What essential gas do red blood cells deliver to body tissues?

Red blood cells are primarily responsible for transporting oxygen from the lungs to various tissues throughout the body. This essential gas is critical for cellular respiration, which is the process by which cells generate energy. Hemoglobin, the main protein found in red blood cells, binds with oxygen in the lungs and carries it through the bloodstream to the tissues that need it. Once it reaches the tissues, oxygen is released, enabling cells to produce energy efficiently.

While carbon dioxide is a byproduct of metabolism that red blood cells also transport back to the lungs for exhalation, it is not the gas delivered to the body tissues. Nitrogen and hydrogen, although they are present in the body, do not play the same critical role in energy production and metabolism as oxygen does. Therefore, the primary purpose of red blood cells in this context is to supply oxygen, making it the correct answer.