Most of the oxygen transported by the blood is:

• A. dissolved directly in plasma
• B. attached to white blood cells
• C. carried as bicarbonate ions
• D. bound to hemoglobin in red blood cells

Answer: (D)

Verified step by step guidance

1. Understand the role of hemoglobin in oxygen transport: Hemoglobin is a protein found in red blood cells that has a high affinity for oxygen. It binds oxygen molecules in the lungs and releases them in tissues where oxygen is needed.
2. Recognize why oxygen is not primarily dissolved in plasma: Oxygen has low solubility in plasma, meaning only a small amount can be transported this way. Most oxygen needs a carrier for efficient transport.
3. Eliminate the incorrect options: White blood cells do not play a role in oxygen transport, and while bicarbonate ions are involved in carbon dioxide transport, they are not the primary method for oxygen transport.
4. Focus on the correct mechanism: Oxygen binds to the iron-containing heme groups in hemoglobin molecules within red blood cells. This binding forms oxyhemoglobin, which is the primary way oxygen is transported in the blood.
5. Conclude that the majority of oxygen in the blood is transported bound to hemoglobin in red blood cells, as this mechanism is efficient and supports the body's oxygen demands.