By Erin Odya, Pat DuPree

Respiration, or the exchange of gases between an organism and its environment, occurs in three distinct processes: breathing, exchanging gases, and cellular respiration. Here’s a breakdown:

  • Breathing: The technical term is pulmonary ventilation, or the movement of air into and out of the lungs. Breathing is comprised of two distinct actions: inspiration and expiration.
  • Exchanging gases: This takes place between the lungs, the blood, and the body’s cells in two ways:
    • Pulmonary, or external, respiration: The exchange in the lungs when blood gains oxygen and loses carbon dioxide
    • Systemic, or internal, respiration: The exchange that takes place in and out of capillaries when the blood releases some of its oxygen and collects carbon dioxide from the tissues
  • Cellular respiration: Oxygen is used in the catabolism of substances like glucose for the production of energy, creating CO2 as a byproduct.

A single respiratory cycle consists of one inhalation followed by an expiration. The regular, restful breathing rate is controlled by the pons, while the medulla oblongata will signal any necessary changes to that pattern.

To complete a normal inhalation, the diaphragm (the broad skeletal muscle that forms the bottom of the thoracic cavity) is triggered to contract. This pushes down on the contents of the abdominal cavity, thus increasing the volume of the lungs. An increase in volume causes a decrease in pressure (known as Boyle’s Law). So, as a result of the diaphragm contracting, the pressure of the air already inside the lungs drops below that of atmospheric pressure (the pressure of the air outside our bodies). Because gasses will naturally diffuse to areas of lower pressure, air flows into the lungs. Thus we do not need to “suck in” air with each normal breath.

For a deeper inhalation, we contract the intercostal muscles (between the ribs), which pull the ribcage out, further increasing the volume and dropping the pressure inside the lungs even lower so more air can come in. Once inside the lungs, gasses can be exchanged between the air and the blood.

Exhalation is a passive process; that is, we don’t tell the lungs to breathe out. We simply stop telling the diaphragm (and the intercostal muscles if they’re engaged) to contract. When they relax, volume decreases and pressure increases. Further, the lungs contain a great deal of elastic tissue. As the muscles relax, the elastic tissue snaps back. This elastic recoil briefly drops the pressure inside the lungs to below atmospheric pressure and air flows out.