Carbon Dioxide Transport and Chloride Movement
(a) In tissues, carbon dioxide enters
     erythrocytes and reacts with
     water to form carbonic acid,
     which dissociates to form
     bicarbonate and hydrogen ions.
     In the chloride shift,
     bicarbonate ions are
     exchanged for chloride ions.
     Hydrogen ions combine with
     hemoglobin. Lowering the
     concentration of bicarbonate and
     hydrogen ions inside
     erythrocytes promotes the
     conversion of carbon
     dioxide to bicarbonate ions.
(b) In the lungs, carbon dioxide
     leaves erythrocytes, resulting in
     the formation of additional
     carbon dioxide from carbonic
     acid.  Bicarbonate and
     hydrogen ions combine to
     replace the carbonic acid.  The
     bicarbonate ions are
     exchanged for chloride ions,
     and the hydrogen ions are
     released from hemoglobin.