Gas Exchange in Humans
W
O
R
K
• Why do we breathe? Think
of all the reasons why we
need a respiratory system.
T
O
G
E
T
H
E
R
Gas exchange by Diffusion
• Some animals simply
allow gases to diffuse
through their skins.
• These animals have a
low metabolic rate.
Why?
• All of these are aquatic
animals. Why?
Specialized structures
• Structures
specialized for gas
exchange include:
• gills (aquatic
animals)
• spiracles
(terrestrial insects)
• lungs (most
terrestrial
vertebrates)
Gas exchange is the interchange of O2 and CO2
between an organism and its environment
-It is also called respiration
What are advantages of gas exchange surfaces in humans?
1. Good ventilation with air
2. Thin
surface
3. Large surface area (alveoli)
4. Good blood supply
Copyright © 2014 Henry Exham
Features of gas exchange
surfaces in humans
• They have a large surface area for diffusion.
• Fluid to help dissolve gases and increase diffusion rate.
• A rich blood supply to maintain a steep diffusion
gradient between the alveoli and the blood.
• Due to both the alveoli and the capillary only having
walls one cell thick (Thin surface) there is a short
diffusion distance between the air and the blood
which increases diffusion rate.
• good ventilation with air (constant ventilation –
breathing in and out)
Lungs




trachea
bronchi
bronchioles
alveoli
capillaries
(circulatory system)
alveoli
Copyright © 2014 Henry Exham
The Gaseous Exchange System
Nose
larynx
Mouth
Lungs
Trachea
Pleural membranes
Bronchi
Ribs
Bronchiole
Intercostal muscles
Alveoli
Diaphragm
Structure of the lungs
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Parts of the respiratory system
Copyright © 2014 Henry Exham
The Gaseous Exchange System
Structure
Function
Trachea
Tube with incomplete rings of cartilage carries air to lungs; lined
with cells making mucus, and cells with cilia to move the mucus
away.
Bronchi
Carries air to lungs
Bronchioles
Carry air to lungs
Alveoli
Tiny air sacs adapted for gas exchange
Diaphragm
Sheet of muscle with a fibrous middle part which is domed; it helps
make breathing movements and separates the thorax from the
abdomen
Ribs
Bones that protect and ventilate the lungs
Intercostal muscles
Move ribs for ventilation
Pleural membranes
Thin moist membranes forming an airtight seal around the lungs
and separating inside of thorax from lungs
State the functions of the cartilage in the
trachea
•At the top of the trachea is a piece of
cartilage called the epiglottis
–This closes the trachea and stops food
going down the trachea when you
swallow.
•The trachea has rings of cartilage around
it which keep it open.
Typical Epithelium in respiratory system
The mechanism of ventilation
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Gas exchange in the alveoli
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Gas Exchange on the Cellular Level
1) When the blood
reaches the cells,
oxygen from
oxyhaemoglobin
diffuses into the
body cells
2) At the same time,
carbon dioxide
from the cells
diffuse into the
plasma to be
exhaled from the
lungs
The job of the LUNGS is to
transfer OXYGEN to the blood
and to remove waste CARBON
DIOXIDE from it.
To do this, the lungs contain
millions of ALVEOLI where GAS
EXCHANGE takes place.
bronchiole
alveoli
capillary network
The alveoli are an ideal exchange surface.
They have:
1. An huge surface area (about 70cm2)
2. A moist lining for dissolving gases.
3. Very thin walls.
4. Copious blood supply.
A=alveoli
B=capillary
C=bronchiole
Composition of inhaled/exhaled air
composition (%)
In one breathing cycle, the air in the lungs loses only some
of its oxygen content. This is why mouth-to-mouth
resuscitation can be effective.
90 78% 78%
80
70
60
50
40
30
20
10
0
N2
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inhaled air
exhaled air
21%
15%
0.04% 4%
O2
CO2
<1% 3%
<1% <1%
H2O
other
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State and Explain the differences in
composition between inspired and expired air
Inspired air
Expired air
Reason for
difference
Oxygen (O2)
21%
16%
Oxygen is absorbed
across the gas
exchange surface,
then used by cells in
respiration
Carbon dioxide
(CO2)
0.04%
4%
Carbon dioxide is
made inside respiring
cells, and diffuses out
across the gas
exchange surface.
Argon and other
noble gases
1%
1%
Water vapour
(humidity)
Variable
Always high
Gas exchange
surfaces are made of
living cells, so must be
kept moist; some of
this moisture
evaporates into the
air.
How to test for CO2?
Based on observations above, what can you conclude?
Use limewater as a test for carbon dioxide to
investigate the differences in composition between
inspired and expired air
The mechanism of ventilation
Negative Pressure Breathing
Breathing = alternation of inhalation (active) and exhalation (passive)
Inspiration
Expiration
The exchange of gases
Aerobic respiration is the process of releasing energy
through the oxidation of glucose molecules:
glucose
+
oxygen 
Oxygen is breathed
into the lungs and is
transported in blood.
carbon
dioxide
+
water ( + energy)
Carbon dioxide and water are formed as waste
products in cells. They can be excreted when we
breathe out.
The respiratory system carries out gas exchange. It is made
up of the lungs, intercostal muscles, diaphragm and airways.
Exercise, respiration and ventilation
Energy is used up by the active
muscles during exercise.
The rate of respiration must
therefore increase to supply
enough energy to the muscles.
This means that more oxygen
must be breathed in and more
carbon dioxide breathed out.
During exercise, why does:
 the heart rate increase
 the rate and depth of breathing increase
 the arteries supplying the muscles dilate?
Connection: The human fetus exchanges
gases with the mother’s bloodstream
• A human fetus depends on
the placenta for gas exchange
• A network of capillaries
exchanges O2 and CO2 with
maternal blood that carries
gases to and from the
mother’s lungs
• At birth, increasing CO2 in the
fetal blood stimulates the
fetus’s breathing control
centers to initiate breathing
Placenta, containing
maternal blood vessels
and fetal capillaries
Umbilical cord,
containing fetal
blood vessels
Amniotic
fluid
Uterus
Connection: Smoking is one of the deadliest
assaults on our respiratory system
• Mucus and cilia in the
respiratory passages
protect the lungs
– Pollutants, including
tobacco smoke, can
destroy these protections
• Smoking kills about
430,000 Americans each
year
Connection: Smoking is one of the deadliest
assaults on our respiratory system
• Smoking causes lung cancer
and contributes to heart
disease
• Smoking also causes
emphysema
– Cigarette smoke
makes alveoli
brittle, causing
them to rupture
– This reduces the
lungs’ capacity
for gas exchange
Disorders of the Respiratory system
• Asthma: a severe allergic reaction in which contraction of the
bronchioles makes breathing difficult
• Bronchitis: an inflammation of the lining of the bronchial
tubes. The passageways to the alveoli become swollen and
clogged with mucus
• Emphysema: lungs lose their elasticity, deterioration of the
lung structure
• Pneumonia: alveoli become filled with fluid. Caused by
bacterial or viral infection
• Lung Cancer: a disease in which tumors form in the lungs as a
result of irregular and uncontrolled cell growth
Why do breathing rate need to
increase with exercise?
1. Supply more O2 (for
aerobic respiration)
2. Respiration releases
energy
3. Remove CO2
All of this is
controlled
by your
brain
Copyright © 2014 Henry Exham
Investigate the effect of exercise on
breathing rate.
• First you must determine your resting
breathing rate.
• Count how many times you inhale in a minute.
• Do this three times and take an average.
• This will make your data more reliable.
Copyright © 2014 Henry Exham
Investigate the effect of exercise on
breathing rate.
• Then you must calculate your breathing rate
carrying out exercise.
• Jog on the spot for one minute and count the
breaths you inhale in that time.
• Then take a minute to rest.
• Once again repeat the experiment
two more times and take the average
of all three results.
Copyright © 2014 Henry Exham
Investigate the effect of exercise on
breathing rate.
• You should find that breathing rate increases
as you exercise.
• This is to provide your muscles with more
oxygen which it can use for aerobic
respiration to release energy.
Investigate and describe the effects of physical activity on rate and depth of
breathing
Explain the link between physical activity and rate and depth of breathing in terms of the increased
carbon dioxide concentration in the blood, detected by the brain, causing an increased rate of breathing
•••
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