Composition of Alveolar Gas

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1) Explain why alveolar gas has a different composition from air. Atmospheric (external) air contains a high percentage of oxygen and nitrogen whereas alveoli gas contains a lower percentage of oxygen and a higher percentage of carbon dioxide. The differences in composition are due to the fact that gaseous exchange is taking place within the alveoli in the lungs. Oxygen is diffusing from the alveoli into the pulmonary artery and carbon dioxide is diffusing back into the alveoli from the pulmonary artery due to the concentration gradients. This is the main reason why external air and alveolar gas have such different compositions.

Another reason why the gas compositions are different is due to the fact that alveoli gas contains a mixture of both atmospheric air which has been inhaled and ‘old’ air which stays in the respiratory track and is not exhaled after each breath therefore giving it a different composition.

Alveolar gas has a larger percentage of water vapour than atmospheric air due to the fact that air is being warmed and moistened as it travels into the respiratory track. This is done by the mucus in the nasal cavity as the air travels past it.

|Gas |% Atmospheric Air |% Alveolar Air | |Nitrogen |78.6 |74.9 | |Oxygen |20.9 |13.7 | |Carbon Dioxide |0.04 |5.2 | |Water |0.46 |6.2 |

(Human anatomy and physiology 6th Ed, Marieb.)

2) Identify the reason for [A] myoglobin and [B] fetal haemoglobin having oxygen binding curves different from...
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