- have a very large internal surface area due to many alveoli: more surface are for gas exchange - alveolis are supplied with close blood vessels/capillaries - concentration gradients are maintained: blood is continuously moved through blood vessels , air is continuously moved in and out of lungs - alveolis are one cell thick: decrease distance for easier diffusion - thin layer of moisture in alveoli for easier gas diffusion - lungs are deep in the body to prevent excess evaportaion