A scratch and a bruise are distinct physical injuries, affecting different layers of the skin. A scratch is an abrasion damaging the outer surface, while a bruise (contusion) involves damage to underlying blood vessels. Scratching can cause bruising, but this is not the typical outcome of a gentle itch. This crossover requires significant force or underlying conditions that make tissues unusually fragile.
Understanding the Skin Damage from Scratching
A typical scratch, or abrasion, involves superficial damage, primarily affecting the epidermis. The epidermis is the outermost layer of the skin, acting as the primary barrier. A mild scratch involves only this layer, which is why it often does not bleed and heals quickly without scarring.
When a scratch is deeper, it may reach the dermis, the layer beneath the epidermis, causing slight bleeding. This trauma results in a linear wound that usually forms a scab as it heals, reflecting damage confined mostly to the skin’s surface.
What Causes a Bruise
A bruise, or ecchymosis, is an injury beneath the skin that does not break the surface. Discoloration occurs when small blood vessels, known as capillaries, are ruptured by impact or pressure. When these vessels tear, blood leaks into the surrounding tissues, a process called extravasation.
The pooling of this escaped blood creates a localized collection, or hematoma, visible through the skin as a black, blue, or purplish mark. As the body heals, the bruise undergoes a predictable color cycle as hemoglobin is broken down, transitioning from dark colors to green, then yellow, before the body reabsorbs the waste products.
How Scratching Can Lead to Bruising
Bruising occurs when mechanical force applied to the skin is strong enough to compress and rupture the underlying capillaries. When a person scratches intensely, the fingernail delivers concentrated pressure below the skin’s surface. This pressure can overcome the structural integrity of small vessels, leading to blood leakage while simultaneously causing the surface-level abrasion.
The vulnerability of the capillaries is a significant factor. Individuals with thin or fragile skin, such as older adults, often bruise more easily because the protective layer of fat beneath the skin has diminished. Certain medications, including blood thinners and long-term corticosteroids, can also weaken capillary walls, making the vessels more susceptible to rupture from aggressive scratching.
In these cases, the injury is a combination wound: a linear scratch mark on the epidermis accompanied by diffuse discoloration underneath.