Gas Exchange In Humans

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Notes

Features of Gas Exchange Surfaces

All gas exchange surfaces share common features to maximise diffusion.
**Large surface area** allows faster diffusion of gases.
**Thin walls** ensure short diffusion distances.
**Good ventilation** maintains diffusion gradients.
**Good blood supply** maintains high concentration gradients.
In humans, the **alveolus** is the gas exchange surface.

The Breathing System

Air enters through the **nose/mouth**, passes through the **trachea**.
The trachea splits into two **bronchi** (one to each lung).
Bronchi branch into smaller **bronchioles**.
Bronchioles end in tiny air sacs called **alveoli**.
The **diaphragm** and **intercostal muscles** help move air in and out.
**Rib cage** protects the lungs.

Investigating Differences in Inspired & Expired Air

Use **limewater** to test for carbon dioxide: clear → cloudy/milky with CO₂.
Setup: inspired air drawn through boiling tube A, expired air blown into tube B.
Tube A (inspired) limewater stays clear; tube B (expired) turns cloudy.
Shows expired air has **higher CO₂ concentration** than inspired air.

Differences in Inspired & Expired Air

Inspired air: ~21% oxygen, ~0.04% carbon dioxide, variable water vapour.
Expired air: ~16% oxygen, ~4% carbon dioxide, ~1% more water vapour.
Expired air is **warmer** due to heat from respiration.
Differences result from gas exchange in alveoli.

Investigating Effects of Physical Activity on Breathing

Exercise increases **frequency** and **depth** of breathing.
Measure breaths per minute and chest expansion at rest.
After $\ge 3$ minutes exercise, measure again.
Both breaths per minute and chest expansion increase after exercise.

Explaining Differences in Inspired & Expired Air (Extended)

Oxygen diffuses from alveoli into blood; carbon dioxide diffuses from blood into alveoli.
Oxygen used in respiration; carbon dioxide produced as a waste product.
Water vapour added from moist surfaces of respiratory tract.
Heat from respiration warms expired air.

Explaining the Link Between Physical Activity & Breathing (Extended)

Muscles respire more during exercise, increasing oxygen demand and CO₂ production.
Increased CO₂ in blood lowers pH (more acidic).
**Receptors in the brain** detect changes in CO₂ and pH.
Brain sends nerve impulses to diaphragm and intercostal muscles.
Rate and depth of breathing increase to supply more O₂ and remove more CO₂.
**Respiration** (chemical reactions) vs **breathing** (mechanical movement of air).

Alveolar cell (type I pneumocyte) is thin for gas exchange.

Practice questions

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1.Which of the following is a feature of gas exchange surfaces that allows rapid diffusion?
Easy
- AThick walls
- BSmall surface area
- CThin walls
- DPoor blood supply
2.What is the approximate percentage of oxygen in expired air?
Easy
- A16%
- B4%
- C21%
- D0.04%
3.Which gas causes limewater to turn cloudy?
Medium
- AOxygen
- BNitrogen
- CCarbon dioxide
- DWater vapour
4.Expired air contains more carbon dioxide than inspired air.
Easy
True or false?
5.The alveoli have thick walls to prevent gas exchange.
Medium
True or false?
6.State two features of the alveoli that make them efficient for gas exchange.
Medium
7.What is the name of the process by which oxygen moves from the alveoli into the blood?
Medium
8.Inspired air contains approximately 21% oxygen. Expired air contains approximately 16% oxygen. Calculate the percentage of oxygen absorbed by the body.
Medium
- A7
- B4
- C6
- D5

Slides

Features of Gas Exchange Surfaces

All gas exchange surfaces share common features to maximise diffusion.
Large surface area allows faster diffusion of gases.
Thin walls keep diffusion distances short.
Good ventilation and blood supply maintain concentration gradients.

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 mouth/nose, passes through the trachea, bronchi, and bronchioles to reach the alveoli.
The alveoli are tiny air sacs where gas exchange occurs.
The diaphragm and intercostal muscles help ventilate the lungs.

The tiny air sacs in the lungs where gas exchange takes place are called ____.

Investigating Differences in Inspired & Expired Air

Limewater turns cloudy when carbon dioxide is bubbled through it.
Inhaled air passed through limewater stays clear; exhaled air turns it cloudy.
This shows exhaled air contains more carbon dioxide than inhaled air.

What happens to limewater when exhaled air is bubbled through it?

It turns blue
It turns cloudy
It remains clear
It turns red

Differences in Inspired & Expired Air

Inspired air: ~21% oxygen, ~0.04% carbon dioxide, variable water vapour.
Expired air: ~16% oxygen, ~4% carbon dioxide, saturated with water vapour.
Expired air is warmer due to heat from respiration.

Which of these are true about expired air compared to inspired air?

Contains more oxygen
Contains more carbon dioxide
Contains more water vapour
Is cooler

Investigating the Effects of Physical Activity on Breathing

Exercise increases both the frequency and depth of breathing.
Count breaths per minute and measure chest expansion at rest and after exercise.
After exercise, breaths per minute and chest expansion increase.

What happens to breathing rate and depth during exercise?

Both increase
Both decrease
Rate increases, depth decreases
Rate decreases, depth increases

Explaining Differences in Inspired & Expired Air (Extended)

Oxygen diffuses from alveoli into blood; carbon dioxide diffuses from blood into alveoli.
This is why expired air has less oxygen and more carbon dioxide.
Water vapour is added as air passes over moist surfaces.

Oxygen diffuses from $the ____$ into the blood, while carbon dioxide diffuses from the blood into $the ____$ .

Explaining the Link Between Physical Activity & Breathing (Extended)

During exercise, muscle cells respire more, producing more carbon dioxide.
Carbon dioxide dissolves in blood plasma, lowering pH.
Receptors in the brain detect the pH change and send nerve impulses to increase breathing rate and depth.

What triggers an increase in breathing rate during exercise?

Low oxygen levels in blood
High carbon dioxide levels in blood
Low carbon dioxide levels in blood
High oxygen levels in blood

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