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
clot is serum.
The muscles which are attached to the skeleton (skeletal muscles) act
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.
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.
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.
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.
The parazoans like sponges, use sea water for transportation. Since
the natural water currents are not reliable, the sponges create their own
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
highly branched and supplies digested food to all the cells directly. In these
animals the excretory system collects wastes from each cell individually.
In these organisms most of the body is occupied by digestive and excretory
systems.
In animals belonging to Nemathelminthes (round worms), the
pseudocoelom has taken up the function of collection and distribution of
materials.