Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 25The Gonads: Development & Function of the Reproductive System 413

phase is remarkably constant at about 14 d, and the variations

seen in the length of the menstrual cycle are due for the most

part to variations in the length of the proliferative phase.

When fertilization fails to occur during the secretory phase,

the endometrium is shed and a new cycle starts.

Normal Menstruation

Menstrual blood is predominantly arterial, with only 25% of

the blood being of venous origin. It contains tissue debris,

prostaglandins, and relatively large amounts of fibrinolysin

from endometrial tissue. The fibrinolysin lyses clots, so that

menstrual blood does not normally contain clots unless the

flow is excessive.

The usual duration of the menstrual flow is 3 to 5 d, but

flows as short as 1 d and as long as 8 d can occur in normal

women. The amount of blood lost may range normally from

slight spotting to 80 mL; the average amount lost is 30 mL.

Loss of more than 80 mL is abnormal. Obviously, the amount

of flow can be affected by various factors, including the thick-

ness of the endometrium, medication, and diseases that affect

the clotting mechanism.

Anovulatory Cycles

In some instances, ovulation fails to occur during the men-

strual cycle. Such anovulatory cycles are common for the first

12 to 18 mo after menarche and again before the onset of the

menopause. When ovulation does not occur, no corpus lu-

teum is formed and the effects of progesterone on the en-

dometrium are absent. Estrogens continue to cause growth,

FIGURE 25–22 Relationship between ovarian and uterine changes during the menstrual cycle. (Reproduced with permission from
Windmaier EP, Raff H, Strang KT: Vander’s Human Physiology: The Mechanisms of Body Function, 11th ed. McGraw-Hill, 2008.)

Follicle

Ovum

Corpus luteum

Follicular Luteal Follicular

Menstrual

Day

Ovarian

phase

Uterine

phase

Endometrial

thickness

Ovarian

event

Proliferative Secretory Menstrual

Ovulation

Estrogen Progesterone

Estrogen

1 5 10 15 20 25 28 5

FIGURE 25–23 Spiral artery of endometrium. Drawing of a
spiral artery (left) and two uterine glands (right) from the endometri-
um of a rhesus monkey; early secretory phase. (Reproduced with
permission from Daron GH: The arterial pattern of the tunica mucosa of the uterus in
the Macacus rhesus. Am J Anat 1936;58:349.)

Myometrium

Uterine lumen