The partial pressure of oxygen in alveoli of lungs is:
A. Equal to that in the blood
B. More than that in the blood
C. Less than that in blood
D. Less than that of carbon dioxide

Hint: Lungs are the organ that helps in breathing and exchange of gases such as carbon dioxide and oxygen. The exchange of gases between lungs alveoli and pulmonary capillaries is known as external respiration. The alveolar wall is very thin and consists of a network of blood capillaries. Because of this, the alveolar wall seems to be a sheet of flowing blood and is called the respiratory membrane.

Complete Answer:
- The respiratory membrane has a limit of gaseous exchange between alveoli and pulmonary blood. This is known as its diffusion capacity. The diffusing capacity may be defined as the volume of gas that diffuses through the membrane per minute for a pressure difference of 1mm of Hg.
- The diffusion capacity has a further dependency on the solubility of the diffusing gases. In the other words, at the particular pressure difference, the diffusion of carbon dioxide is 20 times faster than oxygen and the diffusion of oxygen is two times faster than nitrogen. The partial pressure of oxygen in the alveoli is higher than that in the deoxygenated than that in the deoxygenated blood in the capillaries of the pulmonary arteries.

Therefore the correct answer is B.

Note: The alveoli is the part of the lung which is an essential part of the respiratory system. The wall of alveoli is very thin and has a network of blood capillaries. The partial pressure of oxygen is higher i.e. 104 mm of Hg than that in the deoxygenated blood in the capillaries of the pulmonary arteries which is equal to 95 mm of Hg. As the gases are diffusing out from the higher to a lower concentration, the migration of oxygen is from alveoli to the blood.