WJEC General Certificate of Secondary Education (GCSE) PE Practice Test

Gas exchange with tissues happens mainly across the walls of capillaries. These tiny vessels form dense networks throughout tissues, and their walls are only one cell thick, allowing oxygen to diffuse from the blood into tissue cells and carbon dioxide to diffuse from the cells back into the blood. Arteries carry oxygen-rich blood to tissues but don’t exchange gases there, veins return blood and also don’t maximize gas exchange, and nerves aren’t involved in this process. So the vessels responsible for exchanging oxygen with tissues are capillaries.