gas exchange

• Alveoli give lungs a large internal surface area, allowing large amounts of gas to be exchanged in a relatively short time
• Each alveolus is surrounded by blood vessels which means as much blood as possible is close to the air in the alveolus. Continuous flow of blood maintains a difference in concentration of oxygen and carbon dioxide
• Membrane of alveoli and capillary is only one cell thin, so gas molecules do not have to travel far
• the lungs are positions deep inside body to prevent the excessive evaporation of fluid that covers respiratory surfaces which is needed as gases can only diffuse when dissolved in fluid
• blood is brought to capillaries in lungs by the pulmonary arteries. the blood brought to lungs have already been throughout the body and is low in oxygen as body cells have taken the oxygen for cellular respiration
• oxygen dissolved in the moisture in the alveolus and diffuses through the membranes of the alveolus and capillary wall into the blood
• the blood has also picked up carbon dioxide from body cells which diffuses out of the blood into the alveolus due to the concentration difference between the air and blood
• for diffusion of gases, there must be a concentration gradient which is maintain by having a constant flow of blood through capillaries and constant change of air in lungs through respiration
• as blood flowing through capillaries in the lungs picks up oxygen and loses carbon dioxide, it is replaced by more blood being pumped into capillaries. The new blood is low in oxygen and high in carbon dioxide, maintaining the gradient
• air that has picked up carbon dioxide and lost oxygen is replaced by air high in oxygen and low in carbon dioxide with each breath