Expirated blood is a projected bloodstain pattern resulting from the forceful expulsion of blood from the body’s respiratory system. This expulsion is typically caused by a powerful physiological event, such as a cough, a sneeze, or a forced exhalation. Identifying this type of blood is important because it reveals information about the source of the bleeding and the circumstances of its expulsion.
How Blood Becomes Expirated
The presence of expirated blood signifies that the source of the bleeding is within the respiratory tract, such as the lungs, trachea, or major bronchi, or sometimes the nasal or oral cavity. For blood to be expirated, it must enter the airway and be subjected to the high-pressure airflow generated by a sudden, forceful breath. The expulsion mechanism is most often a reflexive cough, which attempts to clear foreign material from the air passages.
The physical process that makes this blood unique is called aeration, which occurs while the blood is inside the airways. As the blood moves up the respiratory tract, it mixes intensely with air, mucus, and saliva. This turbulent blending incorporates tiny air bubbles into the liquid blood mass.
This internal mixing process differentiates expirated blood from other forms of projected blood spatter, such as arterial spurting. While arterial blood is expelled by heart pressure, expirated blood is propelled by lung pressure. The internal trauma or medical condition must be severe enough to cause blood to breach the respiratory system and accumulate in the airways.
A significant amount of force is necessary to atomize the blood and project it out of the nose or mouth. Peak air velocities during a cough can be high, causing the blood to exit the body as a fine mist or spray. The mixing with other fluids, such as saliva and mucus, also dilutes the blood, changing its physical properties and external appearance.
Identifying Characteristics of Expirated Blood Spatter
The defining characteristic of expirated blood spatter lies in the small, physical details visible within the dried stain itself. Forensic and medical analysts look for the presence of tiny, ruptured air bubbles, known as vesiculation, within the individual droplets or the overall pattern. These air bubbles form when the blood is whipped with air in the lungs and airways before being forced out.
When the blood drops land and dry, the bubbles burst, leaving behind small, pale rings or vacuoles within the bloodstain. The pattern’s color is another distinguishing feature, often appearing lighter, pinkish, or less saturated than pure blood. This color alteration results from the blood mixing with clear airway fluids, such as saliva and mucus, effectively diluting the hemoglobin concentration.
The physical distribution of the spatter offers strong clues. Due to the high velocity and aerosolization caused by the forceful expiration, the resulting pattern is often a fine mist or spray of very small droplets, frequently less than 0.5 millimeters in diameter. This pattern can be similar in size to high-velocity impact spatter, making the internal characteristics of the stains, like the bubble rings, important for differentiation.
The pattern of expelled droplets is typically radial, spreading outward from the source, but it is often concentrated on surfaces immediately adjacent to the person. The bloodstains may exhibit “beading,” where small drops are visibly connected by thin, dried strands of mucus or saliva. The presence of mucus or saliva can be confirmed through specialized forensic testing, such as PCR analysis for human-specific oral bacteria, which helps link the blood to the oral cavity.
Significance in Medical Diagnosis and Forensic Analysis
The identification of expirated blood holds significant meaning in both clinical medicine and forensic investigation. In a medical setting, coughing up blood, or hemoptysis, indicates serious pathology or trauma affecting the lower respiratory tract. The presence of aerated blood suggests a pulmonary hemorrhage, a severe lung infection like tuberculosis, or major trauma to the chest and airways causing internal bleeding.
Recognizing the characteristics of aerated blood helps medical professionals quickly localize the source of the bleeding to the lungs rather than the stomach or upper digestive tract. The finding directs diagnostic efforts toward imaging and procedures focused on the pulmonary system. Prompt recognition can be life-saving, as pulmonary hemorrhage can lead to rapid blood loss and airway obstruction.
In forensic science, the presence of an expirated blood pattern is a powerful tool for crime scene reconstruction. The pattern proves that the individual was alive and capable of forceful breathing after sustaining an injury that caused internal bleeding. This finding helps establish the sequence of events, such as whether a person was injured and then moved or coughed up blood in a specific location before succumbing to their injuries.
Distinguishing expirated blood from other types of spatter, like impact spatter, can be a factor in determining guilt or innocence. For instance, expirated blood found on a suspect’s clothing might suggest they were assisting an injured person, while impact spatter might imply they were the assailant. Testing the stain for the presence of saliva allows investigators to confirm the oral origin of the blood, providing a more complete picture of the events.