1.2. Fisiologia della circolazione feto-placentare Physiology of Fetoplacental Circulation |
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1.2. Physiology of Fetoplacental Circulation |
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1.2.1. Fetal Circulation . |
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Fig.6: The fetal circulation.
Fig.7: Fetal circulation in the sheep:
Circled values = of oxygen saturation,
Remaining values = pressures (max/min) relative to the amniotic pressure.
Fig.8: Changes in the circulation from the fetus to the newborn and the adult.
1.2.2. The Placenta: Morphology and Functions . |
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The system of multivillous exchange of the human placenta is not the most efficient: this is confirmed by
the fact that 1 gram of human placenta feeds only 6 grams of fetus, compared to a ratio 1:20 in the
guinea-pig.
Anyway the multivillous system has an advantage: it allows in a better way the continuous adaptation to
conditions that change heavily during the growth.
Fig.10:
Types of placental exchange system.
The white arrows indicate the fetal flow, the black ones indicate the maternal flow,
the variation in density of points indicates the efficiency.
1.2.3. Anatomy of the Fetal Side . |
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a) the cotyledon: ChP: chorionic plate BP: basal plate T: trunk I-IV: rami. b) peripheral ramification of a cotyledon: 1: stem villus 2: mature intermediate villus 3: immature intermediate villus 4: terminal villi. c) section of two terminal villi. d) section of the placental barrier. |
The vascular venous and arterial structures of the villi are almost symmetrical, like in other organs.
The muscular cells of the vessels are relatively dispersed but they have more intercellular connections:
Nikolov and Schiebler (1973) have interpreted this as a muscular system that could be important for an
independent local regulation.
Following the blood flow from the umbilical arteries to the terminal capillaries, we will introduce a
classification of the placental villi, with reference to Kaufmann (1990) [16] and Arts [25].
Kaufmann has obtained his data analyzing sections of tissue obtained from placentas fixed through
immersion in formaline, without perfusion, with the electronic microscope. Arts has analyzed tissues
after the perfusion of the placentas with polimetilmetacrilate and their dissolution through immersion
in KOH.
Following the description of Arts [25] we can classify the placental vessels as follows:
+ chorionic vessels;
+ vessels of the cotyledons;
+ capillary vessels in the villi;
+ paravascular capillary network.
The villi can be classified in 5 main groups: stem villi, mature and immature intermediate villi,
terminal villi and to mesenchimal villi [20].
* Chorionic vessels:
They are arteries and veins pertaining to the chorionic plate of the placenta, which are originated by
the extensions of the funicular vessels. They originate vessels that exceed the chorionic plate reaching
vascular tree of the cotyledons. They are called vessels of the first order. The average diameter is
about 1,5 millimeters for the arteries and 2 millimeters for the veins, their length is 5-10 millimeter.
Arts [25], with the exception of a frequent anastomosis between the two umbilical arteries at their
insertion to the placenta, did not found any anastomosis between the veins, neither between the
arteries, and excluded that arterovenous connections can be found.
* Vessels of the cotyledons:
At the root of each cotyledon, the trunci connect the villous tree to the chorionic plate. From their
ramifications 4 orders of rami and 10 of ramuli are originated; the first ones are short, wide, deriving
from the trunk near the chorionic plate; the second ones, thinner, span to the periphery of the tree (in
fig.12 respectively T, I-IV, 1-10). Trunci, rami and ramuli constitute the stem villi. Among the stem
villi there are also several particular ramuli connected to the basal plate [20]. The described villi
have the same histological characteristics but differ in the geometric ones: the diameters span from
50-80 µm (smaller ramuli) to 3000 µm (trunci).
The figure 13 shows a stem villus in section. There are 2 main vessels (an arteriola and a venula),
and some small paravascular capillary vessels.
Fig.13: Cross-section of a stem villus.
The function of the stem villi is mechanic, as support for the structures of the villous tree;
their contribution to the exchange of maternal-fetal mass and the endocrine activity is negligible.
* Capillary in the intermediate villi:
The stem villi split into the intermediate villi, distinguished between immature and mature.
The last ones represent the connection between stem villi and terminal villi, and are the great majority
in the mature placenta: from their surface more than 95% of the terminal villi take origin [20].
The diameter of the mature intermediate villi is between 60 and 150 µm, in the immature ones between 60
to 200 µm. An important difference is the presence of a bigger number of terminal villi in the mature
intermediate villi rather than in the immature ones.
From a functional point of view the immature intermediate villi can be considered as the centers for
the development of the villous tree; they represent sites of exchange during the first two trimesters
of pregnancy, after that this task is accomplished by the terminal villi, produced by the mature
intermediate villi (having about the same diameter of the terminal villi).
For this reason in the model the immature intermediate villi are not considered.
* Capillary in the terminal villi:
From the precapillary arteries of the intermediate villi, blood flows into 3 to 5 terminal villi in
series, then into a post-capillary vein.
The terminal villi are the excrescences that fetal capillaries form on the surface of the cotyledons.
They develop from the intermediate villi during the last trimester of pregnancy.
The terminal part of the villous tree with the development of fetal capillaries is represented in figure
16.
The diagram of capillary vessels made by Kaufmann [26] is characterised by a low degree of ramification.
The estimated length of the capillaries is between 3000 and 5000 µm; the length of the correspondent
capillary paravascular network spans from 1000 to 2000 µm.
The measure of the mean diameters of capillary vessels has been executed through the electron microscope
and with a technique, already used by Arts, of measurement of stamps of vessels, obtained with perfusion
of the cotyledon using a polymer. The measured mean diameter is 12,2 µm with the first method and 14,4 µm
with the second one.
There are capillaries with diameters reaching 30 µm, sometimes 50 or even 80 µm; they are called
sinusoids. They are located mainly on the terminal-villous crests, where capillary are folded around
themselves, and at the points of bifurcation and fusion of vessels. Their surface is 30-40% of the total
surface of the villi.
The sinusoids seem to have the task to slow down the blood flow for a better gaseous exchange between
fetus and mother. Probably [27] blood is mixed here, including the central zone of the flow,
otherwise not involved in the exchange.
The capillaries are rather wide but also short. The consequence is that the difference of pressure
between efferent and afferent flow is small.
On the number of placental capillaries there is a mismatch in the values calculated by the researchers.
Assuming for each capillary a diameter of 15 µm and a length spanning between 3000 and 5000 µm, it is
easy to calculate the volume; moreover estimating the volume occupied by the capillary equal
approximately to 45 milliliters [28], the ratio of these volumes is the total number of the capillaries.
The result is 50 to 80 millions of capillaries. Other researchers propose 50 million approximately
(Wilkin and Burzstein).
This description of the terminal villi not absolutely complete, as these structures are extremely
variable in terms of characteristics of the villous surface, of capillary membrane and of tissue.
A schematic diagram of the capillary structure in the single villus is shown in figure 17.
* Capillaries in the mesenchimal villi:
At the end of pregnancy, the mesenchimal villi are in small number, mainly on the surface of the
immature intermediate villi: they compose less than 1% of the total volume of the villi [20].
* Capillary paravascular network:
It is a network of several capillaries connected by anastomosis, with smaller diameter compared to the
capillaries of the villi; they do not originate the sinusoids but are almost linear, 1000-2000 µm long
[20].
According to the hypothesis of Arts the paravascular network task is the nutrition for the stem villi.
This hypothesis is considered improbable by Kaufmann [20]. This network can be considered a residual of
the network present during the early development of the placenta [29]. Due to the development of the
terminal villi during the last trimester of pregnancy, the greater part of the paravascular network is
not working. For this reason it is neglected in our placental model.
Fig.14: Diagram of a peripheral stem villus.
Fig.18: Development of the volume of several the types of villi, versus the gestational age:
SV stem villi 31g 11%
transition to intermediate immature villi 24g 9%
VII intermediate immature villi 9g 3%
VM mesenchimal villi 3g 1%
transition to intermediate mature villi 3g 1%
VIM intermediate mature villi 77g 28%
transition to terminal villi 31g 11%
VT terminal villi 95g 35%
1.2.4. Anatomy of the Maternal Side . |
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The maternal blood of the spiral arteries flows into a system of communicating interstices that encircles
the cotyledons: the intervillous space. Here oxygen and the nourishing substances of the maternal blood
are exchanged with the fetal blood and at the same time the products of the fetal metabolism are acquired.
Then the maternal blood flows into the veins of the basal plate ("physiological" model of Ramsey) [31].
Each villous tree depends on its spiral artery: although the intervillous space is a system rather open,
the disposition of the villi and the gradient of pressure are linked so that the perfusion depends
closely on the original placement of the flows [32]: the occlusion of a spiral artery causes the death
of the tissue of the cotyledon that it used to feed.
The blood pressure at the end of the spiral arteries is approximately 70 mmHg, in the intervillous space
it is 12-15 mmHg [15] and in the endometrial veins, where blood is drained, it is 8 mmHg: the
pressure gap is almost totally in the intervillous space.
The studies of Freese explained that the cotyledons do not grow in uniform way but leave cavities in
front of the orifices of the spiral arteries. The maternal blood from the basal plate enters in this
empty space and, pushed by the pressure, flows towards the chorion (fig. 14), it passes through the
cotyledons not dispersing itself and finally it is drained by the endometrial veins.
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