Question

The atmospheric pressure is 760 mm Hg at the sea level. Which of the following ranges is nearest to the partial pressure of $C{O}_2$ in mmHg?
 A.0.30 -0.31
B.0.60 -0.61
C.3.0 – 3.1
D.6.0 - 6.1

Answer 1

Hint: Partial pressure of a gas is the pressure it exerts in a mixture of a gas and is equal to the total pressure of the mixture divided by the percentage of that gas in the mixture. For instance, if the pressure of atmospheric air at sea level is 760 mm of mercury and oxygen forms 21% of the air, the partial pressure of oxygen will be 21% of 760 mm Hg or 159 mm Hg. In other words the partial pressure of a gas is proportional to its concentration in the mixture.

Complete answer:
Oxygen in the alveoli has a higher partial pressure or concentration than that in the blood.
Hence, oxygen diffuses from the alveoli into the blood through the alveolar epithelium and capillary endothelium.It passes first into the blood plasma and then combines with the haemoglobin in the red blood corpuscles to form oxyhaemoglobin.
Carbon dioxide in the lung capillaries has a higher partial pressure or concentration than that in the alveoli. Therefore, carbon dioxide diffuses from the blood into the alveoli.
The air of the alveoli thus becomes foul and needs renewal.The alveolar capillaries are narrower than the red blood corpuscles so that the latter are squeezed through the capillaries by blood pressure.This exposes more of their surface area to the gaseous exchange surface of the alveolus, allowing greater uptake of oxygen.
When the blood leaves the alveolus it has almost the same partial pressure of carbon dioxide and oxygen as the alveolar air.

Gas	Inspired air	Alveolar air	Venous blood	Arterial blood	Expired air	Tissue cells
Oxygen	159	104	40	95	116	<40
Carbon dioxide	0.3	40	45	40	32	>45

So, the correct answer is option A. 0.3-0.31

Note:
The partial pressure of carbon dioxide of blood reaching the alveolar capillaries is higher than the partial carbon dioxide of alveolar air. Therefore, carbon dioxide diffuses from the blood of alveolar capillaries into the alveolar air.
Blood carries the oxygen from the respiratory organs to the tissue cells for oxidation, and carbon dioxide from the tissue cells to the respiratory surface for elimination. Exchange of gases take place in the alveoli.