Blood plasma is the liquid component of blood, serving as the matrix in which blood cells are suspended and transported throughout the body. It typically presents as a pale yellow or straw-colored liquid, a hue observable once the cellular components have been removed. This fluid constitutes approximately 55% of the total volume of blood, making it the largest single component in the circulatory system. Plasma’s color and composition are direct indicators of its underlying biological state.
The Natural Color and Its Key Components
The straw-colored appearance of healthy plasma results from the dissolved organic and inorganic compounds it contains. Although plasma is roughly 92% water, the remaining 8% consists of solids, primarily proteins, which give the liquid its slight coloration. The most abundant of these proteins is albumin, synthesized by the liver.
The specific yellow pigment responsible for the tint is bilirubin, a normal metabolic byproduct derived from the breakdown of aged red blood cells. When red blood cells are processed, they yield unconjugated bilirubin, which is transported through the plasma, bound mostly to albumin, to the liver for excretion. The concentration of this normal bilirubin imparts the characteristic pale yellow hue to the plasma sample.
Separating Plasma From Whole Blood and Serum
Plasma is obtained from whole blood using centrifugation, a laboratory process that uses rapid spinning to separate components based on density. An anticoagulant must be added before spinning to prevent the sample from clotting. During centrifugation, the denser cellular elements—red blood cells, white blood cells, and platelets—settle at the bottom of the tube.
The pale yellow liquid layer remaining at the top is the plasma, which is then extracted. This method ensures plasma retains all its original components, including clotting factors. This distinguishes it from serum, which is the fluid remaining after blood clots naturally without an anticoagulant. Since clotting consumes proteins like fibrinogen, serum lacks these factors, making plasma the preferred sample for coagulation studies.
What Abnormal Colors Indicate
Laboratory technicians visually inspect plasma samples for deviations from the normal pale yellow color, as an abnormal appearance indicates a physiological problem or a pre-analytical issue. These color variations are categorized into three types: icterus, lipemia, and hemolysis. Each variation points toward a specific biological or handling issue, and their analysis often guides further diagnostic testing.
Icterus
Plasma that appears dark yellow, orange, or greenish is described as icteric, caused by an unusually high concentration of bilirubin. This condition, known as hyperbilirubinemia, corresponds to the clinical symptom of jaundice. Bilirubin levels in icteric plasma are often elevated above the normal concentration of less than 1 milligram per deciliter. Excess bilirubin may stem from liver dysfunction (such as hepatitis or cirrhosis) or from conditions involving excessive red blood cell destruction, where bilirubin is produced faster than the liver can excrete it.
Lipemia
Lipemic plasma has a milky white, opaque, or turbid appearance due to extremely high levels of lipids, primarily triglycerides. This change in clarity is caused by the light-scattering properties of large, fat-rich lipoproteins like chylomicrons and very-low-density lipoproteins (VLDL). While lipemia can temporarily occur if a blood sample is drawn shortly after a high-fat meal, persistent or severe lipemia often indicates metabolic disorders. These disorders include uncontrolled diabetes mellitus or specific genetic hyperlipidemias, where the body struggles to clear fats from the bloodstream.
Hemolysis
When plasma takes on a pink or red tint, it is described as hemolyzed, indicating the presence of free hemoglobin released from ruptured red blood cells. Hemolysis can occur in vivo (within the patient’s body) due to conditions like hemolytic anemia, where red blood cells are destroyed prematurely. More commonly, the pink color is an in vitro artifact caused by poor sample collection technique, such as drawing blood too forcefully or incorrect sample handling. The presence of free hemoglobin interferes with the accuracy of many laboratory tests by chemically reacting with reagents or by altering photometric readings.
Primary Physiological Functions of Plasma
Beyond serving as the solvent for blood cells, plasma performs functions fundamental to maintaining whole-body stability. It acts as the principal transport medium, carrying absorbed nutrients (like glucose, amino acids, and lipids) from the digestive system to tissues. Plasma also transports hormones to target organs and carries waste products (such as urea and carbon dioxide) to the kidneys and lungs for excretion.
Plasma proteins, particularly albumin, maintain the body’s osmotic pressure, regulating fluid distribution between blood vessels and surrounding tissues. This pressure ensures fluid remains within the circulation, supporting blood volume and pressure. Plasma also contains immunoglobulins (antibodies) to neutralize foreign pathogens, and clotting factors, including fibrinogen, ready to be activated to prevent blood loss.