Introduction
Gas exchange in humans is the process by which oxygen enters the body and carbon dioxide is removed. This occurs primarily in the lungs through a system of specialized structures optimized for efficient gas transfer.
Features of Gas Exchange Surfaces
Four Essential Features
For effective gas exchange, the surface must have four key characteristics:
| Feature | Purpose | How it Works |
|---|---|---|
| Large surface area | More area for gas exchange | Many alveoli create large total area (~70 m² in adult lungs) |
| Thin surface | Short diffusion distance | Alveoli and capillary walls are one cell thick (~0.5 μm) |
| Good blood supply | Maintain concentration gradient | Capillaries carry away O₂, bring CO₂ |
| Good ventilation | Maintain concentration gradient | Breathing brings in O₂, removes CO₂ |
How These Features Work Together
- Large surface area = more sites for O₂ and CO₂ to cross
- Thin surface = faster diffusion (shorter distance)
- Good blood supply = blood constantly removes O₂, loads with CO₂
- Good ventilation = fresh air constantly replaces stale air
Breathing System Parts
Main Structures
The breathing system consists of several parts working together to move air into and out of lungs:
| Structure | Location | Function |
|---|---|---|
| Lungs | Chest cavity | Main site of gas exchange |
| Diaphragm | Below lungs | Main muscle for breathing |
| Ribs | Thoracic cavity | Protect lungs, move for breathing |
| Intercostal muscles | Between ribs | Assist rib movement |
| Larynx | Neck | Voice box, connects throat to trachea |
| Trachea | Chest | Airway from larynx to lungs |
| Bronchi | Lungs | Two branches from trachea to each lung |
| Bronchioles | Lungs | Smaller branches leading to alveoli |
| Alveoli | Lungs | Tiny air sacs where gas exchange occurs |
| Capillaries | Lungs | Blood vessels surrounding alveoli |
Ventilation Mechanism
Role of Ribs, Intercostal Muscles, and Diaphragm
Breathing (ventilation) involves air movement into and out of lungs through changes in thoracic volume.
Inbreathing (Inspiration)
- Diaphragm contracts - flattens and moves downward
- Intercostal muscles contract - ribs move up and out
- Thoracic volume increases - chest cavity gets bigger
- Air pressure inside lungs decreases (Boyle’s law)
- Air flows in from higher pressure outside to lower pressure inside
Outbreathing (Expiration)
- Diaphragm relaxes - returns to dome shape
- Intercostal muscles relax - ribs move down and in
- Thoracic volume decreases - chest cavity gets smaller
- Air pressure inside lungs increases
- Air flows out from higher pressure inside to lower pressure outside
Internal and External Intercostal Muscles
| Muscle Type | Location | Function |
|---|---|---|
| External intercostal | Between ribs, front of chest | Contract to lift ribs up for inbreathing |
| Internal intercostal | Between ribs, deeper layer | Contract to push ribs down for forced outbreathing |
Function of Cartilage in Trachea
- Structure: Rings of cartilage around trachea
- Purpose: Keep trachea open
- Reason: Prevents collapse during breathing
- Flexibility: Allows some movement without restricting airflow
Differences in Inspired and Expired Air
Composition Changes
Air changes as it passes through the respiratory system:
| Component | Inspired Air | Expired Air | Change |
|---|---|---|---|
| Oxygen | ~21% | ~16% | Decreased (used in respiration) |
| Carbon dioxide | ~0.04% | ~4% | Increased (waste product) |
| Water vapour | Variable (~1%) | Saturated (~6%) | Increased (from moist lungs) |
Why Changes Occur
Oxygen Decrease
- Oxygen diffuses from alveoli into blood
- Used in cellular respiration for energy
- Lower concentration in expired air
Carbon Dioxide Increase
- CO₂ produced as waste from respiration
- Diffuses from blood into alveoli
- Higher concentration in expired air
Water Vapour Increase
- Lungs are moist (mucous membranes)
- Water evaporates into air spaces
- Exhaled air is saturated with water
Gas Exchange Process
How Gases Move Between Lungs and Blood
Oxygen Movement
- Oxygen concentration higher in alveoli (~14 kPa)
- Oxygen concentration lower in capillary blood (~5 kPa)
- Diffusion: Oxygen moves down concentration gradient
- Binding: Oxygen binds to haemoglobin in red blood cells
Carbon Dioxide Movement
- CO₂ concentration higher in capillary blood (~6 kPa)
- CO₂ concentration lower in alveoli (~5 kPa)
- Diffusion: CO₂ moves down concentration gradient
- Exhalation: CO₂ leaves lungs through breathing
Protective Mechanisms
Goblet Cells
- Produce mucus lining airways
- Traps dust, particles, pathogens
- Prevents them from reaching lungs
Mucus
- Sticky layer lining respiratory tract
- Traps inhaled particles and pathogens
- Keeps airways moist
- Helps warm incoming air
Ciliated Cells
- Hair-like projections on cell surface
- Beat in coordinated waves
- Move mucus (with trapped particles) upward
- Expels mucus through coughing or swallowing
Effect of Physical Activity on Breathing
Breathing Rate and Depth Changes
| State | Breathing Rate | Breathing Depth |
|---|---|---|
| Rest | ~12-20 breaths/min | ~500-1000 ml per breath |
| Exercise | 40-60+ breaths/min | 2000-4000+ ml per breath |
Why Breathing Rate Increases
- Muscles need more oxygen for contraction
- CO₂ production increases from respiration
- Brain detects increased CO₂ in blood
- Breathing rate increases to supply O₂ and remove CO₂
Why Breathing Depth Increases
- More air per breath = more O₂ intake
- More CO₂ removed per breath
- Deeper ventilation = more efficient gas exchange
- Meets higher metabolic demand
Link Between Physical Activity and Breathing Rate
Carbon Dioxide Detection
- Brain detects rising CO₂ levels in blood
- Signals breathing centre to increase rate
- Faster breathing removes excess CO₂
- Prevents acidosis (blood becoming too acidic)
Oxygen Demand
- Active muscles need more O₂
- Faster breathing brings in more O₂
- Increased blood flow delivers O₂ to muscles
Investigation: Limewater Test
Purpose
- Detect differences between inspired and expired air
- Limewater turns cloudy with carbon dioxide
Method
- Set up apparatus: Blow through tubing connected to limewater
- Inspired air test: Draw air through limewater
- Expired air test: Blow air through limewater
- Observe results: Note cloudiness in both cases
Expected Results
| Air Type | Result | Explanation |
|---|---|---|
| Inspired air | Little or no cloudiness | Low CO₂ concentration (~0.04%) |
| Expired air | Significant cloudiness | High CO₂ concentration (~4%) |
Explanation
- Limewater (calcium hydroxide) reacts with CO₂
- Forms calcium carbonate precipitate
- Makes solution cloudy/white
Glossary
| Term | Definition |
|---|---|
| Ventilation | Movement of air into and out of lungs |
| Inspiration | Inbreathing - air moving into lungs |
| Expiration | Outbreathing - air moving out of lungs |
| Diaphragm | Main muscle for breathing |
| Intercostal muscles | Muscles between ribs |
| Alveoli | Tiny air sacs in lungs |
| Capillaries | Tiny blood vessels |
| Haemoglobin | Oxygen-carrying protein in red blood cells |
| Concentration gradient | Difference in concentration between two areas |
| Diffusion | Movement from high to low concentration |
| Limewater | Calcium hydroxide solution for CO₂ detection |
| Goblet cells | Cells that produce mucus |
| Ciliated cells | Cells with hair-like projections |
| Mucus | Sticky substance trapping particles |