a smoker develops damage to several alveoli that can no longer function. how does this affect gas exchange?

Smoking causes the hardening of the alveoli walls. This keeps them from extending or stretching out because of the loss of elasticity. In such a circumstance, it is hard to take up oxygen from the inspired air or to remove carbon dioxide from the circulation system. This would prompt partial or less trapping of the gases in the lungs.  

With this damage due to smoking, the alveoli can not assist the bronchial tubes enough. The tubes also get damaged and cause a blockage or obstruction, which traps air inside the lungs. An excess of air caught in the lungs can give a few patients a barrel-chested appearance.

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