UTERINE BLOOD FLOW 



l6o 3 



fig. 15. Local vasodilating 

 action of estrogen in endome- 

 trium of guinea pig. A, anti- 

 mesometrial, B, lateral; C, mes- 

 ometrial. [From Bacsich & 

 Wyburn (15).] 



volume increases [Orsini (164), Reynolds (199)] 

 coinciding with die period of most rapid uterine 

 enlargement. From this time until near term, there is 

 a period of diminished blood in the vascular bed 

 until term, when a period of partial hemostasis 

 supervenes [Barcroft & Rothschild (21), Reynolds 

 (192, 196)]. These changes are supported by studies 

 of bits of transplanted endometrium to the anterior 

 chamber of the eye [Neumann (160), Krichesky 



('30]. 



When the uterus is in situ, the blood vessels over the 

 conceptus give evidence of hypertrophy and the 

 tortuous course of the uterine arteries progressively 

 changes as they straighten out [Reynolds (199)]. 

 This is associated with local distention of the tissues 

 by the conceptus. Distention is a factor in uterine 

 hypertrophy [Reynolds (198)]. The veins, showing no 

 initial tortuosities, can only adapt by growth, stretch- 

 ing, and proliferation. In the uterus duplex, the 

 vessels in the interconceptus sites show no such 

 changes. Only as the spheroidal conceptuses enlarge 

 and encroach upon the interconceptus sites do the 

 blood vessels there become involved in extension and 

 stretching. These processes continue until a phase of 

 maximum spheroidal size is attained. At this time, 

 vessels that have been crowded from about each 

 conceptus toward the interconceptus sites lie close 

 together; those that lie around a conceptus are 

 stretched and, in any one area, sparse. Within a very- 

 short period of time (in the order of hours), the rapidly 

 enlarging conceptus breaks out of its spheroidal 

 shape as it pushes along the uterine lumen into less 

 distended regions of the uterus. When this happens, 

 the vessels of the interconceptus region slip with the 

 tissues in which they lie over the conceptus, much as a 

 stocking is slipped up a leg. After this, the enlargement 

 of the conceptus is solely by elongation, without 

 further increase in diameter. This elongation con- 



tinues until shortly before term, at which time a second 

 limit of distention is reached and stress is placed upon 

 the circulation for a second time. In any event, at a 

 time when fetal growth and demands upon the cir- 

 culation are great, the uterine blood vessels merely 

 become rearranged so as to minimize the hemo- 

 dynamic work of the maternal circulatory system in 

 supplying the uterus and its contents. 



How is blood flow in the uterus modified as these 

 changes take place? By measuring local circulation 

 times [Reynolds (194, 199)], it has been found that as 

 the spheroidal conceptus enlarges there is a progres- 

 sive decline in the circulation rate until the time of 

 conversion. Just prior to conversion, there is a pro- 

 found hemostasis in the tissues about a conceptus. 

 Upon release of tissue tension by the act of conversion, 

 a sudden increase in local circulation rate takes place, 

 approaching that observed at the start of pregnancy. 

 As gestation nears its end, there is a second decline in 

 blood flow concomitant with the longitudinal stretch- 

 ing of the uterus prior to parturition. 



The flow characteristics described above relate to 

 the flow in the maternal vessels of the uterine wall, 

 not to the other part of the uterine circulation, which 

 goes to the placenta. Here, there must be adequate 

 flow at all times, otherwise the fetus will be 

 endangered. No objective study has been made of the 

 manner by which this is accomplished, but it has been 

 speculated that the governing factor is the changing 

 shape of the pregnant uterus combined with tension 

 in the uterine tissues [Reynolds (195)]. Blood flow is 

 reduced to the tissues of the uterus which are most 

 concerned with change of shape in order to accommo- 

 date products of conception, and at the same time 

 blood is directed toward the placenta, since both 

 parts of the system are supplied by the same arteries at 

 the border of the mesometrium. This is to say that as 

 the peripheral vascular resistance increases in one 



