What is oxyhemoglobin?

• A. Hemoglobin bound to O2, up to four molecules, bright red.
• B. Hemoglobin without oxygen bound (deoxyhemoglobin).
• C. A protein that carries carbon dioxide in plasma.
• D. A deficiency of red blood cells.

Answer: (A)

Oxyhemoglobin is the form of hemoglobin in which oxygen is bound to the heme groups. Each hemoglobin molecule can carry up to four oxygen molecules, and when oxygen is bound in the lungs, the molecule becomes oxyhemoglobin. This binding gives arterial blood its bright red color. As blood delivers oxygen to tissues, the oxygen is released and hemoglobin shifts to the deoxygenated form, which is a darker color. The idea that oxyhemoglobin is simply Hb bound to O2 up to four molecules and that this state gives bright red blood captures the key concept. Other choices describe related but different concepts, such as carbon dioxide transport or red blood cell deficiency, which do not define oxyhemoglobin itself.