Question

A large proportion of oxygen remains unused in the human blood even after it’s uptake by the
body tissues. This Oxygen?
A. acts as a reserve during muscular exercise
B. raises the carbon dioxide of blood to 75 mm of Hg
C. is enough to oxyhemoglobin saturation at 95%
D. helps in releasing more oxygen to the epithelial tissues.

Answer 1

Hint:- The exchange of gases that occur at a low level of the body tissues is called internal
respiration, and they also occur as simple diffusion, due to partial pressure gradient, whereas internal
respiration is similar to external respiration which occurs simply as diffusion.

Complete step-by-step solution:-
1. In tissues, generally, the partial pressure of oxygen is low, which is about the 40mm of hg, this is
because in the tissues oxygen is used continuously for cellular respiration.
2. In the blood, it is a contrast to the tissues, where the partial pressure of oxygen is about 95mm
of Hg.
3. This difference in the partial pressure gradient of oxygen in the tissue and blood causes the
oxygen to dissociate from hemoglobin and it diffuses out of the blood and it crosses the
interstitial space and they enter the tissues.
4. This is the reason why the blood that returns from the tissues to the heart and lungs loss its
brightness and appears a little dull.
5. But tissues utilize only 25% of the oxygen that is obtained from the blood. And about 75% of oxygen
in the blood is not utilized by the tissues, instead, they act as reserves to use during muscle
contraction.
6. At the same time in the tissues, cellular respiration continuously produces carbon dioxide, so
The carbon dioxide is more in tissues than in the blood.
7. The pressure difference of carbon dioxide in blood and tissues cause the diffusion of carbon
dioxide out of the tissues and enter the blood which is carried back to the lungs, and it is carried
either in dissolved plasma or in the converted form.
8. So by the time blood returns to the heart and the lungs partial pressure of oxygen in the blood
falls to 40mm of hg, while the partial pressure of Carbon dioxide has returned to 45mm of Hg.
So the correct option is A.

Note:- To simply say that exchange of gases in the body between the tissues and blood mainly depends on the pressure gradient, and the blood is always oxygenated in the lungs, so the oxygenated blood has more oxygen which diffuses to less oxygen gradient tissues, the tissues utilize only some percent of o2 only, where most of it reserved for future use.