Introduction
A pressure ulcer is a localized injury to the skin and the tissue beneath it that develops when sustained pressure, often combined with friction or shear, reduces blood flow to an area long enough to damage cells. These lesions most often occur over bony prominences such as the sacrum, heels, hips, ankles, and elbows, where soft tissue is compressed between bone and an external surface. The condition primarily involves the skin, subcutaneous tissue, small blood vessels, and in advanced cases deeper muscle and connective tissue. Its defining biological feature is tissue ischemia, followed by cell injury, inflammation, and breakdown of the normal structures that keep skin intact.
The Body Structures or Systems Involved
Pressure ulcers involve several layers of tissue, beginning with the epidermis and dermis and potentially extending into fat, fascia, muscle, and bone. Healthy skin functions as a barrier, regulates water loss, supports temperature control, and protects deeper tissues from the external environment. Beneath the surface, capillaries deliver oxygen and nutrients while removing metabolic waste. This circulation is essential because skin and soft tissue have high metabolic demands and limited energy stores.
The microcirculation is central to the process. Small blood vessels in the compressed region normally maintain continuous perfusion, even as body position changes and local tissues experience minor mechanical stress. The lymphatic system also helps clear fluid and proteins from the interstitial space, preventing edema that could otherwise impair exchange between blood and cells. When these systems function normally, tissue pressure stays within a range that preserves capillary flow and cellular metabolism.
Structural support also matters. Collagen, elastin, and the extracellular matrix provide mechanical strength, while fat pads distribute pressure away from points over bone. Muscles and subcutaneous tissue are especially vulnerable because they have higher metabolic requirements than the superficial skin and can be injured before the surface appears visibly changed. For this reason, a pressure ulcer may begin in deeper tissues and only later become obvious at the skin surface.
How the Condition Develops
The core mechanism is prolonged compression of soft tissue between an internal bony prominence and an external surface such as a bed, wheelchair, cast, or medical device. When external pressure exceeds capillary closing pressure, blood cannot pass effectively through the compressed microvasculature. As perfusion falls, oxygen delivery drops, nutrients become limited, and metabolic wastes accumulate. Cells then shift toward anaerobic metabolism, producing less energy and more lactic acid, which contributes to local acidosis and cellular dysfunction.
Cells deprived of oxygen cannot maintain normal ion gradients across their membranes. Sodium and water move into cells, causing swelling, while energy-dependent pumps fail. Mitochondrial injury develops, and persistent ischemia triggers membrane damage, protein dysfunction, and eventually cell death. Endothelial cells lining the capillaries are particularly sensitive, and their injury increases vascular permeability. Fluid leaks into surrounding tissue, worsening edema and further compressing microvessels in a self-reinforcing cycle.
Pressure alone is not the only force involved. Shear occurs when tissue layers slide in opposite directions, such as when a patient slides down in bed. In this setting, deeper tissues may be displaced over fixed bony structures, distorting and stretching capillaries. This deformation can narrow or occlude vessels even when surface pressure is not extreme. Friction damages the superficial skin barrier, making it easier for injury to begin at the epidermal level. Moisture from sweat, urine, or wound drainage softens the stratum corneum and reduces its resistance to mechanical stress, allowing breakdown to occur more readily.
Because muscle has a greater oxygen demand than skin, the earliest injury may occur in deep tissue rather than at the surface. Cells in muscle undergo metabolic failure first, and the damage can spread outward. This explains why some pressure ulcers appear small at first but conceal extensive internal necrosis beneath relatively intact skin. Once tissue death begins, inflammatory mediators are released, attracting immune cells and amplifying local injury through proteolytic enzymes and oxidative stress.
Structural or Functional Changes Caused by the Condition
As a pressure ulcer develops, the affected tissue undergoes a sequence of structural changes. Early injury may present as reversible erythema or nonblanching redness, reflecting local vasodilation and inflammation in response to ischemia. If pressure continues, the epidermal barrier fails, the dermis loses integrity, and the tissue may break open. In deeper lesions, fat, fascia, and muscle can become necrotic, producing a crater-like defect.
At the microscopic level, the tissue shows endothelial damage, capillary thrombosis, edema, and inflammatory cell infiltration. Collagen fibers may be degraded by matrix metalloproteinases released during inflammation, weakening the extracellular scaffold that normally supports skin repair and mechanical stability. Necrotic tissue loses elasticity and tensile strength, making the region more vulnerable to further injury and expansion of the wound.
Functionally, the skin can no longer serve as an effective protective barrier. Loss of barrier function increases the risk of fluid loss, contamination by microorganisms, and deeper invasion of bacteria into damaged tissue. Because pressure ulcers commonly occur in people with limited mobility, the injury may also interfere with posture, comfort, and tissue tolerance in adjacent areas, creating new sites of stress. In advanced cases, exposed muscle or bone may alter the local mechanical environment and intensify ongoing tissue destruction.
Factors That Influence the Development of the Condition
Whether a pressure ulcer forms depends on the balance between mechanical load and tissue tolerance. Immobility is a major influence because it prolongs exposure to pressure without the normal relief provided by shifting body position. Reduced sensation also matters, because pain ordinarily prompts movement before tissue injury becomes severe. When sensory input is impaired, the body may not generate the protective responses needed to redistribute load.
Circulatory status affects susceptibility. Conditions that reduce perfusion, such as vascular disease, hypotension, anemia, or dehydration, lower the margin of safety for tissue oxygenation. If blood flow is already compromised, even moderate pressure may be enough to cause ischemia. Metabolic factors can also contribute. Malnutrition, especially protein or micronutrient deficiency, impairs collagen synthesis, immune function, and tissue maintenance, making skin less resilient and slower to recover from mechanical stress.
Age influences tissue architecture and repair capacity. Older skin tends to have thinner dermis, less subcutaneous padding, reduced elasticity, and slower regenerative responses. Edema, which increases the distance oxygen must diffuse to reach cells, can compound injury. Moisture from incontinence or wound exudate weakens the epidermal barrier and increases friction. Medical devices can create highly localized pressure because they concentrate force over small contact points, sometimes producing injury in patterns that reflect the shape of the device rather than a bony prominence.
Systemic illness, inflammation, and altered consciousness can further reduce the body’s ability to respond to load. Fever, infection, and hypermetabolic states increase tissue oxygen demand, while sedation, weakness, or paralysis reduce the chance of spontaneous repositioning. The result is an environment in which external compression lasts longer and tissue recovery is less effective.
Variations or Forms of the Condition
Pressure ulcers vary according to depth, tissue involvement, and the pace at which injury develops. Superficial forms involve the epidermis and upper dermis, where the main changes are redness, nonblanching discoloration, and partial-thickness skin loss. More severe forms extend through the dermis into subcutaneous fat, fascia, muscle, or even bone. The deeper the lesion, the more likely it is that injury began in the subcutaneous tissues or muscle before becoming visible externally.
Another variation is the distinction between localized and extensive injury. Some ulcers remain confined to a small area over a single pressure point, while others develop broad zones of undermining or tunneling as necrosis spreads along tissue planes. Deep tissue pressure injury is a related form in which intact skin may mask substantial damage beneath. This pattern occurs because deeper tissues under high compression fail first, while the superficial layer remains temporarily preserved.
Pressure ulcers can also differ in whether the process is acute or chronic. Acute injury may arise after a sudden period of immobility or after prolonged pressure from a device or positioning error. Chronic ulcers persist when the underlying mechanical forces or impaired healing conditions remain in place. In chronic lesions, repeated ischemia, low-grade inflammation, bacterial colonization, and ongoing tissue remodeling can coexist, making the lesion biologically complex rather than simply an unhealed wound.
How the Condition Affects the Body Over Time
If pressure persists, the local injury can progress from reversible ischemia to permanent tissue loss. The damaged area often becomes a site of prolonged inflammation because necrotic tissue and bacteria stimulate immune activity. This sustained inflammatory state can delay repair by keeping proteases elevated and degrading newly formed matrix. As a result, the wound environment may remain unstable for long periods.
Chronic pressure ulcers can alter the local tissue architecture substantially. Scar tissue may form, but scarred tissue does not replicate the strength, elasticity, or vascularity of original skin. Repeated breakdown is therefore common. In deeper ulcers, chronic exposure of fat, muscle, or bone can create a pathway for infection to spread into underlying structures. Osteomyelitis may develop when bacteria invade bone, and surrounding soft tissue can become progressively fibrotic and poorly perfused.
The body may attempt to adapt through inflammation, angiogenesis, and collagen deposition, but these responses are limited if mechanical loading continues. Persistent ischemia prevents full restoration of cellular function, and repeated injury often outpaces repair. In addition, the wound itself can become metabolically demanding, increasing local protein turnover and fluid loss. In severe cases, systemic consequences such as widespread infection or chronic inflammatory burden may arise from what began as a localized pressure-related injury.
Conclusion
A pressure ulcer is a localized injury caused by sustained pressure, often combined with shear and friction, that compromises blood flow and damages skin and deeper tissues. Its biology centers on microvascular compression, tissue ischemia, cellular energy failure, inflammation, and structural breakdown of the skin and its supporting layers. The condition develops most readily where bony prominences create concentrated forces and where mobility, sensation, circulation, or tissue integrity are impaired.
Understanding the anatomy and physiology behind pressure ulcers explains why they can begin below the surface, why they vary in severity, and why chronic mechanical loading leads to progressive tissue loss. The condition is not simply a surface wound; it is the visible outcome of a deeper failure in perfusion, tissue tolerance, and repair.