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Human A&P BIO 304 · American River College ← Course home (Week 6)

BIO 304 . WEEK 6 . THURSDAY . LAB WORKBOOK

Cardiac Conduction System

The intrinsic pacemaker, signal propagation, and how the ECG reads it all.

Print this page. You will draw your own diagrams from the directions below, then hand-label the structures listed. Drawing by hand is the integrity mechanism for this course.

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Part 1 of 2

Anatomy Lab

1A. What you will draw

The heart's rhythm is set by specialized cardiac cells, not by nerves from the brain. Today you'll draw the conduction pathway through the heart, then a normal ECG with its named waves.

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.

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.

1C. Structures to label (14)

After you finish each drawing, label every structure below directly on your sketch.

  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

Part 2 of 2

Physiology Lab

2A. Match ECG component to electrical event

For each ECG feature below, identify the electrical event AND the mechanical event that corresponds to it.

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).

2B. Synthesis questions

Answer each in 2 to 4 sentences. Use the language from this week's lecture and your drawings as evidence.

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.

3. What to submit

Complete both the Anatomy Lab (your own drawings, hand-labeled, plus the structures list) and the Physiology Lab (activity and synthesis questions). Photograph or scan every page and upload to Canvas before the deadline listed on the schedule. Hand-drawn, hand-labeled work is the integrity mechanism for this course. Typed or AI-generated diagrams are not accepted.