Which barrier must o2 and co2 cross to pass between air and blood inside the lungs: ?

Oxygen and carbon iv oxide are respiratory which play important role in the respiratory system. During inhalation oxygen is taken in, which is then taken by pulmonary vein to the heart for pumping to all the body tissues through the aorta then the capillaries. Pulmonary  artery takes the deoxygenated blood to the lungs where carbon iv dioxide is exchanged an the exhaled. Through the whole process these respiratory gases crosses the following barriers; extracellular fluid, capillary walls and the epithelial cells.