8.2 Transport of Oxygen and Carbon Dioxide

Red blood cells transport most of the body’s oxygen using haemoglobin. Oxygen bound to haemoglobin forms oxyhaemoglobin; this is distinct from the smaller amount of oxygen carried dissolved in plasma.

• Biconcave shape: the disc-shaped cell has a large surface relative to its volume → oxygen can reach haemoglobin across a short cell distance → loading and unloading are efficient.
• No nucleus: the mature red cell has no nucleus → more internal space is available for haemoglobin → more oxygen can be carried per cell.
• High haemoglobin content: haemoglobin contains four haem groups, each able to bind one oxygen molecule → one haemoglobin molecule can carry four oxygen molecules → red cells provide the main reversible oxygen-carrying capacity of blood.
• Lung–tissue boundary: in the lungs, oxygen binds to haemoglobin and oxyhaemoglobin forms; in respiring tissues, oxygen can dissociate and enter cells. The oxygen-dissociation curve and Bohr shift explain how conditions tune this exchange in later cards.

Do not equate oxygen transport with oxygen dissolved in plasma: most is haemoglobin-bound. Keep carbon-dioxide forms, the oxygen-dissociation curve and the Bohr shift for their dedicated cards. No image generated or bound.

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.

Carbon dioxide from respiring tissues reaches the blood in three forms. Most is converted inside red blood cells and then carried in plasma as hydrogencarbonate ions; smaller amounts remain dissolved in plasma or bind to haemoglobin as carbaminohaemoglobin.

• Dissolved CO2 — a small fraction (about 5% in the SME comparison): CO2 remains in solution in plasma → it can be carried directly in the blood, but this is not the main transport form.
• Carbaminohaemoglobin — a smaller bound fraction (about 10%): CO2 binds to haemoglobin in red blood cells → this carries CO2 in a reversible haemoglobin-bound form.
• Hydrogencarbonate — the major fraction (about 85%): tissue CO2 diffuses into a red blood cell → carbonic anhydrase catalyses reaction with water and carbonic acid dissociates → HCO3− leaves the red cell and enters plasma → plasma carries it in solution towards the lungs.

Division of labour: red blood cells provide the rapid conversion site and haemoglobin binding; plasma provides the main fluid route for hydrogencarbonate. At the lungs, the transport pathway is reversed so carbon dioxide can be regenerated and removed.

Boundary: the detailed HCO3−/Cl− membrane exchange belongs to the chloride-shift card, and CO2/H+ effects on haemoglobin oxygen affinity belong to the Bohr-shift card.

Do not say that all carbon dioxide is dissolved molecular CO2 in plasma or that plasma formed the hydrogencarbonate without red-cell involvement. The percentages are approximate comparison values from the SME note; no image generated or bound.

The oxygen dissociation curve shows haemoglobin saturation against the partial pressure of oxygen (pO2). Its sigmoidal shape reflects changing haemoglobin affinity as oxygen binds and dissociates.

1. Read the axes: x-axis = oxygen partial pressure; y-axis = percentage saturation of haemoglobin with oxygen. The graph is not a time graph and does not show the total oxygen content of all blood.
2. Explain the low-pO2 region: at low pO2, the first oxygen binds less easily, so affinity and saturation are relatively low; dissociation of the final bound oxygen also becomes slower when few binding sites remain occupied.
3. Explain the steep region: after one oxygen binds, haemoglobin changes conformation → the next oxygen molecules bind more easily → a modest pO2 change can produce a relatively large change in saturation. This is cooperative binding.
4. Explain the high-pO2 plateau: as haemoglobin approaches saturation, few binding sites remain → further pO2 increases produce only a small additional saturation change.
5. Apply the shape: high pO2 in the lungs supports loading and keeps haemoglobin near the plateau; the lower pO2 conditions of respiring tissues lie in the unloading region, where oxygen can be released for cellular respiration.
6. Make a cautious read-off: identify the relevant pO2, project to the curve, then read the approximate saturation; report an estimate rather than inventing precision between graph marks.

Boundary: this card explains the curve and cooperative shape. Card 4609 handles the detailed partial-pressure gradient and loading/unloading application; card 4610 handles the Bohr shift.

A right-hand plateau does not mean no oxygen can be released, and a left/steep region does not mean haemoglobin is absent. Saturation is a percentage at a specified pO2. No image generated or bound.

Oxygen partial pressure (pO2) is the pressure contribution of oxygen within a gas mixture. The local pO2 changes haemoglobin’s loading or unloading demand: high pO2 favours binding, while lower pO2 favours dissociation.

• Lungs — high pO2: haemoglobin encounters a high oxygen partial pressure → affinity and percentage saturation are high → oxygen loads onto haemoglobin and blood leaves the lungs carrying a high oxygen load.
• Respiring tissues — lower pO2: cells use oxygen, so tissue pO2 is lower than in the lungs → haemoglobin’s affinity is lower and saturation falls → oxygen dissociates and becomes available for cellular respiration.
• Steep-region demand: when the tissue pO2 lies on the steep part of the curve, a relatively small fall in pO2 can produce a substantial fall in saturation → a useful amount of oxygen is unloaded without requiring complete deoxygenation.
• Active/respiring tissue: greater respiration tends to maintain a lower local oxygen partial pressure → the loading–unloading system is driven towards release where oxygen demand is present, within the conditions represented by the page.

Application method: identify the location, compare its pO2 with the lung/tissue context, locate the corresponding curve region, then state whether saturation rises or falls and why.

Partial pressure is not simply the percentage of oxygen in the whole atmosphere or the total oxygen amount in blood. This card applies pO2 to location; 4608 explains the curve shape/read-off and 4610 explains the Bohr shift when CO2/H+ conditions alter affinity. No image generated or bound.

The Bohr shift is the change in haemoglobin’s oxygen affinity caused by the higher carbon-dioxide conditions of respiring tissues. It shifts the oxygen dissociation curve to the right, so haemoglobin is less saturated at the same oxygen partial pressure.

• Demand signal: active respiring cells produce more carbon dioxide → carbonic acid dissociates and hydrogen ions are formed.
• Affinity change: hydrogen ions bind to haemoglobin and the higher CO2/H+ conditions reduce haemoglobin’s affinity for oxygen → oxygen is released more readily.
• Curve consequence: the dissociation curve shifts right → at the same tissue pO2, haemoglobin has a lower percentage saturation than it would under lower-CO2 conditions.
• Functional result: more oxygen dissociates from haemoglobin in the respiring tissue where demand is high → oxygen becomes available for cellular respiration.
• Lung-side recovery: in the lungs, CO2 is removed and the high-oxygen environment favours restoration of haemoglobin’s higher oxygen affinity, allowing oxygen loading for the next circuit.

Boundaries: the basic curve and cooperative binding belong to 4608; the partial-pressure location application belongs to 4609; the HCO3−/Cl− charge-balancing exchange is the chloride shift in 4606.

A right shift does not mean haemoglobin contains no oxygen or that oxygen cannot load in the lungs. It is a conditional affinity change linked to CO2/H+ conditions, not the chloride-ion exchange. Staff-only visual brief: paired low-CO2/high-affinity and high-CO2/right-shift curves with one shared pO2 read-off; do not generate or bind an image.