The respiratory cycle consists of inspiration (breathing in), expiration (breathing out) and a very short respiratory pause. In normal adults, the breathing rate is 12 – 18 breaths per minute. A newborn breathes 60 times per minute. It is always 4 – 5 times less than heart rate. Slight increase in CO2 content in blood increases breathing rate.
Inspiration (or inhalation) is the result of increase in the size of thoracic cavity and this increase is due to the combined action of the ribs and the diaphragm. The ribs are moved upward and outward by the muscles (intercostal muscles) stretched between them, thus enlarging the chest cavity all around. The diaphragm is a sheet of muscular tissue, which normally remains arched upward like a dome, towards the base of the lungs. On contraction, it falls or flattens from the dome – shaped outline to an almost horizontal plane and thus contributes to the enlargement of the chest cavity lengthwise.
As the diaphragm flattens, it presses the organs inside the abdomen and with the abdominal muscles relaxed, the abdominal wall moves outwards. When the thoracic (chest) cavity increases in size, its internal pressure is decreased. The lungs expand and as a result, the pressure inside the lungs is lowered below the atmospheric pressure. The outside air, which is at a greater pressure, rushes in to equalize the pressure.