Question

During strenuous exercise, the muscle interstitial fluid $PO_2$ falls to $20\, mm \,Hg$. The oxygen delivered by 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

Answer: (C)

The given graph shows the relationship between partial pressure of oxygen with the amount of oxygen that can bind with haemoglobin. The $PO_2$ 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 blood that leaves the tissues in the veins has a $PO_2$ 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, i.e., for a small decrease in $PO_2$ (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 $PO_2$ , for example the $PO_2$ of the venous blood could drop to $20\, mm \,Hg$. Hence, $D$, $E$ and $F$ shows venous blood in exercise, normal venous blood and normal arterial blood respectively.
As we know, in the resting person, the $PO_2$ 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 $PO_2$ falls to $20\, mm \,Hg$, so the oxygen that unload to the tissue is $15 \,mL/100 \,mL$ of blood, $(20 - 5)$ $mL/100mL = 15\, mL/100mL$, from the graph i.e., three times as much as normal.