Select the best answer or answers from the choices given: Most oxygen carried in the blood is (a) in solution in the plasma, (b) combined with plasma proteins, (c) chemically combined with the heme in red blood cells, (d) in solution in the red blood cells.
The Correct Answer and Explanation is:
The best answer is:
(c) chemically combined with the heme in red blood cells
Explanation:
Oxygen transport in the blood occurs mainly through two mechanisms: oxygen dissolved in plasma and oxygen bound to hemoglobin inside red blood cells.
- Oxygen in Plasma (a and b):
Only a small fraction of oxygen is dissolved directly in the plasma. Oxygen is relatively insoluble in water (and plasma is mostly water), so the amount of oxygen dissolved in plasma is minimal—about 1.5% to 3% of total oxygen content in the blood. This dissolved oxygen is important because it determines the partial pressure of oxygen (pO2) and drives oxygen diffusion into tissues, but it does not carry the bulk of oxygen for transport.
Oxygen does not combine with plasma proteins (b). Some gases like carbon dioxide can bind to plasma proteins, but oxygen’s main transport form is not bound to plasma proteins.
- Oxygen Bound to Hemoglobin (c):
The majority—approximately 97%—of oxygen in the blood is chemically bound to hemoglobin molecules inside red blood cells. Hemoglobin is a complex protein with four heme groups; each heme contains an iron (Fe2+) ion that can reversibly bind one oxygen molecule (O2). This binding is cooperative, meaning once one oxygen binds, hemoglobin’s affinity for oxygen increases, allowing it to pick up more oxygen efficiently in the lungs.
This reversible binding allows hemoglobin to pick up oxygen in the lungs, transport it through the bloodstream, and release it into tissues where oxygen levels are lower. This is why hemoglobin-bound oxygen is critical for meeting the metabolic needs of the body.
- Oxygen in Solution Inside Red Blood Cells (d):
While there is oxygen dissolved inside the red blood cells in the cytoplasm, this amount is also very small compared to that bound to hemoglobin. It’s effectively the same principle as oxygen dissolved in plasma, just inside the cell, and not the primary mode of oxygen transport.
Summary:
- Most oxygen is transported chemically combined with hemoglobin in red blood cells (answer c).
- A very small amount is dissolved in plasma or inside red blood cells.
- Oxygen does not bind significantly to plasma proteins.
This is fundamental to understanding oxygen delivery in physiology and how diseases like anemia or carbon monoxide poisoning affect oxygen transport.