human gas exchange system . oxygen travel from lungs to heart, to all body cells and back to lungs as co2.

air alveoli alveolus anatomy artery atria biology blood breathe bronchi bronchial capillaries capillary carbon cells circulation connective cross cycle diagram dioxide drawing duct education educational erythrocytes exchange gas health human illustration labeled lung lungs medical medicine organ organs oxygen pulmonary red respiration respiratory science section structure system tissue vector vein