Bed sores, also called pressure injuries, are caused by sustained pressure on skin and soft tissue that cuts off blood flow. When external pressure on the skin exceeds about 32 mmHg (roughly the pressure inside your smallest blood vessels), blood can no longer deliver oxygen to the tissue. Just two hours of uninterrupted pressure in a bedridden person is enough to start the process of tissue damage.
But pressure alone doesn’t tell the whole story. Friction, moisture, poor nutrition, and underlying health conditions all play a role in how quickly and severely these wounds develop.
How Pressure Damages Tissue
Your body weight presses soft tissue against bone. In areas like the tailbone, heels, hips, and shoulder blades, there’s relatively little padding between skin and skeleton. When you sit or lie in one position, the tissue trapped between bone and the surface beneath you gets compressed. Blood vessels collapse under that pressure, and the cells they supply begin to starve for oxygen.
Healthy people shift position constantly, even during sleep, without thinking about it. These small movements restore blood flow before damage sets in. People who can’t move independently, whether due to paralysis, sedation, surgery, or severe illness, lose that protective reflex. The tissue stays compressed, inflammation builds, and cells begin to die. What starts as a red, tender patch can progress to an open wound that reaches muscle or bone if pressure continues unrelieved.
Friction and Shear: The Other Mechanical Forces
Pressure gets most of the attention, but two related forces accelerate the damage. Friction occurs when skin drags across a surface, like bedsheets. It strips away the outer layer of skin, creating superficial abrasions that make the area far more vulnerable to deeper injury. This is sometimes called a “top-down” mechanism because the damage starts at the skin surface.
Shear works in the opposite direction. It happens when your skeleton and your skin move in different directions. Picture a patient whose bed is raised at the head: gravity pulls their body downward while the skin on their back stays stuck to the sheet. The tissue layers in between get stretched and torn internally. Shear damages deeper structures like blood vessels and fat beneath the skin, a “bottom-up” injury that can be hard to detect on the surface until significant damage has already occurred.
Why Moisture Makes Everything Worse
Wet skin breaks down faster. Sweat, urine, wound drainage, or fecal incontinence all soften the outer layer of skin through a process called maceration. Softened skin is structurally weaker and tears more easily. Moisture also increases the friction between skin and fabric, meaning the skin is more likely to catch and drag when a person is repositioned or slides in bed.
Temperature plays a role too. Skin trapped against a mattress heats up because body warmth can’t dissipate. Higher skin temperature increases the metabolic demand of tissue that’s already oxygen-starved from compression, speeding up cell death. This is why the materials a patient lies on matter. Fabrics that wick moisture and allow airflow help maintain a healthier skin environment, while waterproof or plastic-backed surfaces trap heat and sweat.
Who Is Most at Risk
Healthcare providers assess bed sore risk using tools like the Braden Scale, which scores six factors: sensory perception (can the person feel discomfort and respond?), moisture exposure, physical activity level, mobility, nutrition, and friction or shear exposure. Scores range from 6 to 23, with anything at or below 18 signaling increased risk. The lower the score, the higher the danger.
Several groups face elevated risk:
- People with limited mobility, including those with spinal cord injuries, stroke, or who are recovering from major surgery.
- Older adults, whose skin is thinner, less elastic, and slower to heal.
- People with diabetes, who often have reduced sensation in their extremities (affecting roughly 40 to 60 percent of diabetic patients) and impaired circulation from peripheral artery disease (affecting about 30 percent). Both conditions mean they’re less likely to feel early warning pain and less able to heal once damage starts.
- People with incontinence, because chronic moisture exposure weakens skin integrity.
- People who are malnourished or underweight, who lack both the nutritional building blocks for tissue repair and the body fat that cushions bony areas.
The Role of Nutrition
Poor nutrition is one of the most underappreciated causes of bed sores. Your body needs protein to build collagen, the structural fiber that holds skin and tissue together. When protein intake is low, collagen production drops, skin becomes fragile, and wounds heal slowly or not at all. People with chronic pressure injuries lose additional protein through wound drainage, creating a cycle where the wound itself worsens the deficiency that caused it.
Several specific nutrients are involved. Vitamin C is essential for collagen formation and immune function; without enough of it, the tiny blood vessels in skin become fragile and the body can’t fight infection at the wound site. Vitamin A supports immune response and the early stages of tissue repair. Zinc drives cell growth and protein building, though supplementation above 40 mg per day can actually cause problems by interfering with copper absorption. Two amino acids, arginine and glutamine, become especially important during the stress of illness or injury. Arginine is the only raw material the body uses to produce nitric oxide, a molecule critical for blood flow to healing wounds. In people with diabetes, nitric oxide production in wound tissue is already reduced, which may partly explain why their pressure injuries are so difficult to treat.
Clinical guidelines recommend that people with moderate to severe pressure injuries consume 1.25 to 1.5 grams of protein per kilogram of body weight daily, and up to 2 grams per kilogram for the most severe wounds.
How Bed Sores Progress
Pressure injuries are classified by depth. In the earliest stage, the skin is intact but shows a persistent red area that doesn’t fade when you press on it. In darker skin tones, the discoloration may appear purple or ashy rather than red, making it harder to spot. At this point, the damage is still reversible if pressure is relieved.
The next stage involves partial loss of the outer skin layers. You might see a shallow open wound with a pink or red base, or a blister filled with clear fluid. More than half of hospital-acquired pressure injuries reach at least this stage before they’re identified.
Deeper injuries extend through the full thickness of skin into the fat layer beneath, or further still into muscle and bone. At the most severe stage, tendons, joints, or bone may be visible in the wound. Some injuries can’t be staged at all because dead tissue covers the wound base, hiding how deep the damage goes. Deep tissue injuries are particularly deceptive: they may appear as a dark bruise on intact skin, then rapidly break open to reveal extensive damage underneath.
How Common They Are
Hospital-acquired pressure injuries occur at a rate of about 5.4 per 10,000 patient-days, with an overall incidence of roughly 8.4 percent among hospitalized patients. Long-term care residents and people in intensive care units face even higher rates. These numbers persist despite decades of prevention efforts, largely because the underlying causes (immobility, frailty, poor nutrition) are difficult to fully eliminate in seriously ill patients.
Prevention Through Repositioning and Support Surfaces
The single most important preventive measure is regular repositioning. The traditional recommendation is every two hours for bed-bound patients, though the ideal interval varies from person to person and no single schedule works for everyone. When pressure-redistributing mattresses are used, some guidelines allow extending the interval to every four hours, which can matter for comfort and sleep quality.
Support surfaces range from simple to sophisticated. Reactive (or “low-tech”) options include specialized foam, gel, air, or water-filled mattresses that conform to the body and spread pressure across a larger area, reducing the force on any single point. Active (or “high-tech”) surfaces go further. Alternating-pressure mattresses use air cells that inflate and deflate in a cycle, periodically relieving pressure at different body sites without requiring the person to move. Air-fluidized beds circulate warm air through fine beads, essentially floating the patient. Low-air-loss beds use a series of air compartments that also manage heat and moisture.
Studies comparing these options consistently show that alternating-pressure mattresses reduce pressure injury rates more effectively than standard hospital foam mattresses. They also appear to outperform reactive mattresses, though the quality of evidence is limited. Even upgrading from a standard foam mattress to a basic pressure-redistributing foam mattress makes a measurable difference. For people at high risk, the choice of surface is not a luxury but a front-line prevention tool, as important as repositioning itself.