What is the difference between hemoglobin in arterial and venous blood?

Hemoglobin is required to carry oxygen and carbon dioxide. Hemoglobin has the ability to combine with oxygen due to the presence of iron in heme. Arterial blood has many bonds with oxygen molecules. As it flows through the body, these bonds with oxygen change to bonds with carbon dioxide. This is how venous blood is formed. Therefore, hemoglobin in arterial blood is saturated with oxygen, and in venous blood – with carbon dioxide. Because of this, venous blood is darker than arterial.