Gas exchange in humans happens in the lungs. The lungs sit in the chest. The ribs protect them. Air goes in through the nose and mouth. It passes down the t...
Question 1 Report
Gas exchange in humans happens in the lungs. The lungs sit in the chest. The ribs protect them. Air goes in through the nose and mouth. It passes down the trachea. The trachea splits into two bronchi. One bronchus goes to each lung. The tubes split again and again. They end in tiny air sacs. These sacs are the alveoli. Oxygen passes from the alveoli into the blood. Carbon dioxide passes the other way. A student was asked how the alveoli suit gas exchange. The student was also asked how we breathe in. The questions below are about the lungs.
(a) Name the gas that passes from the alveoli into the blood. [1]
(b) Describe how oxygen passes from the air in an alveolus into the blood. [2]
(c) Explain three ways in which the alveoli are adapted for efficient gas exchange. [3]
(d) Describe how the actions of the diaphragm and the rib muscles cause air to be breathed in. [4]
(e) State one way in which the air breathed out differs from the air breathed in. [1]
Answer Details
This question is about gas exchange in the alveoli and the mechanism of breathing in.
(a) The gas that passes from the alveoli into the blood is oxygen [1].
(b) Oxygen passes into the blood by diffusion: it diffuses across the thin alveolus wall and the capillary wall [1]; moving down a concentration gradient from a high concentration in the air in the alveolus to a lower concentration in the blood [1].
(c) Any three adaptations of the alveoli [1 each, max 3]: a very large total surface area for diffusion; walls that are thin (one cell thick), giving a short diffusion distance; a good blood supply / many surrounding capillaries which removes oxygenated blood and maintains a steep concentration gradient; a moist lining so that gases can dissolve before diffusing.
(d) Breathing in (inspiration): the diaphragm muscle contracts and flattens / moves downwards [1]; the external intercostal (rib) muscles contract and move the ribs upwards and outwards [1]; together these increase the volume of the thorax (chest) [1]; increasing the volume lowers the pressure inside the chest below atmospheric pressure, so air is pushed in from outside [1].
(e) Any one difference in exhaled air [1]: it contains more carbon dioxide / less oxygen / more water vapour (accept it is warmer) than inhaled air.
Exam tip: for part (d) examiners want the causal chain volume up then pressure down then air in; simply saying "the chest gets bigger" is not enough for full marks.
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