Hemoglobin is made up of four symmetrical subunits and four heme groups. Iron associated with the heme binds oxygen. It is the iron in hemoglobin that gives blood its red color.

Where does hemoglobin bind oxygen and where does it release oxygen?

Hemoglobin with bound carbon dioxide and hydrogen ions is carried in the blood back to the lungs, where it releases the hydrogen ions and carbon dioxide and rebinds oxygen.

Does oxygen bind to hemoglobin at high pH?

In 1904, Danish scientist Christian Bohr noticed that hemoglobin binds oxygen more tightly at high pH than it does at low pH. ... As pH rises, hemoglobin loses hydrogen ions from specific amino acids at key sites in its structure, and this causes a subtle change in its structure that enhances its ability to bind oxygen.