Mechanisms of Inspiration and Expiration

Understanding Inspiration (Breathing in):
- Breathing in, also known as inspiration, is an active process that requires energy.
- It starts when the diaphragm contracts and moves downwards, creating a larger volume in the thoracic cavity.
- At the same time, the intercostal muscles (found between the ribs) contract, causing the rib cage to expand upwards and outwards.
- The increase in volume within the chest cavity lowers the internal air pressure to less than the outside pressure.
- Air from the outside then rushes into the lungs to equalise pressure. This fills the lungs with air, effectively breathing in.
Understanding Expiration (Breathing out):
- Breathing out, termed expiration, is a usually passive process that happens when muscles relax.
- In contrast to inspiration, the diaphragm relaxes and moves upwards, decreasing the volume in the thoracic cavity.
- Alongside this, the intercostal muscles also relax, bringing the rib cage to rest downwards and inwards.
- These relaxing movements decrease the volume within the chest cavity and increase the internal air pressure to more than the outside pressure.
- The air in the inflated lungs is then forced out to the outside, achieving expiration or breathing out.
Factors Affecting Breathing Rate:
- Exercise, anxiety, or excitation lead to the need for more oxygen in the body, and hence a higher breathing rate.
- On the other hand, activities like sleep or meditation, which are relaxing, lower the body’s demand for oxygen and tend to decrease the breathing rate.
Regulation of Breathing:
- The brain controls the breathing rate, specifically through the respiratory centre in the medulla oblongata of the brain stem.
- It does this by constantly receiving signals from chemoreceptors that monitor oxygen and carbon dioxide levels in the blood.
- When carbon dioxide levels in the blood are high, the respiratory centre raises the breathing rate to expel the excess carbon dioxide.

Remember, continuous practise in drawing and labelling diagrams of the respiratory system will help increase understanding of these processes.