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Human Body Study

Capillaries

The smallest blood vessels — so narrow that red cells pass through in single file. They form vast networks linking the smallest arteries to the smallest veins.

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”

Walls just one cell thick — which is what makes them permeable enough for exchange.

What they do

Section titled “What they do”

This is where the cardiovascular system actually does its job: oxygen, nutrients, and hormones pass out to the tissues, and carbon dioxide and wastes pass in.

Key idea

Section titled “Key idea”

Everything else in the cardiovascular system exists to serve the capillaries — no body cell is more than a hair’s width from one.