Substance exchange with the environment and transportation of substances within
the living body are necessary conditions for the existence of life.

The basic method of substance transport in living organisms is diffusion;
diffusion is the intermingling of the molecules of one body/fluid
with the molecules of another body/fluid they come into contact with.

With primitive and smaller organisms, diffusion is sufficient to
distribute oxygen, water, nutrients, throughout the entire body and
to maintain a constant substance exchange with the environment. With
the evolution of living matter, however, the necessity of superior
means of transportation arose. So, the heart and the blood vessels,
forming a convective system, have developed, in order to provide fast
transport of substances and heat around the body.

Approximate Rates of Diffusion

For all protozoans, that are unicellular organisms, the only method of
substance transport is diffusion; the processes of digestion,
respiration and excretion are carried out by diffusion, which takes
place over the surface of the entire body. The first system of
cavities and ducts, called “œgastrovascular system”, that
communicated directly with the environment for substance
transport/intake is found in phylum Coelenterata / Cnidaria (that
contains the hydras, sea anemones, corals and jellyfish). However,
this is not a proper vascular system, carrying blood (or a body
fluid); the first vascular system appears with phylum Nemertini
(“œribbon worms”), where, in the lack of a heart, blood flows with
the aid of the contractive walls of the vessels. Although in the more
evolved organisms, a vascular system is present, diffusion still has
its role, in intracellular transport, as well as transport between
cells.

For inferior organisms that have a diffusive transport system, this is
sufficient, and even suitable for a number of reasons. First of all,
being a passive process, diffusion does not involve energy
consumption. Secondly, the diffusive transport does not require such
a high level of specialization. Thirdly, the rate of transport is
sufficient for the transportation of nutrients, since only a small
area is involved. However, over distances longer than 1mm,
diffusional transport can become hopelessly slow (it would take a
molecule over half a day to cover a distance of 1cm!).

Toget an idea of how diffusion works, here are some typical speeds:

0.1 micrometer is covered in 0.000005s
10 micrometer ..0.05s
1mm ..9.26 min
1 cm .. 15.4 hours

This is the main reason why a more rapid transport system is needed for
larger and more complex organisms.

The convective system is able to transport substances at great distances,
sweeping them along blood vessels, in a stream of fluid that is
actually pumped through them. The pump in this system is the heart,
and its role is to provide the energy input for the blood to flow. An
oxygen molecule coming from the lungs needs only approximately 30 s
to reach the extremity of the limbs in humans, using the convective
system; by comparison, if it were to get there by diffusion, it would
need 5 years!

In superior organisms, the circulatory system actually combines both
methods – convection and diffusion. In the beginning and ending
segments – that is, at the uptake of molecules and over the last
10-20 µm diffusion is used, for the molecules to be taken
on by the fine capillaries into the blood stream or delivered to
destination cells from the blood flow. For the much greater distance
between these points, convection is needed, and molecules are
transported with the speed of the blood stream that carries them.