Final answer:
Normal hemoglobin has a higher affinity for oxygen at high partial pressures of oxygen and possesses a safety zone for oxygen binding to ensure high saturation even with slight drops in PO₂. It binds primarily to oxygen under normal conditions, but the presence of CO and CO₂ can influence binding. The O₂-hemoglobin dissociation curve is S-shaped, reflecting its changing affinity for oxygen with varying levels of saturation.
Step-by-step explanation:
The true statements regarding normal hemoglobin are that hemoglobin indeed has a higher affinity for oxygen when the partial pressure of oxygen (PO₂) is high. Additionally, it has a sort of "safety zone" for oxygen binding, which ensures that even if the PO₂ decreases slightly from the normal level, hemoglobin remains highly saturated with oxygen. Under standard conditions, hemoglobin primarily binds to oxygen throughout the vascular system, although carbon monoxide (CO) and carbon dioxide (CO₂) can also bind under certain circumstances, with CO having a high affinity for hemoglobin.
The unique S-shaped or sigmoidal oxygen-hemoglobin dissociation curve illustrates this phenomenon. As the PO₂ increases, the affinity for O₂ initially increases, allowing hemoglobin to become progressively more saturated with oxygen. The curve shows that it's easier to bind the second and third oxygen molecules to hemoglobin than the first, which is due to the conformational change that occurs in the hemoglobin molecule as oxygen molecules bind. However, after a certain point as the percent saturation of hemoglobin increases at very high levels, its affinity for oxygen decreases due to its decreasing ability to bind additional oxygen molecules.