How Breathing Works

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To understand the cause and symptoms of high altitude pulmonary edema, it is helpful to be familiar with the mechanics of breathing: how exactly we are able to inhale and exhale air to exchange gases necessary to maintain life. Once we understand how this mechanism works, we can further understand what happens to our breathing capability at higher altitudes.

Whether air flows in or out of the lungs is determined by its pressure gradient. Breathing consists of an alternating process of increasing and decreasing the pressure of the lungs. The pressure of a gas is inversely proportional to its volume. When lung volume increases and pressure is decreased, the atmospheric pressure is greater than that of the lungs, compelling air to flow from the atmosphere into the lungs in an effort to equalize the pressure. By the same reasoning, air will flow out of the lungs back into the atmosphere when lung volume decreases and pressure is increased.


Surrounding muscles control the volume (and, therefore, the pressure) of our lungs. During inhalation, contraction of the intercostal muscles (muscles that run in between the ribs) enable the expansion of the thoracic (chest) cavity. The rib cage moves out and up. The diaphragm also contracts downward, helping to reduce air pressure in the lungs. During exhalation, relaxation of these muscles exerts pressure on the lungs, decreasing their volume and forcing air to flow back out into the atmosphere.

Transportation of Oxyen and Carbon Dioxide

Oxygen transport

The oxygen inhaled travels to the alveoli. From the alveoli, oxygen diffuses across into the arteriole, from an area of higher pressure to an area of lower pressure. The vast majority of blood travels binds to hemoglobin molecules, proteins found in red blood cells. A hemoglobin molecule has four heme groups, each of which contains an iron atom that binds oxygen.

Hemoglobin's affinity for oxygen changes based on the partial pressure of oxygen in certain areas of the body. Near tissues that are lacking in oxygen, hemoglobin will have less of an affinity for oxygen, therefore releasing it. In the tissues, carbon dioxide causes hemoglobin to alter its shape, thereby giving up oxygen more easily. Therefore, the blood unloads more oxygen to undergo metabolism to generate even more carbon dioxide, starting the cycle over again.

Movement of oxygen and carbon dioxide in and out of bloodstream

Carbon dioxide transport

A small percentage of carbon dioxide dissolves directly into the blood plasma. Other carbon dioxide molecules bind to hemoglobin (like oxygen), except a different site on the molecule. However, most carbon dioxide molecules diffuse straight into the red blood cells. The enzyme carbonic anhydrase triggers the reaction between water molecules and carbon dioxide molecules to form carbonic acid, which then dissociates into bicarbonate and hydrogen ions. Red blood cell transport membrane proteins move bicarbonate ions into the plasma in exchange for chloride ions - process known as chloride shift. This process keeps blood plasma carbon dioxide levels low so more can diffuse from tissues. Bicarbonate ions will then move back to the lungs via the blood plasma. The low concentration of carbon dioxide in the lungs urges carbon dioxide to diffuse back into the alveolar sacs from the bloodstream. Even the carbon dioxide molecules bound to hemoglobin will re-enter the alveoli due to the pressure gradient.

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