Pressure ulcers form when sustained force on the skin cuts off blood flow to the tissue underneath. When external pressure exceeds about 32 mmHg (roughly the pressure inside your smallest blood vessels), oxygen stops reaching the cells. Toxic waste products build up, and the tissue starts to die. This process can begin in as little as a few hours if pressure isn’t relieved.
But pressure alone doesn’t tell the whole story. A combination of mechanical forces, moisture, nutrition, and individual health factors determines who develops these wounds and how quickly they progress.
How Pressure Starves Tissue of Oxygen
Your capillaries, the tiniest blood vessels in your body, normally deliver oxygen and carry away waste at very low internal pressures. Arterial capillaries operate at around 32 mmHg, and venous capillaries at just 8 to 12 mmHg. When you sit or lie in one position for too long, the weight of your body compresses the tissue between your bones and whatever surface you’re resting on. If that external force exceeds those small internal pressures, blood flow stops.
At first, the tissue simply isn’t getting fresh oxygen. Within hours, waste products from normal cell metabolism accumulate with nowhere to go. The cells begin to swell, break down, and die. This cascade, from compression to oxygen deprivation to cell death, is why even moderate pressure becomes dangerous when it lasts long enough. The areas most vulnerable are spots where bone sits close to the skin surface with little padding in between: the tailbone, heels, hips, shoulder blades, and the back of the head.
Shear and Friction: The Hidden Forces
Pressure pushing straight down is only one part of the problem. Shear forces occur when layers of tissue slide against each other internally. Picture someone propped up in a hospital bed at an angle. Gravity pulls their body downward, but the skin on their back stays in place against the sheets. The deeper tissue near the bone shifts while the surface skin doesn’t, stretching and kinking the blood vessels in between. This internal distortion can cut off circulation even when the surface pressure alone wouldn’t be enough to cause damage.
Friction works differently. It acts on the outer layer of skin, essentially rubbing it raw. Dragging someone across bed linens rather than lifting them is a common example. Friction strips away the protective outer skin barrier, making the tissue underneath far more vulnerable to deeper injury from pressure and shear. These forces rarely act alone. In real-world situations, a person slouching in a chair or being repositioned without proper technique experiences all three simultaneously.
Why Moisture and Skin Temperature Matter
The environment right at the skin’s surface, sometimes called the microclimate, plays a surprisingly large role in whether a pressure ulcer develops. Excess moisture from sweat, urine, or wound drainage softens the outermost layer of skin through a process called maceration. Softened skin is weaker and tears more easily under friction or shear. At the same time, completely dry skin cracks and loses its integrity, creating another entry point for damage.
Temperature is an emerging predictor. Research has found that skin temperature rises by about 1.2°C in the one to four days before a pressure ulcer becomes visible. Higher temperatures increase the tissue’s metabolic demand for oxygen at the exact moment when pressure is restricting the supply. Patients who went on to develop pressure ulcers showed consistently higher temperature differences at vulnerable sites compared to those who didn’t. This means the damage often begins well before anything is visible on the surface.
Nutrition and the Body’s Repair Capacity
Your skin constantly repairs minor cellular damage on its own, but that repair process requires raw materials. Protein is the most critical. Collagen, the structural protein that holds skin together, can’t be rebuilt without adequate protein intake. Guidelines recommend 1.25 to 1.5 grams of protein per kilogram of body weight daily for someone at risk, and up to 2.0 grams per kilogram for those with advanced wounds. For a 150-pound person, that translates to roughly 85 to 100 grams of protein a day. In one study, patients receiving 1.8 grams per kilogram healed at nearly twice the rate of those receiving 1.2 grams per kilogram.
Specific vitamins and minerals also matter. Vitamin C is essential for collagen production and immune function; without enough, the body can’t build new tissue or fight infection at the wound site. Vitamin A supports immune response and collagen formation. Zinc deficiency, common in people with chronic illness or poor absorption, impairs appetite and slows healing. Older adults are especially vulnerable because their baseline protein needs are already higher than once thought, closer to 1.0 gram per kilogram rather than the older recommendation of 0.8.
Immobility and Sensory Loss
The single biggest risk factor is an inability to shift your weight. Healthy people constantly make small adjustments, even during sleep, that relieve pressure on vulnerable areas before damage begins. People who can’t move independently, whether due to spinal cord injury, sedation, stroke, or severe illness, lose this protective instinct.
Sensory perception matters just as much as physical mobility. If you can’t feel the discomfort that signals tissue compression, you won’t shift your position in response. This is why conditions that reduce sensation, such as diabetes-related nerve damage or neurological injuries, significantly raise risk even when a person has some ability to move. Clinical risk assessment tools evaluate six distinct factors: mobility, activity level, sensory perception, nutrition, exposure to friction and shear, and moisture. Each one independently contributes to overall vulnerability.
Medical Devices as a Cause
Not all pressure ulcers come from beds and chairs. In hospitals, about one-third of pressure ulcers that develop during a stay are caused by medical devices. Oxygen tubing pressing against the ears or nose, cervical collars compressing the chin or back of the head, urinary catheters trapped under a thigh, splints, and even pulse oximeter clips can all create enough sustained pressure on a small area to cause tissue breakdown. These injuries are easy to overlook because clinicians may focus on traditional high-risk areas like the tailbone while devices press against less obvious spots.
Device-related ulcers tend to form quickly because the contact area is small, concentrating force on a tiny patch of skin. The tissue beneath has little room to distribute the load, so even relatively light devices can exceed the capillary occlusion threshold if left in place without adjustment.
Who Is Most Vulnerable
Several conditions stack risk factors together. People in intensive care units face immobility, sedation, device exposure, poor nutrition, and often moisture from incontinence or sweating simultaneously. Elderly individuals have thinner skin, reduced blood flow, lower protein reserves, and frequently impaired mobility. Spinal cord injury eliminates both movement and sensation below the level of injury. Diabetes reduces circulation and nerve function at the same time.
The common thread across all these situations is the same: tissue gets compressed, blood flow stops, and the body can’t compensate. Whether the pressure comes from a mattress, a wheelchair seat, or an oxygen mask, and whether the person can’t move, can’t feel, or can’t heal, the underlying biology is identical. Preventing pressure ulcers means addressing as many of these overlapping factors as possible, from repositioning and proper nutrition to managing moisture and checking under every device that touches the skin.