
Model 3: Sequencing Electrical Excitation in the Heart
NOTE: (darkened/gray areas indicate electrical excitation)
7. EXAMINE Model 3 and DESCRIBE what is happening in each of the figures (A-E).
In your descriptions include the names of specific intrinsic electrical system structures when
they are involved.
8. INDICATE which part of the intrinsic electrical system serves as the normal pacemaker for
the heart.
9. FACT: Mechanical events of the heart must be preceded by an electrical event. EXPLAIN
10. FACT: Depolarization and repolarization are the two electrical events of the heart muscle.
a. What do depolarization and repolarization mean?
b. What (general) mechanical event follows each of these electrical events?
11. PREDICT what specific mechanical event follows figure C.
12. PREDICT what specific mechanical event follows figure E.
13. FACT: The part of the intrinsic electrical excitation system with the fastest rate of
spontaneously generated action potentials serves as the pacemaker. If the normal
pacemaker stopped functioning, INDICATE how would heart rate be affected. EXPLAIN
which cells would take over the role of pacemaker.
14. INDICATE why it’s important for impulses from the atria to be delayed at the AV node
before passing to the AV bundle.
15. PREDICT potential consequences of cell death resulting in a “blockage” of the AV node.
Model 4: Electrocardiogram (EKG or ECG)
The electrocardiogram (ECG) records electrical activity in the heart. Scientists and clinicians
make assumptions about mechanical activity based on the ECG, but it does not directly
measure mechanical events. The P wave represents atrial depolarization; the QRS complex
represents ventricular depolarization; the T wave represents ventricular repolarization.
16. NAME the part of the ECG representing:
a. atrial depolarization
b. ventricular depolarization
c. ventricular repolarization
17. FACT: Atrial repolarization is usually not visible on the ECG. PREDICT during which
wave/complex atrial repolarization occurs.
18. PLACE an arrow on the ECG recording in Model 4 to INDICATE depolarization of the SA
Model 5: ECG and Blood Pressure Changes during Two Heart Beats
19. FACT: Model 3 shows an ECG and blood pressure changes during two heart beats (a
partial Wigger’s diagram). NAME the three blood pressure locations shown in Model 5.
20. DESCRIBE the change in left atrial pressure that occurs following the P wave AND its
21. DESCRIBE the change in left ventricular pressure that occurs following the R wave (QRS
complex) AND its cause.
22. DESCRIBE the change in left ventricular pressure that occurs following the T wave AND
its cause.
23. Based on the ECG and aortic pressure, INDICATE which small letter (a-f) in Model 5
represent when the ventricle starts ejecting blood into the aorta, EXPLAINING why.
Voltage in Millivolts
Model 6: ECG and Selected Arrhythmias
ECGs are helpful in determining changes and irregularities (arrhythmias) in the heart’s
excitation-conduction system. Below are 4 ECG recordings: a normal heart rate of 75 beats/min
and three examples of normal changes in SA Node activity.
Time in Milliseconds
24. FACT: Elite athletes, say a cyclist for our example, can have resting heart rates as low as 35
beats/minute. INDICATE which recording on the previous page could belong to this kind of
athlete and WHY.
25. SUPPOSE when our elite athlete is pedaling up a mountainside, their heart rate is 150
beats/minute. INDICATE which recording on the previous page could be this athlete’s
during strenuous exercise and WHY.
26. FACT: Most people have a change in heart rate associated with breathing, e.g. heart rate
often increases with inhalation and decreases with exhalation. INDICATE which recording
on the previous page reflects this.
27. FACT: People have been known to live without atrial depolarization. INDICATE what an ECG
would look like in a person with this condition; write a sentence or draw an ECG to EXPLAIN
your answer.
28. Given that people can live without atrial depolarization, INDICATE if you think people can
live without ventricular depolarization, DEFENDING your answer.
MODEL 7: Ventricular Fibrillation
In 1990, basketball player Hank Gathers collapsed and died during a college basketball game.
The cause of his collapse was an irregular heartbeat. He suffered from exercise-induced
ventricular tachycardia but developed ventricular fibrillation during the game. An automated
external defibrillator (AED) was used to try and treat him for this condition.
29. DESCRIBE how this ECG of ventricular fibrillation compares to the normal sinus rhythm ECG
in Model 6.
30. TRUE/FALSE: The ECG shown in Model 7 can be used to find heart rate. DEFEND your
31. Ventricular fibrillation (V-fib) is a condition of uncoordinated contraction of the cardiac
muscle of the ventricles, making them quiver rather than contract properly. People who are
not health professionals usually cannot feel a pulse. Such an arrhythmia is only confirmed
by an ECG. EXPLAIN why this is a medical emergency that requires prompt treatment.
32. EXPLAIN if V-fib is the same as or different (mechanistically) as a person who doesn’t
experience ventricular depolarization. INDICATE if V-fib and lack of ventricular
depolarization can lead to the same outcome, EXPLAINING your answer.
FYI: An automated external defibrillator or AED is a portable electronic device that
automatically diagnoses the potentially life-threatening cardiac arrhythmias of ventricular
fibrillation and ventricular tachycardia. It is also able to treat them through defibrillation, which
(hopefully) stops the arrhythmia, allowing the heart to reestablish an effective rhythm.
An AED (or paddles
in a hospital) could
work here:
But NOT here: