<big>'''''QUESTA PARTE E' ANCORA DA TRADURRE!'''''</big>
The [[rib cage]] itself is also able to expand and contract to some degree, through the action of other respiratory and accessory respiratory muscles. As a result, air is sucked into or expelled out of the lungs, always moving down its pressure gradient. This type of lung is known as a '''bellows lung''' as it resembles a blacksmith's [[bellows]].
Air enters through the oral and nasal cavities; it flows through the larynx and into the trachea, which branches out into bronchi. In humans, it is the two main bronchi (produced by the bifurcation of the trachea) that enter the roots of the lungs. The bronchi continue to divide within the lung, and after multiple generations of divisions, give rise to bronchioles. Eventually the bronchial tree ends in alveolar sacs, composed of alveoli. Alveoli are essentially tiny sacs in close contact with blood filled capillaries. Here [[oxygen]] from the air [[diffusion|diffuses]] into the blood, where it is carried by [[hemoglobin]], and carried via pulmonary veins towards the [[heart]].
Deoxygenated blood from the heart travels via the [[pulmonary artery]] to the lungs for oxidation.
