Question

Direction: Refer to the given graph and select the correct options for the questions that follow.

During strenuous exercise, the muscle interstitial fluid P$O_2$ falls to 20 mm Hg. The oxygen delivered by the blood that passes through the exercising muscle tissues will be

• A. Five times as much as normal
• B. Double to the normal
• C. Three times as much as normal
• D. None of these.

Answer 1

The correct option is C Three times as much as normal

The given graph shows the relationship between the partial pressure of oxygen with the amount of oxygen that can bind with haemoglobin. The P$O_2$ of the air within alveoli is about 100 mm Hg. As the blood travels through the systemic blood capillaries, oxygen leaves the blood and diffuses into the tissues. Consequently, the bipod that leaves the tissues in the veins has a P$O_2$ that is decreasing (in a resting person) to about 40 mm Hg. Therefore, A depicts oxygenated blood and B depicts systemic blood. The steep portion of the curve from 60 down to 20 mm Hg is ideal for unloading oxygen in the tissue, Le., for a small decrease in P$O_2$ (due to diffusion of oxygen) a large quantity of oxygen can be unloaded to the peripheral tissue capillary. During strenuous exercise, the muscles accelerated metabolism uses more oxygen from the capillary blood and thus decreases the venous blood P$O_2$, for example, the P$O_2$ of the venous blood could drop to 20 mm Hg. Hence, D, E and F show venous blood in exercise, normal venous blood and normal arterial blood respectively. As we know, in the resting person, the P$O_2$ of tissue is 40 mm Hg, which is favourable for oxygen diffusion from arterial blood to the tissues. Oxygen unloaded to the tissues by arterial blood is 5 mL/100 mL of blood (20 mL- 15 mL = 5mL/100mL, from the graph). During strenuous exercise, the muscle interstitial fluid P$O_2$ falls to 20 mm Hg, so the oxygen that unloads to the tissue Is 15 mL/100 ml of blood, (20 - 5) ml/100 mL = 15 mL/100mL, from the graph i.e., three times as much as normal.