Final answer:
The atrium is thinner than the ventricle and beats first, leading to a smaller P wave on the ECG that occurs before the larger QRS complex.
Step-by-step explanation:
The atrium, or upper portion of the heart, is thinner than the ventricle. Thus, it is represented by a smaller wave on the rhythm strip. The atrium beats first, so its corresponding wave on the strip comes first. The atria are the two upper chambers of the heart that receive and pump blood into the ventricles just before they contract. The right atrium gets blood from the systemic circuit and channels it into the right ventricle, while the left atrium receives blood from the pulmonary circuit and pumps it into the left ventricle.
When examining an electrocardiogram (ECG), the atrial depolarization and subsequent contraction are indicated by the P wave. This occurs before the larger QRS complex, which signifies ventricular depolarization. Due to the atria's smaller muscle mass and the lower pressure they generate compared to the ventricles, the P wave is indeed smaller than the waves that represent ventricular activity.