What is transported by blood but is regulated by body functions?

The correct answer emphasizes the regulation of nutrient levels in the bloodstream by various physiological processes. Nutrients, such as glucose, amino acids, and lipids, are absorbed from the digestive system and transported through the blood to provide essential energy and building blocks for cells. The regulation of these nutrients is vital for maintaining homeostasis.

For instance, blood glucose levels are tightly regulated by hormones like insulin and glucagon, facilitating the storage or release of glucose depending on the body's needs. Similarly, the levels of amino acids and fatty acids in the blood can fluctuate based on dietary intake and metabolic activity.

Oxygen and carbon dioxide levels are crucial for respiration, but they are primarily regulated through respiratory processes rather than metabolic regulation typical of nutrients. While waste products are prevalent in the bloodstream and their levels can change based on metabolic activity, their transport does not involve intricate regulatory mechanisms akin to those seen with nutrients. Thus, the dynamic regulation of nutrient levels makes this option the most appropriate choice.