The common, informal phrase “period blood” often leads to the mistaken idea that the monthly flow is simply old, stagnant blood being expelled. This misunderstanding is sometimes reinforced by the notion that the fluid is “dead blood.” This article provides a scientific explanation of menstrual discharge, detailing its true composition and the biological process that creates it.
The Nature of Menstrual Fluid
Menstrual fluid is a complex biological discharge, not standard venous blood. While it has a reddish-brown appearance due to the presence of blood, it is more accurately termed menstrual fluid. The uterine lining contains the enzyme plasmin, which acts as an anticoagulant.
This anti-clotting mechanism prevents the blood from solidifying before it leaves the body, allowing it to flow more easily. If the flow is particularly heavy, the enzyme may be overwhelmed, which is why some people pass small blood clots. The actual blood component only makes up a fraction of the entire discharge, typically 36% to 50% of the total volume.
Components Beyond Blood
The majority of menstrual fluid is composed of elements other than red blood cells, accounting for roughly 50% to 64% of the total volume. This complex mixture contains cellular debris, proteins, and various secretions that distinguish it from simple blood loss. The non-blood components are what give the discharge its unique consistency.
The most significant non-blood material is the endometrial tissue itself, which breaks down into cellular debris and tissue fluid as it is expelled. This tissue fluid is rich in electrolytes, including sodium, calcium, phosphate, and iron. The discharge also incorporates cervical mucus and normal vaginal secretions as the fluid travels out of the body.
The fluid is a unique biological sample, containing over 300 distinct proteins, some of which are not found in peripheral blood. It also contains live cells, including immune cells and mesenchymal stem cells, which researchers are studying for their regenerative properties.
The Biological Process of Shedding
The creation of menstrual fluid is the final stage of the reproductive cycle, where the body prepares the uterus for a potential pregnancy. Each month, the inner lining of the uterus, known as the endometrium, thickens under the influence of hormones like estrogen and progesterone. This thickening is an elaborate preparation to house and nourish a fertilized egg.
If fertilization does not occur, the levels of progesterone drop sharply, triggering the start of menstruation. The drop in progesterone causes the spiral arteries—the blood vessels supplying the functional layer of the endometrium—to constrict, a process called vasoconstriction. This constriction cuts off the blood supply to the superficial layer of the uterine lining.
This lack of blood leads to tissue breakdown and disintegration through cell death, known as apoptosis. The shedding of the upper layer of the endometrium, along with subsequent bleeding from the damaged vessels, forms the basis of the menstrual flow. The process is a highly regulated cycle of tissue injury, shedding, and rapid repair that prepares the uterus to begin the process all over again without scarring.