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8.2 Transport of Oxygen and Carbon Dioxide

Syllabus
9700–2028–2029
Topic
8.2
Level
AS

Learning objectives

8.2.1Red blood cell roles in O2 and CO2 transport• Red blood cell roles in O2 and CO2 transport:- Haemoglobin- Carbonic anhydrase- Formation of haemoglobinic acid- Formation of carbaminohaemoglobin8.2.2Chloride shift and its importance• Chloride shift and its importance8.2.3Plasma role in CO2 transport• Plasma role in CO2 transport8.2.4Oxygen dissociation curve• Oxygen dissociation curve of adult haemoglobin8.2.5Partial pressures and oxygen loading• Importance at partial pressures in lungs and respiring tissues8.2.6Bohr shift and its importance• Bohr shift and its importance

Red cells use reversible reactions to transport both respiratory gases

Red blood cells transport oxygen mainly by reversible binding to haemoglobin and help transport carbon dioxide through enzyme-catalysed conversion and reversible haemoglobin products.

In respiring tissues:

  1. O2 dissociates from oxyhaemoglobin and diffuses to cells.
  2. CO2 diffuses into red cells. Carbonic anhydrase rapidly catalyses CO2 + H2O ⇌ H2CO3; carbonic acid then dissociates to H+ and HCO3-.
  3. H+ binds to haemoglobin to form haemoglobinic acid (HHb), limiting the fall in red-cell pH.
  4. Some CO2 binds directly and reversibly to haemoglobin to form carbaminohaemoglobin (HbCO2).

In the lungs, the reactions reverse. Oxygen binds to haemoglobin, promoting release of H+ from haemoglobinic acid. H+ combines with HCO3- to form carbonic acid, and carbonic anhydrase catalyses its conversion to CO2 and water. Carbaminohaemoglobin also dissociates, and the released CO2 diffuses into alveoli for exhalation.

Haemoglobinic acid is haemoglobin carrying H+, not haemoglobin carrying CO2. Carbonic anhydrase is a catalyst: it speeds the reversible reaction but is not consumed.

The chloride shift keeps carbon-dioxide transport electrically balanced

The chloride shift is the exchange of hydrogencarbonate ions and chloride ions across the red-cell membrane during carbon-dioxide transport. It allows hydrogencarbonate to leave the red cell without leaving an electrical imbalance.

  1. CO2 enters the red cell: in respiring tissues, carbon dioxide diffuses from cells into the blood and then into red blood cells.
  2. Hydrogencarbonate forms: carbonic anhydrase catalyses CO2 + water → carbonic acid; carbonic acid dissociates into hydrogencarbonate ions (HCO3−) and hydrogen ions (H+).
  3. HCO3− leaves: negatively charged hydrogencarbonate ions move out of the red cell through a membrane transport protein into the plasma, where they can be carried in solution.
  4. Cl− enters: chloride ions move into the red cell through the same exchange system → the negative charge leaving as HCO3− is balanced by negative charge entering as Cl− → electrical neutrality is maintained and conversion can continue.
  5. Lung-side reversal: at the lungs, the exchange runs in the opposite direction: HCO3− returns to the red cell and Cl− leaves. The carbon-dioxide chemistry can then be reversed so CO2 is regenerated for removal in exhaled air.

Boundary: this is an ion-exchange mechanism that supports CO2 transport; it is not the Bohr shift, which describes the effect of CO2/H+ conditions on haemoglobin oxygen affinity.

Chloride does not replace carbon dioxide as the transported gas. HCO3− is the carbon-dioxide-derived form carried mainly in plasma; Cl− enters to balance charge. No image generated or bound.

Plasma carries carbon dioxide in solution and mainly as hydrogencarbonate

Plasma transports some carbon dioxide dissolved as CO2 and carries most carbon dioxide-derived material as dissolved hydrogencarbonate ions (HCO3-).

  1. CO2 from respiring tissues diffuses into blood plasma and red blood cells.
  2. A small amount remains as dissolved molecular CO2 in plasma.
  3. In red cells, carbonic anhydrase rapidly forms carbonic acid, which dissociates to H+ and HCO3-.
  4. HCO3- moves into plasma by the chloride shift and is transported there in solution towards the lungs.
  5. At the lungs, HCO3- re-enters red cells; CO2 is regenerated and diffuses through plasma towards alveoli.

The red blood cell is the main rapid conversion site; plasma is the fluid transport route for dissolved CO2 and HCO3-. A further portion of CO2 travels inside red cells as carbaminohaemoglobin, not in plasma.

Do not say that plasma carries all carbon dioxide as molecular CO2, or that HCO3- is formed mainly in plasma. Exact percentages are not required here and vary with conditions.

Cooperative binding makes adult haemoglobin's curve sigmoid

The oxygen dissociation curve for adult haemoglobin plots oxygen partial pressure (pO2) on the x-axis against percentage saturation of haemoglobin with oxygen on the y-axis.

  • Low pO2: saturation is low because few oxygen molecules are available and the first oxygen binds relatively less readily.
  • Steep middle region: binding of one oxygen changes haemoglobin's shape, making further oxygen binding easier. This cooperative binding means a small pO2 rise gives a large saturation rise; in reverse, a small pO2 fall can release much oxygen.
  • High-pO2 plateau: most haem groups are occupied, so further increases in pO2 produce only a small increase in percentage saturation.

To read the graph, choose a pO2 on the x-axis, project to the curve, then project to the y-axis for percentage saturation. Report only the precision supported by the scale. The sigmoid shape describes reversible loading and unloading at equilibrium conditions; it is not a time graph.

Percentage saturation is the proportion of haemoglobin oxygen-binding sites occupied, not the total oxygen content of the whole blood sample. The plateau does not mean oxygen can never dissociate.

One haemoglobin curve supports lung loading and tissue unloading

The local oxygen partial pressure selects an operating point on the adult-haemoglobin dissociation curve: high pO2 in lungs favours loading, while lower pO2 in respiring tissues favours unloading.

  • Lungs: alveolar pO2 is high, so haemoglobin operates on the plateau and becomes highly saturated. The plateau helps loading remain high despite moderate pO2 variation.
  • Respiring tissues: cells consume oxygen, keeping tissue pO2 below that of oxygenated blood. Haemoglobin moves down the steep part of the curve, so a relatively small pO2 fall produces a substantial saturation fall and oxygen release.
  • More active tissue: faster oxygen use can lower local pO2 further, moving the operating point farther down the curve and increasing unloading.

Application routine: identify the location and its relative pO2; locate that pO2 on the x-axis; read percentage saturation; compare the lung and tissue values; interpret the difference as oxygen loaded or unloaded.

Moving to a lower pO2 along the same curve lowers saturation but does not by itself mean haemoglobin's intrinsic affinity has changed. A change in affinity shifts the curve, as in the Bohr effect.

The Bohr shift targets oxygen unloading to respiring tissues

The Bohr shift is a change in the position of the oxygen dissociation curve caused by carbon-dioxide and H+ conditions. Higher CO2 and H+ shift the curve to the right, indicating lower haemoglobin affinity for oxygen.

  1. Active respiring tissues produce CO2.
  2. In red cells, CO2 conversion produces H+, and H+ binds to haemoglobin.
  3. Haemoglobin's shape and oxygen affinity change.
  4. At the same pO2, the right-shifted curve has a lower percentage saturation.
  5. Therefore more oxygen is unloaded where respiration and oxygen demand are high.

In the lungs, CO2 is removed, H+ concentration falls and high pO2 favours oxygen loading. The curve comparison must hold pO2 constant when demonstrating the affinity effect; movement along one curve answers a different question.

The Bohr shift changes haemoglobin oxygen affinity; the chloride shift exchanges HCO3- and Cl- to maintain electrical neutrality. A right shift does not mean haemoglobin cannot load oxygen in the lungs.