Respiratory system. Computer artwork showing the lungs and upper respiratory tract. Air is breathed in through the mouth or nose (upper right). It is passed down the trachea (windpipe, pink, upper centre), which divides into two bronchi, each of which leads to a lung (pink). Inside the lungs (one seen opened, centre right), the bronchi divide further into bronchioles. Bronchioles terminate in alveoli (not seen), air sacs surrounded by capillaries. Here, blood exchanges carbon dioxide for oxygen from the air. The carbon dioxide is exhaled, and the oxygenated blood is pumped around the body.
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