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How we breathe

The primary function of pulmonary ventilation is the exchange of carbon dioxide for oxygen.

Breathing occurs when the contraction or relaxation of muscles in the chest and abdominal areas change the volume inside the air passages found in the lungs' interior. According to Boyle's Law, when the volume inside the lungs changes, so does the pressure inside it.

One of the key concepts on which breathing is based is that the air flows from higher to lower pressure. If two compartments are connected, and there is a difference of pressure between them, the air will always flow towards the one that has the lowest pressure. For the volume of the lungs to change and, therefore, the pressure inside them, two processes need to occur:

  • Inspiration occurs when the diaphragm and the intercostal muscles contract. The contraction of both muscles causes an increase in the thoracic cavity's size, which leads to lung expansion. In response to this, the pressure of the air inside the lungs decreases below the atmospheric pressure, which allows the air to flow in. The air gets slightly warmed up when inhaling. The volume of the gas is adjusted to match the internal body temperature, which will be kept constant, helping in the pressure-volume relationship of Boyle’s law.

  • Expiration occurs when the respiratory muscles relax, causing the lungs to recoil to their original size. As a result, the pressure rises inside the lungs above atmospheric pressure, and, similar to the inspiration process, the air flows out.

Two illustrations of the respiratory system, one on the left saying inspiration and one on the right saying expiration. The atmospheric pressure is 760 millimeters of mercury. The one on the left shows what happens to the lungs and the diaphragm when inspiration occurs. The diaphragm contracts downwards and the lungs expand, creating less pressure inside them, which will allow the air to flow in. The air flowing in, the lungs expanding, and the diaphragm contracting are shown with arrows. The illustration on the right shows the opposite. When expiration happens, the arrows point in another direction, showing how the diaphragm relaxes and goes upwards, the lungs contract, and the air flows to the outside through the mouth due to the higher pressure in the interior.

Figure 1: Illustration of the respiratory system showing what happens when inspiration and expiration occur.

Referred from:

Article
Structure of the respiratory system
Article
Intrapleural cavity
Article
Pressure inside the lungs