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.
C Blood as a transport medium for blood cells (modified from Lüllmann-Rauch, Thieme; 2012)
White blood cells circulate through the blood and as a result are distributed throughout the body. They constantly migrate out of the blood stream and into the connective tissue of the organs where they attack bacteria or cancer cells. Migration (diapedesis) occurs via the leukocyte adhesion cascade. Upon receiving a stimulus, endothelial cells express cell adhesion molecules on their luminal surface. These molecules either come to the surface immediately through vesicles in the cytoplasm or they are synthesized in response. Ligands on the cell membrane of the leukocytes bind to these molecules. This binding (keying) causes the leukocytes to roll along the endothelium. Sometimes they come to a stop and sometimes they disconnect and rejoin the bloodstream. When they stop, the endothelial cells reduce their intercellular cohesion to allow the leukocytes to pass through the gap between them.
D Innate and adaptive immunity
As the blood has access to all organs, it plays an important role in the immune system in defending against infections and malignant cells. The immune system and the blood are thus tightly integrated. The innate immune system responds immediately to an appropriate stimulus and is nonspecific as it must respond to many possible attacks. Cellular components (cells are transported by the blood) and humoral components are differentiated. Humoral (“liquid”) components include complement and cytokines in the blood. Adaptive immunity is specific and is directed toward a specific noxious agent such as a certain virus. The adaptive immune system includes T and B cells, which also circulate in the blood. T cells kill cancer cells or cells infected by viruses by direct contact (cellular immunity); B cells secrete various classes of antibodies (humoral immunity).
E Lifespan facts of certain blood cells
This unit discusses the cells of the blood that can be morphologically distinguished in a normal blood smear. The classic blood smear is colored with Pappenheim stain. Red and white blood cells as well as platelets are readily identifiable. See p. 22 for their normal values in blood.
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.