Blood is a specialized type of connective tissue that flows throughout the body. Like all connective tissues, it is composed of cells suspended in an extracellular matrix, but in blood, this matrix is liquid rather than solid. This fluid component is known as blood plasma, and it serves as the medium for all the formed elements of blood—red cells, white cells, and platelets—to circulate.
Defining Blood Plasma
Blood plasma is the pale, straw-yellow liquid that constitutes the largest single component of whole blood. When blood is separated by centrifugation, the plasma forms the top layer, making up approximately 55% of the total volume. This liquid matrix is responsible for the fluidity of blood, allowing it to move easily through the vast network of blood vessels. Plasma provides the suspension for all the solid components, including red blood cells and white blood cells. Its consistency is slightly thicker than water due to the presence of numerous dissolved proteins and solutes.
The Chemical Makeup of the Matrix
The plasma matrix is overwhelmingly composed of water, which accounts for about 92% of its total volume. This high water content allows plasma to effectively dissolve and transport substances throughout the body. The remaining 8% consists of dissolved solids, with proteins being the most numerous components by weight.
Plasma proteins, synthesized primarily by the liver, make up about 7% of the plasma composition. The three major classes are albumins, globulins, and fibrinogen. Albumin is the most abundant, making up about 60% of the total plasma protein mass.
Globulins account for about 38% of plasma proteins and are divided into alpha, beta, and gamma types. Gamma globulins are the antibodies, produced by immune cells, necessary for defense against pathogens. Fibrinogen constitutes about 4% of the plasma proteins and is the precursor to the fibrin threads required for blood clotting.
Other dissolved solutes make up the final 1% of the plasma. These include electrolytes like sodium, potassium, and chloride ions, which maintain the body’s pH balance and nerve function. Nutrients such as glucose, amino acids, and fats, along with metabolic waste products like urea, are also transported within the plasma matrix.
Primary Functions in the Body
The plasma matrix performs several functions necessary for maintaining homeostasis. Its primary role is to act as a transport medium, carrying dissolved substances and formed elements to every tissue. Hormones, which act as chemical messengers, travel via the plasma from endocrine glands to their target cells.
Plasma is also responsible for maintaining fluid balance, largely through the action of its most abundant protein, albumin. Albumin generates colloid osmotic pressure, a pulling force that helps keep water within the blood vessels. This pressure counteracts the hydrostatic pressure that pushes fluid out of the capillaries, preventing fluid accumulation in the surrounding tissues.
The high water content of the plasma matrix gives it a role in regulating body temperature. Water has a high specific heat capacity, meaning it can absorb and release large amounts of heat with only small temperature changes. As blood circulates, plasma distributes heat generated by active tissues evenly throughout the body.
The plasma matrix is fundamental to the body’s ability to stop bleeding through the process of hemostasis. Fibrinogen is a soluble protein that converts into an insoluble, thread-like protein called fibrin upon injury. These fibrin threads form a meshwork that traps platelets and red blood cells, creating a stable blood clot to seal the damaged vessel wall.