Biology STAAR (State of Texas Assessments of Academic Readiness) Practice Test

The highest concentration of nutrients in the circulatory system is found in capillaries, which are the smallest blood vessels where the exchange of substances occurs. Capillaries are strategically located throughout the body, allowing them to connect arterioles (small arteries) and venules (small veins). Here, oxygen and nutrients are delivered to tissues, while carbon dioxide and metabolic waste are collected.

In the capillary network, the walls are thin, facilitating efficient diffusion of nutrients from the blood into the surrounding tissues. This close proximity allows for the high concentration of nutrients in the capillaries right after they receive blood from the digestive system, particularly following the absorption of nutrients in the intestines.

In contrast, while plasma does carry nutrients, it is diluted with water and other substances, making its concentration lower than in the capillaries. Red blood cells primarily transport oxygen and carbon dioxide, playing a limited role in nutrient transport. Veins, on the other hand, are primarily responsible for returning deoxygenated blood back to the heart and do not have the same concentration of nutrients as capillaries, where nutrient exchange occurs.