Plasma, the liquid matrix of blood, serves as the medium for transporting all the necessary components for life throughout the body. This straw-colored fluid comprises the largest single component of whole blood, and its composition is overwhelmingly dominated by a single substance. The most abundant component of blood plasma is water, which accounts for approximately 90 to 92 percent of its total volume. This high concentration allows plasma to perform its primary function as a universal solvent and transport system for a diverse collection of dissolved molecules and cells.
Defining Plasma and Its Volume
Plasma is the non-cellular portion of blood, remaining after the formed elements—red blood cells, white blood cells, and platelets—have been separated. When whole blood is separated, this liquid layer constitutes about 55 percent of the total blood volume in the body. This volume percentage is significant because it means plasma is the largest single constituent of blood by volume.
The massive amount of water within this plasma volume provides the necessary fluidity for circulation. Water acts as the continuous phase in which all other substances are suspended or dissolved. The remaining 8 to 10 percent of plasma is composed of various solutes, including proteins, electrolytes, nutrients, and waste products.
The Role of Water in Plasma Function
The abundance of water in plasma is directly linked to its role as a transport vehicle and a regulator of bodily processes. Water’s unique property as a highly effective solvent allows it to dissolve a wide array of polar and ionic substances. This solvency enables the continuous carriage of glucose, amino acids, hormones, and metabolic waste products like urea and carbon dioxide to and from cells.
Water is also instrumental in maintaining the overall volume and pressure of the blood within the circulatory system. By ensuring adequate blood volume, water helps maintain the blood pressure necessary for proper circulation and the delivery of oxygen and nutrients to all tissues. Furthermore, water’s high specific heat capacity allows the blood to absorb and distribute heat throughout the body, providing a mechanism for thermoregulation. This capacity helps stabilize body temperature by preventing rapid fluctuations in internal heat. The water in plasma is constantly exchanged with the water in body cells and other extracellular fluids, a process that helps maintain the normal state of hydration across all tissues.
The Essential Solutes of Plasma
The remaining fraction of plasma, making up roughly 8 to 10 percent of its volume, consists of dissolved substances known as solutes. Plasma proteins are the next most abundant component by weight after water, constituting about 7 percent of the total plasma. These proteins are classified into three major groups, each with distinct functions that rely on the water medium for their circulation.
Plasma Proteins
Albumin
Albumin is the most common plasma protein, making up about 60 percent of the total protein content. Synthesized in the liver, its primary role is to create an osmotic effect that prevents excessive water from leaking out of the blood vessels and into surrounding tissues. This function is crucial for maintaining blood volume and pressure. Albumin also acts as a transport vehicle for substances like fatty acids, hormones, and certain drugs.
Globulins
Globulins represent the second major group of plasma proteins, divided into alpha, beta, and gamma types. Alpha and beta globulins primarily serve as carriers, transporting lipids, metal ions, and fat-soluble vitamins throughout the bloodstream. The gamma globulins are known as immunoglobulins or antibodies, which are produced by B lymphocytes and are a fundamental part of the body’s defense against pathogens.
Fibrinogen
Fibrinogen is a large protein essential for hemostasis, the process of stopping blood loss. Upon injury, fibrinogen is converted into insoluble fibrin threads. These threads form a mesh that traps blood cells and platelets to create a stable blood clot.
Other Solutes
Beyond these proteins, the final 1 percent of plasma solutes includes electrolytes, nutrients, and waste products. Electrolytes such as sodium, potassium, and chloride ions help maintain the blood’s pH and osmotic balance. Glucose and amino acids provide energy and building blocks to the cells. Metabolic waste products like urea and creatinine are also carried by the plasma water to the kidneys for excretion.