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Philosophy of the Blood Circulation
A-. The blood circulation
a) The morphologiocal significance of the circulatory system.
Figure 27 shows circulatory system of mammals and birds. It becomes
evident that the atria, on one
hand, and the ventricles,
on the other, constitute independent myocardial units related by a feeble connection, the
A-V rings. There is a clear continuity between both atria, on one side, and between both
ventricles, on the other side but no between the atria and the ventricles. A ventricle can
never be separated from the other ventricle, neither an atrium from the other atrium,
without cutting myocardial fibres but the atria and the ventricles can be easily separated
at their contact points, along the A-V rings, without cutting any myocardial fibres.
The ventricular muscle band, a looping (fig. 10 c and d) of the arterial side of the circulatory system, runs between arteries, the pulmonary artery and the aorta (3 and 4, respectively, in fig. 27). The same happens with the atrial band, which runs between veins since there is also a continuity between the right atrium and the left atriun (RA and LA, in fig. 27) from the root of the caval veins (pointed out with 1 in the same figure) and the root of the pulmonary veins (2).
The
unravelling of the circulatory system explains the above mentioned facts. The atria belong
to the venous side of the circulatory system (fig.
28 D ; compare with fig. 10 b) whereas the ventricles belong
to the arterial side (compare fig. 28 and fig.10).
It becomes evident that the atria of higher animals is the present vestige of the fish
heart.
That is why (because of their different origin) the atrial myocardium and the ventricular myocardium look and feel so different. Furtheremore, differences are also evident at molecular level ; the atrial muscle fibres and the ventricular muscle fibres contain distinct forms of myosin.
It is clear that, in their mental model of the
heart, the classical anatomists forgot the anatomy and thought only about functional
aspects. That model contained right and left units, i.e., the tandem right atrium-right
ventricle, related to the
deoxigenated blood, and the tandem left atriun-left ventricle, related to the oxigenated
blood. That model makes not any sense neither from a morphological nor mechanical point of
view although may be it can be useful, in some cases, for clinicians.
The circulatory system (fig. 29) is a tube that becomes a band at the root of the pulmonary
artery and returns to its tubular form at the root of the
aorta ; this transformation, tube-band-tube, explains the existence of the pulmonary
artery and the aortic tepees (fig. 30 A y
B).
When
the right and the left ventricular cavities are compared it can be seen the semilunar
form of the right one and the circular form of the left one (fig. 31 A and B). The reason of such different
forms is explained by the following anatomical
facts.
Figure 32 shows the successive stages for the dissection of the ventricular band. If after stretching out the band (last specimen of the same figure) it is rolled up again, returning it to its normal configuration (first specimen), and afterwards is made a longitudinal cut base-apex (see straight line of scheme A of fig. 33), the ventricular mass is divided into two halfs (fig. 34).
The posterior
half, schematically reproduced in fig. 33 B, shows, on one hand,
the spatial course along which the ventricular band runs (see arrows) and, on the other
hand, shows the laminary trajectory (pointed aut by small arrows in scheme C ). Such
laminary trayectory is evidenced in this way because, thanks to the previous unwinding of
the band (fig. 32), that virtual trajectory becomes in a real
one as shown by figure 34.
As it can be seen in scheme C of figure 33, the basal loop, constituted by the right segment (rs,
the unique
component of the right free wall, RFW) and the left
segment (ls, one of the two componentes of the left free wall, LFW), encircles the apical
loop, constituted by the ascendent segment (as, the other component of the left free wall)
and the descendent one (ds), which, when crossing their fibres, make up the septum (IVS ;
see also fig. 25 A and B).
That scheme C,
of figure 33, is again reproduced in A of figure
35 and represented in schemes B and C with a change : the septum appears now as a unit
without any separation between its two componentes, the descendent (ds) and ascendent (as)
segments (see scheme C of fig. 33). Such two segments can be
unified, according to the more strict anatomical legality, because both belong to the same
loop, the apical one, but, nevertheless (see again scheme C of fig.
33), the left segment (ls, of the basal loop) and the ascendent segment (as, of the
apical loop) can not be unified because each one of them belong to a different loop.
All what has been mentioned above is adduced
because of one fact. As soon as in diastole the free wall of
the right ventricle remains separated of the septum, as it can be seen in scheme B of figure 35, during systole that wall leans against the septum, as
the scheme C shows, a figure that remembers the unique ventricle of amphibia and reptilia.
The right ventricular cavity can not be considered as a true ventricle but , merely, as a fissure or dehiscence, a cleft opened in the thickness of the ventricular wall. The right ventricular cavity only represents the transformation of the first part of that laminary trajectory, along which is unwinded the ventricular band (such trajectory has been pointed out by two small arrows in scheme C of figure 33 ; it can be seen also in scheme C of figure 35 separating the basal and the apical loops), in a real cavity, the cavity that is considered as the right ventricle.
The heart only has one ventricle and a dehiscence, on one hand, since, on the other hand, the atria, present vestige of the fish heart, do not belong to the motor that pushes the blood in the amphibia, reptilia, birds and mammals.
B-. The electrical circulation.
In the worms (as annelida or nemertina) the circulatory system consists of one tube with a pulmonary and a systemic bed (fig. 10 a). The blood flow is the result of the peristaltic form contractions of the vascular musculature of the vessels, arteries and veins, of this primitive circulatory system, which forms a single circle. Those contractions took place each time the electric wave passes along the succcessive segments of this circulatory system.
In a later phylogenetic step, to fishes, a simple heart appears in the venous side (VS, scheme b of fig. 10), which consists of a single sinus venosus, an atrium and a ventricle. This circulatory system still forms a single circle and the contraction of the heart takes place each time that the electric wave (which preceeds inmediatly to the peristaltic wave) arrives.
But later on, with the amphibia and reptilia and with their descendents, the birds and mammals, appears a new heart that has a ventricular motor to make circulate the blood. As happened in the worms, in the mammals an electric wave, followed by a peristaltic one, runs along the vessels going to the periphery along the arteries and coming back to the heart along the veins. This explains why the right atrium, in which finish the veins, is the first region of the heart to be electrically active in each cardiac cycle.
There is an electrical circulation as well as a blood circulation. The vessels work as pipe lines, in their inside, to conduct blood, and work as electric cables, in their outside, to conduct electricity.
Understanding of the electrical circulation will be valuable to explain the still unknown genesis of the electrocardiogram.