Final answer:
The steepness of the oxygen dissociation curve when PO₂ increases from 20 to 40 is due to hemoglobin saturation, with small increases in PO₂ leading to large increases in oxygen binding until hemoglobin approaches saturation.
Step-by-step explanation:
The question relates to the oxygen dissociation curve in hemoglobin, which is a graphical representation showing how the saturation of hemoglobin with oxygen varies with changes in PO₂, or partial pressure of oxygen. When the PO₂ increases from 20 to 40, the graph becomes very dynamic and steep because at these levels, small changes in PO₂ lead to large changes in hemoglobin saturation due to its cooperative binding nature. Initially, as the first few oxygen molecules bind to hemoglobin, it increases the affinity of hemoglobin for oxygen, making it easier for additional oxygen to bind. This results in a steep section of the curve. However, once hemoglobin is nearly saturated, its affinity for oxygen decreases, and the curve levels off, indicating it is harder to bind more oxygen molecules.
The correct answer to the question why going from a PO₂ of 20 to 40 is very dynamic and steep on a graph is because of hemoglobin saturation (Option A). This part of the oxygen dissociation curve shows a sigmoidal or S-shaped pattern due to hemoglobin's changing affinity for oxygen as more oxygen molecules bind.