Which of the following represents the correct events of a cardiac cycle beginning from the firing of an impulse from the pacemaker of the heart ?

Options

A.
B.
C.
D.

Answer

To solve the question regarding the correct events of a cardiac cycle beginning from the firing of an impulse from the pacemaker of the heart, we will analyze the sequence of events that occur during the cardiac cycle.

### Step-by-Step Solution:

1. **Understanding the Pacemaker:**
- The pacemaker of the heart is the Sinoatrial (SA) node, which is located in the right atrium. It initiates the electrical impulse that triggers the heartbeat.

2. **Phases of the Cardiac Cycle:**
- The cardiac cycle consists of several phases:
- **Atrial Systole:** This is the contraction of the atria, which lasts about 0.1 seconds. During this phase, blood is pushed from the atria into the ventricles.
- **Ventricular Systole:** This is the contraction of the ventricles, lasting approximately 0.3 seconds. Blood is ejected from the ventricles into the large arteries (aorta and pulmonary artery).
- **Joint Diastole:** This is the relaxation phase where both the atria and ventricles are relaxed, lasting about 0.4 seconds. During this time, the heart fills with blood.

3. **Analyzing the Options:**
- **Option 1:** This option suggests that atrial and ventricular contractions occur simultaneously, which is incorrect as atrial contraction occurs before ventricular contraction.
- **Option 2:** This option indicates that the atria relax first, which is incorrect since atrial contraction occurs before relaxation.
- **Option 3:** This option correctly states that atrial contraction occurs first (0.1 seconds), followed by ventricular contraction (0.3 seconds), and then joint diastole (0.4 seconds).
- **Option 4:** This option incorrectly states the timing of contractions for the atria and ventricles.

4. **Conclusion:**
- Based on the analysis, the correct representation of the events in the cardiac cycle beginning from the firing of an impulse from the pacemaker is **Option 3**.