Two G-protein-linked angiotensin II receptors are expressed in the uterus-AT1 and AT2. In nonpregnant women, the AT2 receptor predominates, but the ATI receptor is preferentially expressed in gravidas (Cox, 1993). Angiotensin II binding to the plasma-membrane receptor evokes contraction. During pregnancy, the vascular smooth muscle that expresses the AT2 receptor is refractory to the pressor efects of infused angiotensin II (Chap. 4, p. 63).
Physiology of Labor 41 7
PHASE 4: THE PUERPERIUM
Immediately and for about an hour after delivery, the myometrium remains persistently contracted. This directly compresses large uterine vessels and allows thrombosis of their lumens to prevent hemorrhage. his is typically augmented by endogenous and pharmacological uterotonic agents (Chap. 27, p. 526).
Uterine involution and cervical repair are prompt remodeling processes that restore these organs to the nonpregnant state. hese protect the reproductive tract from invasion by commensal microorganisms and restore endometrial responsiveness to normal hormonal cyclicity.
During the early puerperium, lactogenesis and milk let-down begin in mammary glands (Chap. 36, p. 656). Reinstitution of ovulation signals preparation for the next pregnancy. Ovulation generally occurs within 4 to 6 weeks after birth. However, it is dependent on the duration of breastfeeding and lactationinduced, prolactin-mediated anovulation and amenorrhea.
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