What it means
A vein is a vessel returning blood towards the heart. Veins are thin-walled and operate at low pressure, relying on surrounding muscle contraction and on one-way valves to move blood upwards against gravity.
Most veins carry deoxygenated blood; the pulmonary veins are the exception. The smallest are venules.
Venous disease characteristically causes problems of drainage — swelling, congestion and clot — rather than the supply failure seen in arterial disease.
In practice
Blood samples are taken from veins, usually in the antecubital fossa at the elbow, and intravenous fluids and medicines are given into them.
Failure of the valves produces varicose veins; clot within a deep vein is a deep vein thrombosis.
Why it matters
Because venous blood returns to the heart and then the lungs, a clot dislodging from a leg vein travels to the pulmonary circulation.
This is why a swollen painful calf and sudden breathlessness are treated as related emergencies.
Clinically reviewed by Dr Mohammad Zubair Khan, GMC 7563469
Last reviewed
August 30, 2026