What is defined as a tiny blood vessel where food, oxygen, and other substances pass into the cells?

The definition of a tiny blood vessel where food, oxygen, and other substances pass into the cells refers specifically to capillaries. Capillaries are the smallest blood vessels in the body and are integral to the exchange of materials between the bloodstream and the tissues. Their thin walls allow for the diffusion of oxygen, nutrients, and other essential substances into nearby cells while simultaneously facilitating the removal of waste products, such as carbon dioxide.

In contrast, veins and arteries serve different functions in the circulatory system. Arteries carry oxygen-rich blood away from the heart to the body, while veins return deoxygenated blood back to the heart. Venules are small vessels that collect blood from capillaries and transport it to veins, but they do not function at the level of nutrient and gas exchange like capillaries do. Thus, the unique structure and function of capillaries make them the correct answer to the question regarding the tiny blood vessels responsible for nutrient and gas exchange with cells.