Human A&PBIO 304 · American River College ← Week 06 pre-work

BIO 304 · Week 06 · Interactive Workbook

Heart Physiology: Conduction, ECG & the Cardiac Cycle

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Part 1 of 4 · Recall

Fill in the blanks

Type the term that completes each statement, using the word bank. Pull it from memory first.

Word bank

Phase 0 (upstroke)AV block (2nd degree)Phase 4SympatheticResting tone~100 ms pauseSA nodeAV nodePhase 2 (plateau)QRS durationAllows proper fillingRepolarizationNo true resting potentialT waveRight & left bundle branches

  1. right atrium near SVC; intrinsic rate 60-100; primary pacemaker
  2. interatrial septum; intrinsic rate 40-60; delays signal ~0.1s
  3. down either side of septum
  4. allows atrial systole to finish before ventricular contraction begins
  5. membrane drifts upward toward threshold
  6. K+ channels open
  7. fast Na+ channels open; rapid depolarization
  8. Ca²⁺ influx balances K+ efflux; sustained depolarization — this is unique to cardiac muscle
  9. resting potential restored; ready to fire again
  10. each beat is followed by relaxation and refilling
  11. ventricular repolarization
  12. normally < 0.12 s; widened in bundle branch block
  13. norepinephrine on beta-1 receptors → speeds SA, increases contractility
  14. parasympathetic dominates; intrinsic SA rate without ANS is ~100/min
  15. some atrial beats not conducted

Define it: high-yield vocabulary

Write a clear definition in your own words for each term.

  1. SA node
  2. AV node
  3. Bundle of His
  4. Purkinje fibers
  5. Autorhythmicity
  6. Pacemaker potential
  7. P wave
  8. QRS complex
  9. T wave
  10. Cardiac refractory period
  11. AV block

Part 2 of 4 · Anatomy lab

Draw and label

Box A. Conduction pathway

Directions

  1. Draw a heart silhouette with all four chambers.
  2. At the top of the right atrium, draw a small oval: the SA node (sinoatrial node). Label. Note: the natural pacemaker, fires at 60 to 100 bpm.
  3. Show the signal spreading across both atria (small arrows), causing them to contract.
  4. At the junction of atria and ventricles (in the interatrial septum, near the tricuspid valve), draw the AV node (atrioventricular node). Label. Note: delays the signal by about 0.1 second to let atria empty.
  5. From the AV node, draw the bundle of His (AV bundle) descending through the interventricular septum. Label.
  6. Split into right and left bundle branches.
  7. End in the Purkinje fibers spreading throughout the ventricular walls. Label.
  8. Add arrows showing the direction of signal flow: SA node, atria, AV node, bundle of His, bundle branches, Purkinje fibers, ventricles contract.
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Box B. Normal ECG

Directions

  1. Draw a single normal ECG cycle (one heartbeat). x-axis is time, y-axis is voltage.
  2. Start with a flat baseline. Draw a small upward bump: the P wave. Label.
  3. After P, a brief flat segment (PR segment), then a tall sharp downward-upward-downward complex: the QRS complex. Label Q, R, S.
  4. After QRS, another flat segment (ST segment), then a smaller rounded upward bump: the T wave. Label.
  5. Annotate what each wave represents: P = atrial depolarization (atria contract); QRS = ventricular depolarization (ventricles contract); T = ventricular repolarization (ventricles relax).
  6. Note: atrial repolarization is hidden inside the QRS complex.
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Box C. The cardiac cycle (one beat)

Directions

  1. Draw two heart silhouettes side by side, both showing all four chambers.
  2. Left silhouette: DIASTOLE (ventricles relaxed). Show atria contracting and pushing blood into ventricles. Tricuspid and mitral valves OPEN. Pulmonary and aortic valves CLOSED.
  3. Right silhouette: SYSTOLE (ventricles contracted). Show ventricles squeezing blood out to lungs and body. Tricuspid and mitral valves CLOSED. Pulmonary and aortic valves OPEN.
  4. Below the silhouettes, draw a pressure-time graph. Show LV pressure rising sharply during systole, falling during diastole. Show aortic pressure following LV during systole, holding higher during diastole (because of valve closure).
  5. Label end-diastolic volume (EDV, max ventricle volume), end-systolic volume (ESV, min volume after contraction), and stroke volume (SV = EDV minus ESV).
  6. Note: the heart sounds 'lub-dub' correspond to valve closure: lub = AV valves close at start of systole; dub = semilunar valves close at start of diastole.
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Structures to label

Label each on your drawing.

  1. SA (sinoatrial) node
  2. AV (atrioventricular) node
  3. Bundle of His
  4. Right bundle branch
  5. Left bundle branch
  6. Purkinje fibers
  7. P wave
  8. QRS complex
  9. T wave
  10. PR interval
  11. ST segment
  12. Atrial depolarization
  13. Ventricular depolarization
  14. Ventricular repolarization
  15. Systole
  16. Diastole
  17. End-diastolic volume (EDV)
  18. End-systolic volume (ESV)
  19. Stroke volume

Part 3 of 4 · Physiology lab

Reason it through

A. Match ECG component to electrical event

1. The P wave.
2. The PR interval.
3. The QRS complex.
4. The ST segment.
5. The T wave.
6. A flat line between heartbeats (the baseline).

B. Synthesis

1. A patient is in third-degree (complete) heart block: the atria fire normally, but the signal does not pass through the AV node to the ventricles. The ventricles develop their own slower rhythm. Predict the ECG pattern (relationship between P waves and QRS complexes), and predict the heart rate and the patient's symptoms.
2. Atrial fibrillation: the atria depolarize chaotically at 400+ times per minute. The AV node filters most of these signals. Predict (a) what happens to the P wave on the ECG, (b) the regularity of QRS complexes, and (c) why patients are at high risk for stroke.
3. Ventricular fibrillation: ventricles depolarize chaotically. Unlike atrial fibrillation, this is a cardiac emergency. Explain mechanistically why V-fib is immediately life-threatening but A-fib is not, and why a defibrillator works to reset the heart.

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