Gas Exchange In Humans

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Notes

Features of Gas Exchange Surfaces

All gas exchange surfaces share features that maximise the rate of diffusion.
**Large surface area** – allows faster diffusion of gases across the surface.
**Thin walls** – ensure short diffusion distances.
**Good ventilation** – maintains diffusion gradients.
**Good blood supply** – maintains a steep concentration gradient for faster diffusion.
The **alveolus** is the gas exchange surface in humans.

The Breathing System

Air enters through the **nose/mouth**, passes through the **trachea**, **bronchi**, **bronchioles**, and reaches the **alveoli**.
The **trachea** is supported by rings of **cartilage** to prevent collapse.
The **bronchi** branch from the trachea into each lung.
**Bronchioles** are smaller branches leading to alveoli.
**Alveoli** are tiny air sacs where gas exchange occurs.
The **diaphragm** is a sheet of muscle separating the thorax from the abdomen.
**Intercostal muscles** are located between the ribs; **external** (outside) and **internal** (inside).

Ventilation: Volume & Pressure Changes

**Inspiration (inhalation):** diaphragm **contracts** and flattens; external intercostal muscles **contract**, pulling ribs up and out.
This **increases** thorax volume, **decreases** air pressure inside lungs, drawing air in.
**Expiration (exhalation):** diaphragm **relaxes** and moves up; external intercostal muscles **relax**, ribs drop down and in.
This **decreases** thorax volume, **increases** air pressure, forcing air out.
**Forced exhalation** (e.g., during exercise) involves internal intercostal muscles contracting to pull ribs down more forcefully.
Cartilage in trachea and bronchi keeps airways open during pressure changes.

Differences Between Inspired and Expired Air

**Inspired air:** ~21% oxygen, ~0.04% carbon dioxide, variable water vapour.
**Expired air:** ~16% oxygen, ~4% carbon dioxide, more water vapour.
Nitrogen remains constant at ~78%.
Differences arise because oxygen diffuses into blood and carbon dioxide diffuses out in alveoli.
**Limewater test:** expired air turns limewater **cloudy** (milky) due to higher CO₂; inspired air does not.

Effects of Physical Activity on Breathing

Exercise **increases** both breathing **rate** (breaths per minute) and **depth** (tidal volume).
More oxygen is needed for increased **aerobic respiration** in muscles; more CO₂ is produced.
Increased CO₂ lowers blood pH, detected by **chemoreceptors** in the **medulla oblongata**.
The brain sends nerve impulses to diaphragm and intercostal muscles to increase ventilation.
After exercise, breathing remains elevated to repay **oxygen debt** (remove lactic acid).
Investigation: count breaths per minute and measure chest expansion before and after exercise.

Protecting the Breathing System

Airways are lined with **ciliated epithelial cells** and **goblet cells**.
**Goblet cells** produce **mucus** that traps dust, bacteria, and particles.
**Cilia** beat to move mucus upwards towards the throat, where it is swallowed or expelled.
This prevents pathogens and debris from reaching the lungs.

The human breathing system showing trachea, bronchi, lungs, and diaphragm.

Alveolus surrounded by capillaries; oxygen diffuses into blood, carbon dioxide diffuses into alveolus.

Comparison of inspiration (left) and expiration (right): diaphragm flattens during inspiration, domes during expiration.

Practice questions

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1.Which of the following is a feature of gas exchange surfaces that maximises the rate of gas exchange?
Easy
- ALarge surface area
- BThick walls
- CPoor blood supply
- DLack of ventilation
2.What is the function of cartilage in the trachea?
Easy
- ATo produce mucus
- BTo support the airways and keep them open
- CTo contract and expand the lungs
- DTo trap dust and bacteria
3.During inhalation, the diaphragm:
Easy
- ARelaxes and moves upwards
- BContracts and flattens
- CRemains stationary
- DContracts and moves upwards
4.Which gas has the same percentage in inspired and expired air?
Easy
- AOxygen
- BCarbon dioxide
- CNitrogen
- DWater vapour
5.What is the approximate percentage of carbon dioxide in exhaled air?
Medium
- A0.04%
- B4%
- C16%
- D21%
6.In an investigation using limewater to compare inhaled and exhaled air, which observation indicates a higher carbon dioxide concentration?
Medium
- ALimewater turns cloudy in the tube receiving exhaled air
- BLimewater turns cloudy in the tube receiving inhaled air
- CLimewater remains clear in both tubes
- DLimewater turns red in the tube receiving exhaled air
7.Which muscles contract during forced exhalation?
Medium
- AExternal intercostal muscles only
- BInternal intercostal muscles only
- CBoth external and internal intercostal muscles
- DDiaphragm only
8.What causes the increase in breathing rate during exercise?
Hard
- AIncreased oxygen levels in the blood
- BDecreased carbon dioxide levels in the blood
- CIncreased carbon dioxide levels detected by chemoreceptors
- DDecreased pH in the muscles

Slides

Features of Gas Exchange Surfaces

All gas exchange surfaces share features that maximise the rate of diffusion.
Large surface area – allows faster diffusion of gases.
Thin walls – short diffusion distance.
Good ventilation – maintains diffusion gradients.
Good blood supply – maintains a high concentration gradient.

Which of the following is NOT a feature of gas exchange surfaces?

Large surface area
Thick walls
Good blood supply
Good ventilation

The Breathing System

Air enters through the nose/mouth, passes through the trachea, bronchi, bronchioles, and into alveoli.
The trachea is supported by rings of cartilage to prevent collapse.
The lungs are located in the thorax, protected by the rib cage.
The diaphragm is a sheet of muscle separating the thorax from the abdomen.

The ____ is supported by rings of cartilage to prevent it from collapsing.

Differences Between Inspired and Expired Air

Inspired air: 21% oxygen, 0.04% carbon dioxide, variable water vapour.
Expired air: 16% oxygen, 4% carbon dioxide, more water vapour.
Nitrogen remains at 78% in both.
Expired air has less oxygen and more carbon dioxide due to gas exchange in alveoli.

What is the approximate percentage of oxygen in expired air?

21%
16%
4%
0.04%

Investigating Differences with Limewater

Limewater is a test for carbon dioxide: it turns cloudy/milky when CO₂ is bubbled through.
Inhaled air is drawn through boiling tube A; exhaled air is blown into tube B.
Tube A stays clear; tube B turns cloudy, showing exhaled air has more CO₂.

Limewater turns ____ in the presence of carbon dioxide.

Ventilation: Volume and Pressure Changes

Inhalation: diaphragm contracts (flattens), external intercostal muscles contract → rib cage moves up and out → thorax volume increases → pressure decreases → air flows in.
Exhalation: diaphragm relaxes (domes), external intercostal muscles relax → rib cage moves down and in → thorax volume decreases → pressure increases → air flows out.
During forced exhalation, internal intercostal muscles also contract to pull ribs down further.

Which of these are true about inhalation?

Diaphragm contracts and flattens
Thorax volume decreases
Air pressure inside lungs decreases
External intercostal muscles relax

Effects of Exercise on Breathing

Exercise increases breathing rate and depth (tidal volume).
Muscles respire more, producing more CO₂.
CO₂ lowers blood pH, detected by chemoreceptors in the medulla oblongata.
The brain sends nerve impulses to increase diaphragm and intercostal muscle activity.

What detects increased carbon dioxide levels during exercise?

Chemoreceptors in the nose
Chemoreceptors in the medulla oblongata
Stretch receptors in the lungs
Baroreceptors in the aorta

Protecting the Breathing System

The airways are lined with ciliated epithelial cells and goblet cells.
Goblet cells produce mucus that traps dust, bacteria, and pathogens.
Cilia beat to move mucus up towards the throat, where it is swallowed or coughed out.

____ cells produce mucus that traps particles, and ____ beat to move the mucus away from the lungs.

Role of Cartilage in the Trachea

Rings of cartilage surround the trachea and bronchi.
They provide support and prevent the airways from collapsing when air pressure drops during inhalation.

What is the function of cartilage in the trachea?

To produce mucus
To trap pathogens
To keep the airway open
To increase surface area

Summary of Gas Exchange

Alveoli provide a large surface area, thin walls, good blood supply, and ventilation.
Inspired air: high O₂, low CO₂; expired air: lower O₂, higher CO₂.
Ventilation is driven by diaphragm and intercostal muscles changing thorax volume and pressure.
Exercise increases breathing rate and depth to meet oxygen demand and remove CO₂.
Mucus and cilia protect the lungs from pathogens and dust.

Which of these are features of alveoli that aid gas exchange?

Large surface area
Thick walls
Good blood supply
Good ventilation

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