alveoli

• in humans, gas exchange happens in the alveoli
• each alveoli is made from a thin, flat single layer of cells called alveolar epithelium
• alveoli are air sacs covered in a network of capillaries
• there's lots of alveoli in the lungs, so there's a large surface area for exchanging oxygen and carbon dixoide
• alveoli are surrounded by a network of capillaries
• oxygen diffuses out of the alveoli, across the alveolar epithelium and the capillary endothelium and into haemoglobin in the blood
• oxygen diffuses out of the alveoli and into blood in the capillaries
• carbon dioxide diffuses into the alveoli from the blood as is breathed out
• a thin exchange surface - epithelium is only one cell thick, so a there's a short diffusion pathway which speeds up gas exchange
• a large surface area - the large number of alveoli means there's a large surface area for gas exchange
• there's a steep concentration gradient of oxygen and carbon dioxide between the alveoli and the capillaries, which increases the rate of diffusion. this is maintained by the flow of blood and ventilation
• less alveoli lead to a reduced surface area, so increased distance for diffusion, therefore a reduced rate of gas exchange (alveolar death)
• an oxygen molecule will diffuse across the alveolar epithelium, then across the endothelium of the capillary
• the alveolar epithelium is one cell thick, creating a short diffusion pathway