Respiration

Respiration occurs as the dimensions of the thoracic cavity are altered which changes air pressure within the lungs.

Essentially, to breath in (inspiration), the size of the chest cavity is increased by the intercostal muscles between the ribs moving the ribcage up and out at the same time as the diaphragm, a muscular sheet between the thoracic and abdominal cavities, contracts and flattens.

As these movements increase the chest volume in all three dimensions, the pressure is reduced so air is drawn in, aided by the pressure of the atmosphere outside.

Breathing out (expiration) occurs when the intercostal muscles and diaphragm relax and the lungs deflate to their original dimensions and thus requires no muscular effort when breathing quietly.

Control of breathing is normally automatic but may be overridden to a certain extent (such as holding the breath, deep breathing exercises or speech).

The rate and depth of respiration is controlled by several factors, the most important being the concentration of oxygen and carbon dioxide in the blood.

Muscle Movements During Respiration:

1. Normal Inspiration - The diaphragm and external intercostal muscles contract

2. Forced Inspiration - The scalenes, pectorals, and serratus anterior muscles elevate the ribcage

3. Normal Expiration - The diaphragm relaxes and there is passive recoiling of the lungs

4. Forced Expiration - The internal intercostal and abdominal muscles contract