In breathing movements, air volume can be estimated by

Options

A. stethoscope
B. hygrometer
C. sphygmomanometer
D. spirometer.

Explanation

**Step-by-step solution:**

1. **Understanding Breathing Movements**: Breathing involves two main processes: inspiration (the intake of air) and expiration (the expulsion of air).

2. **Identifying Instruments**: There are various instruments that can measure different physiological parameters. The question lists four instruments: stethoscope, hygrometer, sphagnometer, and spirometer.

3. **Function of Each Instrument**:
- **Stethoscope**: Used primarily for listening to heartbeats and lung sounds, not for measuring air volume.
- **Hygrometer**: Measures the amount of water vapor in the air, irrelevant to breathing volume.
- **Sphagnometer**: This is not a commonly used term in physiology; it may refer to a device used for measuring blood pressure, which is also not related to air volume.
- **Spirometer**: Specifically designed to measure the volume of air inhaled and exhaled during breathing.

4. **Conclusion**: The instrument that accurately measures the air volume during breathing movements (inspiration and expiration) is the spirometer.

5. **Final Answer**: The correct instrument to estimate air volume in breathing movements is the **spirometer**.

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