Blood is a specialized fluid that circulates within the body, carrying oxygen, nutrients, and immune cells. Once it leaves a blood vessel, this liquid immediately begins to change its structure and composition in a process known as hemostasis. This natural response to injury results in a rapid transformation. The physical appearance and color of blood change significantly as it loses moisture and reacts with the external environment. This article explores the terminology and scientific processes that describe this altered, dried state.
Common Names for Dried Blood
The terms used to describe dried blood depend on whether the mass is functioning to stop bleeding or has hardened to protect a healing wound. The initial gelatinous plug formed within a wound is referred to as a blood clot. A clot is formed through the activation of coagulation factors and the protein fibrin, creating a temporary seal to stop blood flow.
The common, hard, protective layer that forms over a skin wound is called a scab. A scab is the dried and hardened form of the initial blood clot, mixed with serum, tissue debris, and immune cells. Wound care professionals may refer to this protective crust as a “crust” to distinguish it from other types of necrotic tissue. This hardened layer adheres to the wound surface, acting as a natural bandage until the underlying tissue is repaired.
Biological and Chemical Changes During Drying
The transition from liquid blood to a dry, solid mass involves two distinct physical changes: coagulation and desiccation. Coagulation is the first step, where platelets and a meshwork of fibrin strands trap red blood cells, solidifying the liquid into a gel-like substance. Desiccation quickly follows, which is the evaporation of water, leading to the hard, shrunken mass observed externally.
The change in color, from bright crimson to dark brown or black, results from chemical oxidation. The red color of fresh blood comes from hemoglobin, which carries oxygen via an iron atom in its ferrous (Fe\(^{2+}\)) state. When blood is exposed to air, the iron atom loses an electron and is oxidized to the ferric (Fe\(^{3+}\)) state, forming methemoglobin. This chemical change is comparable to the rusting of iron, resulting in the dark, non-oxygen-carrying color of dried blood.
Investigative Significance of Dried Blood
Outside wound healing, dried blood residue is a valuable form of trace evidence in forensic science. The dried stains, often called bloodstains, are analyzed for the information they hold about an event. Forensic analysts use Bloodstain Pattern Analysis (BPA) to interpret the physical characteristics of the dried residue.
BPA involves scrutinizing the size, shape, distribution, and location of bloodstains to reconstruct the sequence of events that caused the bloodshed. For instance, the shape of a dried stain indicates the direction of travel, and the spatter pattern reveals the force and angle of impact. The dried blood residue is also a rich source of DNA, which can be extracted to identify individuals involved. Scientists can analyze chemical changes within the stain to estimate the time since the blood was shed.