Which of the following structures has the lowest blood pressure?

The structure that has the lowest blood pressure among the given options is the veins. This is due to the function and anatomy of the circulatory system.

Veins are responsible for returning deoxygenated blood back to the heart after it has circulated through the body. Unlike arteries, which are designed to handle high-pressure blood flow directly from the heart, veins operate under much lower pressure. This lower pressure is necessary because veins have thinner walls and larger diameters, which makes them more compliant and allows them to accommodate larger volumes of blood without the need for high pressure. Furthermore, veins contain one-way valves that help prevent the backflow of blood, ensuring efficient return to the heart without the need for high pressure.

In contrast, arteries, including the aorta, are designed to withstand and maintain high pressure as they carry oxygen-rich blood away from the heart to various parts of the body. Capillaries, while they do have lower pressure compared to arteries, still do not reach the low levels found in veins, as they facilitate the exchange of gases and nutrients at a controlled pressure to optimize diffusion.

This physiological design illustrates the vital role veins play in the circulatory system, relying on low pressure to efficiently collect and return blood to the heart.