How many $O_2$ molecules can bind with a single molecule of $Hb$?

The major fraction of $CO_2$ released during cellular respiration is transported by the blood to the lung capillaries as:

$O_2$ binds with haemoglobin of $RBC$ to form

Oxy-haemoglobin dissociates into oxygen and deoxy-haemoglobin at

The following statements refer to the transportation of gases. Read them carefully and choose the correct option containing the incorrect statements.
$(i)$ After leaving the lungs, each litre of blood contains about $20 \ ml$ of oxygen.
$(ii)$ When $PCO_2$ rises, the affinity of haemoglobin for oxygen is increased.
$(iii)$ Each litre of arterial blood contains about $4 \ ml$ of $CO_2$.
$(iv)$ The role of oxygen in the regulation of respiratory rhythm is quite significant.

The transport of carbon dioxide from tissues to the respiratory surface occurs primarily through ..........