Blood is a fluid that serves as the body’s internal transport system, continuously circulating to deliver oxygen and nutrients while removing metabolic waste. It also regulates body temperature, fluid balance, and protects against foreign invaders. This specialized connective tissue is a complex physiological substance. Analyzing its physical properties reveals why its chemical classification challenges a single scientific definition.
Defining Scientific Mixtures
In chemistry, mixtures are categorized based on the size of the dispersed particles and their behavior within the medium.
A true solution forms when one substance dissolves completely into another, creating a homogeneous mixture. The dispersed particles are atoms, ions, or molecules, generally less than one nanometer in diameter. These particles are too small to scatter light, which is why a true solution, like filtered saltwater, appears transparent.
A colloid contains intermediate particles ranging from one to one thousand nanometers. These particles are larger than those in a solution but remain permanently dispersed without settling, as seen in milk or fog. Colloids are identified because their intermediate-sized particles are large enough to scatter a beam of light, known as the Tyndall effect.
The third category is a suspension, a heterogeneous mixture containing the largest particles, often exceeding one thousand nanometers. These particles are readily visible and will physically separate from the liquid medium if the mixture is left undisturbed. For example, soil particles in muddy water eventually settle to the bottom due to gravity.
The Liquid Matrix: Blood Plasma
Plasma is the straw-colored liquid portion of whole blood, serving as the extracellular matrix where all other components are suspended. Plasma accounts for approximately 55% of the total blood volume and is about 90% water. This high-water content acts as the solvent, allowing for the complete dissolution of small solutes like glucose, amino acids, and ions, characteristic of a true solution.
Within this solvent are large plasma proteins, including albumins, globulins, and fibrinogen, synthesized primarily by the liver. These protein molecules are too large to be true solutes, positioning them within the colloidal particle size range (one to one thousand nanometers). The presence of these large proteins causes plasma to exhibit the properties of a colloid, specifically a hydrocolloid.
Albumin, the most abundant protein, maintains the colloidal osmotic pressure of the blood. This pressure keeps water within the bloodstream rather than leaking into surrounding tissues.
The Cellular Components: Formed Elements
The remaining 45 percent of whole blood consists of the formed elements: erythrocytes, leukocytes, and platelets. Erythrocytes, or red blood cells, are the most numerous elements, accounting for the blood’s characteristic red color and high density. These cells are significantly larger than the proteins found in plasma, placing them within the particle size range of a suspension.
These components are perpetually mixed and circulated throughout the body by the heart’s pumping action. If a blood sample is collected and prevented from clotting, gravity causes these large, dense particles to separate from the liquid plasma over several hours. This sedimentation of the formed elements, with red blood cells settling to the bottom, is the definitive property classifying whole blood as a suspension. Continuous circulation prevents this natural settling from occurring while blood is flowing.
Blood as a Complex Heterogeneous System
The analysis of plasma and formed elements demonstrates that whole blood cannot be assigned to a single category of mixture. It is accurately described as a complex, heterogeneous system that simultaneously exhibits the properties of a solution, a colloid, and a suspension.
The smallest components, such as electrolytes and glucose, are completely dissolved in water, fulfilling the definition of a solution. Plasma proteins are dispersed with particle sizes that prevent them from settling, yet they are large enough to scatter light and maintain osmotic pressure, fitting the criteria for a colloid. Finally, the large cellular components—red and white blood cells—are dense enough to settle out over time, proving the presence of a suspension.
This multi-faceted composition is validated through the laboratory process of centrifugation. When a blood sample is spun at high speed, the denser red blood cells are forced to the bottom of the tube, forming the lowest layer. Above this is a thin, whitish layer called the buffy coat, which contains the leukocytes and platelets. The lightest component, the straw-colored plasma, remains at the top. This layering clearly separates the suspension components from the colloidal and solution components mixed within the plasma layer.