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Red blood cells are the oxygen carriers. As they travel away from the heart, they traverse smaller and smaller arteries, finally arriving at the collections of microscopic blood vessels, known as capillaries. Here, they exchange nutrients and oxygen for cellular waste products. The exchange of oxygen and nutrients between the red blood cells and the surrounding tissues occurs through a process called diffusion.

In diffusion, when capillaries contain a high concentration of oxygen and nutrients, while the surrounding tissues contain a lower concentration, oxygen and nutrients leave the capillaries and enters the tissues.

Conversely, when body tissues contain high concentrations of carbon dioxide and metabolic waste, while the capillaries contain a lower concentration, the waste products diffuse from the tissues into the capillaries, and from there are carried by the venous system back toward the heart.

The waste products are eventually eliminated from the bloodstream through the urinary and respiratory systems.

Review Date: 4/5/2025
Reviewed By: Charles I. Schwartz, MD, FAAP, Clinical Assistant Professor of Pediatrics, Perelman School of Medicine at the University of Pennsylvania, General Pediatrician at PennCare for Kids, Phoenixville, PA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.
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