Bed sores are caused by sustained pressure that cuts off blood flow to the skin and underlying tissue, most often over bony areas like the tailbone, heels, and hips.
The clinical term is pressure ulcer or pressure injury, and it happens when body weight presses skin against a mattress or chair for too long. That pressure compresses tiny blood vessels, starving tissue of oxygen and nutrients. Cell death can begin in as little as two hours of unrelieved pressure. Understanding exactly what causes a bed sore is the first step in preventing one, because once tissue damage starts, healing is slow and complications are real.
The Core Cause: Pressure That Blocks Blood Flow
The single mechanism behind every pressure injury is ischemia — inadequate blood supply to tissue. When you sit or lie in one position, the weight of your body compresses capillaries against bone. Without blood flow, skin cells suffocate and die. The damage starts deep near the bone and works outward, which is why a sore may look minor on the surface while serious injury exists below. The areas most vulnerable are those where bone sits directly under skin with little fat or muscle to cushion it: the tailbone, heels, hips, shoulder blades, elbows, and the back of the head.
Friction and Shear: The Forces That Make Pressure Worse
Pressure is the primary driver, but two mechanical forces accelerate the damage. Friction happens when skin rubs against sheets or clothing — this strips away the outer protective layer, making the skin more vulnerable. Shear occurs when a person slides down in a bed or chair; the tailbone moves downward while the skin stays put, pulling and kinking blood vessels. Together with pressure, friction and shear create the conditions for tissue breakdown much faster than pressure alone.
Anyone caring for someone on bed rest or in a wheelchair needs the right protective gear.
Who Is Most at Risk and Why
Immobility is the single biggest risk factor — anyone who cannot shift their own weight is vulnerable. But several other conditions make a person more susceptible. Skin weakened by moisture from sweat or incontinence breaks down quickly. Poor nutrition, especially low protein intake, reduces the fat padding that naturally protects bony areas. Reduced sensation from nerve damage means pain signals never fire, so the person doesn’t feel the need to move. Age also plays a major role: people over 85 are at the highest risk because aging skin is thinner and more fragile. Circulatory problems like diabetes or atherosclerosis leave tissues with weak blood flow even before pressure is applied.
Common Prevention Mistakes
- Dragging heels or elbows when repositioning someone creates friction and shear damage.
- Wrinkled bedding creates uneven pressure points that concentrate force on small areas.
- Slumping forward in a wheelchair dramatically increases pressure on the hips and tailbone.
- Using doughnut-shaped pads to raise heels — these actually increase pressure by constricting surrounding tissue and impairing circulation.
Do not massage or rub the skin over bony areas to stimulate blood flow; this can damage fragile tissue and make the problem worse.
Prevention Guidelines That Actually Work
For someone on bed rest, change position at least every two hours. For wheelchair users, aim to shift every 30 minutes — even leaning forward or shifting side to side for two minutes relieves enough pressure. Use pillows or foam wedges to keep bony areas from contacting each other (place them between the knees when lying on one side). Raise heels slightly off the mattress by putting a pad under the lower calves, not directly under the heels.
Skin care matters as much as positioning. Keep skin clean and dry; avoid scented soaps that strip natural oils, and skip talcum powder, which dries skin. Moisturize thoroughly after washing. Drink at least two liters of fluid daily and eat a balanced diet with adequate protein, vitamins, and minerals.
If you notice any skin changes — redness that doesn’t fade when pressure is removed, warmth, or a break in the skin — alert a doctor or nurse immediately. Early intervention stops a stage one sore from becoming a much more serious wound.
FAQs
Can a bed sore develop in just a few hours?
Yes. Pressure damage can begin within two hours if pressure is unrelieved over a bony area. This is why regular repositioning is critical for anyone confined to a bed or chair.
What does a stage one bed sore look like?
Stage one appears as a patch of red skin that does not turn white when pressed. On darker skin tones, it may show as a persistent blue or purple discoloration, warmth, or firmness compared to surrounding skin.
Are bed sores always preventable?
Not always, but consistent repositioning, proper nutrition, moisture management, and supportive surfaces dramatically reduce the risk. Some patients with severe circulatory issues or advanced age remain vulnerable even with excellent care.
References & Sources
- Mayo Clinic. “Bedsores (Pressure Injuries) — Symptoms and Causes.” Explains the pressure-ischemia mechanism and key risk factors.
- Cleveland Clinic. “Bedsores (Pressure Injuries).” Covers staging, vulnerable locations, and prevention guidelines.
- Johns Hopkins Medicine. “Bedsores.” Details friction, shear forces, and care protocols for at-risk patients.