8.3 Gaseous Exchange in Humans
Learn how oxygen and carbon dioxide diffuse down partial pressure gradients and are transported throughout the human circulatory system.
Learning Objectives
- Explain how differences in partial pressure drive the diffusion of oxygen and carbon dioxide down a partial pressure gradient.
- Describe the exchange of respiratory gases between the alveoli and lung capillaries, and between tissue capillaries and body cells.
- State the form in which oxygen is transported from the lungs to body tissues.
- Explain the three forms in which carbon dioxide is transported in the blood circulatory system.
- Describe the biochemical sequence of carbon dioxide transport from body cells to tissue capillaries, and from lung capillaries into the alveoli.
Gaseous Exchange and Respiratory Gas Transport in Blood Circulation
The circulatory loop connects the alveoli and body tissues through partial pressure differences, diffusion pathways and molecular conversions of O₂ and CO₂.
Panel 1: Alveolar Interface
Panel 2: Tissue Capillary Interface
Short Explanation
Partial Pressure and Gas Diffusion
Partial pressure is the pressure exerted by an individual gas in a mixture of gases. Gas diffusion depends on the difference in partial pressure between two areas. A gas always diffuses from an area where its partial pressure is higher to an area where its partial pressure is lower — down a partial pressure gradient.
Atmospheric pressure values at sea level
Total atmospheric pressure = \(760\text{ mm Hg}\). The atmosphere contains \(21\%\) oxygen by volume. Therefore, the partial pressure of oxygen \(p\text{O}_2 = 0.21 \times 760\text{ mm Hg} = 160\text{ mm Hg}\). The partial pressure of carbon dioxide \(p\text{CO}_2\) at sea level is \(0.23\text{ mm Hg}\).
Gaseous exchange in the lungs
Blood entering lung capillaries through the pulmonary artery has a low \(p\text{O}_2\) and a high \(p\text{CO}_2\). Oxygen diffuses from the alveolus into the lung capillaries and combines with haemoglobin in erythrocytes to form oxyhaemoglobin. Carbon dioxide diffuses out of the capillaries into the alveolus to be expelled.
Gaseous exchange in body tissues
Cellular respiration releases carbon dioxide and consumes oxygen. The partial pressure of carbon dioxide in cells is higher than in tissue capillaries, so carbon dioxide diffuses from body cells into tissue capillaries. The partial pressure of oxygen in tissue capillaries is higher than in body cells, so oxyhaemoglobin dissociates and oxygen diffuses into body cells.
Transport of carbon dioxide
Carbon dioxide is transported in three forms: \(70\%\) as bicarbonate ions (\(\text{HCO}_3^-\)) in blood plasma, \(23\%\) as carbaminohaemoglobin, and \(7\%\) dissolved as carbonic acid (\(\text{H}_2\text{CO}_3\)) in blood plasma.
Mechanism of Carbon Dioxide Transport
From body cells to tissue capillaries: Carbon dioxide released by body cells enters erythrocytes and binds with water to form carbonic acid. This reaction is catalysed by the enzyme carbonic anhydrase. Carbonic acid breaks down into a bicarbonate ion (\(\text{HCO}_3^-\)) and a hydrogen ion (\(\text{H}^+\)). The bicarbonate ion diffuses out of the erythrocyte into the blood plasma and is transported to the lungs.
From lung capillaries to the alveolus: When bicarbonate ions in the blood plasma reach the lung capillaries, they diffuse back into erythrocytes. Inside the erythrocyte, bicarbonate ions combine with hydrogen ions to form carbonic acid. Carbonic acid breaks down into carbon dioxide and water. Carbon dioxide diffuses through the lung capillaries into the alveolus and is eliminated during exhalation.
Try to Answer First
Answer in your mind, then press “Check Answer”.
What is the partial pressure of oxygen in the atmosphere at sea level?
In what three forms is carbon dioxide transported in the human circulatory system, and what is the percentage of each?
Which enzyme catalyses the reaction between carbon dioxide and water in erythrocytes?
Common Mistakes
- Confusing blood vessels: assuming the pulmonary artery carries oxygen-rich blood. In fact, blood entering the lungs via the pulmonary artery has a low \(p\text{O}_2\) and a high \(p\text{CO}_2\), whereas blood leaving through the pulmonary vein has a high \(p\text{O}_2\) and a low \(p\text{CO}_2\).
- Misattributing the main transport form of \(\text{CO}_2\): thinking carbon dioxide is predominantly transported bound to haemoglobin. Haemoglobin only carries \(23\%\) as carbaminohaemoglobin, whereas \(70\%\) is transported as bicarbonate ions in the plasma.
- Incorrect location of bicarbonate ion formation: forgetting that bicarbonate formation begins inside the erythrocyte (where carbonic anhydrase is present) before the bicarbonate ions diffuse out into the blood plasma.
Concept Misunderstandings
Oxygen and carbon dioxide move across capillary membranes via active transport.
Both gases move strictly by simple diffusion down their respective partial pressure gradients without requiring metabolic energy.
Bicarbonate ions remain inside erythrocytes throughout their entire journey to the lungs.
Bicarbonate ions diffuse out of erythrocytes into the blood plasma to be carried to the lungs, and only diffuse back into erythrocytes once blood reaches the lung capillaries.
Summary
- Gas diffusion is driven from an area of higher partial pressure to an area of lower partial pressure.
- In alveoli, high \(p\text{O}_2\) causes oxygen to diffuse into lung capillaries, combining with haemoglobin to form oxyhaemoglobin.
- In tissues, low \(p\text{O}_2\) and high \(p\text{CO}_2\) cause oxyhaemoglobin to break down, releasing oxygen into body cells, while \(\text{CO}_2\) enters the blood.
- Oxygen is carried to tissues primarily as oxyhaemoglobin.
- Carbon dioxide is transported as bicarbonate ions (\(70\%\)), carbaminohaemoglobin (\(23\%\)), and dissolved carbonic acid (\(7\%\)).
- Carbonic anhydrase in erythrocytes catalyses the conversion of \(\text{CO}_2\) and \(\text{H}_2\text{O}\) into \(\text{H}_2\text{CO}_3\), which dissociates into \(\text{HCO}_3^-\) and \(\text{H}^+\). The process reverses in the lung capillaries to release \(\text{CO}_2\) into the alveoli.
Short Activity
Objective: test your understanding of gaseous exchange and respiratory gas transport.
A. Objective Quiz
What is the primary form in which carbon dioxide is transported through the human blood circulatory system?
Which of the following correctly describes the blood entering the lungs through the pulmonary artery?
B. Fill in the Blanks
Inside erythrocytes, the reaction between carbon dioxide and water to form carbonic acid is catalysed by the enzyme .
At sea level, the partial pressure of oxygen in the atmosphere is mm Hg.
C. Matching / Drag and Drop
Match each respiratory gas transport form or parameter with its correct textbook description or percentage. On a phone, tap a choice first, then tap the matching box. Tap the Answer Choices area to return a selected item.
C. Match the Form / Parameter to Its Description / Value
Write the correct form or parameter next to each description.
| Description / Value | Form / Parameter |
|---|---|
| 1. Form accounting for 70% of transported carbon dioxide | |
| 2. Form resulting from carbon dioxide combining directly with haemoglobin (23%) | |
| 3. Form in which 7% of carbon dioxide is dissolved and carried | |
| 4. Compound formed when oxygen combines with haemoglobin to be carried to tissues |
Answer choices: Bicarbonate ion • Carbaminohaemoglobin • Carbonic acid • Oxyhaemoglobin
