In humans

• Alveoli and gas exchange: lungs contain millions of microscopic air sacs where gas exchange occurs - alveoli

  alveoli are surrounded by a network of capillaries
• Alveoli structure:

  the wall of each alveolus is made from a singular layer of thin, flat cells called alveolar epithelium

  the walls of the capillaries are made from capillary endothelium
• The walls of the alveoli contain a protein called elastin - is elastic

  helps the alveoli to return (recoil) to their normal shape after inhaling and exhaling air
• Movement of oxygen and carbon dioxide through the gas exchange system:

  air (containing oxygen) moves down the trachea, bronchi and brochioles into the alveoli.

  movement happens down a pressure gradient

  oxygen moves into the blood where it can be transported around the body - this movement happens down a diffusion gradient
• carbon dioxide moves down its own diffusion gradient and pressure gradient but in opposite direction to oxygen so it can be breathed out
• Gas exchange in the alveoli:

  oxygen diffuses out of the alveoli across alveolar epithelium

  across capillary endothelium into compound called haemoglobin in the blood

  carbon dioxide diffuses into the alveoli from the blood
• Factors affecting rate of diffusion:

  1. thin exchange surface - alveolar epithelium is only one cell thick - short diffusion pathway - speeds up diffusion
  2. large surface area - millions of alveoli - large surface areas for gas exchange
  3. steep conc. gradient of oxygen and carbon dioxide between alveoli and capillaries - increases rate of diffusion
  4. steep conc. gradient constantly maintained by flow of blood and ventilation