Question

: When $P_50$ value in blood rises, the affinity of respiratory pigment to combine with $O_2$ will-
A) Remain same
B) Rise
C) Fall
D) First rise and then fall

Answer 1

Hint: Respiratory Pigment is haemoglobin. Oxygen combines with haemoglobin in the blood and is transported as oxyhaemoglobin. The maximum amount (97%) of oxygen is transported by this method.

Complete answer:
Oxygen combines with haemoglobin only as a physical combination. Haemoglobin accepts the oxygen molecule whenever the partial pressure of oxygen in the blood is more. Haemoglobin gives out the oxygen molecule whenever the partial pressure of oxygen in the blood is less. $P_50$ is the partial pressure of oxygen at which haemoglobin saturation (haemoglobin combines with oxygen molecule) with oxygen is 50%. When the partial pressure of oxygen is 25 to 27 mm Hg, the haemoglobin is saturated by about 50%. That is, the blood contains 50% oxygen. If the value increases from 25mm Hg then it will lead to dissociation of oxygen from haemoglobin.So, When the $P_50$ value in the blood rises, the affinity of respiratory pigment to combine with $O_2$ will fall.

Hence the correct answer is option ‘B’ i.e, Rise.

Note:Oxygen combines with the iron in the heme part of haemoglobin. Each molecule of haemoglobin contains 4 atoms of iron and each iron atom combines with one molecule of oxygen making it 8 oxygen atoms combined with one haemoglobin molecule.