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A Handbook of Biology

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Circulation is the transport of nutrients, oxygen, CO2, and excretory

products to the concerned tissues or organs. For circulation, simple

organisms (Sponges, Coelenterates, etc.) use water from their

surroundings. Complex organisms use body fluids (blood and lymph) for

circulation.

ÇÏRÇÜLÅTØRÝ PÅTHWÅÝS

Circulatory system is of 2 types: Open and Closed.

Open circulatory system:

Here, the blood pumped by

the heart passes through

large vessels into open

spaces or cavities called

sinuses. E.g. Arthropods and

molluscs.

Closed circulatory system:

Here, the blood pumped by

the heart is always

circulated through blood

vessels. This system is more

advantageous as the flow

of fluid can be more

precisely regulated. E.g.

Annelids and chordates

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All vertebrates have a muscular chambered heart

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Fishes : 2 chambered heart (an atrium + a ventricle).

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Amphibians : 3 chambered heart (2 atria + a ventricle).

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Reptiles ( except crocodiles ) : 3-chambered heart (2 atria + a ventricle).

Ventricle is incompletely partitioned.

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Crocodiles, birds & mammals : 4-chambered heart

Single Circulation in Fish: Heart pumps out the

deoxygenated blood which is oxygenated by the

gills and it is supplied to body parts from where

deoxygenated blood is returned to heart.

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Incomplete double circulation in amphibians & reptiles: Left atrium

receives oxygenated blood from the gills/lungs/skin and right atrium

gets the deoxygenated blood from other body parts. However, they get

mixed up in the single ventricle which pumps out mixed blood.

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Double circulation in Birds & Mammals : Oxygenated and deoxygenated

blood received by the left and right atria respectively passes on to

ventricles of the same sides. The ventricles pump it out separately

without any mixing up.