Vascular system of humane uterus during labor

Three special blood circuits with their morphological distinctions are functioning in uterus during labor. Even the most up-to-date guides and obstetrics literature unfortunately do not contain many essentials about the functional morphology of each of the blood circuits typical for the last period of pregnancy and especially for labor.

Personal mural blood supply system of myometrium of the uterus body, its lower segment and cervix, ensuring trophic needs of the tissue cells is subject to adaptive hypertrophy equivalent to the rise of their volume and mass and consists of a common set of elements of a vasculature: arteries, arterioles, capillaries, venules, and veins. The essentially important specific feature of the development of the blood system of uterus during pregnancy is the emergence and progressive development of a venous system of special kind with its emergence and development not directly related to ensuring trophic needs of the organ tissue that take place in the muscle shell stratum of the uterus body, by the end of pregnancy – in the lower segment, and only by the very beginning of labor – in cervix. This venous system consists of thin-walled venous lacunas stripped of their own muscle shell as well as an endothelial lining in some areas. By anastomosing between each other the lacunas make up a basis for a dividing venous sinus located between the external and internal muscular layers of the uterus body as well as pierce through the stratum of the internal muscular layer thus turning it into corpus cavernosum. With the beginning of labor a similar venous system is developed in the cervix that also turns into corpus cavernosum. This huge venous network is able to deposit the blood coming from the myometric and uteroplacental circuits of blood circulation and deliver it in the extraorganic veins that drain the venous system of uterus. The essentially important fact is that the shells of the functional contracting modules of myometrium become the “muscular shell” of these lacunary transformed veins of the dividing venous sinus and the internal muscular layer of uterus. The walls of this kind of lacunas in cervix are supported by the subject elements of its connective-tissue matrix.

The uteroplacental blood circuit starts with large-sized (diameter up to 2 mm) arcuate arteries located between the muscular layers of uterus that have radial (spiral) arteries going aside in the area of the placental bed; intervillous space, formed by the placental bed of uterus and maternal surface of placenta, and end with placental veins that anastomose with the lacunary transformed veins of the internal mascular layer of uterus. During pregnancy spiral arteries are subject to “gestational restructuring”. The main point is that as a result of two subsequent waves of trophoblast invasion the muscular layer of arteries disappears, the arteries in the physical sense turn into a “soft pipe”; the myometrium segment of the spiral artery turns out to be surrounded by the smooth muscle tissue of myometrium as well as the uteroplacental veins. A priori, it can be stated that neither uteroplacental vein nor uteroplacental artery possesses their own system of lumen regulation. The essential distinctions in the morphostructure of each of the blood circuits of uterus during labor give basis for believing that these distinctions may well determine some peculiarities of their functioning.

Concise discussion

Contracting system of human uterus during labor is morphologically homogeneous due to the fact that it consists of single-type functional contracting units – contracting modules that, in their turn, consist of single-type primary cylindrical beams; the basis for which are morphologically absolutely identical smooth muscle cells of a single type. During labor two layers in the myometrium of uterus can be singled out morphologically – external and internal. As a huge part of functional contracting modules move from one layer to another thus forming a single-type grid three-dimensional structure, the division of myometrium into the two layers has generally morphological reasons. The basis for the external muscular layer of uterus is functional contracting modules that do not have fixation points in the lower segment. Their fixation points are located in the supporting and fixating ligamentous apparatus of uterus. The anatomical fixation points tightly glue the external muscular layer of the uterus body to the pelvic bones. Each of the functional contracting modules being simultaneously located by their different parts both in the layers of the uterus body and lower segment form a special semi-half-loop structure in the area of the anatomic internal pharynx of uterus (contraction ring) that during labor divides uterus into two parts – the body and lower segment. Functional contracting modules acquire a horizontal course only in the area of the contraction ring, while their upper (located in the uterus body) and lower (located in the lower segment) parts have an oblique-longitudinal or oblique-diametrical direction.

The second morphologically essential distinction of the external and internal layers of the uterus shell is the development of a unique (in a volume and structure respect) venous system in the latter consisting of thin-walled lacunary transformed vessels. The ssentially important is that by the beginning of labor a similar venous system is developed in cervix. It consists of lacunary transformed veins turning the cervix into corpus cavernosum.

Conclusion. The above-mentioned documentary information as well as our idea of the peculiarities of the actual functional morphology of human uterus during labor have not been officially disproved during the passed three decades, despite it stands in evident contradiction to the data and ideas that are still disseminated in obstetrics text books and guides. Our data do not anyhow blend with the ideology of the biomechanics of uterine contraction based on the classic idea. Unfortunately for the two-century history of scientific obstetrics no one from recognized authority in this area has raised a question of what is the reason for the presence of the essentially important venous system in the body, lower segment and its cervix with its function that cannot be defined only by the drainage needs of organ. The volume of lacunary transformed vessels as well as the peculiarities of their morphology are obviously redundant and unreasonable if considered from the perspective of the “regular” function of vessels which serve for blood withdrawal from the organ tissue. Despite the information access and credibility of the materials devoted to resolving the issue it has not gained a discussion status yet in spite of being raised thirty years ago. There has been no attempts to evidentially disprove our propositions but there have been attempts of more progressive researchers to independantly solve the issue. Silence on the part of the specialists concerned can only be treated as an acceptance of our propositions and desire to gain more accurate and exhaustive information on the issue. This monograph will contribute to reaching this goal.

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