Vein

A vein is a blood vessel that carries blood toward the heart.

Most veins carry oxygen-poor blood, but the pulmonary veins carry oxygen-rich blood from the lungs to the heart. Veins generally have thinner, less muscular walls than arteries and can contain valves that help prevent backward flow of blood, particularly in the limbs.

Veins collect blood from smaller vessels and return it to the heart.

QUICK FACT

Like arteries, veins are defined by the direction of blood flow, not by oxygen content.

COMMONLY CONFUSED WITH

SEE ALSO

Clinical & Educational Context

Discussed in Health Articles & Guides

Explore how "Vein" is covered across our evidence-based editorial guides:

1 Article
The Hard Parts of Medicine

Polypharmacy: What Really Happens When You Take Multiple Medicines

Polypharmacy, the use of multiple medicines by one person is becoming increasingly common as people live longer and more people...