01 / Location & anatomical relationships
In networks returning blood from tissues toward the heart.
02 / Structure & organization
Veins generally have thinner walls and larger lumens than comparable arteries. Their capacitance allows them to hold a substantial fraction of circulating blood. Limb valves and surrounding muscle activity help return blood against gravity.
03 / Function
Many veins have valves that help blood flow in one direction.
04 / Mechanism
Small veins collect blood from capillaries and join into larger vessels. In many parts of the body, valves and the squeezing of nearby muscles help blood return. The pulmonary veins bring oxygen-rich blood from the lungs.
Working together: circulatory system
Your heart pumps blood to your lungs to collect oxygen. Blood then delivers oxygen to body cells and carries carbon dioxide back to your lungs.
Meet this body system →FOR DEEPER STUDY
Connect structure with function
Veins are defined by direction toward the heart. Pulmonary veins carry oxygen-rich blood, demonstrating why oxygenation alone does not classify a vessel.
Check your understanding
Locate the vein in relation to its neighbors. Explain how the structural features described above support its function, then distinguish it from a nearby structure with a different role.
Keep learning
Further reading: NHLBI: How the Heart Works.
Continue with OpenStax Anatomy and Physiology for system-level study. This atlas is educational and has not undergone independent medical peer review.