Question

What is the approximate normal composition of alveolar air?
(a) $14\%$ oxygen, $6\%$ carbon dioxide, $80\%$ nitrogen
(b) $21\%$ oxygen, $2\%$ carbon dioxide, $77\%$ nitrogen
(c) $16\%$ oxygen, $3\%$ carbon dioxide, $81\%$ nitrogen
(d) $10\%$ oxygen, $8\%$ carbon dioxide,$82\%$ nitrogen

Answer 1

Hint: The composition of gases in alveoli depends on several factors and two basic activities. The two activities are ventilation and perfusion. The solubility of gases in blood and its partial pressure also plays a role in this case.

Complete answer:
The approximate normal air composition of alveolar air has $14\%$ oxygen, $6\%$ carbon dioxide, and $80\%$ nitrogen in total. The oxygen level is usually higher than that of carbon dioxide but nitrogen is the highest in amount.
Therefore, the correct answer is $14\%$ oxygen, $6\%$ carbon dioxide, $80\%$ nitrogen.

Additional Information: The normal composition of the alveolar air is determined by the balance between ventilation and perfusion of gases. Ventilation is the process involving the refreshing of air in the alveolar space with the external fresh air taken by the nostrils. The process in which carbon dioxide diffuses out in the alveolar space in conjunction with oxygen diffusing into the blood from the alveolar space is known as perfusion. These two basic processes determine the composition of alveolar air.
Ventilation doesn't occur in bulk, instead, the structure of our lung allows bulk airflow to end at the terminal bronchioles. Upon inspiration, the air moves only up to the point of the terminal bronchiole,s and in expiration, only air within the terminal bronchiole and large bronchi are expelled from the lung. When ventilation increases the rate of fresh air delivered to the terminal bronchioles increases and the rate of carbon dioxide-rich air expelling out of the terminal bronchioles also increases. This modifies the composition of alveolar air so that the oxygen tension reduces and the carbon dioxide tension rises.
The perfusion also plays an important role in determining the composition of alveolar air as it controls the rate of carbon dioxide getting added to the alveolar space and oxygen carried away from it.
The normal composition of the alveolar air remains $14\%$ oxygen, $6\%$ carbon dioxide, and $80\%$ nitrogen.

Gase	Percentage of the total composition
Nitrogen (N2)	74.9
Oxygen (O2)	13.7
Water (H2O)	6.2
Carbon dioxide (CO2)	5.2

Note: According to Henry's law, we know that the concentration of a gas in a liquid is directly proportional to the partial pressure and solubility of the gas. Although nitrogen is the highest present gas in the atmosphere and a great amount of it stays in the alveoli, a very little amount of it dissolves in the blood. It is because it's solubility is very low. Hence, the relative concentration of gases in alveolar air is nitrogen > oxygen > water vapor> carbon dioxide.