Chapter 10: Blood: Cells & Hemopoiesis

The plasma is composed mostly of water and proteins, including albumin, which maintains crucial colloid osmotic pressure, globulins, which include antibodies for immune function and carrier proteins, and fibrinogen, essential for blood coagulation. The formed elements include erythrocytes (red blood cells), which are anucleate, biconcave discs filled with hemoglobin (Hgb) to maximize gas transport; the structural integrity of these cells relies on a specialized cytoskeleton anchored by proteins like spectrin and band 3, and defects can lead to disorders like hereditary spherocytosis. Clinically significant details include the hematocrit (HCT) measurement, the operation of the ABO and Rh blood group systems, and the diagnostic use of glycated hemoglobin (HbA1c) levels in managing diabetes. Leukocytes (white blood cells) are subclassified into granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes). Neutrophils, the most abundant, are potent phagocytes that migrate out of circulation through diapedesis using adhesion molecules, responding rapidly to injury, where they kill microbes primarily via oxygen-dependent mechanisms like the respiratory burst (involving the phox system and myeloperoxidase, MPO). Eosinophils contain cytotoxic proteins, notably major basic protein (MBP), important in defending against helminthic parasites and moderating allergic reactions. Basophils, functionally related to mast cells, release heparin and histamine, playing a key role in hypersensitivity responses. Monocytes are the largest leukocytes and serve as precursors to tissue macrophages, which function as antigen-presenting cells (APCs). Lymphocytes, the key cells of the adaptive immune system, include T cells (cell-mediated immunity), B cells (antibody production), and Natural Killer (NK) cells. Thrombocytes (platelets) are membrane-bound cytoplasmic fragments derived from bone marrow megakaryocytes, vital for hemostasis by forming primary plugs and participating in coagulation cascades. The entire process of blood cell formation, or hemopoiesis, occurs in the red bone marrow of adults, starting with the Hemopoietic Stem Cell (HSC) which differentiates into Common Myeloid Progenitor (CMP) and Common Lymphoid Progenitor (CLP) lines; the maturation of erythrocytes (erythropoiesis) is regulated by erythropoietin, while neutrophil release is managed by the CXCL12-CXCR4 axis, demonstrating the tight control exerted by transcription factors, cytokines, and growth factors over lineage commitment and cell kinetics.