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Circulatory System of a Mammal
Why large organisms need a transport system
All organisms need to exchange materials between themselves and the

As an organism gets bigger, the surface area to volume ratio decreases to a point where
the needs of the organism cannot be met by the body…

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Features of Transport Systems
Any large organism encounters the same problems in transporting materials within itself.

The transport systems of many organisms have common features:

A suitable medium in which to carry E.g. Blood
materials This is normally a liquid based on water
because water readily dissolves substances
and can…

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Transport Systems in Mammals
Mammals have a low surface area to volume ratio, so they need a specialised
transport system to carry materials from the specialised exchange organs to their body

The system is closed and blood is contained in the vessels.

A muscular pump called the heart circulates…

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Substances are then delivered to the rest of the body quickly, which is
necessary as:

Mammals have a high temperature
High rate of metabolism

Blood transports:

Respiratory gases
Products of digestion
Metabolic wastes

The vessels that make up the circulatory system of a mammal are divided
into three types:…

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There are two circuits:
1. Takes blood from the heart to the lungs then back to the heart
2. Takes blood around the rest of the body

Page 6

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Blood Vessels and their
Blood circulates in a series of different kinds of blood vessels as it circulates round the body

Heart Aorta Arteries Arterioles Capillaries Venules Veins Vena Cava Heart

A blood vessel is a tube with a space in the centre called the lumen, where blood

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Arteries, arterioles and veins all have the same basic layered structure.

From the outside inwards, these layers are:

Structure What it does
Tough outer layer Resists pressure changes from both within
and outside

Muscle layer Can contract and so control the flow of

Elastic layer Helps maintain blood pressure…

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Artery Structure related to its function

Function Transport blood rapidly
under high pressure from the
heart to the tissues.

Muscle layer Walls are thick and muscular This means smaller arteries can be
constricted and dilated in order to
control the volume of blood passing
through them.
Allows them to withstand…

Page 9

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Arteriole Structure related to its function

Function Carry blood, under lower
pressure than arteries, from
arteries to capillaries.
Also control the flow of blood
between the two.

Muscle layer Walls are relatively thicker The contraction of this muscle layer
than in arteries allows constriction of the lumen of
the arteriole.…

Page 10

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Vein Structure related to its function

Function Transport blood slowly,
under low pressure, from the
tissues to the heart.

Muscle layer Walls are relatively thin Veins carry blood away from tissues
compared to arteries and therefore their constriction and
dilation cannot control the flow of
blood to the tissues.



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