1. Initiation of exercise
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Due to neurological alterations that send excitatory impulses to the dorsal respiratory group (DRG) of the medullary respiratory center in the medulla, respiration increases abruptly when exercise is first initiated. The alterations are as follows: (a) stimulation of limbic system due to the expectation of the activity, (b) motor impulses from the precentral gyrus, and (c) sensory impulses from proprioceptors in muscles, tendons, and joints.
2. Moderate exercise
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Breathing gradually gets deeper during moderate exercise than it did when it first started because of chemical and physical changes in the bloodstream. These changes include (a) a decrease in PO2 caused by an increase in oxygen consumption, (b) an increase in PCO2 caused by an increase in CO2 production from the contraction of muscle fibers, and (c) an increase in temperature caused by the release of heat from more oxygen usage.
3. Abrupt ascent into high altitudes
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The total atmospheric pressure and partial pressure of oxygen decreases in increasing altitudes; hence, an abrupt ascent to high altitudes would cause shortness of breath due to a lower level of oxygen in the bloodstream.
4. Paralysis of phrenic nerve
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Paralysis of the phrenic nerve would cause breathing difficulties because it would no longer be able to send nerve impulses to the diaphragm. The diaphragm is the main muscle for inhalation.
5. Severe anemia
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Since oxygen and carbon dioxide are released and taken in by the body via blood circulation, severe anemia causes breathing issues which result from the lungs attempting to make up an effort to take in more oxygen. Hemoglobin deficiency prevents the different parts of the body from receiving enough oxygen.
10. Advanced pregnancy
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An advanced pregnancy may hinder the diaphragm, the most important muscle for inhalation, to descend completely or properly contract; therefore, causing respiratory difficulties in terms of taking in oxygen.