A Composition of blood

Blood is a unique tissue in that it is liquid. Yet like every other tissue it consists of an extracellular matrix (plasma) and cellular components (red and white blood cells as well as platelets). A transport and communication organ, it is contained within a closed vascular system that interconnects all organ systems. Its functions are accordingly diverse: transport of gases and substances, protection, thermoregulation, regulation of pH value, and coagulation. Coagulation prevents all of the blood from draining out of the cardiovascular system when its walls have been damaged. A protein-containing fluid (plasma) accounts for 50–63% of the blood volume; 37–50% is blood cells. The ratio of the volume of red blood cells to the total volume of blood is referred to as hematocrit. Plasma is obtained by centrifuging whole blood which has been prevented from coagulating by the addition of a substance such as heparin. Serum is obtained by initially allowing the blood to coagulate and then centrifuging it. Serum is thus plasma minus clotting factors. About 90% of plasma consists of water; the rest includes proteins, electrolytes, and low-molecular-weight substances of metabolism and metabolic regulation (hormones). Most plasma proteins are synthesized by the liver. Accounting for a full 99% of the hematocrit, the vast majority of blood cells (see p. 24) are the non-nucleated red blood cells. Their cytoplasm is filled with hemoglobin, which serves to transport oxygen and buffer the blood. The white blood cells have a protective function; the platelets promote blood coagulation. All blood cells come from stem cells of the red bone marrow (see p. 27) where they are constantly produced.

B Phases of blood formation during development

Blood is required before birth even prior to the development of red bone marrow. As a result, blood is initially formed elsewhere: in the yolk sac (insular), in the liver (hepatic), in the spleen (splenic), and finally in the secondary bone marrow (medullary). Malignant systemic diseases of the blood and the immune system “remember” these sites that provide favorable conditions for growth, and certain forms of these diseases then colonize the liver and spleen.

A Red blood cells (erythrocytes)

Erythrocytes (approx. 5 million/μL) are large, biconcave cells measuring about 7.5 μm in diameter. In mammals they contain no nucleus or cytoplasmic organelles. Their lack of organelles and their specially reinforced cell membrane allow erythrocytes to adapt particularly well to different flow characteristics in blood so that they can even squeeze through narrow capillaries. This adaptability allows them to survive in the blood about 120 days. After this, erythrocytes are eliminated by macrophages in the liver and spleen. Because they lack mitochondria, they must obtain their energy from anaerobic glycolysis. As a result they are dependent on glucose as a source of energy. Ninety-five percent of the interior of erythrocytes is filled with the protein hemoglobin, which binds O₂ and, to a lesser extent, CO₂. Erythrocytes are created in a series of morphologically distinct stages from a precursor cell containing a nucleus (see page 27). Reticulocytes represent the stage immediately preceding mature erythrocytes. They can be demonstrated with Cresyl violet stain. This stain binds to the RNA of the rough endoplasmic reticulum of erythrocytes, whose precursor cells ejected their nuclei 1–2 days previously and still contain residual traces of the endoplasmic reticulum. About 2.5 million reticulocytes leave the bone marrow per second. They mature into erythrocytes within a day. About 1% of erythrocytes are reticulocytes. As a result, reticulocytes are particularly suitable for monitoring erythropoiesis. When the number of reticulocytes increases, as can occur after acute bleeding, it is referred to as a reticulocyte crisis. This indicates that the bone marrow has responded to the blood loss by increasing the production of new erythrocytes. Here, the term crisis is used in a positive sense to indicate the regenerative output of the bone marrow.

Note: As the average diameter of erythrocytes is a reliably constant 7.5 μm, it can be used in histologic sections as an intrinsic scale for the size of a histologic structure.