Because there is more oxygen in an alveolus (sac in the lung) than in the blood around it, oxygen diffuses

Because there is more oxygen in an alveoli sac than in the blood around it, oxygen diffuses from alveoli into capillaries. 

Wetlands form where water and land meet. a lake with a wetland fringe can fill in with decayed plant and animal material to become a wet meadow. a pond can fill in to become a bog or fen. sediment can fill in a river marsh to become a swamp.flooding of coastal lowlands from rising sea levels has created broad coastal marshes that are protected from wave action by barrier islands or reefs. coastal wetlands also form when rivers deposit sediment as they reach the ocean. plants then take root and hold the soil firm against the forces of tides and waves.