Under normal conditions, the rate of breathing of a normal person is 15 to 18 times per minute. During vigorous exercise, the rate of breathing increases by about 20 to 25 times per minute.
Reason: During vigorous exercise, the demand of oxygen increasesto release more energy for extra work. Hence, there is increase in breathing rate to supply the extra oxygen needed.

In tissues oxygen is used up in respiration and carbon dioxide is released. So, there is less concentration of O₂ and more concentration of carbon dioxide. On the contrary, blood coming from lungs has more concentration of oxygen and less concentration of carbon dioxide.
Due to difference in concentration, Oxygen diffuses from blood into tissues and Carbon dioxide from tissues into blood.

Carbon monoxide (CO) binds very strongly with haemoglobin of red blood cells which is main carrier of O₂ from lungs to the body cells. When all the haemoglobin of red blood cells gets bonded with carbon monoxide, there is none left for the transport of oxygen. Lack of oxygen in the body or cells can cause death.

Breathing comprises of inspiration and expiration of air.

1. Inspiration: During inspiration the volume of the thorax is increased by two movements, for intake of air.
(a) The muscles of the diaphragm contract and becomes flattenned.
(b) The lower ribs are raised upward and outwards by the contraction of the intercostal muscles which run obliquely from one rib to the next rib. The chest cavity enlarges, the air pressure in the lungs is decreased, air rushes into the lungs.
2. Expiration: Breathing out of air, results mainly from relaxation of muscles of the ribs and diaphragm. The chest cavity is reduced and air is forced outward through the nose and trachea.