Basically, breathing is ventilation. Ventilation is the mechanical act of moving air in and out of your lungs.

Respiration is commonly confused with ventilation. Respiration takes place at the cellular level when oxygen diffuses on to the red blood cells and carbon dioxide diffuses into the lung to be exhaled. When you inhale (breathe in), fresh air enters your lungs. The lungs take oxygen from the air and add carbon dioxide to the air.

When you exhale (breathe out), you force the air from your lungs back into the environment. You do not, however, force all the air out of your lungs when you exhale. A person takes in about 500 ml of air when he inhales normally and exhales the same amount. After a normal exhale, the lungs will still contain about 2300 ml of air.

a. Oxygen.

The oxygen diffused from the air by the lungs is absorbed by the red blood cells in the blood and taken to all parts of the body. Diffusion is the movement of molecules from an area of higher concentration (the air) to an area of lower concentration (the blood cells). The body cells use the oxygen to change stored energy in the form of sugars and fats into usable energy. In addition to producing energy, the process produces certain waste products, including carbon dioxide.

b. Carbon Dioxide.

Carbon dioxide (CO2) is a byproduct of cellular respiration and is carried in the blood stream as carbonic acid from the cells to the lungs. When the carbon dioxide reaches the lungs, it has a higher concentration than the air and it diffuses out of the blood to be exhaled in to the environment.