Hemoglobin is classified as a ___________ that binds O2 ___________ in the lungs.

Hemoglobin is classified as a tetramer because it is composed of four polypeptide subunits, typically two alpha and two beta chains. This quaternary structure allows hemoglobin to efficiently bind oxygen (O2) molecules.

In the lungs, hemoglobin binds O2 tightly due to its high affinity for oxygen when the pO2 is high, such as in the alveoli where oxygen is plentiful. This tight binding ensures efficient loading of oxygen onto hemoglobin for transport to tissues throughout the body. The cooperative binding mechanism of hemoglobin also enhances this affinity; as one O2 molecule binds, the conformation of hemoglobin changes, increasing the likelihood of additional O2 binding.

This combination of tetrameric structure and tight binding in the lungs is essential for effective oxygen transport and delivery, making this classification accurate for hemoglobin's function and behavior in the respiratory system.