A scrape (abrasion) is superficial skin damage resulting from friction, affecting the outermost layers and creating an open wound that may bleed slightly. A bruise (contusion) occurs beneath the skin, where blood vessels are damaged but the surface remains intact. The appearance of both injuries is not a coincidence; they are simultaneous consequences of the same physical impact, revealing how mechanical force affects the body’s tissues.
Understanding Surface Damage
Scraping involves a tangential force that removes the skin’s superficial layers. This injury is typically limited to the epidermis and sometimes extends into the upper dermis beneath it. Because the damage often does not penetrate deeply, abrasions frequently heal without leaving a scar. While an abrasion might cause minor external bleeding from tiny surface capillaries, the visible wound shows how the energy that caused the scrape also affected deeper structures.
How Force Transmission Ruptures Capillaries
The blunt force trauma that creates a surface abrasion also transmits energy downward through the layers of skin. This impact generates a compressive or shearing force that travels past the damaged epidermis and dermis. Underlying tissue structures, including small blood vessels, are stressed by this mechanical energy. The delicate walls of capillaries and venules are particularly vulnerable to this transmitted force. When the impact is sufficient, these vessels rupture beneath the skin’s surface. This tearing allows blood to leak into the surrounding tissue, where it pools and forms the visible discolored mark known as a bruise. The abrasion is the external evidence of the force, while the bruise is the internal evidence of the same force causing hemorrhage in the deeper vascular network.
The Sequence of Bruise Coloration
Once the capillaries rupture, the leaked blood begins the biological process of reabsorption and breakdown, which causes the bruise’s changing colors. Initially, the bruise appears reddish or purplish because the pooled blood is rich in oxygenated hemoglobin. Within one or two days, as the blood loses oxygen and clots, the color changes to a blue, purple, or black hue.
Immune cells, called macrophages, then begin metabolizing the trapped blood components to clean up the injury. Hemoglobin, the iron-containing protein in red blood cells, is broken down into its constituent parts, causing subsequent color transitions, usually occurring between five and ten days after the injury.
The hemoglobin is first converted into biliverdin, a green pigment. The bruise then takes on a yellowish color as the biliverdin is processed into bilirubin. Finally, the bruise may appear light brown or yellowish-brown before fading entirely, often due to hemosiderin, an iron-storage complex. This entire cycle usually takes about two weeks as the body reabsorbs the blood products.
Individual Factors Affecting Bruising Severity
Several biological factors influence capillary fragility and affect bruising severity from an equivalent injury. With increasing age, the skin naturally thins and loses some of the protective layer of subcutaneous fat that cushions blood vessels. The blood vessels themselves also become more fragile, making them more susceptible to rupture from minor trauma.
The location of the injury also plays a role, as areas with less underlying muscle or fat padding, such as the arms and legs, are more prone to visible bruising. Furthermore, certain medications can increase the likelihood of bruising by affecting the blood’s ability to clot or by weakening blood vessel walls. Blood thinners, such as aspirin and warfarin, or long-term use of corticosteroids, can significantly raise the risk of a noticeable bruise from a seemingly minor scrape.