2.38 explain how alveoli are adapted for gas exchange by diffusion between air in the lungs and blood capillaries

2.38 •huge number maximises surface area

        •moist lining for gasses to dissolve in

        •one cell thick walls, minimising travel distance 

        •great blood supply, maintaining a high concentration gradient 

        •the alveoli walls are permeable, so gases can diffuse across easily