Breathing, a process which is so normal and universal that most of time we don't even think about it unless you have a sneeze, cough, or hiccup.
People need to take Oxygen into the body and get rid of the inside carbon dioxide all the time and the respiratory system, which our breathing takes place, get the job done.

You speak, you smell and you laugh, however, before all of these happen, you breathe.
Respiration is a team work completed by the nose, mouth, trachea, lungs and diaphragm. Oxygen enters your nose and mouth, and then passes through the a tube called trachea which spreads to the chest cavity. And there, the trachea splits into two smaller tubes called the bronchi (meaningful enough by the suffix 'chi') and each bronchus has its own divided bronchial tubes and these bronchial tubes go into the lungs directly and again, there they divided into many smaller tubes which connect to numerous alveoli (looks like tiny spongy sacs) which surrounded by capillaries. The inhaled oxygen passes into the alveoli and then diffuses into the arterial blood through the capillaries. And at the same time, carbon dioxide was released into the alveoli from the veins which filled with waste-rich blood. The carbon dioxide follows the same path but opposite direction out when you breathe out.
The diaphragm, pumping out the carbon dioxide and pulling in fresh air with rich Oxygen, is a sheet of muscles that lies across the bottom of your chest cavity. When it relaxes, you breathe out and when it contracts, you breathe in.

We use abdominal breathing method which is different from deep breathing. Going with the whole tai chi sequence (each movement has a certain process of inhale and exhale), you are slowly aware of your breathing and after a certain period, you ignore it again but this time, your breathing has already been 'trained' and follow the nature rules.