In which vessels does gas exchange take place?

The human vascular system consists of arteries, veins and capillaries. Oxygen-rich blood (arterial) flows through the arteries from the lungs to all organs and tissues. Large arteries branch into smaller ones, and those, in turn, into capillaries. Capillaries release O2 through their walls into the tissue, and get CO2 back. Blood with a high content of CO2 (venous), through the veins, returns back to the lungs, where O2 is again enriched in the pulmonary capillaries. Thus, gas exchange takes place in the capillaries.