The liquid portion of the blood, the plasma, is a complex solution containing more than 90 percent water. The water of the plasma is freely exchangeable with that of body cells and other extracellular fluids and is available to maintain the normal state of hydration of all tissues. Water, the single largest constituent of the body, is essential to the existence of every living cell.

The major solute of plasma is a heterogeneous group of proteins constituting about 7 percent of the plasma by weight. The principal difference between the plasma and the extracellular fluid of the tissues is the high protein content of the plasma. Plasma protein exerts an osmotic effect by which water tends to move from other extracellular fluid to the plasma. When dietary protein is digested in the gastrointestinal tract, individual amino acids are released from the polypeptide chains and are absorbed. The amino acids are transported through the plasma to all parts of the body, where they are taken up by cells and are assembled in specific ways to form proteins of many types. These plasma proteins are released into the blood from the cells in which they were synthesized. Much of the protein of plasma is produced in the liver.

The major plasma protein is serum albumin, a relatively small molecule, the principal function of which is to retain water in the bloodstream by its osmotic effect. The amount of serum albumin in the blood is a determinant of the total volume of plasma. Depletion of serum albumin permits fluid to leave the circulation and to accumulate and cause swelling of soft tissues (edema). Serum albumin binds certain other substances that are transported in plasma and thus serves as a nonspecific carrier protein. Bilirubin, for example, is bound to serum albumin during its passage through the blood. Serum albumin has physical properties that permit its separation from other plasma proteins, which as a group are called globulins. In fact, the globulins are a heterogeneous array of proteins of widely varying structure and function, only a few of which will be mentioned here. The immunoglobulins, or antibodies, are produced in response to a specific foreign substance, or antigen. For example, administration of polio vaccine, which is made from killed or attenuated (weakened) poliovirus, is followed by the appearance in the plasma of antibodies that react with poliovirus and effectively prevent the onset of disease. Antibodies may be induced by many foreign substances in addition to microorganisms; immunoglobulins are involved in some hypersensitivity and allergic reactions. Other plasma proteins are concerned with the coagulation of the blood.

Many proteins are involved in highly specific ways with the transport function of the blood. Blood lipids are incorporated into protein molecules as lipoproteins, substances important in lipid transport. Iron and copper are transported in plasma by unique metal-binding proteins (transferrin and ceruloplasmin, respectively). Vitamin B₁₂, an essential nutrient, is bound to a specific carrier protein. Although hemoglobin is not normally released into the plasma, a hemoglobin-binding protein (haptoglobin) is available to transport hemoglobin to the reticuloendothelial system should hemolysis (breakdown) of red cells occur. The serum haptoglobin level is raised during inflammation and certain other conditions; it is lowered in hemolytic disease and some types of liver disease.

Lipids are present in plasma in suspension and in solution. The concentration of lipids in plasma varies, particularly in relation to meals, but ordinarily does not exceed 1 gram per 100 millilitres. The largest fraction consists of phospholipids, complex molecules containing phosphoric acid and a nitrogen base in addition to fatty acids and glycerol. Triglycerides, or simple fats, are molecules composed only of fatty acids and glycerol. Free fatty acids, lower in concentration than triglycerides, are responsible for a much larger transport of fat. Other lipids include cholesterol, a major fraction of the total plasma lipids. These substances exist in plasma combined with proteins of several types as lipoproteins. The largest lipid particles in the blood are known as chylomicrons and consist largely of triglycerides; after absorption from the intestine, they pass through lymphatic channels and enter the bloodstream through the thoracic lymph duct. The other plasma lipids are derived from food or enter the plasma from tissue sites.

Some plasma constituents occur in plasma in low concentration but have a high turnover rate and great physiological importance. Among these is glucose, or blood sugar. Glucose is absorbed from the gastrointestinal tract or may be released into the circulation from the liver. It provides a source of energy for tissue cells and is the only source for some, including the red cells. Glucose is conserved and used and is not excreted. Amino acids also are so rapidly transported that the plasma level remains low, although they are required for all protein synthesis throughout the body. Urea, an end product of protein metabolism, is rapidly excreted by the kidneys. Other nitrogenous waste products—uric acid and creatinine—are similarly removed.

Several inorganic materials are essential constituents of plasma, and each has special functional attributes. The predominant cation (positively charged ion) of the plasma is sodium, an ion that occurs within cells at a much lower concentration. Because of the effect of sodium on osmotic pressure and fluid movements, the amount of sodium in the body is an influential determinant of the total volume of extracellular fluid. The amount of sodium in plasma is controlled by the kidneys under the influence of the hormone aldosterone, which is secreted by the adrenal gland. If dietary sodium exceeds requirements, the excess is excreted by the kidneys. Potassium, the principal intracellular cation, occurs in plasma at a much lower concentration than sodium. The renal excretion of potassium is influenced by aldosterone, which causes retention of sodium and loss of potassium. Calcium in plasma is in part bound to protein and in part ionized. Its concentration is under the control of two hormones: parathyroid hormone, which causes the level to rise, and calcitonin, which causes it to fall. Magnesium, like potassium, is a predominantly intracellular cation and occurs in plasma in low concentration. Variations in the concentrations of these cations may have profound effects on the nervous system, the muscles, and the heart, effects normally prevented by precise regulatory mechanisms. Iron, copper, and zinc are required in trace amounts for synthesis of essential enzymes; much more iron is needed in addition for production of hemoglobin and myoglobin, the oxygen-binding pigment of muscles. These metals occur in plasma in low concentrations. The principal anion (negatively charged ion) of plasma is chloride; sodium chloride is its major salt. Bicarbonate participates in the transport of carbon dioxide and in the regulation of pH. Phosphate also has a buffering effect on the pH of the blood and is vital for chemical reactions of cells and for the metabolism of calcium. Iodide is transported through plasma in trace amounts; it is avidly taken up by the thyroid gland, which incorporates it into thyroid hormone.

The hormones of all the endocrine glands are secreted into the plasma and transported to their target organs, the organs on which they exert their effects. The plasma levels of these agents often reflect the functional activity of the glands that secrete them; in some instances, measurements are possible though concentrations are extremely low. Among the many other constituents of plasma are numerous enzymes. Some of these appear simply to have escaped from tissue cells and have no functional significance in the blood.

Blood cells

There are four major types of blood cells: red blood cells (erythrocytes), platelets (thrombocytes), lymphocytes, and phagocytic cells. Collectively, the lymphocytes and phagocytic cells constitute the white blood cells (leukocytes). Each type of blood cell has a specialized function: red cells take up oxygen from the lungs and deliver it to the tissues; platelets participate in forming blood clots; lymphocytes are involved with immunity; and phagocytic cells occur in two varieties—granulocytes and monocytes—and ingest and break down microorganisms and foreign particles. The circulating blood functions as a conduit, bringing the various kinds of cells to the regions of the body in which they are needed: red cells to tissues requiring oxygen, platelets to sites of injury, lymphocytes to areas of infection, and phagocytic cells to sites of microbial invasion and inflammation. Each type of blood cell is described in detail below.

The continuous process of blood cell formation (hematopoiesis) takes place in hematopoietic tissue. In the developing embryo, the first site of blood formation is the yolk sac. Later in embryonic life, the liver becomes the most important red blood cell-forming organ, but it is soon succeeded by the bone marrow, which in adult life is the only source of both red cells and the granulocytes. In young children, hematopoietic bone marrow fills most of the skeleton, whereas in adults the marrow is located mainly in the central bones (ribs, sternum, vertebrae, and pelvic bones). Bone marrow is a rich mixture of developing and mature blood cells, as well as fat cells and other cells that provide nutrition and an architectural framework upon which the blood-forming elements arrange themselves. The weight of the marrow of a normal adult is 1,600 to 3,700 grams and contains over 1,000,000,000,000 hematopoietic cells (18 × 10⁹ cells per kilogram). Nourishment of this large mass of cells comes from the blood itself. Arteries pierce the outer walls of the bones, enter the marrow, and divide into fine branches, which ultimately coalesce into large venous sacs (sinusoids) through which blood flows sluggishly. In the surrounding hematopoietic tissue, newly formed blood cells enter the general circulation by penetrating the walls of the sinusoids.

In the adult the bone marrow produces all of the red cells, 60 to 70 percent of the white cells (i.e., the granulocytes), and all of the platelets. The lymphatic tissues, particularly the thymus, the spleen, and the lymph nodes, produce the lymphocytes (comprising 20 to 30 percent of the white cells). The reticuloendothelial tissues of the spleen, liver, lymph nodes, and other organs produce the monocytes (4 to 8 percent of the white cells). The platelets are formed from bits of the cytoplasm of the giant cells (megakaryocytes) of the bone marrow.

Both the red and white cells arise through a series of complex transformations from primitive stem cells, which have the ability to form any of the precursors of a blood cell. Precursor cells are stem cells that have developed to the stage where they are committed to forming a particular type of new blood cell. By dividing and differentiating, precursor cells give rise to the four major blood cell lineages: red cells, phagocytic cells, megakaryocytes, and lymphocytes. The cells of the marrow are under complex controls that regulate their formation and adjust their production to the changing demands of the body. When marrow stem cells are cultured outside the body, they form tiny clusters of cells (colonies), which correspond to red cells, phagocytic cells, and megakaryocytes. The formation of these individual colonies depends on hormonal sugar-containing proteins (glycoproteins), referred to collectively as colony-stimulating factors (CSFs). These factors are produced throughout the body. Even in minute amounts, CSFs can stimulate the division and differentiation of precursor cells into mature blood cells and thus exert powerful regulatory influences over the production of blood cells. A master colony-stimulating factor (multi-CSF), also called interleukin-3, stimulates the most ancestral hematopoietic stem cell. Further differentiation of this stem cell into specialized descendants requires particular kinds of CSFs; for example, the CSF erythropoietin is needed for the maturation of red cells, and granulocyte CSF controls the production of granulocytes. These glycoproteins, as well as other CSFs, serve as signals from the tissues to the marrow. For instance, a decrease in the oxygen content of the blood stimulates the kidney to increase its production of erythropoietin, thus ultimately raising the number of oxygen-carrying red cells. Certain bacterial components accelerate the formation of granulocyte CSF, thereby leading to an increased production of phagocytic granulocytes by the bone marrow during infection.

In the normal adult the rate of blood cell formation varies depending on the individual, but a typical production might average 200 billion red cells per day, 10 billion white cells per day, and 400 billion platelets per day.