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4.2Unit 1: Breathing and respiration

Gas exchange — Cambridge Checkpoint Biology 0893 Revision Notes & Topical Questions

5 min read 9 parts Lesson 2 of 4 2 quizzes

📚 Cambridge Checkpoint Biology 0893 Hub / Topical Revision
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Lesson summary

4.2 Gas exchange — CHECKPOINT BIOLOGY 0893 notes: Describe the structure of an air sac, explain gas exchange by diffusion, and compare inspired and expired air.The lungs are mostly holes called air sacs, also called alveoli.Many tiny blood vessels called capillaries are wrapped a…

  • Haemoglobin
  • Inspired air
  • Many tiny blood vessels called capillaries are wrapped around the air sacs
  • The wall of an air sac is a single layer of very thin cells
  • The capillary is pressed tightly against the air sac, and its wall is also a single layer of very thin cells
  • Only two very thin cells separate the air from the blood

Section 1 of 9Air sacs

1
The lungs are mostly holes called air sacs, also called alveoli.
2
Many tiny blood vessels called capillaries are wrapped around the air sacs.
3
The wall of an air sac is a single layer of very thin cells.
4
The capillary is pressed tightly against the air sac, and its wall is also a single layer of very thin cells.
5
Only two very thin cells separate the air from the blood.

Section 2 of 9Gas exchange

Gas exchange
Gas exchange oxygen from the air goes into the blood and carbon dioxide from the blood goes into the air.
The blood in the capillary comes from the body organs.
  • Cells in those organs use up oxygen and make carbon dioxide.
  • So the blood has little oxygen and a lot of carbon dioxide.
The air in the air sac comes from outside the body.
  • It has a lot of oxygen and only a small amount of carbon dioxide.

Section 3 of 9Diffusion of the gases

Diffusion
Diffusion the movement of particles from where there are many of them to where there are fewer of them.
Gas particles move freely, so oxygen particles move from the air, through the thin cell walls, into the blood.
Carbon dioxide has the higher concentration in the blood, so it diffuses from the blood into the air in the air sac.
Oxygen dissolves in the blood and goes into the red blood cells, where it combines with haemoglobin.

Section 4 of 9Adaptations of air sacs

AdaptationHow it helps gas exchange
Very large surface area from millions of air sacsMore gas can diffuse at once
Very thin walls, one cell thickGives a short distance for diffusion
Rich supply of blood capillariesKeeps a steep concentration difference by carrying oxygen away and bringing carbon dioxide
Moist liningLets gases dissolve

Section 5 of 9Why air sacs are small and many

Why air sacs are small and many
An agar jelly model with holes and coloured dye represents the lungs and the oxygen in the air sacs.
  • Holes are an analogy for air sacs, and the dye is an analogy for oxygen.
  • Many small holes with the same total volume as a few large holes let the dye spread through the jelly faster.
Many very small air sacs are better than a few large ones because they give a larger surface area for the same volume.
The model is limited, because jelly is not a living tissue and has no blood flow.

Section 6 of 9Comparing inspired and expired air

  • Inspired air is the air that is breathed in, and expired air is the air that is breathed out.
  • Limewater goes cloudy (milky) when carbon dioxide bubbles through it.
  • Method:
1
Set up two test tubes of limewater, A and B, connected by tubing to one mouthpiece.
2
Arrange the glass tubes so that breathing in draws air through one tube of limewater and breathing out pushes air through the other.
3
Breathe in and out gently through the mouthpiece.
4
Continue until the limewater in one tube goes cloudy and note which tube it is.
Safety
  • Safety the tubing must be clean, the mouthpiece must not be shared, and no limewater must reach the mouth.
Result
  • Result the limewater bubbled by expired air goes cloudy first, showing that expired air contains more carbon dioxide than inspired air.

Section 7 of 9Gases in inspired and expired air

GasInspired airExpired air
OxygenAbout 21%About 16%
Carbon dioxideAbout 0.04%About 4%
Water vapourLessMore
TemperatureCoolerWarmer
  • Nitrogen stays about 78%.

Section 8 of 9Key terms

TermMeaning
Gas exchangeThe movement of oxygen into the blood and carbon dioxide out of the blood in the air sacs.
DiffusionThe movement of particles from a region where they are many to where they are fewer.
CapillaryA tiny blood vessel wrapped around an air sac.
HaemoglobinThe red pigment in red blood cells that combines with oxygen.
Inspired airAir that is breathed in.
Expired airAir that is breathed out.
LimewaterA solution that turns cloudy when carbon dioxide is bubbled through it.

Section 9 of 9Exam technique

1
Saying oxygen is used up completely in the lungs — expired air still contains about 16% oxygen.
2
Saying air sacs have thick walls — they are one cell thick.
3
Saying gas exchange moves gases by pumping — it happens by diffusion.
4
Saying expired air has no oxygen — it contains less oxygen but still some.
5
Using limewater without checking that the tubing is clean and not shared.

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