The lungs are like big pink sponges that are protected by the rib cage. The diaphragm is a muscle that sits underneath the ribcage, it moves up when it relaxes and down when it contracts. This movement helps to get air in and out of your lungs. The air you breath in goes through the trachea, this splits into two tubes called the bronchi (each one is a bronchus) one going into each lung. The bronchi split into smaller tubes called the bronchiholes. The bronchioles end at small air sacs in the lungs called alveoli. These are where gas exchange takes place.
Air is inhaled into the lungs. Some of the oxygen in the inhaled air passes into the bloodstream to be used in respiration. Carbon dioxide is a waste product of respiration. In the lungs it passes out of the blood and is then breathed out. The gases pass into or out of the bloodstream by diffusion, where a substance moves from where there's lots of it to where there's less of it. The lungs are well adapted for gas exchange as they: are moist, have a good blood supply and the alveoli give the lungs big inside surface area.
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