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OCR Biology AS Level (7)
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Chapter 7: Exchanged surfaces and breathing7.1: Specialised exchange surfaces
They need for specialised exchange surfaces
In microscopic organisms such as Amoeba all the oxygen needed by the organism, and the waste carbon
dioxide produced, can be exchanged with the external environment by diffusion through the cell surface.
The distances the substances must travel are very small.
There are two main reasons why diffusion alone is enough to supply the needs of single-celled organisms:
o The metabolic activity of a single-celled organism is usually low, so the oxygen demands, and carbon
dioxide production of the cell are relatively low.
o The surface area to volume ratio of the organism is large
As organisms get larger, they can be made up of millions or even billions of cells arranged in tissues, organs,
and organ systems.
Their metabolic activity is usually much higher than most single-celled organisms.
The amount of energy used in moving through the water means the oxygen demands of the muscle cells
deep in the body will be high and they will produce lots of carbon dioxide.
The distance between the cells where the oxygen is needed, and the supply of oxygen is too far lor effective
diffusion to take place.
What's more, the bigger the organism, the smaller the SA:V ratio.
So, gases can't be exchanged fast enough or in large enough amounts for the organism to survive.
Surface area: volume ratio - modelling an organism
The SA:V ratio is important in many areas of biology.
A sphere is a useful shape for modelling cells or organisms.
A series of simple calculations show clearly how the SA:V ratio changes as the organism gets bigger, and why
size matters so much.
Specialised exchange surfaces
Large, multicellular organisms have evolved specialised systems for the exchange of the substances they
need and the substances they must remove.
All effective exchange surfaces have certain features in common.
Summary of the characteristic features of effective exchange surfaces, along with some examples:
o Increased surface area
provides the area needed for exchange and overcomes the limitations of the SA:V ratio of
larger organisms.
Examples include root hair cells in plants and the villi in the small intestine of mammals.
o Thin layers
these mean the distances that substances must diffuse are short, making the process fast
and efficient.
Examples include the alveoli in the lungs and the villi of the small intestine.
o Good blood supply
the steeper the concentration gradient, the faster diffusion takes place. Having a good blood
supply ensures substances are constantly delivered to and removed from the exchange
surface. This maintains a steep concentration gradient for diffusion.
For example, the alveoli of the lungs, the gills of a fish and the villi of the small intestine.
o Ventilation to maintain diffusion gradient
for gases, a ventilation system also helps maintain concentration gradients and makes the
process more efficient
For example, the alveoli and the gills of a fish where ventilation means a flow of water
carrying dissolved gases
7.2: The mammalian gaseous exchange system
The human gaseous exchange system
Mammals are relatively big - they have a small SA:V ratio and a very large volume of cells.
They also have a high metabolic rate because they are active and maintain their body temperature
independent of the environment.
As a result, they need lots of oxygen for cellular respiration and they produce carbon dioxide, which needs to
be removed.
, This exchange of gases takes place in the lungs.
Key structures
Nasal cavity
The nasal cavity has several important features:
o a large surface area with a good blood supply, which warms the air to body temperature
o a hairy lining, which secretes mucus to trap dust and bacteria, protecting delicate lung tissue from
irritation and infection
o moist surfaces, which increase the humidity of the incoming air, reducing evaporation from the
exchange surfaces.
After passing through the nasal cavity, the air entering the lungs is a similar temperature and humidity to the
air already there.
Trachea
The trachea is the main airway carrying clean, warm, moist air from the nose down into the chest. It is a wide
tube supported by incomplete rings of strong, flexible cartilage, which stop the trachea from collapsing.
The rings are incomplete so that food can move easily down the oesophagus behind the trachea.
The trachea and its branches are lined with a ciliated epithelium, with goblet cells between and below the
epithelial cells
Goblet cells secrete mucus onto the lining of the trachea, to trap dust and microorganisms that have
escaped the nose lining.
The cilia beat and move the mucus, along with any trapped dirt and microorganisms, away from the lungs.
Most of it goes into the throat and is swallowed and digested.
One of the effects of cigarette smoke is that it stops these cilia beating.
Bronchus
In the chest cavity the trachea divides to form the left bronchus, leading to the left lung, and the right
bronchus leading to the right lung.
They are similar in structure to the trachea, with the same supporting rings of cartilage, but they are smaller.
Bronchioles
In the lungs the bronchi divide to form many small bronchioles.
The smaller bronchioles have no cartilage rings.
The walls of the bronchioles contain smooth muscle.
When the smooth muscle contracts, the bronchioles constrict (close).
When it relaxes, the bronchioles dilate (open).
This changes the amount of air reaching the lungs.
Bronchioles are lined with a thin layer of flattened epithelium, making some gaseous exchange possible.
Alveoli
The alveoli are tiny air sacs, which are the main gas exchange surfaces of the body.
Alveoli are unique to mammalian lungs.
Each alveolus has a diameter of around 200–300pm and consists of a layer of thin, flattened epithelial cells,
along with some collagen and elastic fibres
These elastic tissues allow the alveoli to stretch as air is drawn in. When they return to their resting size, they
help squeeze the air out. This is known as the elastic recoil of the lungs.
The main adaptations of the alveoli for effective gaseous exchange include:
o Large surface area
there are 300–500 million alveoli per adult lung, The alveolar surface area for gaseous
exchange in the two lungs combined is around 50-75m”. The average floor area of a 4-
bedroom house in the UK is only 67m. If the lungs were simple, balloon-like structures, the
surface area would not be big enough for oxygen needed to diffuse into the body. This
demonstrates again the importance of the SA:V ratio
o Thin layers
both the alveoli and the capillaries that surround them have walls that are only a single
epithelial cell thick, so the diffusion distances between the air in the alveolus and the blood
in the capillaries are very short.
o Good blood supply
the millions of alveoli in each lung are supplied by a network of around 280 million
capillaries. The constant flow of blood through these capillaries brings carbon dioxide and
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