A pressure sore (or bed sore) is an injury to the skin and tissue under it. Pressure sores are usually caused by unrelieved pressure. If you sit or lie in the same position for a long time, the pressure on a small area of the body can squeeze shut tiny blood vessels that normally supply tissue with oxygen and nutrients. If tissue is starved of these "fuels" for too long, it begins to die, and a pressure sore starts to form.
Pressure sores are called pressure ulcers and decubitus ulcers as well as bed sores. How serious they are depends on the amount of damage to skin and tissue. Damage can range from a change in the color of unbroken skin (Stage I) to severe, deep wounds down to muscle or bone (Stage IV). In light-skinned people, a Stage I sore may change skin color to a dark purple or red area that does not become pale under fingertip pressure. In dark-skinned people, this area may become darker than normal. The affected area may feel warmer than surrounding tissue.
Information provided by National Library of Medicine (NLM).
pressure mattress / low air loss mattress for prevention / treatment
of pressure sores.