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Name: Daniel B.
Status: student
Age:  20s
Location: N/A
Country: N/A
Date: 2000-2001


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.


Replies:
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


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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