The regulation of heartbeat in mammals is due to

Options

A. The volume of blood in the circulatory system
B. The presence of excess of oxygen in blood
C. The presence of thyroxine in blood
D. The presence of pacemaker in the heart

Answer

**Step-by-Step Solution:**

1. **Understanding Heartbeat Regulation**: The heartbeat in mammals is regulated by specific structures within the heart that generate electrical impulses. These impulses control the contraction and relaxation of the heart muscles.

2. **Identifying the Pacemaker**: The primary structure responsible for initiating the heartbeat is the Sinoatrial (SA) node, also known as the natural pacemaker of the heart. It is located in the upper part of the right atrium.

3. **Function of the SA Node**: The SA node consists of a cluster of specialized cells that generate electrical impulses. These impulses spread throughout the heart, causing the heart muscles to contract and pump blood.

4. **Evaluating Other Options**:
- The volume of blood in the circulatory system does not directly regulate the heartbeat.
- The presence of excess oxygen in the blood influences respiration more than heartbeat regulation.
- Thyroxine, a hormone, can affect heart rate but is not the primary regulator of heartbeat.

5. **Conclusion**: The regulation of heartbeat in mammals is primarily due to the presence of the pacemaker, specifically the SA node, which generates the electrical impulses that control heart contractions.

**Final Answer**: The regulation of heartbeat in mammals is due to the presence of the pacemaker in the heart.

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