gas exchange/ high to low concentration
You take in a deep breath of air which goes into your lungs and fills the alveoli. This air is oxygen rich, as a matter of fact there is much more oxygen in the air in the alveoli than there is in the blood of the capillaries surrounding the alveoli. So, the oxygen will want to move out of the alveoli and in to the blood in the capillaries. Likewise, if the air in the alveoli is oxygen rich, that means it doesn't have much carbon dioxide... so the carbon dioxide from the blood in the capillaries will move out of the blood and into the alveoli so when we exhale it gets out of the body... so the blood in the capillaries has now gotten rid of its carbon dioxide and picked up oxygen... it will now go back to the right atrium of the heart via the pulmonary veins.