Blood is a complex biological substance that functions as the body’s primary transport system, circulating nutrients, oxygen, and waste products. When analyzed using the principles of chemistry, blood is unambiguously defined as a mixture. This classification is based on how its numerous components interact and how they can be physically separated.
Understanding Chemical Classifications
To understand why blood is classified as a mixture, it helps to establish the differences between the three main classifications of matter. An element is the simplest form of matter, consisting only of atoms with the same number of protons, and it cannot be broken down by chemical means.
A compound is formed when two or more different elements chemically bond together in a fixed, definite ratio. Water (\(\text{H}_2\text{O}\)) is a classic example, and breaking a compound requires a chemical reaction.
A mixture involves two or more substances that are physically combined but not chemically bonded. Each substance retains its individual chemical identity and properties, such as salt and water in saltwater.
Why Blood is Classified as a Mixture
Blood’s classification as a mixture is confirmed by its ability to be separated into its constituent parts using physical processes. When whole blood is placed in a centrifuge and spun at high speeds, the components separate based on their varying densities. This physical separation is the defining characteristic of a mixture.
The dense red blood cells are forced to the bottom, while the lighter plasma remains at the top. A thin layer known as the buffy coat, containing white blood cells and platelets, forms between the two. The components can be isolated without any chemical reaction, proving they were never chemically bonded.
The substances within blood maintain their original chemical structures and functions while circulating together. This retention of individual properties is the strongest evidence for blood being a mixture, specifically a heterogeneous suspension that also contains a solution.
The Liquid Matrix (Plasma)
The single largest component of this mixture is the liquid matrix known as plasma, which typically accounts for about 55% of the total blood volume. Plasma itself is a complex solution, a specific type of mixture where one substance is dissolved evenly throughout another. It is a yellowish fluid that serves as the suspension medium for all the blood cells and fragments.
The vast majority of plasma (91% to 92%) is the compound water, which acts as the solvent. Dissolved within this water are countless compounds, including plasma proteins like albumin, globulins, and fibrinogen, which collectively account for 7% to 8% of the plasma volume.
The remainder of the plasma is composed of various other dissolved compounds, such as mineral ions (electrolytes) like sodium, potassium, and chloride. Plasma also transports nutrients, metabolic waste products, and chemical messengers such as hormones. The ability of these compounds to remain dissolved and chemically distinct confirms that plasma is a solution.
The Suspended Components
The final 45% of the blood mixture consists of the suspended solid components, often referred to as the formed elements. These components are floating within the liquid plasma, reinforcing blood’s classification as a suspension.
The most numerous of these are the red blood cells (erythrocytes), which give blood its characteristic red color. These cells are rich in the protein hemoglobin, a compound responsible for binding and transporting oxygen from the lungs to the body’s tissues.
The other cellular components include white blood cells (leukocytes), which make up less than 1% of the total blood volume. These cells are part of the immune system, defending against infectious agents and foreign materials.
Platelets (thrombocytes) are the third type of formed element. They are small cell fragments whose primary role is in blood clotting, aggregating at the site of a vessel injury to initiate the formation of a plug.