A scar is the body’s natural attempt to repair damaged tissue, replacing the complex structure of normal skin with fibrous, collagen-based tissue. This repair process leaves behind tissue that functions differently from the surrounding skin. When a scar changes color, particularly turning purple in the cold, it is a visible sign of this difference. This color change is a physiological reaction rooted in the structure of the scar tissue and the chemistry of the blood flowing through it.
The Vascular Anatomy of Scar Tissue
Scar tissue does not possess the same organized structure as the original skin, which impacts the blood vessels that supply it. During the initial healing phase, the body rapidly creates a dense, disorganized network of small blood vessels to deliver nutrients, which is why new scars often appear pink or red. The capillaries within a scar are less efficient and less integrated than the micro-circulation in healthy skin. The dense deposition of collagen fibers can also compress the vessels, complicating blood flow dynamics. Furthermore, the outer layer of the scar tissue may be thinner, allowing changes in the blood beneath the surface to be seen more clearly.
The Body’s Response to Cold
The body maintains its core temperature through thermoregulation; exposure to cold triggers a reflex action to conserve heat. When the skin surface cools, the sympathetic nervous system signals superficial blood vessels to contract, a process known as vasoconstriction. This action narrows the vessels, diverting warm blood away from the skin to protect the internal organs. In scar tissue, this physiological command can produce an exaggerated or inefficient reaction because of its compromised vascular network. The disorganized capillaries may constrict disproportionately, leading to a significant reduction in circulation, and this localized restriction of blood flow is the mechanical precursor to the visible color change.
Why Oxygen Depletion Causes the Purple Color
The purple or bluish hue that appears in the scar is a manifestation of localized cyanosis, a direct consequence of restricted blood flow. Blood color is determined by the state of the hemoglobin protein inside red blood cells; hemoglobin fully loaded with oxygen (oxyhemoglobin) is bright red. When the blood slows down due to intense vasoconstriction, the surrounding scar tissue extracts a higher percentage of the available oxygen. The hemoglobin molecules that have released their oxygen become deoxygenated hemoglobin, which absorbs more red light and reflects less, making it appear much darker, closer to a bluish-purple color. Because the scar tissue is less opaque than normal skin, this darker, oxygen-depleted blood becomes highly visible just beneath the surface, resulting in the purple appearance.
The Timeline of Scar Color Change
The sensitivity of a scar to temperature changes is most pronounced during its immature phase, which typically lasts 12 to 18 months post-injury. During this period, the scar is actively remodeling, and its vascularity is highest, leading to more frequent color shifts. As the scar matures, the vascular density naturally decreases (vascular regression), and the collagen fibers reorganize and stabilize. As the blood vessel network regresses and the scar fades, the purple reaction to cold usually becomes less frequent or stops entirely. This temporary color change is considered a benign sign of the tissue’s ongoing maturation.