Lungs Mammalian lungs 

The lungs of mammals have a spongy & soft texture & are honeycombed with epithelium, having a much larger surface area in total than the outer surface area of the lung itself. The lungs of humans are a typical example of this type of lung.

Breathing is largely driven by the muscular diaphragm at the bottom of the thorax. Contraction of the diaphragm pulls the bottom of the cavity in which the lung is enclosed downward, increasing volume & thus decreasing pressure, causing air to flow into the airways. Air enters through the oral & nasal cavities it flows through the pharynx, then the larynx & into the trachea, which branches out into the main bronchi & then subsequent divisions. During normal breathing, expiration is passive & no muscles are contracted . The rib cage itself is also able to expand & contract to some degree, through the action of other respiratory & accessory respiratory muscles. As a result, air is transported into or expelled out of the lungs. This type of lung is known as a bellows lung as it resembles a blacksmith's bellows.