Ask A Scientist© Zoology Archive

name         Daniel B.
status       student
age          20s

Question -   Why is it that with the evolution of the lungs from gills
the circulatory system became double. Surely if blood can go from the
heart to the gills and then straight on to the body in fish why can't it
go from our heart to our lungs then straight to our bodies (or other
mammals). Sharks and fish are often larger than people so it cannot be a
size issue.

If I understand your question you are asking why mammals and birds have
evolved a four chambered heart with the lungs receiving blood from the right
heart...This, is generally thought to be a consequence of increased oxygen
demand and the increased efficiency of a four chambered heart and a complete
separation of oxygenated and deoxygenated blood.

pf

Peter Faletra Ph.D.
Office of Science
Department of Energy
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The complete process within the development of a living creature shows quite
some similarity with goal-directed movements. From this point of view we
tend to say that we have a leg in order that we can walk. The development of
the knee makes sense in order that we can bend our knee. From this point of
view a new kind of anatomy was created, the so-called functional anatomy.
Using already known phenomena, we made comparisons between those and what
happens in our body. A nice example is comparing the condyls of the femur
with the two wheels of an airplane. Only our leg does not function as a part
of an airplane and neither does our knee. Within the science of functional
anatomy we tend to say "we have this part of our anatomy in order that we
are able to do this or that. The question however is how reliable is this
kind of statement, how much truth is there in these kind of comparable
observations?

I want to give you an example to demonstrate how easy the word "in order
that" can be seen as a fairytale. We all know the story of Little Red
Riding-hood and the wolf. Everyone who ever heard or read this fairytale
knows that at a certain moment the little girl arrives at the house of her
grandmother. She enters the house and asks her lovely grandmother the
following questions: "Grandmother, why do you have these big ears?" The
answer of the wolf, disguised as grandmother, goes as follows: "I have these
big ears in order that I can better hear you. The next question of Little
Red Riding-hood is: "And why do you have such big eyes?" Answer of the wolf:
"in order that I can better hear you". The answer on the question why her
grandmother has such a big mouth is quite simply by the fact that he has
this big mouth in order that he can better eat her which he does
immediately. As easily as the big mouth of functional anatomy claims
functions for structures we can shut his mouth by saying that we do have
these structures because they simply have grown this way during development.

It is a fact that growth factors induce later performances and that
embryological achievements on a regular basis are the precursors of future
functions. However, it still does not answer the question why we have these
structures and their specific functions. Sure we have a structure called
stomach where food is digested in a certain way. But how does our organism
in development knows it needs this so called stomach in order that it can
digest. Since its nourishment so far comes by different ways than the mouth.
Sure we have a knee that makes it possible to bend our leg, which makes
walking much easier. But how does our organism in development it needs this
knee in order that it can bend its leg. It is for sure that intra-uterine
the little human does not walk around like a hamster in a wheel cage.

In the end we can conclude that the science of functional anatomy has given
us a false believe of truth, which we cannot longer hold. We observe
different structures by identifying its morphological features. What is
their purpose, what is their function? Do we know? My conviction is that we
can only come closer to its functional meaning by seeing it in the
complexity of the total human being.

This changing in structure implicates a prerequisite for future development
of blood vessels. Due to the influences of the mesoderm the ectoderm changes
its structure in developing a three-sided configuration which is called a
canalisation zone. A zone where vascular paths are developed. The
development of the neural groove is at the same time a predevelopment for
the vascularization process along the longitudinal axis of the embryo. The
three sides of this zone consists of the inward rolled neural groove
material, the outward rolled ectoderm and the endoderm. They provide the
space for the development of the first dorsal aorta on each side of the
embryo. Within this space we have a canalisation of fluids long before
vessels can be distinguished. The intercellular fluid in these canalisation
zones prepares the way for the blood vessels. The principle of a
canalisation zone as the anlage for vessels is an important principle that
veins as arteries as well seem to follow during their development.

Whether the vessel will become a vein or an artery depends on the local
metabolic gradient. The need for nourishment in a prior developed field and
its positional relationship to the nutritional source are important factors
in the determination of the kind of vessel. Early veins arise from the
coalescing of fluid vacuoles that form in intercellular spaces. Early
arteries give rise to endothelial sprouts that grow into the cell
interstices.

Characteristics of vessels for future development
Blood vessels develop within certain circumstances from spatial ordered
fluid paths. The motor behind the dynamic features of these paths lies
within a larger metabolic gradient. Thereby tissue become nourished, can
grow and develop. We can see blood vessels as structural fluid paths which
have same characteristics. However, after a blood vessel has grown
sufficiently, it then has a restraining function on the structures it
supplies as well.

In observing the development of blood vessels, the following statement can
be made without any exception:

The longitudinal growth of each blood vessel stem is retarded when compared
to the total length of its numerous branches.

In this manner, any blood vessel has the capacity to cause growth as growth
resistance as well. This growth resistance is directed against the organ it
supplies.

The blood flow passes from proximal towards distal. The velocity of the flow
differs according to the area it appears. The velocity of the blood flow
decreases as the fluid passes from the proximal stem to the distal branches.
This causes a change in the characteristics of the fluid. Due to the lower
flow in the distal parts, the nutritive substances can permeate through the
vascular wall and thereby leave the blood stream. Leaving the blood stream
means entering the inner tissue and becoming a part of the intercellular
substances nourishing the connected tissues. In the proximal part of the
blood vessels the velocity is to high and therefore the substances can not
leave the blood stream. They parmeate along the endothelial lining of the
blood vessel. Therefore the growth rate of the vessel in this area is
comparatively to the distal part slow. The retarded growth of the stems
manifests itself as a restraining function.

This throws an interesting light for example on the bending of the embryo as
a whole. It seems that this bending is not so much caused by the extreme
development of the ectodermal material. The restraining function of the so
called dorsal aortae is much more likely the initial cause. Of course the
lengthening of the neural tube can and will deliver additional
characteristics which will aid the bending of the whole embryo...

Sincerely yours

Max Girardin Evolutionary Osteopath
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