A pressure ulcer is a sore on the skin that is caused by putting too much pressure on it. The skin gets red and tender because the pressure stops the blood and other fluids from flowing correctly. It can be caused by sitting or lying in one place for too long, or if someone wears a cast that stops the blood from flowing correctly. People who are bedridden or in wheelchairs are at risk for these sores. It is important to keep the skin as clean and dry as possible and to change positions or turn over frequently to avoid them.