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flowing out of them at different junctions. Such materials are to |
be collected and sent back into blood circulation. |
We know that blood circulates in the blood vessels, pushed by |
the heart. From the heart it flows into the arteries and finally into |
the capillaries. To supply nutrients to the cells (tissues), the liquid |
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portion of the blood with nutrients flows out of the capillaries. |
This |
This is called tissue fluid. |
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The tissue fluid which is present in the tissues should be |
transported into the blood vessels again. Some portion of the tissue |
fluid enters into the venules, which in turn form the veins, which |
carry blood to the heart. What about the remaining tissue fluid? To |
transport the tissue fluid in to the main blood stream, a separate |
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system is present. That is called lymphatic system. In latin lymph |
means water. |
Lymph is the vital link between blood and tissues by which |
essential substances pass from blood to cells and excretory |
fig-12: Lymphatic products from cells to blood. The lymphatic system is a parallel |
system system to venous system which collects tissue fluid from tissues |
and transports it to the venous system. |
60 X Class Transportation - The circulatory system |
Blood is a substance which contains solid and liquid particles. Lymph |
is the substance that contains blood without solid particles. Tissue fluid is |
lymph present in the tissues. The liquid portion after formation of blood |
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A |
clot is serum. |
The muscles which are attached to the skeleton (skeletal muscles) act |
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AN |
as pumps when they contract and help in pushing the lymph flowing in |
lymphatic vessels and the blood flowing in veins towards the heart. |
The valves that are present in the lymphatic vessels and veins stop the |
reverse flow of blood. We shall read about this as the system of lymph |
circulation in detail in higher classes. |
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NG |
Evolution of the transport (circulatory) system |
When the unicellular organisms separated themselves from the sea |
with the formation of the limiting membrane, the problem of transportation |
arose. The nature has found the solution, by creating a microscopic ocean |
which has its own currents. |
LA |
In unicellular organisms like Amoeba the protoplasm shows natural |
movements. These movements are called Brownian movements, because |
of which the nutrients and oxygen are distributed throughout the protoplasm |
equally. |
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This simplest intracellular transportation system, present in unicellular |
animals has been retained in multicellular animals including humans. The |
protoplasm of any cell in our body is mobile and protoplasmic currents |
exist even in the nerve cells. |
The multicellular animals have to develop more complicated system |
for transportation of materials. |
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The parazoans like sponges, use sea water for transportation. Since |
the natural water currents are not reliable, the sponges create their own |
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currents by beating of flagella that are present in their body. |
The cnidarians which are better evolved than sponges (e.g. Hydra and |
jelly fish) have developed blind sac like gastrovascular cavity, which has |
taken up the function of digestion and transportation of nutrients to each |
and every cell of the body. |
In platyhelmenthes (e.g. Fasciola hepatica), the digestive system is |
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highly branched and supplies digested food to all the cells directly. In these |
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