11 Gas exchange in humans

Key objectives l the functions of cartilage in the trachea

The objectives for this chapter are to revise: l the role of the ribs, intercostal muscles and
l the features of gaseous exchange surfaces in diaphragm in ventilation of the lungs
humans l how to explain differences in composition
l structures associated with the breathing between inspired and expired air
system l how to explain the link between physical
l differences in composition between inspired activity and the rate and depth of breathing
and expired air, and the test for carbon dioxide l how to explain the role of goblet cells,
l the effects of physical activity on the rate and mucus and ciliated cells in protecting the
depth of breathing breathing system

Gas exchange in humans

This process involves the passage of gases such as oxygen into and carbon
dioxide out of cells or a transport system. First, air needs to be in contact
with the gaseous exchange surface. This is achieved by breathing. Figure 11.1
shows the breathing system of a human.
larynx (voice box)

trachea

alveoli section through

rib
bronchiole intercostal muscle

bronchus left lung Revision activity

To help you remember the
sequence of structures
through which air passes
when you breathe in,
imagine an apple tree:
l Trunk Trachea
l Bough Bronchus
diaphragm l Branches Bronchioles
l Apples Alveoli

 Figure 11.1 The human breathing system

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Sample question
A new, alternative treatment for diabetes is being developed that involves
inhaling insulin into the lungs as a spray.
Teacher’s comments
Suggest the path the spray would take from the mouth to enter the
alveoli. [3] The student has named
all the parts involved, but
Student’s answer bronchioles and bronchi
are the wrong way round,
The spray would pass through the trachea, then through the bronchioles, then the so only 2 marks are
bronchi to the alveoli. ✓✓ awarded.

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 11 Gas exchange in humans

Features of respiratory surfaces air in

and out
direction of
blood flow
Gaseous exchange relies on diffusion. To be efficient, the gaseous
exchange surface must: wall of
alveolus
l be thin – a short distance for gases to diffuse moist
surface
l have a large surface area – for gases to diffuse over
l have good ventilation with air – this creates and maintains a
concentration gradient
l have a good blood supply – to transport oxygen to respiring tissues and red blood
bring carbon dioxide from those tissues cell wall of blood
capillary
Key
The gaseous exchange surfaces in mammals are the alveoli in the lungs. oxygen
Figure 11.2 shows the features supporting gaseous exchange in an carbon
alveolus. Figure 11.3 shows the blood supply of the alveoli. dioxide

 Figure 11.2 Features for

The composition of inspired and expired air gaseous exchange in an
alveolus
You need to be able to state the percentages shown in Table 11.1.
to
However, the explanations are needed only for the extended paper. pulmonary
q Table 11.1 Composition of inspired and expired air vein

Inspired Expired air passage

from capillary
Gas air/% air/% Explanation pulmonary
Nitrogen 79 79 Not used or produced by body artery
processes
Oxygen 21 16 Used up in the process of respiration, alveolus
but the system is not very efficient, so
only a small proportion of the oxygen
available is absorbed from the air
Carbon dioxide 0.04 4 Produced in the process of respiration  Figure 11.3 Blood supply of the
alveoli
Water vapour Variable Saturated Produced in the process of respiration;
moisture evaporates from the surface breathe in and out
of the alveoli here

Testing for carbon dioxide

Limewater can be used to test for carbon dioxide – it changes colour from
colourless to milky when the gas is bubbled through.
A B
You need to be able to describe how to investigate the differences in
composition between inspired and expired air, using limewater as a test for
carbon dioxide. Using the apparatus in Figure 11.4 is one way of doing this.
When you breathe in through the apparatus, air is sucked in through
boiling tube A. When you breathe out, air is blown through boiling tube B.
limewater
The limewater reacts with any carbon dioxide in the air bubbled through it
and changes from colourless to milky. Expired air makes limewater change
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colour more quickly than inspired air because there is more carbon dioxide  Figure 11.4 Apparatus used
present in expired air. to test for carbon dioxide

Skills
Calculating percentage change
The formula for percentage change is: Applying these values to the equation, the percentage
change change in oxygen between breathing in and out is:
× 100
starting value −5
× 100 = −23.8%
For example, in Table 11.1 the percentage of 21
oxygen in inspired air is 21%. This falls to 16% in
expired air. Therefore, the change is 21 – 16 = –5%.

© Dave Hayward 2022 73

 Sample question

Effects of physical activity on breathing

Both breathing rate and depth increase during exercise. The volume
of air breathed in and out during normal, relaxed breathing is about
0.5 litres. This is the tidal volume. Breathing rate at rest is about 12
breaths per minute.
During exercise, the volume inhaled (depth) increases to about 5 litres
(depending on the age, sex, size and fitness of the person). The maximum
amount of air breathed in or out in one breath is the vital capacity.
Breathing rate can increase to over 20 breaths per minute.

For limbs to move faster during exercise, aerobic respiration in the

skeletal muscles increases. Carbon dioxide is a waste product of aerobic
respiration (see Chapter 12). As a result, carbon dioxide builds up in
the muscle cells and diffuses into the plasma in the bloodstream more
rapidly. The brain detects increases in carbon dioxide concentration
in the blood and stimulates the breathing mechanism to speed up,
increasing the rate of expiration of the gas.

You need to be able to describe how to investigate the effects of physical

activity on the rate and depth of breathing. This can be done with an
instrument called a spirometer. It records, digitally or on paper, the
volume of air being inspired and expired by a person breathing through
the apparatus. From the data produced, the volume of air inspired and
expired per breath can be calculated. The number of breaths per minute
and the volume of oxygen used by the person can also be worked out.

Ventilation of the lungs

Figure 11.5 shows the relationship between the intercostal muscles, diaphragm and ribcage that is
required to achieve ventilation of the lungs.
3 air drawn in 3 air expelled
contracted external contracted internal
intercostal muscles intercostal muscles
trachea
relaxed internal pleural membranes 1 ribs return
intercostal muscles
pleural
2 lungs return to
fluid spinal
original volume
column
2 lungs
expanded

rib
1 diaphragm
1 diaphragm relaxes and
pulled down contracted returns to its relaxed
muscle of domed shape muscle of
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diaphragm diaphragm
(a) inhaling (exaggerated) (b) exhaling

 Figure 11.5 The intercostal muscles, diaphragm and ribcage

The movement of the ribcage is brought about by You need to be able to identify the external
the contraction of two sets of intercostal muscles, and internal intercostal muscles in images and
which are attached to the ribs. The external diagrams.
intercostal muscles are attached to the external
The diaphragm is a tough, fibrous sheet at the
surface of the ribs; the internal intercostal
base of the thorax, with muscle around its edge.
muscles are attached to the internal surface.

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 11 Gas exchange in humans

Inspiration (breathing in) potentially dangerous if not actively removed. Two

types of cell provide mechanisms to help achieve
l When the external intercostal muscles this (Figure 11.6).
contract, they move the ribcage upwards and
mucus
outwards, increasing the volume of the thorax. cilia

l When the diaphragm muscle contracts, the

diaphragm moves down, again increasing the
volume of the thorax. ciliated cell
goblet cell
l This increase in volume reduces the air
pressure in the thoracic cavity.
 Figure 11.6 Epithelial lining of the respiratory tract
l As the air pressure outside the body is higher, air
rushes into the lungs through the mouth or nose. l Goblet cells are found in the epithelial lining
of the trachea, bronchi and some bronchioles
Expiration (breathing out) of the respiratory tract. Their role is to secrete
mucus. The mucus forms a thin film over
l The opposite of inspiration happens when
the internal lining. This sticky liquid traps
breathing out.
pathogens and small particles, preventing
l During forced exhalation, the internal them from entering the alveoli, where they
intercostal muscles contract and the diaphragm could cause infection or physical damage.
muscles relax, pulling the ribs downwards.
l Ciliated cells (Figure 2.5, p. 14) are also present
l Thoracic volume decreases, so air pressure in the epithelial lining of the respiratory tract.
becomes greater than outside the body. Their continual flicking motion moves the
l Air rushes out of the lungs to equalise the mucus, secreted by the goblet cells, upwards
pressure. and away from the lungs. When the mucus
reaches the top of the trachea, it passes down
Protection of the gas exchange the gullet during normal swallowing.
system from pathogens and particles
Pathogens (e.g. bacteria) and dust particles
are present in the air we breathe in, and are

Exam-style questions
1 State how each feature labelled on the diagram of an alveolus
(Figure 11.2) makes the process of gaseous exchange efficient. [5]
2 Calculate the percentage change in volume of carbon dioxide
between inspired air and expired air. [2]
3 a The composition of the air inside the lungs changes during
breathing.
i State three differences between inspired and expired air. [3]
ii Gaseous exchange in the alveoli causes some of the changes
to the inspired air. Describe three features of the alveoli that
assist gaseous exchange. [3]
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b i State what is meant by anaerobic respiration. [2]

ii Where does anaerobic respiration occur in humans? [1]

4 Write out the series of events involved in breathing in and

breathing out as a set of bullet points or as a flowchart linked
with arrows. [6]

© Dave Hayward 2022 75

 12 Respiration

Key objectives
The objectives for this chapter are to revise:
l definitions of the key terms
l the uses of energy in living organisms
l the balanced chemical equations for aerobic
l investigations into the effect of temperature on
respiration and for anaerobic respiration in
respiration in yeast yeast
l the word equations for aerobic and anaerobic
l the way in which an oxygen debt builds up
respiration in muscles and how it is removed during recovery
l that anaerobic respiration releases far less
energy per glucose molecule than aerobic
respiration

Key terms
Term Definition
Aerobic respiration The chemical reactions in cells that use oxygen to break down nutrient molecules to
release energy
Anaerobic respiration The chemical reactions in cells that break down nutrient molecules to release energy
without using oxygen

Respiration
Most of the processes taking place in cells in the body need energy to
make them happen. Examples of energy-consuming processes in living
organisms are:
l the contraction of muscle cells – for example, to create movement of
the organism
l synthesis (building up) of proteins from amino acids
l the process of cell division (Chapter 17) to create more cells, to replace
damaged or worn-out cells, or to make reproductive cells
l the process of active transport (Chapter 3), involving the movement of
molecules across a cell membrane against a concentration gradient
l growth of an organism through the formation of new cells or a
permanent increase in cell size
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l the conduction of electrical impulses by nerve cells (Chapter 14)

l maintaining a constant body temperature in warm-blooded animals
(Chapter 14) to make sure that vital chemical reactions continue at a
steady rate, even when the surrounding temperature varies
This energy comes from the food that cells take in. The food mainly used
for energy in cells is glucose. Respiration, which releases the energy,
is a chemical process that takes place in cells and involves the action
of enzymes. Do not confuse it with breathing – the physical process of
ventilating the lungs to obtain oxygen and remove carbon dioxide.

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 12 Respiration

The effect of temperature on yeast respiration

You need to be able to describe how to investigate the effect of
temperature on respiration in yeast. One investigation uses the apparatus
shown in Figure 12.1.

gas syringe

Skills
yeast and glucose Experiments involving
solution
changes of temperature
Whenever a biology
 Figure 12.1 Investigating the effect of temperature on respiration in yeast experiment involves
changing the temperature,
The water bath is used to control the temperature and the gas syringe it is important to allow
collects the carbon dioxide produced as the yeast respires over a chosen the reacting mixture or
amount of time. The experiment is repeated over a range of temperatures – organism to acclimatise
for example, 10°C, 20°C, 30°C, 40°C, 50°C. to the new temperature
before taking any
As the temperature increases, the volume of carbon dioxide produced
readings, otherwise the
also increases, with an optimum at around 35–40°C. Higher temperatures data obtained may not be
slow down the rate of gas production because the enzymes involved in reliable.
respiration start to denature (see Chapter 5).

Aerobic respiration
In humans, energy is usually released by aerobic respiration. However,
the cells must receive plenty of oxygen to maintain this process. Revision activity
The word equation for aerobic respiration is: You may have noticed
glucose + oxygen → water + carbon dioxide that the word equation
for aerobic respiration
The breakdown of one glucose molecule releases 2830 kJ of energy. is the same as the
word equation for
It is possible to carry out experiments using invertebrates and photosynthesis, but the
germinating seeds and measure the oxygen uptake of the organisms: the other way round. So, if
faster the uptake, the faster the rate of aerobic respiration. Germinating you learn one equation,
seeds do not need energy for movement, so their respiration rate tends to you will remember both.
be lower than that of animals. Note that respiration
uses oxygen and glucose,
If you are following the extended curriculum, you need to be able to so they are to the left
state the balanced chemical equation for aerobic respiration: of the reaction arrow.
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C6H12O6 + 6O2 → 6H2O + 6CO2 Photosynthesis produces

oxygen and glucose, so
If you write a symbol equation, you must make sure that the formulae they are to the right of the
are correct and that the equation is balanced. reaction arrow.
When carrying out experiments using germinating seeds to investigate
the effect of temperature on respiration rate, the rate of oxygen uptake
is used to indicate respiration rate. As temperature increases, so does the
rate of oxygen uptake and, therefore, the respiration rate. This is because
respiration is controlled by enzymes. An increase in temperature increases
the kinetic energy of the molecules, so the reaction rate increases.

© Dave Hayward 2022 77

 Anaerobic respiration

Anaerobic respiration Revision activity

Anaerobic respiration does not require oxygen. When tissues are respiring If you are studying the
very fast, the oxygen supply is not fast enough to cope, so tissues such extended curriculum,
as muscles start to respire anaerobically instead. However, this is a you also need to learn
less efficient process than aerobic respiration, so much less energy is the balanced chemical
produced. equations for aerobic
respiration and anaerobic
The breakdown of one glucose molecule by yeast releases only 118 kJ of respiration in yeast.
energy.
When you write these
The word equation for anaerobic respiration in yeast is: down, make sure you
glucose → ethanol + carbon dioxide have the same number
of carbon, oxygen
Anaerobic respiration in yeast does not produce water. and hydrogen atoms
on both sides of each
The word equation for anaerobic respiration in muscles is: equation. For example,
glucose → lactic acid in the aerobic respiration
equation there are 6
Anaerobic respiration in muscles does not produce carbon dioxide or water. carbon, 18 oxygen and 12
hydrogen atoms on each
The balanced chemical equation for anaerobic respiration in yeast is: side of the reaction arrow.
C6H12O6 → 2C2H5OH + 2CO2

Oxygen debt
Muscles respire anaerobically when exercising vigorously because the
blood cannot supply enough oxygen to maintain aerobic respiration.
However, the formation and build-up of lactic acid in muscles causes
cramp (muscle fatigue). An oxygen debt is created because oxygen is
needed for aerobic respiration to convert lactic acid back to a harmless
chemical (pyruvic acid). This happens in the liver.
At the end of a race, a sprinter has to pant to get sufficient oxygen to
the muscles to repay the oxygen debt. Breathing remains deep and fast
to supply enough oxygen for the aerobic respiration of lactic acid. The
heart rate remains fast to transport lactic acid in the blood from the
muscles to the liver.
Long-distance runners judge their pace, not running too fast, to prevent
the muscles respiring anaerobically. Muscle cramp would stop the athlete
running.

Sample question
Explain why the breathing pattern changes after a period of vigorous exercise. [3]
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Student’s answer Teacher’s comments

The breathing rate increases ✓ because muscles build up an oxygen debt ✓ This is an excellent answer,
when they respire anaerobically ✓. Oxygen is needed to break down the lactic gaining maximum marks. The
acid produced to prevent muscle fatigue. last sentence also contains
creditworthy statements.

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 12 Respiration

Exam-style questions
1 Make a table to compare aerobic and anaerobic respiration in
humans, using the headings shown below. [6]

Comparative amount of
Type of respiration Requirement(s) Product(s) energy released
Aerobic
Anaerobic

2 a State what gas is produced when yeast respires. [1]

b Suggest two reasons why yeast may die if left in a solution of
glucose in a sealed container for 48 hours. [2]
3 a Suggest why a runner taking part in a long-distance race
avoids sprinting until near the end of the race. [3]
b Explain why runners breathe faster during a run than before
they start the run. [2]

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© Dave Hayward 2022 79