What organs make up the circulatory system?

The circulatory system, responsible for transporting blood, nutrients, gases, and waste products throughout the body, is primarily composed of the heart and blood vessels. The heart functions as the central pump, propelling oxygenated blood to various tissues and returning deoxygenated blood to the lungs for oxygenation. Blood vessels, which include arteries, veins, and capillaries, form a vast network that facilitates this transport. Arteries carry blood away from the heart, while veins bring blood back. Capillaries, the smallest vessels, enable the exchange of gases and nutrients at the cellular level.

In contrast, the other choices represent systems or organs that do not comprise the circulatory system. For example, the lungs and trachea are part of the respiratory system, responsible for gas exchange. The brain and spinal cord make up the central nervous system, facilitating nerve function and communication throughout the body. The kidneys and liver are integral to the excretory and digestive systems, respectively, each playing a role in waste removal and metabolism, but they do not constitute components of the circulatory system. Therefore, understanding the core elements of the circulatory system clarifies why the combination of the heart and blood vessels is the correct answer.