Question

What will be the $p{O_2}$and $pC{O_2}$ in the atmospheric air compared to those in alveolar air?
a. $p{O_2}$ lesser, $pC{O_2}$ higher
b. $p{O_2}$ higher, $pC{O_2}$ lesser
c. $p{O_2}$ higher, $pC{O_2}$ higher
d. $p{O_2}$ lesser, $pC{O_2}$ lesser

Answer 1

Hint: Partial pressure is the most important criteria for the movement of the gases. When two systems are joined with each other, gas transmits from the system having higher partial pressure to the system having lower partial pressure to equalize the pressure among them.

Complete answer:
Partial pressure is defined as the pressure of a gas in a mixture of gases. For example in our atmosphere many gases are present like carbon dioxide, oxygen etc and each gas possesses a partial pressure independent of other gases present. The sum of the partial pressures of all the gases is known as the total pressure.

Oxygen in the air will have higher partial pressure or concentration than in the alveolar air. Hence oxygen diffuses from the alveoli to blood by the help of alveolar epithelium and capillary endothelium. It passes first into the blood plasma and then combines with the haemoglobin to form oxyhaemoglobin.

Carbon dioxide in the lung capillaries has a higher partial pressure or concentration than the alveoli but the partial pressure of carbon dioxide will be less in air than the alveoli.
The normal composition of the alveolar air is determined by the balance between ventilation and perfusion to gases. Ventilation helps in the refreshing of the air in alveolar space with the external air taken by 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 perfusion.

The partial oxygen in the atmospheric air is $159$mmHg whereas in the alveolar air it is around $109$mmHg. The partial carbon dioxide in the atmospheric air is around $0.3$mmHg whereas in the alveolar air the partial carbon dioxide is $40$mmHg.

Therefore the partial oxygen is more in atmospheric air compared to alveolar air whereas partial carbon in the atmospheric air is less than the alveolar air.
Hence, the correct answer is option (B).

Note: According to the Henry law the concentration of gas in a liquid is directly proportional to the partial pressure and solubility of the gas. Nitrogen is mostly found in the atmosphere and a great amount of it stays in the alveoli. Hence the relative concentration of gases in alveolar air is nitrogen, oxygen, water vapour and carbon dioxide.