Which statement about oxygen transport is true?

• A. Most oxygen is carried bound to hemoglobin in red blood cells.
• B. Most oxygen is dissolved in plasma.
• C. Hemoglobin carries carbon dioxide primarily.
• D. Oxygen is not transported by blood.

Answer: (A)

Oxygen transport in the blood occurs mainly by binding to hemoglobin inside red blood cells, with only a small portion dissolved in plasma. Hemoglobin has four heme sites, each capable of binding one O2 molecule, so most oxygen carried in the blood is in the form oxyhemoglobin. This binding massively increases the amount of oxygen that can be transported because dissolved O2 in plasma is limited by its low solubility. In arterial blood, the vast majority of O2 is bound to hemoglobin (roughly 97–98%), with only a tiny fraction dissolved in plasma. As blood flows to tissues where PO2 is lower, oxygen is released from hemoglobin and then diffuses into cells to support metabolism. The other statements are inconsistent with how oxygen actually travels in the bloodstream; most oxygen is not dissolved in plasma, carbon dioxide is not carried primarily by hemoglobin, and oxygen is indeed transported by blood.