Post-surgery, the fresh blood containing oxygen from the lungs goes directly to the surgical area. The old blood then has an issue leaving the surgical area and getting back to the lungs and heart. The old blood causes the area to become cold and dark in color.
Leeches are attached to the discolored area to suck out the old blood, allowing fresh blood to enter the area. The leeches can feed anywhere from 10 minutes up to an hour. They suck 1 to 2 teaspoons of blood during a feeding. Once the leeches are done feeding, they will fall off of the area. The leeches are not reused and are placed in a container of alcohol to die.