**Core Concept**
The "gatekeeper of the heart" refers to the **atrioventricular (AV) node**, which plays a crucial role in regulating the heartbeat. This term is used to describe the structure that controls the flow of electrical signals from the atria to the ventricles. The AV node acts as a critical checkpoint, ensuring that the ventricles contract at the right time.

**Why the Correct Answer is Right**
The AV node is responsible for delaying the electrical signal from the atria, allowing the ventricles to fill with blood before contracting. This delay is essential for maintaining a normal heart rhythm. The AV node receives signals from the **sinoatrial (SA) node** and transmits them to the **bundle of His**, which then activates the ventricles.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not related to the heart's electrical conduction system.
**Option B:** This option is also incorrect as it does not describe the AV node's function.
**Option D:** Similarly, this option is incorrect as it is not associated with the regulation of heartbeat.

**Clinical Pearl / High-Yield Fact**
The AV node's function can be affected by certain medications, such as **beta blockers** and **calcium channel blockers**, which can slow down the heart rate.

**Correct Answer:** D. Atrioventricular node