UTERINE BLOOD FLOW 



'599 



tions or hormone treatments will produce menstrua- 

 tion are related to the nature of the vasculature of the 

 endometrium. This is affected by the previous 

 menstrual history of the organism since each cycle 

 brings about some residual changes through growth of 

 the vascular tree which persist to affect the next 

 cyclic bleeding [Phelps (173)]. 



In addition to the idea that changes in the uterine 

 vasculature affect and modify the menstrual process, 

 Smith (220) and Smith & Smith (221) hold to the 

 view that a toxic substance is produced within the 

 endometrium, secondary to the premenstrual 

 ischemia, and that this toxin leads to the breakdown 

 of the tissues and the ensuing menstrual discharge. 



Prostigmin, a vasodilating substance, has been 

 shown to cause uterine bleeding in nonpregnant 

 women [Soskin et al. (223)], but not in pregnant 

 women. Kaiser (123) failed to observe a similar 

 response in the rhesus monkey. This drug also failed 

 to affect estrogen-induced hyperemia in endometrial 

 ocular transplants in the rabbit [Kaiser (125)]. 



In the nonpregnant endometrium of both rabbits 

 and monkeys, there are rhythmic constrictions and 

 dilations of the minute vessels which are independent 

 of the nervous system [Markee (142, 144)]. Under the 

 influence of estrogen there is persistent hyperemia of 

 the endometrium [Pompen (176), Markee (143)]. 

 The significance of the rhythmic vascular changes, 

 both as to cause and as to function, are unknown. 

 They are, apparently, unique to endometrial vessels, 

 although estrogens do have profound effects on 

 somatic minute vessels [see Reynolds (198)], especially 

 integumentary, in rabbits and humans [Reynolds & 

 Foster (206, 208)], and in the nasal mucosa [Mac- 

 kenzie (139)], as well. The retinal circulation is also 

 modified in women, manifesting itself by scotomata 

 that change in position with change in posture [Evans 

 (72)]; this is marked in the last half of the menstrual 

 cycle. 



Of importance to the vascular architecture in the 

 endometrium is the seldom emphasized fact that the 

 tissue in which these structures lie is loose and spongy. 

 The vascular elements are developed out of all pro- 

 portion to the immediate vascular needs of the tissue 

 [Reynolds (196)]. It is clear that the vascular arrange- 

 ment is adapted to the future needs of supplying and 

 invading the implanting trophoblast [Hasner (98), 

 Bartelmez (25)]. This instance is not unique in 

 developmental biology where nature has repeatedly- 

 contrived to anticipate future needs by prior organi- 

 zation of mechanisms. When a trophoblast fails to 

 develop, the complex vascular structure ot the endo- 

 metrium breaks down since it cannot be maintained 



in the face of the requirements of cyclic endocrine 

 activity in which ovulation is the focal point of the 

 pattern. This endometrial cycle occurs, even though 

 ovulation may not occur. Moreover, most uteri, 

 even including those of some primates [Kaiser (120), 

 Hamlett (93), Goodman & Wislocki (85)], do not 

 manifest endometrial sloughing even though they 

 exhibit microscopic bleeding; instead, the endometrial 

 vessels undergo an ebb and flow of cyclic growth and 

 regression unaccompanied by profound menstrual 

 process. In either event, the local vasculature changes 

 cyclically. In some species, such as rats, hamsters, 

 and guinea pigs, the cyclic occurrence of localized 

 areas of hyperemia within the uterus is evident (see 

 below) . 



HORMONES AND THE UTERINE VASCULATURE 



The endometrial hyperemia, indeed the entire 

 uterine hyperemia, that occurs periodically has a 

 metabolic basis under endocrine control. Estrogen 

 augments the amount of acetylcholine found in the 

 uterus [Reynolds (193, 203)] and in the nasal mucosa 

 as well [Reynolds & Foster (207)]. However, it was 

 later found that it is the change in cholinesterase 

 which accounts for this [Everett & Sawyer (73), 

 Herschberg (103)]. It is also reported that the hypere- 

 mia is associated with alterations in the amount of 

 histamine or histamine-like substances in the uterus 

 [Kaiser (122)]. It appears that one can only say that 

 there is a change in vasoreactive tissue constituents 

 under the influence of estrogen and it is probable that 

 more than one substance is involved. 



UTERINE CONTRACTION AND BLOOD FLOW 



The circulation in the uterus, like that in all mus- 

 cular viscera, functions in the face of contraction and 

 relaxation of the muscular components of the organ. 

 It must serve with great efficiency as the uterus under- 

 goes great change in size and shape during pregnancy. 

 The consequences of contractions, growth, and dis- 

 tention upon blood flow in the uterus require con- 

 sideration. Certainly, clamping of the blood supply 

 to the uterus elicits uterine contractions, as Rorhrig 

 showed many years ago [see Reynolds (198)]. In 

 this respect, myometrium is no different from intestine 

 or other smooth muscle. More delicate, however, is 

 the observation that low arterial blood pressure is 

 associated with an increase in frequency and ampli- 

 tude of uterine contractions [Kunisima (132), Robson 



