How is hemoglobin's affinity for oxygen affected by the presence or absence of oxygen?

Explanation:

Haemoglobin is a red protein found in red blood cells of vertebrates responsible for transporting oxygen in the blood. Each haemoglobin has capacity to carry four oxygen molecules. When oxygen is available or present, the molecules of oxygen bind to the iron of the heme prothestic group and it's tend to release oxygen when oxygen level is low in the environment. Haemoglobin carries oxygen in lungs and releases it to tissues.