What is heartbeat? Name the instrument used by the doctors to listen to our heartbeats .

Explanation

### Step-by-Step Solution:

1. **Definition of Heartbeat**:
- A heartbeat is defined as the pulsation of the heart. It consists of a complete cycle of contraction and relaxation of the heart muscles. When the heart contracts, it pumps blood out, and when it relaxes, it allows the heart to fill with blood again. This rhythmic cycle is what we refer to as a heartbeat.

2. **Understanding the Heart's Function**:
- The heart is made up of muscles that work in a coordinated manner. During each heartbeat, the heart muscles contract (systole) and then relax (diastole). This cycle is essential for circulating blood throughout the body, delivering oxygen and nutrients to tissues.

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