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Physiology of Breathing

Nose breathing is normal for humans, and each nostril functions independently and synergistically to filter, warm, moisturize, dehumidify and smell the air. It’s like having two noses in one. Normal respiration follows a gentle wave pattern with 10 to 12 breaths per minute, providing five to six liters of air per minute. Mouth breathers often have a respiration rate above 12 breaths per minute, and those with asthma and serious medical conditions have rates of 20 respirations per minute or higher.1 


Breathing delivers oxygen to the cells of the body and removes excess carbon dioxide. The body requires approximately two to three percent oxygen, and the atmospheric level is 21 percent, so there is no need to store oxygen. The body’s requirement for carbon dioxide is 6.5 percent, and the atmospheric content is 0.03 percent, so the body has to produce and store carbon dioxide in the lungs and blood. Carbon dioxide is produced as a byproduct of exercise and digestion of food. Carbon dioxide facilitates release of oxygen from hemoglobin, triggers when to take the next breath, maintains blood pH by buffering with bicarbonate or carbonic acid and prevents smooth muscle spasms.2


All of these functions are reduced or impaired in mouth breathers.

Oxygen is absorbed on the exhale, not on the inhale. The back pressure created in the lungs with the slower exhale of nose breathing compared to mouth breathing allows more time for the lungs to transfer oxygen to the blood. The exchange of oxygen in the blood requires the presence of carbon dioxide. Approximately 98 percent of oxygen is carried in hemoglobin. Carbon dioxide levels need to be at five percent in the alveoli and arterial blood before the oxygen molecules are released from hemoglobin to reach brain and muscle cells. Lower than five percent carbon dioxide levels lead to an elevation in blood pH, and the oxygen “sticks” to the hemoglobin. This is the Bohr Effect, first described in 1904 by physiologist Christian Bohr.2


Nasal breathing brings nitric oxide into the equation, leading to an increase in oxygen absorption. Nitric oxide is released in the nasal cavity and inhaled into the lungs with nose breathing. Nitric oxide increases the efficiency of oxygen exchange, leading to as much as 18 percent more oxygen being absorbed.3,4 Mouth breathing bypasses the nitric oxide advantage.


References

1. Schatz P. OpenStax College, Anatomy and Physiology. OpenStax CNX. 2012.

2. Brinkman JE, Sharma S. Physiology, respiratory drive. Treasure Island, Florida: StatPearls Publishing, 2018. 

3. Lundberg J, Settergren G, Gelinder S, et al. Inhalation of nasally derived nitric oxide modulates pulmonary function in humans. Acta Physiologica Scand. 1996; 158:343-7.

4. Bian K. Murad F. Nitric oxide (NO) biogeneration, regulation and relevance to human diseases. Frontiers in Bioscience: a journal and virtual library. Front Biosci. 2003; 8:d264-78.

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