Arteries and veins (also called blood vessels) are tubes of muscle that your blood flows through. Arteries carry blood away from the heart to the rest of the body. Veins push blood back to your heart. You have a complex system of connecting veins and arteries throughout your body. Blood vessels are small tube-like structures that transport blood inside your body. How do you know if a blood vessel is an artery, vein, or capillary? Several characteristics can help you distinguish between the three.
Illustration on an artery wall
The wall of an artery consists of three layers. The innermost layer, the tunica intima (also called tunica interna), is simple squamous epithelium surrounded by a connective tissue basement membrane with elastic fibers. The middle layer, the tunica media, is primarily smooth muscle and is usually the thickest layer. It not only provides support for the vessel but also changes vessel diameter to regulate blood flow and blood pressure. The outermost layer, which attaches the vessel to the surrounding tissue, is the tunica externa or tunica adventitia. This layer is connective tissue with varying amounts of elastic and collagenous fibers. The connective tissue in this layer is quite dense where it is adjacent to the tunic media, but it changes to loose connective tissue near the periphery of the vessel

Your largest veins are the superior and inferior vena cava. Your superior vena cava carries blood from your upper body to the heart. Your inferior vena cava carries blood from everywhere below your heart. Like arteries, these two veins branch off into many other veins throughout your body.

Unlike arteries, veins generally need to work against gravity to push blood back to your heart. Veins have valves to help with this. These are one-way pairs of flaps inside a vein. They open for blood that’s heading upwards toward the heart, and close to keep blood from flowing back downwards.‌

Muscle surrounds most veins in your body. When you walk, run, or otherwise use your muscles, they make a squeezing motion. These squeezes push against the vein and force the blood upwards toward your heart.

1. Pulmonary Veins: These veins carry oxygenated blood from the lungs to the heart. They are unique because most veins carry deoxygenated blood, but pulmonary veins are an exception.
2. Systemic Veins: These veins carry deoxygenated blood from the body back to the heart. Systemic veins include major veins like the superior and inferior vena cava, which return blood from the upper and lower body, respectively.

Medically Reviewed by James Beckerman, MD, FACC on February 14, 2024Written by WebMD Editorial Contributors

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