Lung tissue and blood cells, coloured scanning electron micrograph (SEM). The blood cells visible here are red blood cells (erythrocytes, red) and white blood cells (leucocytes, brown). The lungs are where gas exchange (in the alveoli) takes place between the air and the red blood cells. This brings oxygen to the body and releases the respiration waste product carbon dioxide. White blood cells are part of the body's immune system, helping to defend against infection.