WEBVTT 1 00:00:05.100 --> 00:00:08.899 In humans, as in numerous other air breathing animals 2 00:00:08.900 --> 00:00:12.565 gas exchanges occur thanks to the lungs. 3 00:00:12.566 --> 00:00:16.632 During inspiration, Oxygen rich air penetrates the lungs 4 00:00:16.633 --> 00:00:19.399 via the respiratory passages. 5 00:00:19.400 --> 00:00:23.065 It passes through the oral cavity or the nasal cavities 6 00:00:23.066 --> 00:00:26.032 then descends down the trachea and the bronchi. 7 00:00:26.033 --> 00:00:32.532 The air then fills the bronchioles which terminate in several million pulmonary alveoli. 8 00:00:32.533 --> 00:00:38.532 Each of these is surrounded by a network of very small blood vessels: the capillaries. 9 00:00:38.533 --> 00:00:42.232 The assemblage of alveoli makes for a very large surface 10 00:00:42.233 --> 00:00:46.065 for the exchange of air between the blood and the exterior. 11 00:00:46.066 --> 00:00:50.499 Indeed, if one unfolded all the alveoli in the lungs of an adult 12 00:00:50.500 --> 00:00:55.399 they would cover an area of more than 100 square meters 13 00:00:55.400 --> 00:00:59.265 that is the same area as half a tennis court. 14 00:00:59.266 --> 00:01:04.599 Oxygen from the air passes through the very thin walls of the alveoli into the blood. 15 00:01:04.600 --> 00:01:10.665 It is then transported by the circulation of the blood to the organs, where it will be used. 16 00:01:12.666 --> 00:01:15.699 In return, these organs produce Carbon Dioxide 17 00:01:15.700 --> 00:01:18.865 which is a waste product that has to be removed. 18 00:01:19.633 --> 00:01:21.965 It is via the circulation of the blood 19 00:01:21.966 --> 00:01:26.765 that the Carbon Dioxide is transported to the pulmonary alveoli. 20 00:01:28.733 --> 00:01:31.699 It passes from there into the air in the lungs, which expel it to the exterior, during expiration. 21 00:01:31.700 --> 00:01:34.700 which expel it to the exterior, during expiration.