Blood is a specialized fluid composed of cellular components suspended within plasma, a liquid matrix that is approximately 92% water. When blood leaves the circulatory system and is exposed to the environment, it rapidly undergoes physical and chemical changes, transforming it from a flowing liquid into a solid, often dark-colored residue. The resulting dried blood stain is a complex, concentrated matrix determined by the parallel processes of water loss and structural rearrangement.
The Transformation from Liquid to Solid
The transformation of liquid blood into a solid stain is driven by two simultaneous and distinct physical processes: desiccation and coagulation. Desiccation is the physical evaporation of the plasma’s water content into the surrounding air, which causes the remaining biological materials to concentrate and contract. This water loss is rapid initially, and its rate is heavily influenced by external factors like ambient temperature and humidity.
Coagulation, or clotting, is the complex biological process that provides the structural integrity of the final dried mass. This cascade involves a series of protein interactions culminating in the conversion of soluble fibrinogen protein into insoluble strands of fibrin. These fibrin strands form a dense, three-dimensional net that physically traps the blood’s cellular components, including red blood cells and white blood cells, creating a gelatinous mass that is resistant to flow. As water continues to evaporate from this fibrin-cellular matrix, the entire structure shrinks and hardens, leaving behind the characteristic dark, solid stain.
The Persistent Components of Dried Blood
The solid residue of dried blood is a highly concentrated composite of the original blood’s non-water components. The vast majority of this residual material is hemoglobin, the protein responsible for oxygen transport within red blood cells. As the blood dries, the iron within the hemoglobin molecule oxidizes, causing the stain’s color to shift progressively from bright red to dark reddish-brown, and eventually to a deep black color over time.
Concentrated plasma proteins, such as albumins and globulins, also remain and contribute to the dried stain’s composition and structure. Crucially, the cellular components are preserved within the desiccated matrix.
Red blood cells are numerous but lack a nucleus, contributing little genetic material. However, the far less numerous white blood cells contain intact nuclei. These nuclei shelter deoxyribonucleic acid (DNA), which remains stable and accessible within the dried cellular matter, making it a persistent and identifiable component of the stain.
How Environment Affects Stability and Longevity
Temperature and Humidity
The longevity and stability of the biological components in a dried blood stain are dependent on the surrounding environmental conditions. Elevated temperatures accelerate the degradation rate of proteins and nucleic acids by increasing chemical reaction speeds. Conversely, colder temperatures dramatically slow these decomposition processes, which is why samples are often stored frozen. Humidity is another factor, as moisture reintroduces water activity necessary for degradation. High relative humidity promotes the growth of microorganisms, such as bacteria and fungi, that consume and destroy the organic components of the stain, including the DNA.
Light Exposure
Exposure to ultraviolet (UV) light, particularly from direct sunlight, causes photodamage to the DNA structure, leading to strand breakage and chemical cross-linking. These environmental stresses compromise the integrity of the biological material, imposing limits on how long the stain remains useful for analysis.
Scientific and Medical Uses of Dried Blood
Medical Applications
The stability of dried blood, particularly when collected intentionally, has made it invaluable in clinical applications using the technique of Dried Blood Spots (DBS). DBS involves collecting a few drops of capillary blood on specialized filter paper. This method is routinely used worldwide for neonatal screening to identify congenital disorders, allowing for early intervention. DBS is also employed for therapeutic drug monitoring and viral load testing, offering advantages in sample collection and transport due to reduced biohazardous risk and elimination of the need for refrigeration.
Forensic Applications
In forensic science, dried blood stains found at a scene are a source of evidence. The persistent nature of the nuclear DNA within the residual white blood cells allows for the generation of genetic profiles. These profiles can link an individual to the stain and provide definitive identification for criminal investigations.