Breathing 10.6 KS3 The breathing system includes ribs, rib muscles, diaphragm, lungs, trachea, bronchi,
bronchioles and alveoli.
The windpipe (trachea) splits into two branches called bronchi, one going to each lung.
The bronchi divide repeatedly into smaller branches called bronchioles, which end in a
very large number of alveoli.
FT The breathing system takes air into and out of the body so that oxygen from the air can
and diffuse into the bloodstream and carbon dioxide can diffuse out of the bloodstream into the
HT air. The lungs are in the upper part of the body (thorax), protected by the ribcage and
separated from the lower part of the body (abdomen) by the diaphragm.
To make air move into the lungs the ribcage moves out and the diaphragm becomes flatter.
These changes are then reversed to make air move out of the lungs. The movement of air
into and out of the lungs is called ventilation.
HT To inhale:
muscles between the ribs contract, pulling the ribcage upwards;
at the same time the diaphragm muscles contract causing the diaphragm to flatten;
these two movements cause an increase in the volume of the thorax;
the consequent decrease in pressure results in atmospheric air entering the lungs.
The alveoli provide a very large, moist surface, richly supplied with blood capillaries so
that gases can readily diffuse into and out of the blood.
THE BREATHING SYSTEM
THE HUMAN BREATHING SYSTEM
The breathing system is located in the THORAX, which is the upper half of the body.
The breathing system includes:
Ribs These make a bony cage which protects the soft tissue in the thorax.
Rib muscles These are attached to the ribs, so that contraction and relaxation of the muscles can
move the ribcage up and down.
Diaphragm This is a flat sheet of muscle tissue that divides the thorax from the abdomen.
The heart and lungs are in the thorax, above the diaphragm, and the digestive
system is in the abdomen, below the diaphragm.
Lungs There are two lungs – left and right.
Trachea This tube carries air from the mouth and nose into the lungs, and out again.
(Windpipe) It is kept open by rings of stiff cartilage. You can feel these if you run your finger
down your neck, over your trachea.
Bronchi The trachea splits into two bronchi – left bronchus to the left lung, and right
bronchus to the right lung,
Bronchioles Each bronchus divides again and again into smaller branches called bronchioles.
Alveoli The bronchioles end in little sacs called alveoli.
DIAGRAM SHOWING THE HUMAN BREATHING SYSTEM
The breathing system takes air into and out of the body.
When air is taken in, oxygen from the air diffuses from the lungs into the blood stream and carbon
dioxide diffuses from the bloodstream into the lungs.
The oxygen is needed for the process of respiration, which is going on in the mitochondria of all the
living cells in the body.
The carbon dioxide is produced as a waste material from respiration, and must therefore be got rid of.
All gas exchange surfaces have certain characteristics in common:
1. Moist - gases diffuse best when in solution.
2. Thin - to allow diffusion to occur quickly.
3. Large surface area - to allow diffusion to occur quickly.
4. Transport system - multicellular arganisms need an efficient transport system to carry gases
around the body.
DIAGRAM SHOWING HOW GAS EXCHANGE HAPPENS
BREATHING IN AND BREATHING OUT
Breathing in is also called inhaling or inhalation, or inspiration.
Breathing out is also called exhaling or exhalation, or expiration.
The air we breathe in contains the normal atmospheric gases at the normal concentrations.
The air we breathe out has lost some oxygen and gained some carbon dioxide and water vapour.
COMPARISON OF INHALED AND EXHALED AIR
GAS INHALED AIR EXHALED AIR
OXYGEN 21% 16%
CARBON DIOXIDE 0.03% 4%
WATER VARIABLE VARIABLE (HIGHER)
NITROGEN 78% 78%
TEMPERATURE VARIABLE VARIABLE
VENTILATION OF THE LUNGS
In order to make air move into the lungs, the rib cage is moved up and down. You can see this happening
if you lie on your back.
This ventilation is carried out by two sets of muscles i.e. the diaphragm and the rib muscles.
Summary of what happens when you breathe IN.
1. Rib muscles contract, pulling the ribcage upwards.
2. Diaphragm muscles contract, causing the diaphragm to flatten.
3. As a result of (1) and (2), the volume inside the ribcage increases.
4. When the volume increases, the pressure inside the ribcage decreases.
5. Since the pressure inside the ribcage is lower than atmospheric pressure, air flows into
the lungs in order to equalise the pressures inside and outside the ribcage.
Summary of what happens when you breathe OUT.
This is the opposite of breathing in.
Draw two flow charts.
On the left, show what happens when we breathe in. On the right show what happens when we breathe
BREATHING IN BREATHING OUT
TO SHOW THAT INHALED AIR CONTAINS MORE OXYGEN THAN
A candle needs oxygen to burn. The more oxygen present, the longer the candle will burn.
We can make use of the principle to show that inhaled air contains more oxygen than exhaled air.
1. Place a gas-jar on a flat surface over a burning candle and record how long it takes for the candle to
2. Repeat this procedure three times and record the average time.
3. Fill a second gas-jar (the same size as the first gas jar) with air from the lungs. To do this, fill the jar
with water then stand the jar in a trough, then displace the water from the jar by blowing air into it
from the lungs via a tube.
4. Remove the jar containing expired air from the water, with a lid over the opening to keep the air in,
then place the jar over a burning candle.
5. Time how long it takes for the candle to go out. Repeat this procedure three times and record the
average time as before.
Time for candle to go out (secs)
1st trial 2nd trial 3rd trial average
There are several possible explanations for the results you obtain, e.g.
1. The carbon dioxide in the expired air put the candle out.
2. There was less oxygen in the expired air.
3. The wet air in the jar put the candle out.
UNDERLINE which of the above explanations is most likely to be true.
TO SHOW THAT EXHALED AIR CONTAINS MORE CARBON DIOXIDE THAN
Diagram of apparatus
1. Set up the apparatus as shown in the diagram, where the tubes are so arranged that if we blow, air
form the lungs goes through the right hand tube, and if we suck, air from the atmosphere goes through
the left hand tube. Each tube should contain equal volumes of limewater, which turns cloudy in the
presence of carbon dioxide.
NB If you set it up incorrectly, you will get a mouthful of limewater!
2. Suck and blow through the breathing tube for a few minutes and observe what happens to the
limewater in each tube.
Appearance of limewater
Boiling-tube A (inhaled
air passes through
Boiling-tube B (exhaled
air passes through
What do the results of this experiment demonstrate?
Explain why there is a difference in appearance of the limewater in tubes A and B.
HOW MUCH AIR DO WE BREATHE?
The maximum volume of air we can breathe in when we inhale as much as we can, i.e. when we
completely fill the lungs, is called the vital capacity of the lungs. The volume we breathe in and out at
any time is called the tidal volume. At rest we breathe in a relatively small amount of air, and we breathe
slowly. During exercise we breathe in a greater volume of air and we breathe faster. In other words, at
rest, the tidal volume of air breathed in and out is less than the tidal volume breathed in and out during
The aim of this experiment is to determine the vital capacity, the tidal volume at rest and the tidal
volume after exercise. This can be done simply by using a spirometer bag, which has a scale on it to
A. Measuring Vital Capacity
Fill your lungs as fully as possible, then blow the air into the spirometer bag, emptying your lungs. Roll
the bag up so that the air fills part of the bag and read off the volume of air in the bag, using the scale.
Record this volume in a table like the one shown below.
B. Measuring Tidal Volume At Rest
Make sure that the spirometer bag is completely empty. Sit quietly for a minute or two, then breathe out a
normal breath of air into the bag. Do not breathe out more air than you would normally breathe at rest.
Measure this volume of air and record the figure in the table.
C. Measuring Tidal Volume Immediately After Exercise
Make sure that the bag is empty. Then exercise vigorously for 1 minute. (Your teacher will tell you what
form of exercise you can do safely. Immediately breathe a breath of air into the spirometer bag, making
sure that it is the volume of air you would breathe after such an exercise. Measure and record the volume
of air breathed out.
HOW FAST DO WE BREATHE?
Sit quietly until your breathing rate is normal, then count the number of breaths you take in 1 minute.
Record this number of breaths in the results table like the one below.
Exercise vigorously (as before) for 1 minute, then record the number of breaths you take in 1 minute.
Record this number of breaths in the results table.
Volume (cm3) Breaths Per Minute
Vital Capacity At rest
Tidal Volume At Rest After Exercising
Tidal Volume After
1. State the vital capacity of the subject you studied.
2. State what the tidal volume of the subject was, at rest and after exercise.
3. Suggest a reason why the tidal volume increased after exercise.