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VCE Biology Unit 1
Gas exchange in animals
Gas exchange refers to the process by which organisms obtain oxygen from their surroundings and remove
excess carbon dioxide.
Why must an organism carryout gas exchange?
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Respiration is necessary for cells to release the energy from molecules such as glucose. Respiration
that uses oxygen (aerobic respiration) is the most efficient type of respiration; it generates the more
ATP than anaerobic respiration.
Organisms that carry out the process of aerobic respiration need oxygen.
In carrying out aerobic respiration, the cells of organisms produce carbon dioxide this gas must be
removed from the organism.
Elimination of Carbon Dioxide is important since its accumulation inside of an organism alters pH and
this will affect the normal functioning of enzymes
Since carbon dioxide is a metabolic waste its removal of is a form of EXCRETION
Respiratory Surfaces
Respiratory surfaces are surfaces across which gases pass in order to enter or leave the internal environment
of an organism. These include:
Amoeba: body surface:
Frog: skin:
Axolotl: external gills:
Fish: gills:
Insects: tracheal system:
Mammals: lungs
Regardless of the type of respiratory surface they all have several characteristics in common:
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The surface must be very thin(short diffusion pathway=faster diffusion)
The surface must be moist. This is due to the fact both oxygen and carbon dioxide must
be dissolved to cross the exchange surface
It must be able to maintain concentration gradient along which gases can diffuse
A large surface area over which gas exchange can occur.
In large multicellular organisms such as humans, this surface must be in contact with a transport system
that will be able to carry these gases to and from the cells. The surface must be well ventilated.
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VCE Biology Unit 1
Some respiratory surfaces are external others are internal.
Examples of externals respiratory surfaces:
Land living, air breathing organisms such as humans, have their respiratory surfaces inside the body. Why do
you think this is the case? _______________________________________________________________
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How do gases cross the respiratory surfaces?
The external side of the respiratory surface has a high concentration of oxygen compared to the internal side.
Therefore Oxygen will diffuse across the surface and into the transport system. The opposite is true of carbon
dioxide.
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So each gas diffuses down its concentration gradient
The larger the surface area of the respiratory surface, the faster the rate of diffusion of gases across it.
Gas exchange in mammals: Human example
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Ventilation – rhythmic breathing movements of an animal’s muscles and skeleton that will increase
gaseous exchange across a respiratory surface.
In mammals more precisely it is the exchange of respiratory gases (O2 and CO2) between the
atmosphere and the lungs; in other words inhalation and exhalation.
Inhalation and exhalation involved changes in the volume of the lungs that create pressure gradients
that move air into or out of the respiratory tract.
Air will flow from an area of higher pressure to an area of lowers pressure.
Inhalation
Inhalation
Exhalation
Action
Diaphragm muscles action
and resulting diaphragm
movement
Intercostal muscles action
Rib cage movement
Volume in thoracic cavity
air pressure in thoracic cavity
resultant air movement along
pressure gradient
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Exhalation
VCE Biology Unit 1
The Human Respiratory System: Bits for Breathing
The human respiratory system consists of Lungs and the systems of air tubes that carry air to and from the
lungs.
Lungs
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The gas exchange organs in humans and other vertebrate animals.
Consist of small chambers called alveoli that are surrounded by blood capillaries.
Passage of Air through the Respiratory System
Nose  Pharynx  Larynx  Trachea  Bronchus  Secondary Bronchus  Bronchioles ( terminal then
respiratory)  Alveolar ducts  Alveolar sacs  Alveoli
Nose
Air normally enters the respiratory tract through the nostrils; it then enters spaces called the nasal passages.
With respect gas exchange, the nose has three main functions. For each function suggest how the nose is
structured to suit it to the function:
1. Filter the air:
2. Moisten the air:
3. Warm the Air:
Pharynx and the Larynx
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Air leaving the pharynx (throat) enters the larynx also called the voice box. The larynx contains the
vocal cords. The vocal cords are two membranes stretched over the pharynx that vibrate as air
passes over them. The epiglottis prevents food from entering the larynx when we swallow
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VCE Biology Unit 1
Trachea
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Also called the windpipe
12 cm long, 2.5 cm in diameter
What is the function of the trachea?
How does its structure suit it to its function?
Bronchi and Bronchioles
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Branch in a treelike fashion into
numerous bronchioles which branch
into increasingly smaller and thinner
tubes
As they get smaller
o cartilage decreases
o cilia decrease
o Epithelium gets thinner
Alveoli
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Each bronchiole ends in alveolar duct
which lead to alveolar sacs. Each is
made up of tiny air sacs called alveoli
There are thought to be 300 million
alveoli in the lungs: S/A 70m 2
How does the structure of the alveoli suit their
function?
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VCE Biology Unit 1
GAS EXCHANGE AND TRANSPORTION OF GASES
Note the following diagram shows the main way of transporting oxygen and carbon dioxide only. To be accurate it will need to be added to.
N
Note: binding of gases to Hb is reversible.
Oxygen passes across the surface of the alveoli into the blood by diffusion: How is the oxygen transported by the blood to body cells?
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~ 98% of oxygen(MOST):
~2% of oxygen:
Carbon dioxide is produced in body cells during aerobic respiration. It diffuses from cells into the blood. What happens to carbon dioxide from this point?
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~70% of carbon dioxide(MOST):
~23% of carbon dioxide
~7% of carbon dioxide
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VCE Biology Unit 1
WEB QUEST
There is considerably more oxygen in air than in water. So why do fish die when taken out of water and place in air?
If fish can take in gases from the water, then why can’t humans- why do humans drown when under water for too long?
What are the hiccups and what causes them? Do they serve a purpose?
Why do we yawn? Is yawning really contagious?
Why might some athletes opt to train at a higher altitude prior to competing in world class athletics competitions? Is there any
regulation about this practise?
Divers coming to the surface too quickly after a deep dive are likely to experience the bends. What is the bends? What is
suggested treatment for this condition?
Add one question to the list:
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VCE Biology Unit 1
Partial pressure:
mmHg is a unit of
measurement for pressure;
1 mmHg is the fluid
pressure exerted by a 1mm
column of mercury
The air we breathe is not a single gas but a mixture of gases. (~ 79%N2, 21%O2, 0.5% H2O,
0.04%CO2). Atmospheric pressure is 760 mmHg. This represents the combined effects of collisions
involving each type of molecule in air. At any given moment 78.6% of those collisions involved
Nitrogen, 20.9% involve oxygen and so on. Thus each of the gases present contributes to the total
pressure in proportion to its relative abundance. (Dalton’s law)
The partial pressure (P) of a gas is the pressure contributed by a single gas within a mixture of gases. All the partial pressures added
together equal the total pressure exerted by a gas mixture.
In the case of the atmosphere PN2+ PO2+PH2O+PCO2=760 Hg mm.
Since we know the % of each gas in the atmosphere the partial pressures can be e easily calculated.
Eg. Oxygen is 20.9% of 760 mmHg, which is 20.9/100 x 760 = 159 mmHg (158.84)
Differences in pressure move gas molecules from one place to another. The greater the difference in partial pressure of a gas between
two regions, the faster the rate of gas diffusion. At a given temperature the amount of a particular gas in solution is directly proportional to
the partial pressure of that gas.
So the higher the partial pressure of a gas the higher the concentration of that gas, the lower the partial pressure of a gas the lower the
concentration of that gas. So we can think about diffusion rates of gases in terms of partial pressure differences or the size of the
concentration gradient and it is really the same thing.
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