Which is not a type of blood vessel

There are three kinds of blood vessels: arteries, veins, and capillaries. Each of these plays a very specific role in the circulation process. Arteries carry oxygenated blood away from the heart. They're tough on the outside but they contain a smooth interior layer of epithelial cells that allows blood to flow easily

Explanation:Tunica interna

The tunica interna (innermost layer) is a single layer of squamous (flat) epithelial cells called the endothelium; this smooth lining in direct contact with the blood offers little resistance to blood flow (Marieb and Hoehn, 2015). The endothelial cells can easily be damaged by hypertension, toxins such as cigarette smoke, or hyperglycaemia; this damage can result in atherosclerosis. These delicate cells rest on a thin layer of connective tissue made of elastin and collagen (elastic and structural support fibres) that anchors the tunica interna to the tunica media. The endothelium regulates blood flow and prevents clotting; it produces chemicals such as nitric oxide that help regulate blood flow by relaxing the smooth muscle within blood vessels.

Tunica media

The tunica media (middle layer) takes up most of the arterial vessel wall and is composed of smooth muscle fibres and elastin. This is where an activated sympathetic nervous system can stimulate the smooth muscle fibres to contract, provoking blood vessel narrowing (vasoconstriction) and decreasing blood flow (Marieb and Hoehn, 2015). When the sympathetic nerves are inhibited, the muscle fibres of the tunica media relax, the blood vessels increase in diameter (vasodilation) and blood flow increases.

Tunica externa

The tunica externa (outer layer) consists mainly of connective tissue fibres that protect the blood vessels and attach them to any surrounding tissues. In larger blood vessels, additional small vessels – vasa vasorum – supply blood and nutrients to the tunica externa and tunica media.