Respiration is the body process of inhaling air, exchanging gases between air and blood, and exhaling those gases out of the body. This is accomplished during three phases:

• External respiration. The exchange of oxygen and carbon dioxide between the atmosphere and the blood. It takes place in the (large) capillaries in the lungs.
  
  
• Internal respiration. The exchange of oxygen and carbon dioxide between the blood and the cells of the body.
  
  
• Cellular respiration. The process by which body cells use the oxygen taken from the bloodstream as fuel for their work. As the cells work, the metabolic activity produces carbon dioxide. If this carbon dioxide were allowed to build up to high levels, it could damage cells, organs and body tissue. Therefore, the blood picks up the carbon dioxide and brings it to the lungs to be exhaled into the atmosphere.

The engine that drives respiration is the diaphragm, a muscle that makes up the floor of the thoracic cavity (chest) that houses the lungs. As the diaphragm contracts, the rib cage rises, causing a negative pressure in the chest cavity that forces the lungs to expand and air to be drawn in. When the diaphragm expands, the rib cage falls and compresses the lungs, forcing air out of the body.

This process happens 10 to 16 times a minute when the body is at rest. The breathing rate changes with the needs of the body, and is regulated by a respiratory center in the brain called the medulla. Breathing is generally an involuntary act, but people can control the diaphragm to a limited degree.

The normal breathing cycle begins when air enters the body through the nose. During vigorous exercise, a person may open his or her mouth to bring air into the body, a process known as mouth breathing. However, the usual starting point of respiration is the nose and nasal passages.

By the time air has reached the lungs, the mucous membrane lining and cilia have warmed the air to a relative humidity of about 75 degrees. This is important, because cold air can damage delicate lung tissues. The mucus membranes and cilia also help moisten the air and filter out contaminants.

When the air reaches the alveoli, gases are exchanged on the cellular level. The lungs contain about 300 million of these air sacs, which are surrounded by capillaries, the smallest blood vessels in the body.

Here, the hemoglobin within red blood cells exchanges carbon dioxide for fresh oxygen to be delivered back to the heart, which pumps the oxygen-rich blood throughout the body to nourish the tissues.

Blood rich in carbon dioxide travels back to the heart through the veins. The blood is then pumped to the lungs, where carbon dioxide is delivered into the alveoli for exhalation.