What critical function does hemoglobin serve in the body?

Hemoglobin plays a vital role in the body primarily as the protein in red blood cells responsible for transporting oxygen from the lungs to tissues and organs. Once oxygen is bound to hemoglobin, it is delivered to cells, where it is used for metabolic processes that generate energy. This function is crucial for maintaining cellular respiration and overall energy production in the body, as oxygen is essential for the conversion of nutrients into usable energy.

While the other listed functions (carrying nutrients, storing fat, and regulating body temperature) are important for overall health and body function, they do not pertain to the primary role of hemoglobin. Nutrient transport is primarily carried out by other components of the blood, such as plasma proteins. Fat storage is the function of adipose tissue, and body temperature regulation involves various physiological mechanisms, including blood flow and perspiration. Therefore, the key function related to hemoglobin specifically is its role in oxygen transport, which is why this choice is correct.