Question

Which of the following statements is not true?
The partial pressure of
A. Oxygen in deoxygenated blood is $40$ mm Hg
B. Oxygen in deoxygenated blood is $95$ mm Hg
C. Oxygen in alveolar air is $104$ mm Hg
D. Carbon dioxide in the alveolar air is $40$ mm Hg

Answer 1

Firstly we need to know about oxygenated blood and deoxygenated blood. The oxygenated blood is the blood that is brought back to the heart by the pulmonary veins which enter the left atrium of our heart. From the left atrium this blood flows into the left ventricle. The left ventricle pumps the blood to the aorta and then the aorta will distribute the oxygenated blood to all parts of our body.

Complete answer:
The interchange of gases between blood and tissues is influenced by the partial pressure of gases and in oxygenated blood the partial pressure of oxygen is $95$ mm Hg. In deoxygenated blood the partial pressure of oxygen is $40$ mm Hg whereas in alveolar air the partial pressure of oxygen is $104$ mm Hg. The corresponding standards for carbon dioxide are$40$, $40$ and $46$ mm of Hg. As carbon dioxide is more soluble in water a slighter difference in partial pressure is enough to cause a larger exchange of the gas as compared to oxygen.

So, the correct answer is option B “Oxygen in deoxygenated blood is $95$ mm Hg”.

Additional Information:
Option A - For the deoxygenated blood of the pulmonary arteries the oxygen pressure is $40$ mm Hg.
So, option A is a true option. So, this option is not the correct answer.
Option C - Oxygen in alveolar air is $104$ mm Hg: In alveolar air the partial pressure of oxygen is about $104$ mm Hg.
So, option C is a true option. So, this option is not the correct answer.
Option D - Carbon dioxide in the alveolar air is $40$ mm Hg: The partial pressure of $C{O_2}$ in the blood of the capillary is about $45$ mm Hg, while its partial pressure in the alveoli is about $40$ mm Hg.
So, option D is a true option. So, this option is not the correct answer.

Note:
Partial pressure of $C{O_2}$ in alveolar air is $40$ mm Hg and its value in deoxygenated blood is $45$ mm Hg. So, $C{O_2}$ moves from arterial blood to alveoli. Like this, according to partial pressure gaseous exchange takes place. $C{O_2}$ is more soluble than ${O_2}$.