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Rheological and physiological consequences of conversion of the maternal spiral arteries for uteroplacental blood flow during human pregnancy.

Burton GJ, Woods AW, Jauniaux E, Kingdom JC - Placenta (2009)

Bottom Line: Physiological conversion of the maternal spiral arteries is key to a successful human pregnancy.We speculate that the high momentum will damage villous architecture, rupturing anchoring villi and creating echogenic cystic lesions as evidenced by ultrasound.The retention of smooth muscle will also increase the risk of spontaneous vasoconstriction and ischaemia-reperfusion injury, generating oxidative stress.

View Article: PubMed Central - PubMed

Affiliation: Centre for Trophoblast Research, University of Cambridge, Cambridge, UK. gjb2@cam.ac.uk

ABSTRACT
Physiological conversion of the maternal spiral arteries is key to a successful human pregnancy. It involves loss of smooth muscle and the elastic lamina from the vessel wall as far as the inner third of the myometrium, and is associated with a 5-10-fold dilation at the vessel mouth. Failure of conversion accompanies common complications of pregnancy, such as early-onset preeclampsia and fetal growth restriction. Here, we model the effects of terminal dilation on inflow of blood into the placental intervillous space at term, using dimensions in the literature derived from three-dimensional reconstructions. We observe that dilation slows the rate of flow from 2 to 3m/s in the non-dilated part of an artery of 0.4-0.5mm diameter to approximately 10 cm/s at the 2.5mm diameter mouth, depending on the exact radius and viscosity. This rate predicts a transit time through the intervillous space of approximately 25s, which matches observed times closely. The model shows that in the absence of conversion blood will enter the intervillous space as a turbulent jet at rates of 1-2m/s. We speculate that the high momentum will damage villous architecture, rupturing anchoring villi and creating echogenic cystic lesions as evidenced by ultrasound. The retention of smooth muscle will also increase the risk of spontaneous vasoconstriction and ischaemia-reperfusion injury, generating oxidative stress. Dilation has a surprisingly modest impact on total blood flow, and so we suggest the placental pathology associated with deficient conversion is dominated by rheological consequences rather than chronic hypoxia.

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a) Graphs showing the relationship between the flow volume and the radius of a spiral artery, assuming a length of 1 cm and a pressure drop of 80 mmHg. The upper curves correspond to a viscosity of 3 mPa s and the lower curves to a viscosity of 6 mPa s. b) Graphs showing the relationship between the speed of flow and the radius of a spiral artery, assuming a constant radius and no dilation.
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fig4: a) Graphs showing the relationship between the flow volume and the radius of a spiral artery, assuming a length of 1 cm and a pressure drop of 80 mmHg. The upper curves correspond to a viscosity of 3 mPa s and the lower curves to a viscosity of 6 mPa s. b) Graphs showing the relationship between the speed of flow and the radius of a spiral artery, assuming a constant radius and no dilation.

Mentions: If the radius is constant along the length of the artery then the pressure gradient is constant, and in this case Fig. 4a illustrates the flow rate as a function of the radius. Curves are given for blood viscosities of 3 mPa s (upper curve) and 6 mPa s (lower curve), which span the measurements provided for the end of pregnancy [8]. We infer from this figure that for an artery with a radius in the order of 0.2–0.25 mm (diameter 0.4–0.5 mm), as depicted by Harris and Ramsey (Fig. 1c) [16], the flow rate will be about 0.2–0.4 ml/s. With smaller arteries, the flow rate decreases rapidly given the dependence of the flow on the fourth power of the radius. The mean flow speed in the artery is in the order of 1–2 m/s, given the flow rate and cross-sectional area, as indicated in Fig. 4b.


Rheological and physiological consequences of conversion of the maternal spiral arteries for uteroplacental blood flow during human pregnancy.

Burton GJ, Woods AW, Jauniaux E, Kingdom JC - Placenta (2009)

a) Graphs showing the relationship between the flow volume and the radius of a spiral artery, assuming a length of 1 cm and a pressure drop of 80 mmHg. The upper curves correspond to a viscosity of 3 mPa s and the lower curves to a viscosity of 6 mPa s. b) Graphs showing the relationship between the speed of flow and the radius of a spiral artery, assuming a constant radius and no dilation.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2697319&req=5

fig4: a) Graphs showing the relationship between the flow volume and the radius of a spiral artery, assuming a length of 1 cm and a pressure drop of 80 mmHg. The upper curves correspond to a viscosity of 3 mPa s and the lower curves to a viscosity of 6 mPa s. b) Graphs showing the relationship between the speed of flow and the radius of a spiral artery, assuming a constant radius and no dilation.
Mentions: If the radius is constant along the length of the artery then the pressure gradient is constant, and in this case Fig. 4a illustrates the flow rate as a function of the radius. Curves are given for blood viscosities of 3 mPa s (upper curve) and 6 mPa s (lower curve), which span the measurements provided for the end of pregnancy [8]. We infer from this figure that for an artery with a radius in the order of 0.2–0.25 mm (diameter 0.4–0.5 mm), as depicted by Harris and Ramsey (Fig. 1c) [16], the flow rate will be about 0.2–0.4 ml/s. With smaller arteries, the flow rate decreases rapidly given the dependence of the flow on the fourth power of the radius. The mean flow speed in the artery is in the order of 1–2 m/s, given the flow rate and cross-sectional area, as indicated in Fig. 4b.

Bottom Line: Physiological conversion of the maternal spiral arteries is key to a successful human pregnancy.We speculate that the high momentum will damage villous architecture, rupturing anchoring villi and creating echogenic cystic lesions as evidenced by ultrasound.The retention of smooth muscle will also increase the risk of spontaneous vasoconstriction and ischaemia-reperfusion injury, generating oxidative stress.

View Article: PubMed Central - PubMed

Affiliation: Centre for Trophoblast Research, University of Cambridge, Cambridge, UK. gjb2@cam.ac.uk

ABSTRACT
Physiological conversion of the maternal spiral arteries is key to a successful human pregnancy. It involves loss of smooth muscle and the elastic lamina from the vessel wall as far as the inner third of the myometrium, and is associated with a 5-10-fold dilation at the vessel mouth. Failure of conversion accompanies common complications of pregnancy, such as early-onset preeclampsia and fetal growth restriction. Here, we model the effects of terminal dilation on inflow of blood into the placental intervillous space at term, using dimensions in the literature derived from three-dimensional reconstructions. We observe that dilation slows the rate of flow from 2 to 3m/s in the non-dilated part of an artery of 0.4-0.5mm diameter to approximately 10 cm/s at the 2.5mm diameter mouth, depending on the exact radius and viscosity. This rate predicts a transit time through the intervillous space of approximately 25s, which matches observed times closely. The model shows that in the absence of conversion blood will enter the intervillous space as a turbulent jet at rates of 1-2m/s. We speculate that the high momentum will damage villous architecture, rupturing anchoring villi and creating echogenic cystic lesions as evidenced by ultrasound. The retention of smooth muscle will also increase the risk of spontaneous vasoconstriction and ischaemia-reperfusion injury, generating oxidative stress. Dilation has a surprisingly modest impact on total blood flow, and so we suggest the placental pathology associated with deficient conversion is dominated by rheological consequences rather than chronic hypoxia.

Show MeSH
Related in: MedlinePlus