Gas Exchange and the Respiritory
System
How does the human body exchange gases?
• Aerobic respiration requires
oxygen (O2) and produces carbon
dioxide (CO2).
• Organisms have special surfaces to
take in O2 and get rid of CO2.
• Since O2 enters and CO2 exits the
body, such surfaces are for gas
exchange.
• The lungs comprise the human gas
exchange surface, the place where
gas exchange happens.
• These surfaces have to be
permeable to O2 and CO2 so that
the gases can move easily.
What are the features of a gas exchange surface?
• Gas exchange surfaces need to have
the following features:
• Thin tissues for efficient diffusion
of gases
• An efficient transport system
close by to deliver and take away
gases
• Large surface area so that large
volumes of gas can diffuse at once
• A constant supply of oxygen
Do the human lungs also have these features?
Each lung is filled with hundreds of
millions of tiny air sacs called alveoli.
• The surface of the alveoli is the gas
exchange surface.
•
•
O2 diffuses into the blood and CO2 diffuses
out.
Capillary beds cover the alveoli to
deliver a constant supply of blood.
• The walls of the alveoli and capillaries
are both only one cell thick.
• The total surface area of the alveoli in
the lungs is over 100 m2.
• The airways of the lungs deliver a
constant supply of O2.
•
What cell types are found in the respiratory system?
Air first enters the body through the
nose and mouth.
• Goblet cells in the nose produce mucus
to moisten air and protect against
pathogens.
• Dust and pathogens that enter the
respiratory tract are trapped in mucus.
• Tiny, hair-like projections called cilia
continuously move with a sweeping
motion.
• Ciliated cells sweep dust and pathogens
towards the throat so it doesn’t block
the lungs.
•
What are the parts of the respiratory system?
• From the nose, air passes through the
voice box, also called the larynx.
• Air then passes through the largest
airway, the windpipe or trachea.
• Rings of cartilage physically support the
trachea
• The trachea extends into the thorax,
the chest cavity containing the heart
and lungs.
• The trachea branches off into two
smaller passages called bronchi.
• Bronchi branch off into many even
smaller passages called bronchioles.
• Bronchioles branch off into alveoli, the
gas exchange surface.
What are the parts of the respiratory system?
The constant movement of air in and
out of the lungs is ventilation.
• Movements of muscles and bones
cause the lungs to be well ventilated.
• The diaphragm is a dome-shaped
muscle under the lungs.
•
What are the parts of the respiratory system?
The constant movement of air in and
out of the lungs is ventilation.
• Movements of muscles and bones cause
the lungs to be well ventilated.
• The diaphragm is a dome-shaped
muscle under the lungs.
• The intercostal muscles sit between the
ribs surrounding the lungs.
•
•
Internal and external
These muscles work together to change
the size of the thorax.
• Changing the size of the thorax to move
air in and out of the lungs is breathing.
•
How does our body move when we breathe in?
• When we breathe in (inspiration), the
diaphragm contracts and flattens.
• The external intercostal muscles also
contract, pulling the ribs upwards and
outwards.
• Both of these movements cause the
volume of the thorax to increase.
• Air pressure in the lungs becomes lower
than atmospheric air pressure.
• Air moves from higher pressure outside
to lower pressure inside the lungs.
• Air flows through the trachea and
bronchi into the alveoli.
How does our body move when we breathe in?
• The diaphragm and the intercostal
muscles relax to cause breathing out
(expiration).
• When the diaphragm relaxes, it takes on
its normal dome-shape.
• When the intercostal muscles relax, the
rib cage moves back down and inwards.
• The volume of the thorax decreases,
causing the air pressure in the lungs to
increase.
• The air pressure in the lungs is higher
than atmospheric air pressure.
• When coughing or exercising, the
internal intercostals contract to force air
out.
Gas exchange at the alveolus
• When inspired (breathed in) air
reaches the alveoli (air sacs) it
contains a lot of oxygen.
• Oxygen dissolves in the water lining
each alveolus.
• It then diffuses through the wall of
the alveolus and through the
capillary wall into the blood.
• Although this involves diffusing
through two cells, the distance is
very small.
Gas exchange at the alveolus
• Each alveolus has a network of
capillaries around it.
• Oxygen molecules from the alveolus
diffuse into the red blood cells and
combine with haemoglobin.
• The blood cells can then transport
this oxygen to the body tissues.
Gas exchange at the alveolus
• There is a lot of carbon dioxide in the
capillary.
• It has been carried there from the
respiring tissues in the blood plasma.
• It diffuses in the opposite direction,
through the capillary wall across the
alveolar wall into the space inside
the alveolus.
• From here it is breathed out.
Gas exchange at the alveolus
• Alveoli are surrounded by elastic
tissue.
• This stretches when you breathe in
and recoils when you breathe out to
help remove air from the lungs.
• The air we breathe out is saturated
with water vapour that has
evaporated from the moist walls of
the alveoli.
What’s the difference between inspired and expired air?
As we breathe in, air is drawn into
the tube on the left.
• As we breathe out, air flows into
the tube on the right.
• What gas in the air would react
with limewater to form a
precipitate?
• Carbon dioxide (CO2) in the air
would react to form a precipitate.
• Which air contains more CO2, the
air you breathe in or the air you
breathe out?
•
What’s the difference between inspired and expired air?
• Which tube will contain more
precipitate after breathing through
the tubes?
• Your body uses O2 and makes CO2 in
the process of aerobic respiration.
• Oxygen content in inspired air is
higher than oxygen content in expired
air.
• Carbon dioxide content in inspired air
is lower than carbon dioxide content
in expired air.
How does exercise affect the breathing rate?
•
When you exercise vigorously, your muscles quickly
use up oxygen for aerobic respiration.
•
The more your muscles work, the more oxygen they
need. You breathe more quickly.
•
Your lungs can’t take in oxygen quickly enough to
supply your muscles adequately.
•
Muscles in your legs would need more energy than
they can get with just aerobic respiration.
•
To get the extra energy they need, your muscles will
also use anaerobic respiration.
•
Both forms of respiration are used in your muscles
at the same time.
•
The product of respiration in human cells is lactic
acid.
How does exercise affect the breathing rate?
•
As you exercise, lactic acid collects in your muscles
and blood.
•
Your body needs to eliminate this lactic acid using
aerobic respiration in the liver.
•
By using anaerobic respiration for energy in your
muscles, you built up an oxygen debt.
•
Your cells “bought” energy that they needed
immediately with oxygen they would have later.
•
The debt goes away as your cells “pay” with oxygen
to get rid of the lactic acid.
•
Your breathing and heartrate stay elevated after
exercise until the oxygen debt is paid off.
How does exercise affect the breathing rate?
•
When lactic acid or CO2 build up in the blood,
they lower the pH.
•
When the brain detects a blood pH that is too
low, it causes respiratory muscles to contract.
•
The diaphragm and intercostal muscles
contract more often and with more force.
•
The rate and depth of breathing increases to
inspire more O2 and expire more CO2.
•
More O2 means more aerobic respiration or
lactic acid metabolism.
•
Less CO2 means less carbonic acid in the blood,
keeping blood pH to normal.