Oxygen and carbon dioxide in the lungs
The oxygen needed in this complicated process in the cells comes mainly from the lungs to the blood. The alveoli in the lungs are tightly wrapped in small blood vessels, capillaries. Capillary walls are so thin that gases like oxygen, carbon dioxide and water molecules can pass through them.
During inhalation the oxygen from the air will pass through the alveolar-capillary barrier, where it is bound to the red blood cells, the erythrocytes. The blood then flows through the pulmonary veins to the left part of the heart, the atrium sinister, leaving the heart through the aorta and from there reaches the cells by passing smaller and smaller veins, alveolars and capillaries.
During exhalation the process is reversed and the waste products from the cells will be transported in the blood stream, to the alveolar-capillary barrier and then leave the body. That way we excrete water and carbon dioxide coming from the cell energy production process.