As in, how do they know how much blood is lost during surgery?
They suction it up as they go along, and then weigh any gauze that’s saturated with blood if needed
Well doctors use a few methods. They might look at all the blood that’s been collected during the surgery, and make a rough estimate based on how much they can see. They can also weigh the blood they’ve collected, so they can get a more exact number. They may also check the patient’s blood before and after the surgery, to see if there’s been a noticeable drop in the red blood cells, which would indicate a loss of blood. And they keep a close eye on the patient’s heart rate and blood pressure. If either one drops, it’s a sign that they may have lost too much blood.
We guess based on the number of sponges used plus what is suctioned. We can also weigh the sponges but very few places use that system. It’s an estimate and accuracy can vary quite a bit. Usually it doesn’t matter because we don’t give blood based on the estimated loss. We decide to give blood based on how the patient is doing.
Blood loss leads to reduced blood pressure, so they that as a stand-in for total blood volume. If the patient is losing too much blood, they will give them donated blood to keep the patient alive.