Lung

The lungs are a pair of breathing organs situated within the chest of most humanoids, on either side of the heart. Spongy and sac-like, they are a crucial part of the respiratory system. They are connected to the nose and mouth by the pharynx, the trachea and the bronchi, and house the alveoli clusters which are so important to respiration. The lungs are coated by two layers of a delicate, folded membrane called the pleura.

The lungs are the means by which oxygen is delivered to the blood and carbon dioxide is expelled from it. Air from the atmosphere is drawn into the lungs through the nose and mouth, where it enters the many alveoli. Through those, oxygen is extracted and filtered into capillaries lining the alveoli and the walls of the lungs, while carbon dioxide is sent back up to the nose and mouth to be exhaled. During this process, the lungs expand and contract. Additionally, the lungs serve as cushions which protect the heart, and are able to filter out gas micro-bubbles and small blood clots which may form in the veins.

Though most humanoids possess lungs, there are variations found between species. In Humans, the right lung is usually slightly larger than the left. Klingons have three lungs. Ferengi have upper and lower lungs as opposed to left and right. Some species even possess genetically modified lungs, such as the Suliban whose lungs were altered to allow them to process atmospheres which would otherwise have been lethal to them.