Initiation of exercise
It takes more oxygen to support muscular activity when transitioning from a relaxed state to one of increased physical activity. When starting an exercise, breathing rate increases to allow more oxygen to enter the lungs and subsequently be pumped into the blood to power the muscles.
Moderate exercise
Moderate exercises require more oxygen to fuel increased muscle activity due to higher intensity of physical activity. Similar to the previous case, increased respiratory rate occurs as oxygen demand increases.
Abrupt ascent into high altitudes
Higher altitudes have lower air pressure, which results in less oxygen available for breathing. Abrupt ascent into high altitudes causes the body to compensate for the sudden lack of oxygen through increased respiratory rate.
Paralysis of phrenic nerve
Diaphragm movement is controlled by the phrenic nerve. Paralysis of the phrenic nerve may lead to paralyzed and inefficient diaphragm activity which causes difficulties in breathing.
Severe anemia
Individuals with severe anemia have low hemoglobin levels and insufficient RBCs to transport oxygen throughout the body. Increased breathing takes place in an effort to restore normal oxygen levels.
Advanced pregnancy
As pregnancy progresses, the baby grows bigger and shifts position, pressing the uterus up against the diaphragm. This compresses the lungs, making it harder to expand and take in oxygen when breathing.