eli5: Period is active bleeding?

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Okay so I learned back in 3rd grade or whatever that all month long, your body was building up blood and tissue in your uterus to hopefully implant an egg and get pregnant. When that fails to happen, you shed that blood and tissue.

In my mind, that stuff is kinda “ on the shelf”, yeah, it’s blood and tissue, but not like ACTIVE blood pumping through your heart, right?

So why, when I’m having my period, do people say, “ you’re losing a lot of blood, take it easy”. I thought all that blood has been siphoned off all month, and it shouldn’t make a difference? I mean, it’s not like I need a tourniquet for an artery- even if it’s obnoxiously heavy sometimes.

Is it built up all month?

Or like an active wound?

I’m a 40 year old woman who should know this shit. Help.

In: Biology

It is not like an open wound. It’s not something you could cauterize and ‘stop’. Your uterus builds up a nice, cosy lining for a potential fertilized egg, and when that egg exits, so does the lining. Its more like the lining is a celebrity entourage, and the egg is the celebrity. They all gather around it, and leave when it does…..

So while the body builds the layers of tissue, it is actively taking blood from your blood stream (all the new cells being made need food and oxygen!). Then you get to the stage where the body is expected to ovulate and the lining is at its peak. Juicy, bloody cushion for the ovum.

And then it goes downhill. When no fertilized ovum is received, the uterus fails to send a message to the ovary that you’re pregnant. The ovary then continues with the default mode of making hormones that tell the uterus “get rid of this lining, we’re starting over”. The blood vessels in the lining contract and start cutting off the blood supply to the lining. Remember you have just short of two weeks between expected fertilization and the start of your period. Ample time to make sure there’s a nice demarcation line and avoid bleeding out

When people say you are losing blood and should take it easy, you are correct in thinking that blood has mostly already been cut off from your overall blood volume and you shouldn’t feel it much. There is some active blood loss when uterine contractions make a slightly larger blood vessel tear, but most of it is “old blood”. This is also why period blood tends to be darker shades of red/brown/black instead of bright red if it were active arterial bleeding.

You do however get tired during your period due to the contractions and hormonal changes, which people falsely interpret as tiredness from blood loss. The common thought of women losing iron during their periods isn’t completely wrong, but the loss is spread out during the month as the blood gets accumulated in the uterus and eventually shed during menstruation