Red blood corpuscles contain chloride in oxygenated form. Oxygen is released on either side of the RBC, the oxygen is picked up by the lung alveoli on inspiration, on expiration carbon dioxide is released. Oxygen is circulated to the tissues, picking up CO2 and releasing oxygen.
To make the conversion from oxygen to carbon dioxide, chloride - which is connected with the hemoglobin - is traded for bicarbonate ion. This has the polarity to pick up carbon dioxide and transporting it to the lung alveoli again. The process helps maintain the blood pH. The shift of chloride to bicarbonate ion is called the chloride shift.
