Menstrual phase bleeding is due to

The correct option is "vasoconstriction and ischaemia".

To understand why menstrual bleeding occurs, we can look at the physiological changes that take place in the uterus during the late secretory phase of the menstrual cycle:

1. Withdrawal of Progesterone:
Towards the end of the menstrual cycle, if fertilization of the ovum does not occur, the corpus luteum degenerates. This degeneration leads to a rapid decline in the levels of ovarian hormones, primarily progesterone and estrogen.

2. Vasoconstriction of Spiral Arteries:
The decrease in progesterone levels triggers the release of local vasoconstrictors (such as prostaglandins) in the endometrium. This causes intense, spasmodic constriction (vasoconstriction) of the spiral arteries, which supply blood to the functional layer (stratum funktionale) of the endometrium.

3. Ischaemia and Tissue Necrosis:
As a direct result of vasoconstriction, the blood flow to the functional endometrial tissue is severely restricted. This state of oxygen and nutrient deprivation is known as ischaemia. The prolonged ischaemia leads to cell death and necrosis of the functional endometrial lining.

4. Bleeding and Shedding:
Eventually, the constricted spiral arteries relax and open up again. As blood rushes back into the damaged, necrotic vessel walls, they rupture. This causes bleeding, and the shed blood, along with the necrotic endometrial tissue, is discharged as menstrual flow.

Therefore, the primary physiological mechanism that initiates the bleeding of the menstrual phase is vasoconstriction and subsequent ischaemia of the endometrial spiral arteries.