Bed sores, also called pressure injuries, are caused by sustained pressure that cuts off blood flow to the skin and underlying tissue. When external pressure exceeds the normal pressure inside your capillaries (roughly 12 to 32 mmHg), oxygen stops reaching the compressed area. If that pressure isn’t relieved, a pressure injury can begin forming in as little as two hours.
How Pressure Starves the Tissue
Your skin and the tissue beneath it depend on a constant flow of blood through tiny capillaries. When you sit or lie in one position, your body weight presses the skin against the surface beneath you, squeezing those capillaries shut. The tissue downstream loses its oxygen supply. In healthy people, discomfort signals the brain to shift position, restoring blood flow almost automatically, even during sleep. But when someone can’t move easily or can’t feel that discomfort, the compression continues uninterrupted.
Without oxygen, cells begin to die. Waste products accumulate. The damage starts in the deeper layers of tissue closest to the bone and works outward, which means the injury can be more severe underneath than it appears on the surface. A pressure wound can develop in as few as two hours of unrelieved pressure, though the typical window is three to four hours before noticeable damage sets in.
Shear and Friction: The Hidden Forces
Pressure alone isn’t the only mechanical force at work. Shear and friction contribute significantly, and they often go unrecognized.
Friction is what happens when skin drags across a rough surface, like bedsheets. It strips away the outermost layer of skin, creating superficial abrasions that make the area far more vulnerable to deeper injury. This commonly occurs when a person is pulled across a bed rather than lifted.
Shear is more destructive and harder to see. It happens when the body’s internal structures and the skin move in opposite directions. Picture someone propped up in a hospital bed: gravity pulls the skeleton downward toward the foot of the bed, but the skin on the back stays stuck against the sheets. Over the tailbone, the bone slides one direction while the skin stretches the other way. This opposing force tears at blood vessels and tissue deep beneath the surface, reducing blood flow and causing damage that may not be visible until it’s already severe. Research identifies a threshold of roughly 9 to 15 kilopascals of shear force as the range where pressure injury formation becomes likely.
Why Moisture Makes Everything Worse
Skin that stays wet is dramatically more fragile. Prolonged contact with sweat, urine, or stool over-hydrates the outermost skin layer, disrupting its natural barrier. Once that barrier breaks down, irritants penetrate more easily, and the skin becomes far more susceptible to friction and shear damage.
Incontinence poses a particular risk. Urine raises the skin’s pH, making it more alkaline. Digestive enzymes in stool become more active in that alkaline environment, accelerating skin breakdown. When urine and stool are both present, the combined effect is more harmful than either one alone. This is why people with bladder or bowel incontinence develop bed sores at much higher rates, especially over the buttocks and sacrum.
Nutritional Deficiencies That Weaken Skin
Your body needs adequate protein to maintain and repair skin tissue. When protein levels drop, the skin loses its ability to tolerate even moderate pressure. In one study of intensive care patients, 90% of those assessed had below-normal albumin levels (a key protein marker in the blood), and researchers found a strong correlation between lower albumin and higher pressure ulcer risk. Low total protein showed a similarly significant relationship.
Poor nutrition also impairs immune function and slows collagen production, both of which are essential for keeping skin intact and healing minor damage before it escalates. People who are malnourished, whether from illness, poor appetite, or difficulty eating, face a substantially higher risk of developing bed sores and a harder time healing the ones they get.
Who Is Most at Risk
The biggest single risk factor is immobility. Anyone who can’t reposition themselves, whether from paralysis, sedation, extreme weakness, or post-surgical recovery, is vulnerable. But several other factors lower the threshold for injury:
- Spinal cord injury leaves people unable to move and unable to feel the pain signals that would normally prompt a position change. Prolonged wheelchair or bed contact creates ideal conditions for tissue breakdown.
- Diabetes impairs wound healing broadly, making even minor skin damage more likely to progress into a full pressure injury.
- Heart failure and peripheral vascular disease reduce blood flow to the skin. They also cause fluid buildup (edema) in the legs and lower body, which stretches and weakens the skin.
- Malnutrition compromises immune defenses and the body’s ability to produce collagen, the structural protein that gives skin its strength.
Healthcare providers use the Braden Scale to assess pressure injury risk. It scores six factors: sensory perception, moisture exposure, physical activity level, mobility, nutrition, and friction/shear. Scores range from 6 to 23, with anything at 18 or below flagging a person as at risk. A lower score means higher danger.
How Aging Changes the Equation
Older adults are especially vulnerable for reasons that go beyond simply being less mobile. As people age, the outer layers of skin thin out. The layer of fat and muscle beneath the skin, which normally acts as a cushion absorbing pressure, shrinks. Blood vessels become fewer and more fragile, rupturing more easily under compression. All of these changes mean that the same amount of pressure that a younger person’s body handles without trouble can cause tissue death in an older person. Wounds also heal more slowly with age, so a minor injury that might resolve on its own in a younger person can progress into a serious ulcer.
Where Bed Sores Form
Bed sores develop over bony prominences, the places where bone sits closest to the skin surface and concentrates pressure into a small area. The specific location depends on body position. For someone lying on their back, the sacrum (base of the spine), heels, and back of the head are the most common sites. Side-lying positions put the hips and ankles at risk. For people who spend long hours in a wheelchair, the tailbone and the bony bumps at the base of the pelvis (the ischial tuberosities) bear the most pressure.
Any spot where skin is compressed between bone and a firm surface is a potential site. Less obvious locations include the ears, shoulder blades, elbows, and the backs of the knees. In people with medical devices like oxygen tubing or casts, the pressure from the device itself can create an injury wherever it presses against the skin.
The Combination Effect
What makes bed sores so common in certain populations is that these causes rarely appear in isolation. A person recovering from a stroke might be immobile, incontinent, malnourished from difficulty swallowing, and elderly, all at the same time. Each factor lowers the skin’s tolerance to pressure, and together they create a situation where even a few hours without repositioning can cause a wound that takes weeks or months to heal. Understanding that bed sores result from this combination of forces, not just “lying in one spot too long,” is key to recognizing and preventing them early.