BRAINCAKECapillaries
Capillaries are the smallest blood vessels. They link arterioles to venules and are the only vessels where substances are exchanged between the blood and the tissues. Their structure is adapted for this exchange:
Very thin wall. The wall is made only of endothelium, one cell thick. There is no muscle or elastic tissue, so the diffusion distance between the blood and the surrounding cells is very short.
Narrow lumen. The lumen is only about 7 to 10 µm across, roughly the width of a red blood cell. Red blood cells squeeze through in single file, pressed against the wall. This slows them down and reduces the diffusion distance for oxygen even further.
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Key terms in this lesson
- endothelium
- The single layer of thin, flattened cells lining the inside of blood vessels, which is smooth to reduce friction with the flowing blood.
More in Circulatory Systems
- The Blood Clotting Cascade
- Blood Vessels
- Arteries
- Arterioles
- Veins and venules
- Tissue Fluid
- Oncotic Pressure
- Hydrostatic Pressure
All 33 lessons in Circulatory Systems · All Edexcel AS-level Biology topics