A bruise, medically termed a contusion or ecchymosis, is a common injury where damage occurs beneath the skin without breaking the surface. This damage allows blood to leak from vessels, creating the characteristic discoloration. Although the initial injury is localized, the mark often appears to grow larger or shift in location over the following days. This raises the question of whether the bruise actually spreads, or if this apparent change is simply a normal phase of the body’s healing process.
What Causes a Bruise to Form
Bruising begins with blunt force trauma, such as a bump or fall, which damages the small blood vessels located just under the skin’s surface. These delicate vessels, known as capillaries, rupture under the pressure of the impact, causing blood to spill out into the surrounding soft tissues.
The pooled blood, which collects in the interstitial spaces, is called a hematoma. This collection of trapped red blood cells creates the immediate dark, often reddish-blue, mark. The body’s clotting mechanisms work quickly to seal the damaged capillaries and stop the internal bleeding.
The Physics Behind Apparent Spreading
The visible size and shape of a bruise often change dramatically a few days after the initial injury, giving the impression that the damage is spreading. This phenomenon is not due to the injury worsening, but rather the physical migration of the existing pooled blood. The initial hematoma diffuses into the less dense tissues surrounding the injury site.
The physical law of gravity plays a significant role in this visible shift, especially on limbs. Fluid, including the trapped blood and inflammatory fluid, follows the path of least resistance through the body’s connective tissue planes. For example, a bruise sustained high on the thigh may appear lower down near the knee or shin as the fluid tracks downward. The appearance of a larger or shifted mark is simply the original blood deposit spreading out and becoming visible over a broader area.
Decoding the Color Changes
The shifting colors of a healing bruise indicate the body is actively cleaning up the trapped blood cells through a biochemical process. Macrophages, a type of white blood cell, are mobilized to engulf and break down the extravasated hemoglobin. Hemoglobin is the protein in red blood cells responsible for carrying oxygen, and its breakdown products possess distinct colors.
Initially, the bruise appears red or purplish-blue, reflecting the intact hemoglobin. The body then begins to dismantle the hemoglobin molecule. The first major breakdown product is biliverdin, a green pigment, which causes the bruise to take on a greenish hue a few days after the injury.
Biliverdin is then chemically reduced into bilirubin, which is responsible for the yellow coloration seen in the later stages of healing. Finally, the iron component of the heme molecule is stored as hemosiderin, which can impart a golden or brownish-yellow color before the blood products are fully reabsorbed. This color progression, from blue to green to yellow, confirms the body’s natural repair mechanism is working.
When to Seek Medical Attention
While most bruises are harmless and resolve naturally within two weeks, certain characteristics warrant a medical evaluation. Bruising that happens frequently without a clear cause, or unexplained bruising, particularly on the trunk, head, or abdomen, may indicate an underlying condition. These patterns can signal issues with blood clotting or a reaction to certain medications.
Seek attention for a bruise accompanied by severe swelling, intense pain lasting several days, or the inability to move a nearby joint. A hard, raised lump in the area, known as a hematoma, may require draining if it is large or persistent. Any bruise that does not fade or disappear after two weeks should be checked by a healthcare professional.