Blood is a heterogeneous mixture whose classification depends on which component is being examined. It cannot be answered with a single term. A complete understanding requires classifying both whole blood and its liquid matrix, plasma, based on the physical size of their components.
Defining Solutions Colloids and Suspensions
Mixtures are classified into three categories based on the size of the dispersed particles. A true solution is a homogeneous mixture where the solute particles, such as sugar or salt dissolved in water, are extremely small, typically less than one nanometer (nm) in diameter. These particles are too small to be seen and do not scatter light, meaning a beam of light passes straight through.
Colloids represent an intermediate state, containing dispersed particles that range in size from one to 1,000 nanometers. These particles are not large enough to settle out due to gravity, but they are large enough to scatter light, a phenomenon known as the Tyndall effect. This light scattering makes the path of a light beam visible when it passes through the colloid.
Suspensions are heterogeneous mixtures where the dispersed particles are larger than 1,000 nanometers, or one micrometer. These large particles do not dissolve and will eventually settle out of the mixture if left undisturbed. The components of a suspension can often be separated through simple filtration or centrifugation.
The Primary Components of Blood
Blood is a type of connective tissue composed of a liquid base and various cellular elements. The liquid, straw-colored portion is known as plasma, which accounts for approximately 55% of the total blood volume. Plasma is predominantly water, but it also contains a variety of dissolved substances.
The remaining 45% of blood volume consists of the formed elements, which are suspended within the plasma. These elements include red blood cells (erythrocytes), which are the most numerous and transport oxygen. White blood cells (leukocytes) and cell fragments called platelets are also present, though they make up a much smaller percentage of the total volume.
Classifying Blood Plasma as a Colloidal Solution
Blood plasma, the liquid component, is a complex mixture that acts as both a solution and a colloid. It functions as a true solution for small dissolved molecules like glucose, electrolytes, and gases, whose particle sizes are less than one nanometer. These substances are completely dissolved and uniformly distributed throughout the water.
Plasma is classified as a colloid due to the presence of large plasma proteins, such as albumin, globulins, and fibrinogen. These protein molecules are much larger than simple ions, placing them within the colloidal size range of one to 1,000 nanometers. Albumin, the most abundant plasma protein, is important in maintaining the osmotic balance of the blood.
Because these proteins are dispersed but not truly dissolved, plasma exhibits the properties of a colloidal dispersion. The proteins are too large to pass through a semipermeable membrane but too small to be separated by gravity. This colloidal nature is responsible for maintaining the fluid balance between the blood and the surrounding tissues.
Classifying Whole Blood
When considering whole blood, including all formed elements, the mixture must be classified as a suspension. Red blood cells, the most numerous formed elements, have a typical diameter of about 7.8 micrometers (7,800 nanometers). This size is significantly larger than the 1,000 nanometer cutoff for a colloid, pushing the mixture into the suspension category.
The classification as a suspension is demonstrated because the formed elements can be easily separated from the plasma. When a blood sample is left to stand or is spun in a centrifuge, the denser cells settle to the bottom in a process called sedimentation. Therefore, whole blood is best described as a complex, heterogeneous mixture: a suspension of large cellular elements suspended in a colloidal solution of plasma proteins, which itself contains a true solution of small molecules.