Question

The partial pressure of oxygen is maximum in
a. Alveolar air
b. Arterial blood
c. Venous blood
d. Expired air

Answer 1

Hint: Partial pressure is the most important criteria in understanding the movement of the gases. When two systems are connected together, gas moves from the system having higher partial pressure to the system having lower partial pressure to equalize the pressure between them. Also, the movement of gases will be faster if the difference between the partial pressure of the systems is more.

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 such as nitrogen, oxygen, carbon dioxide, etc. and each gas possesses a partial pressure independent of the other gases present. The sum of the partial pressures of all the gases is known as Total pressure.

According to the partial pressure they exert, gases dissolve, diffuse and react. During respiration, the partial pressure of oxygen sets the range for the amount of oxygen that is required and the amount that can be toxic. Therefore, the oxygen concentration is different at different regions.

The partial pressure of oxygen in the alveoli (alveolar air) is approximately 104 mmHg, whereas the partial pressure of oxygen in the venous blood is 40-50 mmHg. The partial pressure of oxygen in the tissue is also very low, about 40 mm Hg, and in the arterial blood is about 95-100 mmHg. Expired air has the maximum partial pressure of 116 mmHg because during exhalation, the extra oxygen that could not be inspired earlier also moves out making its partial pressure more.

Hence, the correct answer is option (D).

Additional information:
Partial pressure of different gases in the blood is called Blood gas tension. During respiration, gases are exchanged between the atmosphere and the body. Air moves inside the body during inhalation and reaches lungs. Through bronchi, air reaches alveoli and the gas exchange occurs at the alveoli. Oxygen enters the bloodstream and carbon dioxide moves out. Carbon dioxide is then removed from the body through exhalation.

The gases behave differently when they come in contact with blood. According to Henry's Law, the concentration of a gas in a liquid (blood) is directly proportional to the partial pressure of the gas and its solubility in the liquid.

Note: As mentioned above, the partial pressure of oxygen in the alveolar air is 104 mmHg, whereas in venous blood it is 40-50 mmHg. This huge difference causes a high pressure gradient and as we know, gas moves from an area of high partial pressure to the area of low partial pressure even faster when the gradient is more. Thus, oxygen moves very fast across the membrane from the alveoli to the blood.