11.1 Gas Exchange in Humans - Revision Notes
1. Overview
Gas exchange is the biological process by which oxygen is absorbed from the environment and carbon dioxide is expelled as a waste product of respiration. In humans, this occurs in the lungs and is essential for providing cells with the oxygen needed for aerobic respiration to produce energy.
Key Definitions
- Gas Exchange: The exchange of oxygen and carbon dioxide across a respiratory surface.
- Ventilation (Breathing): The muscular movement of air into and out of the lungs to maintain steep concentration gradients.
- Alveoli: Tiny air sacs in the lungs where gas exchange takes place.
- Thorax: The chest cavity where the lungs and heart are located.
- Trachea: The "windpipe" that connects the larynx to the bronchi.
Core Content
Features of Gas Exchange Surfaces
To maximize the rate of diffusion, human gas exchange surfaces (the alveoli) have four specific adaptations:
- Large Surface Area: Millions of alveoli provide a massive area for gas molecules to pass through.
- Thin Surface: The walls of the alveoli and the surrounding capillaries are only one cell thick, providing a short diffusion distance.
- Good Blood Supply: A dense network of capillaries ensures that oxygen-rich blood is carried away and carbon dioxide-rich blood is brought to the lungs quickly.
- Good Ventilation: Constant breathing maintains a high concentration of oxygen and a low concentration of carbon dioxide in the alveoli (steep concentration gradient).
Anatomy of the Breathing System
Composition of Inspired vs. Expired Air
| Gas | Inspired Air (%) | Expired Air (%) | Reason for difference |
|---|---|---|---|
| Oxygen | ~21% | ~16% | Absorbed into blood for respiration |
| Carbon Dioxide | ~0.04% | ~4% | Produced by cells as a waste product |
| Water Vapour | Variable (Lower) | High (Saturated) | Evaporates from the moist lining of alveoli |
| Nitrogen | ~78% | ~78% | Not used or produced by the body |
Testing for Carbon Dioxide
- Procedure: Use a delivery tube to bubble expired air into a test tube of limewater.
- Result: The limewater turns from clear to cloudy/milky.
- Comparison: Expired air turns limewater cloudy much faster than inspired air because it contains roughly 100 times more CO2.
Physical Activity and Breathing
During exercise, the rate (breaths per minute) and depth (volume per breath) of breathing increase. This is because the muscles are respiring faster and require more oxygen, while producing more carbon dioxide that must be removed.
Extended Content (Extended Only)
Cartilage in the Trachea
The trachea contains C-shaped rings of cartilage. Its function is to keep the airway open during the pressure changes of breathing and prevent the trachea from collapsing when the pressure inside falls.
The Mechanism of Ventilation
Ventilation relies on changing the volume of the thorax to create pressure differences compared to the outside atmosphere.
Inhalation (Breathing In):
- External intercostal muscles contract; internal intercostal muscles relax.
- The ribcage moves upwards and outwards.
- The diaphragm contracts and flattens.
- The volume of the thorax increases.
- The pressure inside the thorax decreases below atmospheric pressure.
- Air is forced into the lungs.
Exhalation (Breathing Out):
- Internal intercostal muscles contract; external intercostal muscles relax (during forced exhalation).
- The ribcage moves downwards and inwards.
- The diaphragm relaxes and moves up into a dome shape.
- The volume of the thorax decreases.
- The pressure inside the thorax increases above atmospheric pressure.
- Air is forced out of the lungs.
Control of Breathing Rate
During exercise, the concentration of carbon dioxide in the blood increases. This is detected by the brain. The brain sends nerve impulses to the diaphragm and intercostal muscles to increase the rate and depth of breathing to expel the CO2 and take in more O2.
Protection of the Breathing System
The respiratory tract is lined with two specialized cells:
- Goblet Cells: Secrete sticky mucus which traps pathogens (bacteria/viruses) and dust particles.
- Ciliated Cells: Have tiny hair-like projections called cilia that beat in a rhythmic wave to move the mucus up towards the throat, where it is swallowed and destroyed by stomach acid.
Key Equations
- Breathing Rate: $$\text{Breathing Rate (breaths/min)} = \frac{\text{Total number of breaths}}{\text{Time (minutes)}}$$
- Minute Ventilation: $$\text{Minute Ventilation} = \text{Tidal Volume (volume per breath)} \times \text{Breathing Rate}$$
Common Mistakes to Avoid
- ❌ Wrong: Saying we "breathe in oxygen and breathe out carbon dioxide."
- ✓ Right: We breathe in air containing more oxygen, and breathe out air containing more carbon dioxide.
- ❌ Wrong: Confusing "respiration" with "breathing."
- ✓ Right: Breathing (ventilation) is the mechanical movement of air; Respiration is the chemical reaction in cells.
- ❌ Wrong: Saying the diaphragm "pushes" the air out.
- ✓ Right: The diaphragm relaxes, decreasing volume, which increases pressure, forcing air out.
Exam Tips
- Command Words: If asked to "State," give a brief name or factor. If asked to "Explain," you must give a reason (e.g., "Alveoli are one cell thick so that the diffusion distance is short").
- Diagrams: Be ready to identify the internal vs. external intercostal muscles. The external muscles are on the outside of the ribcage.
- Calculations: You may be given a graph of breathing during exercise. Always look for the "resting" value (usually around 12–16 breaths per minute) to compare against active values.
- Real-world context: Questions often focus on how smoking or asthma affects the gas exchange surfaces by reducing surface area or clogging airways with mucus.