Phase 1 · Understand the Body — Why the disease behaves the way it does.
The One-Line Truth
The whole disease in one chain
Everything on this page exists to explain this line.
Pressure squashes tiny blood tubes in the skin, so oxygen stops reaching the spot and the skin dies.
Pinched garden hose stops the flow
A pressure injury is like stepping on a garden hose and cutting off the water. The hose is your capillaries — tiny, delicate tubes that carry life-giving oxygen-rich blood. Your body weight pressing skin against bone is like your foot on the hose — it flattens the tube and water cannot flow through. The grass beyond the pinch (the skin cells) starts to wilt and yellow because it gets no water (oxygen). If you keep your foot there for hours instead of seconds, the grass dies completely instead of bouncing back. Just like dead grass turns brown and rots, dead skin tissue turns black or gray and becomes infected if bacteria move in.
National Pressure Injury Advisory Panel (NPIAP) 2016 staging system · Agency for Healthcare Research and Quality (AHRQ) pressure ulcer prevention guideline · Wound Ostomy and Continence Nurses Society (WOCN) 2016 guideline · Cochrane systematic reviews on pressure ulcer prevention and treatment · reviewed 2026-07-07