Introduction
Pressure ulcer develops when sustained pressure, often combined with friction or shear, reduces blood flow to a localized area of skin and the tissues underneath it. The result is ischemic injury: cells are deprived of oxygen and nutrients, waste products accumulate, and the affected tissue breaks down. In practical terms, the condition is usually caused by prolonged immobility, impaired sensation, and reduced tissue tolerance, but the biological pathway is more specific than a simple “skin breakdown.” It involves disrupted microcirculation, mechanical deformation of soft tissue, inflammation, and eventually cell death. The major causes and contributing factors can be grouped into direct mechanical causes, patient-related risk factors, interacting systemic influences, and underlying medical disorders that reduce the body’s ability to maintain tissue integrity.
Biological Mechanisms Behind the Condition
Healthy skin and the tissues beneath it depend on uninterrupted capillary blood flow. Small blood vessels deliver oxygen and glucose, remove carbon dioxide and metabolic waste, and support the repair of cells exposed to everyday wear. When external pressure compresses tissue for long enough, capillaries collapse and perfusion falls. If the pressure exceeds the pressure within the vessels, blood cannot adequately reach the area. The effect is especially damaging over bony prominences such as the sacrum, heels, hips, ankles, and elbows, where little soft tissue separates skin from bone.
The injury is not caused only by lack of blood flow. Mechanical deformation of cells also contributes. Soft tissue compressed between a hard surface and bone is stretched and distorted, which damages cell membranes, cytoskeletal structures, and the extracellular matrix. Shear forces make this worse by pulling skin layers in opposite directions, bending and twisting blood vessels so that circulation is impaired even more deeply than surface appearance suggests. This is why a pressure ulcer can begin in deeper tissue before obvious redness or skin loss appears.
Once oxygen delivery falls, cells shift to anaerobic metabolism, producing less energy and more acid. Ion pumps fail, sodium and water accumulate in cells, and swelling increases local pressure further. At the same time, inflammatory mediators are released, attracting immune cells and increasing vascular permeability. This inflammatory response can become self-perpetuating: edema raises tissue pressure, which further reduces perfusion. If the insult continues, tissue necrosis develops, and the skin may eventually open into an ulcer.
Primary Causes of Pressure ulcer
The most important cause is prolonged pressure. Constant compression on a localized area blocks capillary flow and prevents tissues from receiving enough oxygen. The longer the pressure remains, the greater the risk that the tissue will cross from reversible ischemia to irreversible damage. People who cannot change position regularly are therefore at highest risk. This is why pressure ulcers are common in individuals who are confined to bed, sit in wheelchairs for long periods, or are unable to shift weight because of paralysis, severe weakness, sedation, or critical illness.
Shear is another major cause. Shear occurs when the skin stays relatively fixed while deeper tissues move, such as when a person slides down in bed and the skeleton moves downward while the skin catches on the sheets. This distorts blood vessels, making them narrow or kinked, and can injure subcutaneous tissue even when the skin surface appears intact. Shear is particularly harmful when the head of the bed is elevated because gravity encourages the body to slide, increasing stress on the sacrum and buttocks.
Friction contributes by abrading the outer skin layers. Although friction alone usually does not cause a deep ulcer, it weakens the epidermal barrier and makes the tissue more vulnerable to pressure and moisture. Repeated rubbing can strip the protective outer layers, allowing breakdown to start more easily. In combination with pressure and shear, friction can accelerate ulcer formation.
Moisture also plays a causal role. Skin exposed to sweat, urine, feces, or wound drainage becomes overhydrated and fragile. Excess moisture softens the stratum corneum, reducing its resistance to friction and increasing the likelihood of maceration. Enzymes and bacteria present in incontinence-associated moisture can further irritate the skin and weaken the barrier. Once the surface is compromised, pressure-induced injury develops more readily.
Reduced mobility is a direct cause because it removes the body’s main defense against sustained pressure: spontaneous repositioning. Healthy individuals instinctively shift weight when pressure becomes uncomfortable or circulation is reduced. When mobility is impaired, that protective cycle fails. This allows pressure to persist long enough for tissue ischemia and cell injury to begin.
Impaired sensation also leads to pressure ulcer because the discomfort that normally triggers movement is absent or blunted. Neurologic disorders, spinal cord injury, diabetic neuropathy, and anesthetic or sedative states can all reduce awareness of pressure. Without sensory feedback, a person may remain in one position for hours, even while tissue damage is accumulating.
Contributing Risk Factors
Several factors do not cause pressure ulcer by themselves but lower the threshold for tissue injury. Age is important because aging skin is thinner, has less collagen, reduced elasticity, and slower repair capacity. Older adults often have less subcutaneous fat over bony areas as well, which means less cushioning against compression.
Malnutrition increases susceptibility by limiting the raw materials needed for tissue maintenance and repair. Inadequate protein intake reduces collagen synthesis and delays wound healing. Deficiencies in calories, vitamin C, zinc, and other micronutrients can weaken the skin barrier and impair immune function. Dehydration has a similar effect by reducing tissue perfusion and making skin less resilient.
Poor circulation from vascular disease, heart failure, or low blood pressure lowers the amount of oxygen delivered to tissue even before pressure is applied. If a person already has borderline perfusion, only a modest amount of external compression may be enough to trigger ischemic injury.
Smoking is another important lifestyle factor. Nicotine causes vasoconstriction, and carbon monoxide reduces oxygen delivery by binding to hemoglobin. Smoking also impairs immune response and collagen production, making tissue less able to resist and recover from damage.
Fever, infection, and systemic inflammation can increase metabolic demand. When tissues need more oxygen but circulation is already compromised, the mismatch between demand and supply becomes greater. Moist skin and inflammatory byproducts also weaken the epidermis, encouraging ulcer formation.
Genetic influences are less direct than mechanical factors, but inherited differences in connective tissue structure, inflammatory signaling, and skin resilience may affect tissue tolerance. Some individuals may have skin that is inherently more fragile or a vascular response that is less effective under stress. Genetic variation does not usually act alone, but it can shape how strongly a person responds to pressure or injury.
Hormonal changes may also contribute indirectly. Conditions that alter cortisol, thyroid hormone, estrogen, or insulin can influence skin thickness, protein turnover, wound healing, and vascular function. For example, chronic elevated cortisol can thin the skin and impair repair, while diabetes-related hormonal dysregulation affects circulation and immune defense.
How Multiple Factors May Interact
Pressure ulcer usually develops through the interaction of several mechanisms rather than a single isolated cause. Prolonged pressure may be mild enough that healthy tissue could tolerate it for a time, but if the person is malnourished, dehydrated, elderly, or has poor circulation, the threshold for injury drops significantly. Likewise, a person with normal sensation may change position before damage occurs, whereas someone with neuropathy may not recognize the need to move. In that setting, immobility and sensory loss reinforce one another.
Mechanical factors and systemic factors also interact. For example, friction damages the skin surface, moisture weakens the barrier, and pressure collapses deeper vessels. At the same time, low protein intake or infection can reduce healing capacity, allowing the initial injury to progress rather than resolve. The final ulcer is therefore the product of layered failure: impaired perfusion, structural deformation, tissue fragility, and inadequate repair.
Variations in Causes Between Individuals
The causes of pressure ulcer differ from person to person because the balance between external stress and tissue tolerance varies widely. Some people can withstand brief pressure better because they have better muscle mass, thicker subcutaneous padding, more intact circulation, and normal sensory feedback. Others develop ulcers under lower levels of pressure because their tissues are already compromised.
Age changes the cause profile by reducing skin resilience and mobility. Genetics may influence collagen quality, inflammatory response, and vascular reactivity. Health status matters because diabetes, neuropathy, vascular disease, kidney failure, and malnutrition each alter how tissues respond to compression. Environmental exposure also matters: a person in a hot, humid environment may have more moisture-related skin fragility, while someone with limited access to repositioning support or proper cushioning faces more sustained mechanical stress.
As a result, two individuals exposed to similar pressure may have very different outcomes. One may experience only temporary redness, while the other develops deep tissue injury. The difference lies in the interaction between external forces and the individual’s physiological reserve.
Conditions or Disorders That Can Lead to Pressure ulcer
Several medical conditions are strongly associated with the development of pressure ulcer because they impair mobility, sensation, circulation, or healing. Spinal cord injury is one of the most important. It reduces motor control and sensation below the level of injury, preventing repositioning and eliminating the normal warning signs of pressure.
Stroke, Parkinson disease, multiple sclerosis, and advanced dementia can also contribute by limiting movement, awareness, or the ability to respond to discomfort. These disorders often lead to prolonged sitting or lying in one position, increasing tissue compression over bony areas.
Diabetes mellitus is another major contributor. Diabetic neuropathy decreases sensation, so pressure is not felt. Diabetes also damages microvascular circulation and impairs immune function, making tissue more vulnerable and healing slower once injury begins.
Peripheral arterial disease and other circulatory disorders reduce oxygen delivery to the skin and underlying tissue. When perfusion is already limited, even short periods of pressure can be enough to cause ischemia. Congestive heart failure may have a similar effect by reducing overall tissue perfusion.
Kidney disease, cancer, and severe chronic illness often contribute through a combination of malnutrition, anemia, weakness, and inflammation. Anemia lowers oxygen-carrying capacity, meaning that tissue receives less oxygen even if blood flow is preserved. Cancer and chronic inflammatory disorders can also increase catabolism, leading to loss of muscle and fat padding over pressure points.
Acute conditions such as sepsis or critical illness can trigger pressure ulcer development in hospitalized patients because they combine immobility, vasopressor-related blood vessel constriction, fever, edema, and poor tissue perfusion. In these settings, pressure injury may progress rapidly because multiple physiologic systems are compromised at once.
Conclusion
Pressure ulcer develops when sustained mechanical stress overwhelms the body’s ability to maintain tissue perfusion and structural integrity. The core biological causes are prolonged pressure, shear, and friction, often worsened by moisture and immobility. These forces reduce capillary blood flow, distort soft tissue, damage cell membranes, and trigger inflammation and necrosis. Risk is increased by factors that lower tissue tolerance, including aging, malnutrition, poor circulation, dehydration, smoking, impaired sensation, and systemic disease. The condition often arises from the interaction of several influences rather than a single event. Understanding these mechanisms explains why pressure ulcer occurs in vulnerable individuals and why the same external pressure can have very different effects depending on underlying health and physiology.