The microorganisms are already present in the gastrointestinal tract. They simply continue with their aerobic activity. Once the heart has ceased to beat, the delivery of fresh oxygen from the outside environment stops and the microorganisms quickly consume the oxygen left in the body. This causes bloating, because gases from the bacteria cause the cadaver to swell, much like a balloon is blown up. This causes cadaver juices to be pushed from every orifice, such as the anus, the nose, and the mouth. Rupturing of the skin may also occur, opening the muscle tissues to fresh oxygen. Flies lay their eggs in these openings in the skin. Maggots (baby flies) feed off the skin and underlying muscle tissue, causing what is known as active decay, where the greatest mass loss is incurred from the cadaver. Microorganism activity is rampant throughout each of the stages of decay, turning the substance of the deceased body back into nutrients and substances that will be absorbed back into the Earth.