Which of the following is NOT a cellular constituent of blood?

Blood consists of both cellular and non-cellular components. The cellular constituents of blood include red blood cells, white blood cells, and platelets, all of which play crucial roles in various physiological functions.

Red blood cells are primarily responsible for transporting oxygen from the lungs to the rest of the body and carbon dioxide back to the lungs for exhalation. White blood cells are essential for the immune system, providing defense against pathogens and foreign substances. Platelets play a key role in blood clotting, helping to prevent excessive bleeding when injury occurs.

Hormones, on the other hand, are not cellular constituents of blood; rather, they are chemical messengers secreted by glands and travel through the bloodstream to regulate various bodily functions. They influence processes such as metabolism, growth, immune responses, and mood, but do not have a cellular structure like the other components mentioned in the question. This distinction makes hormones the correct choice as it explicitly states a component that is not part of the cellular makeup of blood.