What do erythrocytes primarily transport?

• A. Glucose and amino acids only.
• B. Oxygen and carbon dioxide.
• C. Hormones.
• D. Immune complexes.

Answer: (B)

Erythrocytes are specialized for carrying respiratory gases that cells need for energy production. Oxygen is picked up in the lungs, binds reversibly to hemoglobin, and is released in tissues where it’s needed. Carbon dioxide, a waste product of metabolism, travels from tissues back to the lungs to be exhaled. Hemoglobin helps with CO2 transport in two main ways: some CO2 binds directly to hemoglobin as carbaminohemoglobin, and the majority is converted inside red cells to bicarbonate by carbonic anhydrase and then released into plasma; the chloride shift helps maintain charge balance as this exchange happens. In the lungs, the process reverses so CO2 is expelled and O2 binding increases again for the next round of circulation. Other molecules mentioned, like glucose, amino acids, hormones, and immune complexes, are carried by plasma or specialized carriers rather than primarily by red blood cells.