Unlike fish (except a few), mammals have an internal gas exchange organ. This is called the lung. Mammals solved the problem of dessication by placing their gas exchange system inside the body. The mouth and nose are attached to the trachea, a pipe that has cartilage rings in it to hold it open. The trachea branches into 2 bronchi, one leading to each lung. The bronchi branch continuously into bronchioles which end in small hollow sacs called alveoli. The continuous branching and many alveoli provide a massive surface area for gas exchange. The alveoli are the site of gas exchange and, if you were to lay out a human gas exchange surface it would be the size of a tennis court. Breathing in mammals is controlled mainly by the muscles of the ribcage and diaphragm. When the diaphragm contracts, the volume of the thoracic cavity decreases, creating a n environment with a higher pressure system than that of the atmosphere, so air is sucked out. When the diaphragm relaxes, the volume of the chest cavity increases, meaning the pressure in the atmosphere is greater than in the lung, so air is drawn into the lung. As air moves down the bronchioles and to the alveoli, the oxygen in the air becomes dissolved in a fluid in the alveoli. This oxygen then diffuses through the alveoli into the blood supply, and then around the body. Alvioli are very thin and have a high supply of blood vessels over them, so the gas does not have far to travel to get into the blood.
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