Arginine Vasopressin Also known as antidiuretic hormone (ADH), AVP is the second major secretory product of the posterior pituitary (Fig. 7.7). It is synthesized primarily by neurons with cell bodies in the supraoptic nuclei (Fig. 7.8). Its major function is the regulation of circulating blood volume, pressure, and osmolality (52). Specific receptors throughout the body can trigger the release of AVP. Osmoreceptors located in the hypothalamus sense changes in blood osmolality from a mean of 285 mOSM/kg. Baroceptors sense changes in blood pressure caused by alterations in blood volume and are peripherally located in the walls of the left atrium, carotid sinus, and aortic arch (53). These receptors can respond to changes in blood volume of more than 10%. In response to decreases in blood pressure or volume, AVP is released and causes arteriolar vasoconstriction and renal free-water conservation. This in turn leads to a decrease in blood osmolality and an increase in blood pressure. Activation of the renal renin–angiotensin system can also activate AVP release.
In the normal menstrual cycle, orderly cyclic hormone production and parallel proliferation of the uterine lining prepare for implantation of the embryo. Disorders of the menstrual cycle and, likewise, disorders of menstrual physiology, may lead to various pathologic states, including infertility, recurrent miscarriage, and malignancy.
The normal human menstrual cycle can be divided into two segments: the ovarian cycle and the uterine cycle, based on the organ under examination. The ovarian cycle may be further divided into follicular and luteal phases, whereas the uterine cycle is divided into corresponding proliferative and secretory phases (Fig. 7.9). The phases of the ovarian cycle are characterized as follows: 1. Follicular phase—hormonal feedback promotes the orderly development of a single dominant follicle, which should be mature at midcycle and prepared for ovulation. The average length of the human follicular phase ranges from 10 to 14 days, and variability in this length is responsible for most variations in total cycle length.
Figure 7.9 The menstrual cycle. The top panel shows the cyclic changes of follicle-stimulating hormone (FSH), luteinizing hormone (LH), estradiol (E2), and progesterone (P) relative to the time of ovulation. The bottom panel correlates the ovarian cycle in the follicular and luteal phases and the endometrial cycle in the proliferative and secretory phases.
2. Luteal phase—the time from ovulation to the onset of menses has an average length of 14 days.
A normal menstrual cycle lasts from 21 to 35 days, with 2 to 6 days of ﬂow and an average blood loss of 20 to 60 mL. However, studies of large numbers of women with normal menstrual cycles showed that only approximately two-thirds of adult women have cycles lasting 21 to 35 days (54). The extremes of reproductive life (after menarche and perimenopause) are characterized by a higher percentage of anovulatory or irregularly timed cycles (55,56).
Hormonal Variations The relative pattern of ovarian, uterine, and hormonal variation along the normal menstrual cycle is shown in Fig. 7.9.
1.
At the beginning of each monthly menstrual cycle, levels of gonadal steroids are low and have been decreasing since the end of the luteal phase of the previous cycle.
2.
With the demise of the corpus luteum, FSH levels begin to rise, and a cohort of growing follicles is recruited. These follicles each secrete increasing levels of estrogen as they grow in the follicular phase. The increase in estrogen, in turn, is the stimulus for uterine endometrial proliferation.
3.