Breathing is the process by which oxygen is inhaled into the body also known as inspiration and carbon dioxide is released outside the body also called as expiration.

Mechanism of breathing involves two processes which are:

A. Inspiration

B. Expiration

Inspiration: It is the process of taking in external air or oxygen into the lungs.

1. It starts when the diaphragm contracts.

2. This leads to increased volume of thoracic chamber.

3. After which the intercostal muscles contracts and leads to the pulling of ribs and sternum which also increases the volume of the thoracic chamber.

4. Now, the increased volume of thoracic chamber also increases the pulmonary volume.

5. This increased pulmonary volume decreases the intra pulmonary pressure less than the atmospheric pressure.

6. Which further causes the entry of external air or oxygen into the lungs also known as inspiration.

Expiration:

1. It starts when the diaphragm relaxes.

2.This leads to decreased volume of thoracic chamber.

3.After which the intercostal muscles relaxes and leads to the relaxation of ribs and sternum which also decreases the volume of the thoracic chamber.

4.Now, the decreased volume of thoracic chamber also decreases the pulmonary volume.

5. This decreased pulmonary volume increases the intra pulmonary pressure less than the atmospheric pressure.

6.Which further causes the exit of external air or carbon dioxide into the atmosphere also known as expiration.