Human skeleton anatomy

If the ovum is not fertilized, about 2 days

before the end of the monthly cycle, the corpus luteum

in the ovary suddenly involutes, and the ovarian

hormones (estrogens and progesterone) decrease to

low levels of secretion,

Menstruation follows.

Menstruation is caused by the reduction of estrogens

and progesterone, especially progesterone, at the

end of the monthly ovarian cycle. The first effect is

decreased stimulation of the endometrial cells by these

two hormones, followed rapidly by involution of the

endometrium itself to about 65 per cent of its previous

thickness. Then, during the 24 hours preceding the

onset of menstruation, the tortuous blood vessels

leading to the mucosal layers of the endometrium

become vasospastic, presumably because of some

effect of involution, such as release of a vasoconstrictor

material—possibly one of the vasoconstrictor types

of prostaglandins that are present in abundance at this

time.

The vasospasm, the decrease in nutrients to the

endometrium, and the loss of hormonal stimulation

initiate necrosis in the endometrium, especially of the

blood vessels. As a result, blood at first seeps into the

vascular layer of the endometrium, and the hemorrhagic

areas grow rapidly over a period of 24 to 36

hours. Gradually, the necrotic outer layers of the

endometrium separate from the uterus at the sites

of the hemorrhages until, about 48 hours after the

onset of menstruation, all the superficial layers of

the endometrium have desquamated. The mass of

desquamated tissue and blood in the uterine cavity,

plus contractile effects of prostaglandins or other

substances in the decaying desquamate, all acting

together, initiate uterine contractions that expel the

uterine contents.

During normal menstruation, approximately 40

milliliters of blood and an additional 35 milliliters of

serous fluid are lost. The menstrual fluid is normally

nonclotting because a fibrinolysin is released along

with the necrotic endometrial material. If excessive

bleeding occurs from the uterine surface, the quantity

of fibrinolysin may not be sufficient to prevent clotting.

The presence of clots during menstruation is often

clinical evidence of uterine pathology.

Within 4 to 7 days after menstruation starts, the loss

of blood ceases because, by this time, the endometrium

has become re-epithelialized.

Leukorrhea During Menstruation. During menstruation,

tremendous numbers of leukocytes are released

along with the necrotic material and blood. It is probable

that some substance liberated by the endometrial

necrosis causes this outflow of leukocytes. As a result

of these leukocytes and possibly other factors, the

uterus is highly resistant to infection during menstruation,

even though the endometrial surfaces are

denuded. This is of extreme protective value.