Breathing Breathing- (aka ventilation), The process through which the respiratory system moves air into and out of the lungs. In contrast, Respiration refers to cellular respiration, a complex metabolic activity during which the energy needed to support life activities is released. During respiration, the body uses Oxygen to break the bonds of a sugar molecule. The waste products of respiration are carbon dioxide and water.

Both the respiratory and the circulatory systems are needed to support respiration During inhalation, the diaphragm, a broad, thin muscle separating the chest cavity from the abdomen, contracts and moves downward. This increases the size of the chest cavity. The intercostal, or rib, muscles also contract and cause the ribs to move upward and outward.

During exhalation, the diaphragm and the intercostal muscles relax, the volume of the chest cavity decreases, and internal air pressure increases. This forces air out of the lungs. Oxygen and Carbon Dioxide are exchanged between the lungs and the bloodstream by diffusion. Oxygen is moved into the lungs during inhalation and carbon dioxide and water are removed during exhalation.

The actual exchange of these gases in the lungs takes place in clusters of alveoli, or grape-like clusters of air sacs.

The lungs are snugly enclosed by the pleura, a double-layered membrane. Between the two layers is a narrow space called the pleural cavity, which is filled with pleural fluid. This fluid helps the lungs glide easily against the chest wall during breathing.