AQA AS Biology Unit 1: The Lungs

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  • Created on: 22-08-13 17:01
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All aerobic organisms require a constant supply of oxygen to release energy in the form of
ATP during respiration.
The carbon dioxide produced in the process needs to be removed as its build-up could be
harmful to the body.
The volume of oxygen that has to be absorbed and the volume of carbon dioxide that
must be removed are large in mammals because:
- They are relatively large organisms with a large volume of living cells
- They maintain a high body temperature and therefore have high metabolic and
respiratory rates
As a result mammals have evolved specialised surfaces, called lungs, to ensure efficient gas
exchange between the air and their blood.
Mammalian Lungs
The lungs are the site of gas exchange in mammals.
They are located inside the body because:
- Air is not dense enough to support and protect these delicate structures
- They would otherwise lose a great deal of water a dry out
The lungs are supported and protected by a bony box called the rib cage.
The ribs can be moved by the muscles between them.
This enables the lungs to be ventilated by a tidal stream of air, thereby ensuring that the
air within them is constantly replenished.
The main parts of the human gas-exchange system and their structure and functions are
described below:

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Part Function
- Pair of lobed structures
Lungs -
Made up of a series of branched tubules, called bronchioles
Bronchioles end in tiny air sacs call alveoli
- Flexible airway
Trachea -
Supported by rings of cartilage
Cartilage prevents the trachea collapsing as the air
pressure inside falls when breathing in
- The tracheal walls are made up of muscle lined with
ciliated epithelium and goblet cells
- The goblet cells produce mucus that traps dirt particles and
bacteria from the air breathed…read more

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The Mechanism of Breathing
To maintain diffusion of gases across the alveolar epithelium, air must be constantly
moved in and out of the lungs.
We call this process breathing, or ventilation.
When the air pressure of the atmosphere is greater than the air pressure inside the
lungs, air is forced into the alveoli.
This is called inspiration.
When the air pressure in the lungs is greater than that of the atmosphere, air is forced
out of the lungs.
This is called expiration.…read more

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Breathing is an active process; it uses energy and occurs as follows:
The external intercostal muscles contract
The internal intercostal muscles relax
The ribs are pulled upwards and outwards
Volume of the thorax increases
The diaphragm muscles contract causing it to flatten
Volume of the thorax increases
The increased volume of the thorax results in reduction of pressure in the lungs
Atmospheric pressure is now greater than pulmonary pressure
Air is forced into the lungs…read more

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Breathing out is a largely passive process ­ it does not require much energy and occurs as
The internal intercostal muscles contract
The external intercostal muscles relax
The ribs move downwards and inwards
Volume of the thorax decreases
The diaphragm muscles relax causing it to return to its upwardly doomed position
Volume of the thorax decreases
The decreased volume of the thorax results in increase of pressure in the lungs
Pulmonary pressure is now greater than that of the atmosphere
Air is forced…read more

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Pulmonary Ventilation
To know how much air is taken in and out of the lungs in a given time we use a measure
called pulmonary ventilation.
Pulmonary ventilation is the total volume of air that is moved into the lungs during one
To calculate it we multiply together two factors:
- Tidal volume ­ the volume of air normally taken in at each breath when the body is
at rest. This is usually around 0.…read more

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Exchange of Gases in the Lungs
The site of has exchange in mammals is the epithelium of the alveoli.
These alveoli are minute air sacs and are situated in the lungs.
To ensure a constant supply of oxygen to the body, a diffusion gradient must be
maintained at the alveolar surface.…read more

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Role of the Alveoli in Gas Exchange
Each alveolus is lined mostly with epithelial cells.
Around each alveolus is a network of pulmonary capillaries, so narrow that red
blood cells are flattened against the thin capillary walls in order to squeeze through.
These capillaries have walls that are only a single layer of cells thick.…read more


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