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Human Body Study

Veins

The vessels that carry blood back toward the heart.

The closed-loop circulation A four-chambered heart at the centre, with a pulmonary loop to the lungs above and a systemic loop to a body capillary bed below; red vessels carry oxygen-rich blood, blue vessels oxygen-poor. A separate inset shows blood at a vessel cross-section. Each label links to the article for that part. Blood RA LA RV LV Heart Arteries Capillaries Veins The closed-loop circulation — schematic, not to scale.

Structure

Section titled “Structure”

Thinner, less muscular walls than arteries, and a wider channel; many contain one-way valves. Small venules collect blood from the capillaries and merge into veins.

What they do

Section titled “What they do”
  • Return blood to the heart, including against gravity from the lower body.
  • Serve as a reservoir — most of the body’s blood sits in the veins at any moment.

How blood moves

Section titled “How blood moves”

Pressure here is low, so flow relies on the valves plus the squeezing of nearby muscles to push blood upward.

Key idea

Section titled “Key idea”

Veins are low-pressure return vessels and the body’s main blood reservoir; their valves prevent backflow.