Which of the following is NOT classified as a blood vessel?

The correct answer, which is ganglion, is not classified as a blood vessel because it refers to a cluster of nerve cell bodies. Ganglia are part of the nervous system and serve as relay points for nerve signals, connecting different pathways and facilitating communication between the brain and various parts of the body.

In contrast, veins, capillaries, and arteries are all integral components of the circulatory system responsible for the transport of blood throughout the body. Veins carry deoxygenated blood back to the heart, arteries transport oxygenated blood away from the heart, and capillaries are the smallest blood vessels where the exchange of oxygen, carbon dioxide, nutrients, and waste occurs between blood and tissues. Consequently, the classification of ganglion as a nerve structure firmly establishes it outside of the category of blood vessels.