A bruise, or contusion, is a visible injury where small blood vessels called capillaries break beneath the skin due to external trauma, causing blood to leak into the surrounding tissue. In contrast, a blood clot, or thrombus, is a semi-solid mass of blood cells and proteins that forms inside a blood vessel. A clot blocks blood flow and is distinct from a bruise.
The Mechanism of Bruising
A bruise forms immediately following an external impact, such as a bump or a fall, which ruptures the tiny, fragile capillaries located just under the skin’s surface. This injury allows red blood cells to escape from the damaged vessel and pool in the surrounding soft tissue, creating the characteristic discolored mark. Initially, the bruise appears reddish or purple because the hemoglobin in the leaked blood is still oxygen-rich.
As the body begins the natural process of healing and clearing the trapped blood, the color of the bruise changes over several days. Hemoglobin breaks down into various compounds, first turning the area dark blue or black as oxygen is depleted. Later, the bruise progresses to a greenish hue as the hemoglobin is converted into biliverdin, and finally to a yellowish-brown color from bilirubin and hemosiderin before fully fading. This predictable color sequence is a direct sign of the body processing the blood that has collected outside the vessel wall.
What Defines an Internal Blood Clot
An internal blood clot, or thrombosis, occurs when blood transforms into a gel-like plug within a vein or artery, obstructing the normal flow of circulation. The most common and concerning type is Deep Vein Thrombosis (DVT), where a clot forms in a large, deep vein, most often in the leg. This condition is typically caused by damage to the vessel lining, slow-moving blood flow (stasis), or a state where the blood is prone to clotting (hypercoagulability).
DVT symptoms are primarily those of blockage and inflammation, not surface bleeding, and they tend to develop over hours or days. Signs include swelling, often in just one leg, persistent pain or tenderness that feels like a severe cramp, and skin that is unusually warm to the touch. The skin over the area may also appear reddish or bluish due to impaired circulation, but this discoloration is fundamentally different from a bruise. The most significant danger of DVT is the possibility that the clot may break free and travel to the lungs, causing a potentially fatal pulmonary embolism (PE).
Why Bruises and Clots Are Different Conditions
The distinction between a bruise and an internal blood clot rests entirely on the location of the blood involvement. A bruise is defined as extravascular bleeding, meaning the blood has exited the circulatory system and pooled in the soft tissues due to physical trauma. A thrombus, or internal clot, is a problem of intravascular obstruction, where the blood mass remains inside the vessel, blocking the flow of blood. While a bruise is a static collection of blood that changes color as it heals, a DVT is a dynamic circulatory event that can cause swelling and systemic issues.
Situations Where Clots Mimic Bruising
Despite their differences, certain clot-related conditions can produce visual symptoms that lead to confusion with a bruise. Superficial thrombophlebitis, which is a clot and inflammation in a vein just beneath the skin’s surface, is the primary example. This condition can cause a red, darkened area, often accompanied by a firm, cord-like sensation along the affected vein, which may be mistaken for a large, painful bruise.
This discoloration results from the inflammation and the blood pooling in the superficial vein, not from blood leaking into the tissue like a contusion. In rare cases, underlying blood clotting disorders that cause frequent or spontaneous bruising, known as purpura or petechiae, may also be associated with problems of deep clot formation elsewhere in the body. If any suspicious mark is accompanied by significant swelling, warmth, or persistent pain that does not improve, it warrants immediate medical evaluation to rule out a deeper venous issue.