Anaes · Applied cardiovascular & respiratory physiology
Carbon dioxide transport
Also known as CO2 transport · Carbon dioxide carriage · Haldane effect · Chloride shift · CO2 dissociation curve · Bicarbonate buffer
Carbon dioxide is the waste product of metabolism and the chemical driver of ventilation, and its transport has a subtlety oxygen transport lacks: the CO2 dissociation curve is steep and near-linear, and the Haldane effect lets oxygenation in the lung unload CO2. The framework rests on five exam-critical ideas: CO2 is carried in three forms — as bicarbonate (about 70 percent), as carbamino compounds on haemoglobin (about 20 percent) and dissolved (about 10 percent); bicarbonate is made in the red cell by carbonic anhydrase and exported in exchange for chloride (the chloride shift, via the band-3 anion exchanger); the CO2 dissociation curve is steep and near-linear (so CO2 content tracks PCO2 closely, unlike the sigmoid oxygen curve); the Haldane effect — deoxygenated haemoglobin carries more CO2 than oxygenated — is the dominant mechanism of CO2 uptake in the tissues and release in the lung; and CO2 is the acid load of the body, buffered by the bicarbonate system (Henderson-Hasselbalch), with large body stores that equilibrate slowly. Built on the mechanistic CO2-transport-in-blood model (O'Neill 2017), the CO2-derived-variables review (Mallat 2025), the venous-to-arterial CO2-content study (Ospina-Tascon 2025), the haemoglobin-electrolyte interaction study (Valsecchi 2025), the membrane-oxygenator gas-exchange model (Monsefi 2025), and the band-3 chloride-bicarbonate exchanger study (Fawaz 2012).
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- CO2 is carried as bicarbonate about 70 percent, carbamino compounds about 20 percent, dissolved about 10 percent — most CO2 is converted to bicarbonate in the red cell, not carried dissolved.
- The Haldane effect — deoxygenated haemoglobin carries more CO2 than oxygenated — is the dominant mechanism of CO2 exchange: oxygenation in the lung releases CO2, deoxygenation in the tissues takes it up.
- The chloride shift (Hamburger phenomenon): bicarbonate made in the red cell leaves in exchange for chloride via the band-3 anion exchanger, so venous red cells carry more chloride than arterial.
- The CO2 dissociation curve is steep and near-linear over the physiological range, so small PCO2 changes move CO2 content a lot — the basis of the arterial-to-venous CO2 content gap as a perfusion marker.
- CO2 is the body's acid load and is buffered by bicarbonate; a raised PCO2 (respiratory acidosis) is rapidly compensated by bicarbonate retention over hours to days via the kidney.
Meet the patient
A 55-year-old man is in PACU after a laparoscopic cholecystectomy. His end-tidal CO2 is 52 and rising despite adequate ventilation. The capnograph trace shows a normal shape but an elevated plateau. Before you reach for a diagnosis you are already running the CO2 balance: production, elimination, and rebreathing. Every anaesthetist reads capnography as continuous CO2 physiology at the airway — this topic is the physiology behind the number on the monitor.[1]
The question that organises everything: how is CO2 carried, and what moves it from tissue to lung? Master the three forms, the chloride shift, the Haldane effect, and the shape of the dissociation curve, and every capnograph abnormality becomes readable.[1]
References6ShowHide
- [1]O'Neill DP, Robbins PA. A mechanistic physicochemical model of carbon dioxide transport in blood J Appl Physiol (1985), 2017.PMID 27881667
- [2]Mallat J. Use of CO(2)-derived variables in critically ill patients Ann Intensive Care, 2025.PMID 40999252
- [3]Ospina-Tascón GA, De Backer D, Aldana JL, et al. Regional venous-to-arterial carbon dioxide pressure and content differences during endotoxemic shock: influence of hydrogen ion accumulation vs. Haldane effect Intensive Care Med Exp, 2025.PMID 40921906
- [4]Valsecchi C, et al. In vitro characterization of hemoglobin oxygen dissociation curves and electrolyte shifts in human blood under varying PCO(2) Front Med (Lausanne), 2025.PMID 41601791
- [5]Monsefi Estakhrposhti SH, et al. A Validated CFD Model for Gas Exchange in Hollow Fiber Membrane Oxygenators: Incorporating the Bohr and Haldane Effects Membranes (Basel), 2025.PMID 41002903
- [6]Fawaz NA, et al. dRTA and hemolytic anemia: first detailed description of SLC4A1 A858D mutation in homozygous state Eur J Haematol, 2012.PMID 22126643