When a blood sample is taken and allowed to settle or is processed in a laboratory, it visibly separates into distinct layers. The dense, red cellular components sink to the bottom of the collection tube, leaving a clear, straw-colored liquid floating above them. This yellow substance is the non-cellular portion of blood, a fluid packed with vital information about the body’s chemistry. This liquid fraction is formally known as either plasma or serum, depending on the preparation method, and it is the focus of most diagnostic testing.
The Components of Whole Blood
Whole blood is a complex substance that circulates throughout the body, acting as a transportation system. It is composed of a liquid matrix and suspended cellular elements. The liquid portion makes up approximately 55% of the total volume, while the remaining 45% consists of the solid components, which include red blood cells, white blood cells, and platelets.
To separate the blood components, technicians use a machine called a centrifuge, which spins the sample at high speeds. This force drives the heavier cellular elements to the bottom of the tube due to their greater density. The red blood cells form the deepest, thickest layer, while the white blood cells and platelets settle just above them in a thin, pale layer known as the buffy coat. The lightest component, the yellow liquid, rises to the top, isolated from the cells for analysis.
Distinguishing Plasma from Serum
The specific name given to the yellow liquid depends entirely on how the blood was collected and processed. If the blood sample is collected in a tube containing an anticoagulant, a substance that prevents clotting, the resulting liquid is called plasma. Plasma contains all the components of whole blood except the cells, meaning it still retains the clotting factors, most notably a large protein called fibrinogen.
Conversely, if the blood is allowed to clot naturally before being centrifuged, the liquid that separates is called serum. When clotting occurs, the fibrinogen and other clotting factors are consumed to form the solid blood clot, which settles with the red cells. Therefore, serum is essentially plasma without the clotting material. Plasma is the liquid part of unclotted blood, while serum is the liquid part of clotted blood.
What Makes the Liquid Component Yellow
The natural, pale yellow color of plasma and serum is primarily due to the presence of bilirubin, a metabolic waste product. Bilirubin is produced when the body breaks down old or damaged red blood cells and processes the hemoglobin they contain. The fluid also contains small amounts of pigments called carotenoids, which are absorbed from the diet and contribute to the faint yellow hue.
The coloration can sometimes vary, providing immediate visual clues about a patient’s health. If the fluid appears a deeper yellow or even orange, it may indicate elevated levels of bilirubin, a condition known as hyperbilirubinemia, which is associated with jaundice. If the liquid looks cloudy or milky, a condition called lipemia, it signals an abnormally high concentration of fats, such as triglycerides, in the bloodstream.
The Role of Plasma and Serum in Diagnostic Testing
Isolating the yellow liquid is a procedural necessity because this fluid carries the vast majority of substances that doctors test for. Plasma and serum are essentially the body’s delivery and waste disposal system, containing hormones, antibodies, electrolytes, glucose, and metabolic byproducts. These components offer a comprehensive snapshot of the body’s current physiological state.
The choice between testing plasma or serum is determined by the specific diagnostic test required. Plasma is used for coagulation studies, which analyze how quickly a patient’s blood clots, because the test requires the presence of fibrinogen. Serum, lacking fibrinogen, is the preferred sample for most routine biochemical tests, such as liver function panels, kidney function tests, and cholesterol level measurements. This distinction is paramount for ensuring the accuracy and validity of the final test results.