Blood is mostly made up of water, as are our organs and muscles. To read specifically about edema, swelling caused by fluid retention where excess fluid is trapped in the body's tissues, see our article Edema: Causes, Symptoms and Treatments.
If you use the Amsoil Ea15K01 and the ASM oil (For 3rd gen Prius), then you can safely go 15,000 miles between oil changes. I drive 25,000 miles per year in my Prius. Amsoil backs up their products by a written warranty.
Changing the oil in a 2004 Prius. There are many good reasons to change oil yourself. The most important is that you can control the quality and quantity of oil that goes into the car. Too many dealers are overfilling the 2004 Prius, which can hurt gas mileage and possibly damage the engine. Also, changing the oil when hot makes sure all of the crud in the oil system is still in the oil, rather than possibly settled onto the oilpan.