# Gas transport in blood

> CIE A-Level Biology · 9700 (2022-2024)
> Source: https://www.owlsprep.com/study/cie-9700-u8-gas-transport-in-blood/

This sub-topic covers how oxygen and carbon dioxide are transported in mammalian blood, focusing on the role of haemoglobin, red blood cell adaptations and key processes like the chloride shift and Bohr effect. You will learn how gas transport matches the metabolic demands of respiring tissues.

**Prerequisites:** [Structure of haemoglobin](https://www.owlsprep.com/study/cie-9700-u8-haemoglobin-structure/); [Gas exchange at alveoli](https://www.owlsprep.com/study/cie-9700-u7-gas-exchange-mammals/)

## Learning objectives

- Explain how oxygen is loaded, transported and unloaded by haemoglobin
- Describe the role of carbonic anhydrase in CO₂ transport
- Explain the chloride shift and buffering effect of haemoglobin
- Compare the three forms of carbon dioxide transport in blood
- Interpret oxygen dissociation curves and the Bohr effect

## Oxygen Transport by Haemoglobin

**Oxyhaemoglobin** — A reversible compound formed when oxygen binds to the haem group of haemoglobin in red blood cells. Binding occurs in the lungs where $pO_2$ is high.

*Notation:* HbO_2

*Example:* Each haemoglobin molecule can bind up to 4 oxygen molecules, forming $Hb_4O_8$.

Haemoglobin has a high affinity for oxygen when $pO_2$ is high, which occurs in the alveoli of the lungs. Oxygen diffuses down its concentration gradient from the alveoli into red blood cells, where it binds to haemoglobin for transport. In respiring tissues, $pO_2$ is low, so haemoglobin releases (unloads) oxygen, which diffuses into tissue cells to support respiration.

**Worked example:** Explain why an increase in the rate of cellular respiration leads to more oxygen being unloaded from haemoglobin to muscle tissue.

1. 1. Increased respiration increases oxygen consumption, so $pO_2$ in muscle tissue falls.
2. 2. Higher respiration also produces more $CO_2$, which lowers blood pH (the Bohr effect).
3. 3. Lower pH reduces haemoglobin's affinity for oxygen.
4. 4. This shifts the oxygen dissociation curve to the right, meaning more oxygen is unloaded at a given $pO_2$ to meet the increased demand of the muscle.

## Forms of Carbon Dioxide Transport

Carbon dioxide is transported in the blood in three different forms, in varying proportions:

- ~5% dissolved directly in blood plasma
- ~10% bound to the amino groups of haemoglobin, forming carbaminohaemoglobin
- ~85% transported as hydrogencarbonate ions ($HCO_3^-$) in plasma

**Carbonic anhydrase** — A zinc-based enzyme found in high concentrations in red blood cells that catalyses the reversible reaction between carbon dioxide and water to form carbonic acid.

**Worked example:** Calculate how much CO₂ is transported as hydrogencarbonate ions in 1 dm³ of blood that contains 2.5 cm³ of dissolved CO₂ and 0.5 cm³ of CO₂ bound as carbaminohaemoglobin, if total blood CO₂ is 25 cm³.

1. 1. Total CO₂ in the sample is given as 25 cm³.
2. 2. Add CO₂ transported by other methods: $2.5 + 0.5 = 3$ cm³.
3. 3. Subtract to get CO₂ as $HCO_3^-$: $25 - 3 = 22$ cm³.
4. 4. Check the proportion: $\frac{22}{25} \times 100 = 88\%$, which matches the expected ~85% range.

## Chloride Shift and Buffering Action

When carbon dioxide diffuses into red blood cells, carbonic anhydrase catalyses its combination with water to form carbonic acid ($H_2CO_3$). Carbonic acid quickly dissociates into hydrogen ions ($H^+$) and hydrogencarbonate ions ($HCO_3^-$):

$$CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$$

Hydrogencarbonate ions diffuse out of the red blood cell into the plasma down their concentration gradient. To maintain electrical neutrality, chloride ions ($Cl^-$) diffuse from the plasma into the red blood cell: this movement is called the chloride shift.

The hydrogen ions produced would lower the pH of the red blood cell if they were not buffered. Haemoglobin acts as a buffer by binding to the hydrogen ions to form haemoglobinic acid, which prevents a damaging drop in intracellular pH.

**Worked example:** Explain why the chloride shift is necessary for continued CO₂ transport in blood.

1. 1. When CO₂ enters the red blood cell, it dissociates into $H^+$ and $HCO_3^-$.
2. 2. Negatively charged $HCO_3^-$ leaves the red blood cell down its concentration gradient.
3. 3. This loss of negative charge leaves the red blood cell with a net positive charge.
4. 4. Negatively charged chloride ions move into the cell from plasma to balance the net charge, allowing $HCO_3^-$ to continue diffusing out and CO₂ to keep entering the cell.

## The Bohr Effect

The Bohr effect describes how increasing $pCO_2$ (and decreasing pH) reduces haemoglobin's affinity for oxygen. This shifts the oxygen dissociation curve to the right, meaning that haemoglobin releases more oxygen at a given $pO_2$.

This is a key adaptation that matches oxygen supply to tissue demand: highly active tissues (e.g. contracting muscle) produce more $CO_2$, which lowers pH, triggering more oxygen unloading exactly where it is needed most.

**Check your understanding**

Test your understanding of the Bohr effect:

1. Which of the following correctly describes the effect of high $pCO_2$ on haemoglobin?

   - Increased affinity for oxygen, more unloading
   - Decreased affinity for oxygen, more unloading
   - Increased affinity for oxygen, less unloading
   - Decreased affinity for oxygen, less unloading

   *Answer:* Decreased affinity for oxygen, more unloading

   *Why:* Correct: High $pCO_2$ lowers pH, reduces affinity, so more oxygen is released to tissues.

2. Where would you expect the most right-shifted dissociation curve?

   - Alveoli
   - Resting leg muscle
   - Active heart muscle
   - Brain at rest

   *Answer:* Active heart muscle

   *Why:* Correct: Active heart muscle has very high CO₂ production and low $pO_2$, so maximum shift right.

## Common pitfalls

- **Wrong:** Stating that CO₂ is mostly carried as carbon dioxide in plasma.
  - Why it fails: Only 5% of CO₂ is dissolved in plasma; 85% is carried as hydrogencarbonate ions.
  - Correct: State that 85% of carbon dioxide is transported as hydrogencarbonate ions in plasma.
- **Wrong:** Claiming the chloride shift moves chloride ions out of red blood cells.
  - Why it fails: Hydrogencarbonate moves out; chloride moves in to balance charge.
  - Correct: Chloride ions diffuse into red blood cells during the chloride shift.
- **Wrong:** Saying haemoglobin binds to CO₂ to make oxyhaemoglobin.
  - Why it fails: Oxyhaemoglobin is haemoglobin bound to oxygen. CO₂ binds to haemoglobin to form carbaminohaemoglobin.
  - Correct: Oxygen binds to haem groups to form oxyhaemoglobin; CO₂ binds to amino groups to form carbaminohaemoglobin.
- **Wrong:** Thinking the Bohr effect shifts the dissociation curve to the left.
  - Why it fails: A left shift means higher affinity for oxygen, which does not match the needs of active tissues.
  - Correct: Increased CO₂ reduces affinity, shifting the curve to the right, increasing oxygen unloading.
- **Wrong:** Stating that carbonic anhydrase only catalyses reactions in respiring tissues.
  - Why it fails: The reaction is reversible, and the enzyme catalyses the breakdown of carbonic acid to CO₂ and water in the lungs too.
  - Correct: Carbonic anhydrase catalyses the reversible interconversion of CO₂ + water and carbonic acid in both tissues and lungs.

## Cheatsheet

| Process | Key Location | Key Fact |
| --- | --- | --- |
| Oxygen loading | Alveoli (lungs) | High $pO_2$ → Hb binds O₂ → oxyhaemoglobin |
| Oxygen unloading | Respiring tissues | Low $pO_2$ + high $pCO_2$ → Hb releases O₂ |
| Most CO₂ transport | Plasma | ~85% as hydrogencarbonate ions ($HCO_3^-$) |
| Carbonic anhydrase | Red blood cells | Catalyses $CO_2 + H_2O \rightleftharpoons H_2CO_3$ |
| Chloride shift | RBC membrane | $Cl^-$ moves into RBC to balance charge |
| Bohr effect | Active tissues | High $pCO_2$ → lower pH → lower Hb affinity → right shift |
| pH buffering | Red blood cells | Hb binds $H^+$ to prevent damaging pH drop |

## What's next

Understanding gas transport in blood connects alveolar gas exchange to cellular respiration, and explains how the body adapts to changing metabolic demand during exercise. The principles of haemoglobin oxygen affinity also underpin comparisons between fetal and adult haemoglobin, and adaptations to high altitude. This sub-topic completes the core understanding of blood function in mammalian transport, and leads into the structure of blood vessels and regulation of the circulatory system that make up the rest of this unit. Mastering this content is also critical for exam questions on gas exchange and respiration across multiple topics.

- [Gas Exchange](https://www.owlsprep.com/study/cie-9700-u9-overview/)
- [Gas exchange surface properties](https://www.owlsprep.com/study/cie-9700-u9-gas-exchange-surface-properties/)

---

From [OwlsPrep](https://www.owlsprep.com) — free study guides for A-Level, IB, AP and IGCSE, written against the official syllabus. Canonical page: https://www.owlsprep.com/study/cie-9700-u8-gas-transport-in-blood/