Period blood comes from the inner lining of your uterus, called the endometrium. Each month, the upper two-thirds of this lining builds up with blood-rich tissue, then sheds when pregnancy doesn’t occur. The fluid that leaves your body is a mix of blood, tissue from that lining, immune cells, and vaginal secretions. It travels from the uterus through the cervix and out the vagina.
The Endometrium: Where It All Starts
Your uterus has a muscular wall, and the inside is lined with a layer of tissue called the endometrium. This lining has two distinct parts. The lower third, closest to the muscle, is the basal layer. It stays put and never sheds. The upper two-thirds is the functional layer, and this is the part that breaks down and becomes your period.
Throughout your cycle, the functional layer transforms dramatically. In the first half, it thickens and develops a rich network of blood vessels, essentially building a soft, nutrient-dense surface where a fertilized egg could implant. If no embryo arrives, that entire upper layer becomes unnecessary, and the body breaks it down and flushes it out. After shedding, the basal layer regenerates a fresh functional layer, and the whole process begins again.
What Triggers the Shedding
Two hormones drive the cycle: estrogen and progesterone. Estrogen dominates the first half, stimulating the lining to grow. After ovulation, a small structure on the ovary called the corpus luteum pumps out progesterone, which stabilizes the thickened lining and prepares it for a potential pregnancy. Both hormones peak around the middle of the second half of your cycle.
If no pregnancy occurs, the corpus luteum breaks down. Estrogen and progesterone drop sharply. That withdrawal of progesterone is the key trigger. Without it, the functional layer loses its support. The tissue begins to break down through a cascade of inflammation, reduced oxygen supply, and cell death. Specialized blood vessels in the lining constrict, cutting off blood flow to the upper tissue, which accelerates the breakdown. The lining then detaches in a piecemeal fashion rather than all at once, which is why periods typically last several days instead of happening in a single moment.
The Role of Spiral Arteries
The functional layer of the endometrium contains unique blood vessels called spiral arteries. These coiled vessels supply the lining with blood during the cycle, but they also play a central role in menstruation. When progesterone drops, the spiral arteries constrict tightly, starving the tissue above them of oxygen. This triggers the tissue to break apart and shed.
Once shedding begins and the artery walls are disrupted, blood flows into the uterine cavity and mixes with the detaching tissue. Near the base of these arteries, a ring of muscle acts like a valve. After the functional layer has shed, this muscular segment constricts to stop the bleeding, which is how your body naturally limits how much blood you lose each period.
What’s Actually in Menstrual Fluid
What comes out during your period isn’t pure blood. It’s a complex fluid made up of several components: blood from the disrupted spiral arteries, cells shed from the endometrial lining, immune cells that were part of the tissue, and vaginal secretions that mix in as the fluid passes through the cervix and vaginal canal.
One thing people often notice is that menstrual fluid doesn’t clot the way a cut on your skin would. That’s because your body actively prevents it. During menstruation, the endometrium ramps up production of enzymes that dissolve clots as they form, keeping the fluid liquid so it can flow out easily. This system works like a built-in anticoagulant specific to the uterus. On heavier days, though, the clot-dissolving system can be overwhelmed, which is why small clots or clumps of tissue sometimes appear. That’s generally normal.
How It Leaves Your Body
Once the lining detaches into the uterine cavity, it needs a way out. The cervix connects the uterus to the vagina. It’s a muscular, tunnel-like structure that sits 3 to 6 inches deep inside the vaginal canal. It has two openings: one facing the uterus (internal) and one facing the vagina (external), connected by a narrow channel.
During menstruation, the cervix opens slightly wider than usual to allow the fluid to pass through. The blood and tissue flow downward through this channel, into the vaginal canal, and out of the body. Mild contractions of the uterine muscle help push the fluid out, and these contractions are what cause menstrual cramps.
How Much Blood You Actually Lose
The average total volume of menstrual fluid over a full period is about 80 to 87 mL, roughly a third of a cup. That includes everything: blood, tissue, and secretions. The range varies widely from person to person, anywhere from 15 mL to over 250 mL. A volume over about 170 mL is generally considered higher than normal and may be worth discussing with a provider, especially if it’s accompanied by fatigue or other signs of iron loss.
Because the lining sheds gradually rather than all at once, most people bleed for three to seven days. Flow is typically heaviest in the first two days, then tapers off as less tissue remains to shed and the basal layer begins regenerating underneath.
When Blood Flows the Wrong Direction
In most cycles, menstrual fluid flows downward through the cervix. But in some cases, a portion of the blood travels upward through the fallopian tubes and into the abdominal cavity. This is called retrograde menstruation, and it’s surprisingly common. The blood and endometrial cells that enter the abdomen are usually cleared by the immune system without causing problems. In some people, however, those displaced cells can implant on organs like the intestines, bladder, or the lining of the abdominal wall. When this tissue responds to hormonal cycles and grows outside the uterus, it’s known as endometriosis.