CIE iGCSE Co-ordinated Sciences-B11. Gas exchange in humans- Study Notes- New Syllabus
CIE iGCSE Co-ordinated Sciences- Gas exchange in humans – Study Notes
CIE iGCSE Co-ordinated Sciences- Gas exchange in humans – Study Notes -CIE iGCSE Co-ordinated Sciences – per latest Syllabus.
Key Concepts:
Core
- Identify in diagrams and images the following parts of the breathing system: lungs, diaphragm, ribs, intercostal muscles, larynx, trachea, bronchi, bronchioles, alveoli and associated capillaries
- Investigate the differences in composition between inspired and expired air using limewater as a test for carbon dioxide
- Describe the differences in composition between inspired and expired air, limited to: oxygen, carbon dioxide and water vapour
- Investigate and describe the effects of physical activity on the rate and depth of breathing
- Describe the features of gas exchange surfaces in humans, limited to: large surface area, thin surface, good blood supply and good ventilation with air
Supplement
- Explain the differences in composition between inspired and expired air
- Explain the link between physical activity and the rate and depth of breathing in terms of: an increased carbon dioxide concentration in the blood, which is detected by the brain, leading to an increased rate and greater depth of breathing
CIE iGCSE Co-Ordinated Sciences-Concise Summary Notes- All Topics
Breathing System – Key Structures to Identify
🌬️ Main Parts to Identify
- Lungs
Two large spongy organs in the chest.
Filled with bronchi, bronchioles, and alveoli.
Function: Main site of gas exchange. - Diaphragm
Dome-shaped sheet of muscle under the lungs.
Contracts → flattens → increases chest volume (inhalation).
Relaxes → curves upward → decreases volume (exhalation). - Ribs
Bones forming rib cage around lungs and heart.
Protect lungs + assist in breathing (move up and out during inhalation). - Intercostal Muscles
Muscles between ribs.
Work with diaphragm to expand/contract chest.
External intercostals: help inhale.
Internal intercostals: help forceful exhale. - Larynx (Voice Box)
Located at top of trachea.
Contains vocal cords. - Trachea (Windpipe)
Tube carrying air from throat → bronchi.
Lined with cartilage rings (to keep it open). - Bronchi
Two branches from trachea → each lung.
Carry air into lungs. - Bronchioles
Smaller branches of bronchi inside lungs.
Spread air to alveoli. - Alveoli
Tiny air sacs at ends of bronchioles.
Thin walls → surrounded by capillaries.
Site of gas exchange (O₂ in, CO₂ out). - Associated Capillaries
Fine blood vessels covering alveoli.
Allow diffusion of gases between blood and air.
Oxygen → blood, CO₂ → alveoli.
📊 Summary Table
| Structure | Location | Function | Diagram Feature |
|---|---|---|---|
| Lungs | Chest cavity | Gas exchange | Two spongy sacs |
| Diaphragm | Below lungs | Breathing movements | Dome-shaped muscle |
| Ribs | Surround chest | Protection + movement | Bony arcs |
| Intercostal muscles | Between ribs | Expand/contract chest | Thin bands |
| Larynx | Top of trachea | Voice + air passage | Bulge at throat |
| Trachea | Neck → chest | Air passage | Tube with cartilage rings |
| Bronchi | From trachea → lungs | Air conduction | Two branches |
| Bronchioles | Inside lungs | Air distribution | Fine tubes |
| Alveoli | Ends of bronchioles | Gas exchange | Tiny sacs |
| Capillaries | Around alveoli | Gas diffusion | Web around sacs |
⚡ Quick Recap
Big picture: Air enters via trachea → bronchi → bronchioles → alveoli → capillaries.
Muscles involved: Diaphragm + intercostal muscles.
Protective framework: Ribs.
Special structures: Larynx = voice box.
Exchange site: Alveoli + capillaries (like grapes wrapped in a net).
Investigating Inspired vs Expired Air with Limewater
🌱 Introduction
We can compare inspired (inhaled) air and expired (exhaled) air to see how their gas composition changes in the lungs.
The simplest school experiment uses limewater as a test for carbon dioxide (CO₂).
🧪 Principle of the Test
- Limewater = dilute calcium hydroxide solution.
- CO₂ present → limewater turns milky/cloudy (due to calcium carbonate formation).
Equation:
\[
\text{Ca(OH)}_{2} \,(aq) + \text{CO}_{2} \,(g) \;\rightarrow\; \text{CaCO}_{3} \,(s) + \text{H}_{2}O \,(l)
\]
🔧 Apparatus
- Two test tubes or beakers with equal amounts of fresh limewater
- Drinking straws / tubing (clean, separate for each test)
- Clamp stand / rack
🧭 Method
- Fill two containers with equal volumes of limewater.
- Inspired air (control for room air):
Bubble normal room air through limewater (using a syringe or pump, or simply blowing in gently with a straw).
Note how long it takes to turn milky. - Expired air:
Exhale gently into the second limewater container using a clean straw.
Observe how quickly it turns milky. - Compare the results between the two.
- Repeat to improve reliability.
🎯 Observations
- Room/inspired air: Limewater turns milky slowly or very faintly (low CO₂).
- Expired air: Limewater turns milky much faster (high CO₂).
🧠 Explanation
In the alveoli:
- Oxygen diffuses into blood.
- Carbon dioxide diffuses out of blood.
So expired air has:
- Less oxygen
- More carbon dioxide
- More water vapour (air gets humidified in the lungs)
📊 Summary Table
| Gas | Inspired (inhaled) air | Expired (exhaled) air | Limewater test |
|---|---|---|---|
| Oxygen | ~21% | ~16% | Not tested here |
| Carbon dioxide | ~0.04% | ~4% | Limewater turns milky faster |
| Water vapour | Variable | Higher | Not tested here |
📝 Conclusion
Expired air has more CO₂ than inspired air.
This is proven by the limewater test: it becomes milky much quicker when bubbled with expired air.
The difference is due to gas exchange during respiration.
⚡ Quick Recap
Test used: limewater → CO₂ turns it milky.
Expired air → limewater cloudy fast (more CO₂).
Inspired air → limewater slower change (less CO₂).
Explains: O₂ taken in, CO₂ + H₂O breathed out.
Inspired vs Expired Air (Composition Differences)
📌 Introduction
Air changes its composition as it passes through the lungs because gas exchange occurs in the alveoli. Oxygen is absorbed, carbon dioxide is released, and air becomes more moist.
🌬️ Key Differences
- Oxygen (O₂)
Inspired air: ~21%
Expired air: ~16%
Reason: Oxygen diffuses from alveoli → blood for respiration. - Carbon dioxide (CO₂)
Inspired air: ~0.04% (almost negligible)
Expired air: ~4%
Reason: Carbon dioxide diffuses from blood → alveoli (waste of respiration). - Water Vapour (H₂O)
Inspired air: Variable (depends on environment → dry air has less, humid air has more)
Expired air: Always high → because air is moistened in respiratory tract and alveoli.
📊 Summary Table
| Gas | Inspired Air | Expired Air | Reason |
|---|---|---|---|
| Oxygen (O₂) | ~21% | ~16% | Used by body cells for respiration |
| Carbon dioxide (CO₂) | ~0.04% | ~4% | Produced by respiration, exhaled |
| Water vapour (H₂O) | Variable | Always higher | Air becomes moist in lungs |
⚡ Quick Recap
O₂ decreases (taken up by blood).
CO₂ increases (released from blood).
Water vapour increases (air humidified in lungs).
👉 Simple memory trick: “In with O₂, out with CO₂ + H₂O.”
Effects of Physical Activity on Breathing
🌱 Introduction
During physical activity, muscles need more energy for contraction. This energy comes from increased respiration, which uses more oxygen and produces more carbon dioxide.
To meet this demand, our breathing pattern changes.
🔬 What to Investigate
We can measure:
- Breathing rate → number of breaths per minute.
- Breathing depth → how much air is taken in/out per breath (tidal volume).
📊 Method (School Investigation Idea)
- Count breaths per minute at rest (normal breathing).
- Perform moderate exercise (e.g. jogging on the spot for 1–2 min).
- Immediately count breaths per minute again.
- Observe the depth of breathing (shallow vs deep chest movements).
- Compare results.
🎯 Observations
- At rest:
Breathing rate = slower (about 12–18 breaths/min).
Breathing depth = shallow/normal. - During exercise:
Breathing rate = faster (can double or triple).
Breathing depth = much deeper (lungs take in more air per breath). - After exercise:
Breathing rate stays high for a short time, then gradually returns to normal.
🧠 Biological Explanation
- Exercise increases muscle activity → more ATP needed.
- Respiration rate rises, consuming more O₂ and releasing more CO₂.
- Brain (medulla) detects ↑ CO₂ levels in blood → sends signals to breathing muscles.
- Intercostal muscles + diaphragm contract faster and stronger → breathing becomes faster and deeper.
This ensures:
- More O₂ uptake into blood.
- Faster removal of CO₂.
📊 Summary Table
| Condition | Breathing Rate | Breathing Depth | Reason |
|---|---|---|---|
| Rest | Slow (12–18/min) | Shallow/normal | Low O₂ demand |
| Exercise | Fast (↑↑) | Deep (↑ tidal volume) | High O₂ demand, more CO₂ removal |
| Recovery | Gradually decreases | Deep at first, then normal | Body repays oxygen debt |
⚡ Quick Recap
Physical activity → faster + deeper breathing.
Reason = more O₂ needed, more CO₂ produced.
Controlled by brain’s breathing centre responding to CO₂ levels.
Recovery period = “oxygen debt repayment.”
👉 Memory hook: “Run more → breathe more (fast + deep).”
Features of Gas Exchange Surfaces in Humans
📌 Introduction
In humans, gas exchange happens in the alveoli of the lungs. To be efficient, the surface must allow rapid diffusion of gases (O₂ in, CO₂ out).
🌬️ Key Features
- Large Surface Area
Millions of tiny alveoli (like clusters of grapes).
Combined surface area ≈ 70 m² (about the size of a tennis court).
More area → more oxygen and carbon dioxide can diffuse at the same time. - Thin Surface
Alveolar wall is just 1 cell thick.
Capillary wall also 1 cell thick.
Short diffusion pathway → gases move quickly in/out. - Good Blood Supply
Each alveolus is surrounded by a dense network of capillaries.
Maintains a concentration gradient:
– Oxygen constantly taken away into blood.
– Carbon dioxide constantly brought in from blood. - Good Ventilation with Air
Breathing movements (diaphragm + intercostal muscles) continuously refresh air in alveoli.
Maintains steep O₂ in / CO₂ out gradient between alveoli and blood.
📊 Summary Table
| Feature | Adaptation in humans | Benefit |
|---|---|---|
| Large surface area | Millions of alveoli | More diffusion at once |
| Thin surface | Alveolar + capillary walls 1 cell thick | Short diffusion path |
| Good blood supply | Dense capillary network | Maintains steep conc. gradient |
| Good ventilation | Breathing in & out | Keeps fresh O₂, removes CO₂ |
⚡ Quick Recap
Alveoli are perfect for diffusion because they are:
– Big area (lots of alveoli).
– Thin walls (1 cell).
– Rich blood supply (capillaries).
– Well ventilated (breathing keeps air fresh).
👉 Memory trick: “BTVV → Big, Thin, Vessels, Ventilation.”
Differences in Composition of Inspired vs Expired Air
📌 Introduction
When air passes through the lungs, gas exchange occurs at the alveoli. Oxygen is absorbed into the blood, and carbon dioxide + water vapour are released. This changes the composition of air we breathe out compared to what we breathe in.
🌬️ Key Differences
- Oxygen (O₂)
Inspired air: ~21%
Expired air: ~16%
Reason: Oxygen diffuses into blood and is used in respiration. - Carbon dioxide (CO₂)
Inspired air: ~0.04% (tiny amount)
Expired air: ~4%
Reason: CO₂ is a waste product of respiration and diffuses out of blood into alveoli. - Water vapour (H₂O)
Inspired air: Variable (depends on humidity of surroundings).
Expired air: Always higher (lungs add moisture and warmth). - Nitrogen (N₂)
Inspired air: ~78%
Expired air: ~78% (unchanged)
Reason: Nitrogen is not used in respiration.
📊 Summary Table
| Gas | Inspired Air | Expired Air | Reason |
|---|---|---|---|
| Oxygen (O₂) | ~21% | ~16% | Taken into blood for respiration |
| Carbon dioxide (CO₂) | ~0.04% | ~4% | Released as waste of respiration |
| Water vapour (H₂O) | Variable | Higher | Air humidified in lungs |
| Nitrogen (N₂) | ~78% | ~78% | No role in respiration |
⚡ Quick Recap
O₂ decreases (used by body).
CO₂ increases (waste from respiration).
Water vapour increases (lungs moisten air).
N₂ stays same (not used).
👉 Memory tip: “In: O₂, Out: CO₂ + H₂O”
Physical Activity and Breathing Control
📌 Introduction
During exercise, breathing becomes faster and deeper. This is controlled automatically by the brain in response to changes in the blood.
🌬️ Step-by-Step Link
- Exercise increases respiration
Muscles respire more to release energy for contraction.
This produces extra CO₂ as a waste product. - CO₂ concentration rises in blood
Carbon dioxide dissolves in blood → forms carbonic acid.
Blood becomes slightly more acidic. - Detection by brain
Special receptors in the brain (medulla) sense the increase in CO₂ / acidity. - Response sent to breathing muscles
Brain sends nerve signals to the diaphragm and intercostal muscles. - Breathing rate and depth increase
Faster breathing = more breaths per minute.
Deeper breathing = greater volume of air per breath. - Effect
More oxygen enters alveoli → diffuses into blood.
More carbon dioxide is removed from blood → exhaled.
Restores normal gas levels in blood.
📊 Summary Table
| Stage | Event | Result |
|---|---|---|
| Exercise | ↑ Respiration in muscles | ↑ CO₂ produced |
| Blood | ↑ CO₂ concentration | Blood more acidic |
| Brain detects | Medulla senses CO₂ | Sends signals to muscles |
| Breathing response | Faster + deeper | ↑ O₂ in, ↑ CO₂ out |
⚡ Quick Recap
Exercise → more CO₂ in blood.
Brain detects CO₂ rise.
Signals → diaphragm + intercostals.
Breathing becomes faster + deeper.
Ensures: more O₂ supply + CO₂ removal.
👉 Memory hook: “CO₂ tells the brain → breathe more and breathe deeper.”