The intima is the inner layer of the artery, which is lined with a smooth tissue called endothelium.

The media is a layer of muscle that allows arteries to handle the heart’s high pressures.

The adventitia is a connective tissue that connects arteries to surrounding tissues.

Tunica externa. This is the thinnest and most outer layer of the vein wall. Connective tissue makes up the majority of it. The tunica externa also contains vasa vasorum, which are microscopic blood vessels that feed blood to the vein walls.

Tunica media. The tunica medium is the layer in the middle. It’s really thin and has a lot of collagen in it. Collagen is a protein that is found in connective tissue.

Tunica intima. This is the most inner layer. A single layer of endothelial cells and connective tissue makes up this structure. One-way valves can be found in this layer, especially in the veins of your arms and legs. Blood cannot flow backward through these valves.