Complex sentence: Capillaries carrying deoxygenated blood from the pulmonary artery run across the alveoli-they are also extremely thin so the total distance gases must diffuse across is only around 2-cells thick.
Split into simpler sentences: Capillaries carrying deoxygenated blood from the pulmonary artery run across the alveoli. They are also extremely thin so the total distance gases must diffuse across is only around 2-cells thick.