Blood Contents and Production

• Blood is a specialized connective tissue containing plasma, cells, and platelets
• Blood contains:

  • Cells: RBCs and WBCs (and platelets)
  • Extracellular matrix: plasma
  • Extracellular proteins: plasma proteins
• The typical total blood volume for a person depends on body size:

  • 5 - 6 L (range for an adult male)
  • 4 - 5 L (range for an adult non-pregnant female)
• Plasma is more than half of the volume of blood and consists of water, dissolved substances, and proteins
• Plasma is considered an extracellular fluid, but it has far more protein than other extracellular fluids in the body
• The non-protein components of plasma constantly circulate and mix with other extracellular fluids
• Formed elements in blood help carry out its various functions
• White blood cells

  • The % of formed elements in the volume of blood has some variation by gonadal sex (37 - 47% in females, and 40 - 54% in males), mostly due to differences in the number of RBCs
• Functions of formed elements in blood:

  • Defence against fluid loss
  • Defence against pathogens
  • Defence against toxins
  • Transport of dissolved gases
  • Wastes
• Haemostasis refers to all the physiological processes that limit or halt blood loss through damaged blood vessels
• Clotting or coagulation is a major part of haemostasis
• Haemostasis is divided into three components (or phases) which usually overlap in time
• Vascular Phase:

  • Contraction ('vascular spasm') and increased endothelial 'stickiness'
• Platelet Phase:

  • Platelets aggregate at exposed endothelial surfaces to plug the broken vessel
  • Activated platelets change shape and release chemicals that attract other platelets and help them stick to each other
• The only phase of blood clotting that is a positive feedback loop is the platelet phase
• Interaction between activated platelets and chemicals that attract more platelets leads to the accumulation and aggregation of platelets in a positive feedback loop
• In the coagulation phase of haemostasis, a fibrin mesh network forms around platelets, producing a clot
• The ultimate effect of coagulation (secondary haemostasis) is to create strands of insoluble fibrin, a protein that binds aggregated platelets (and blood cells) into a clot
• Coagulation phase involves a cascade of enzymes that catalyze the formation of fibrin from soluble fibrinogen, triggered by tissue damage or exposed connective tissue
• Coagulation involves many clotting factors: enzymes linked in a complex cascade that produces fibrin
• Activation of one clotting factor enzyme catalyzes activation of another enzyme in the blood clotting cascade
• Two distinct sets converge on the common pathway: Factor X → Factor Xa catalyses Prothrombin (Factor II) → Thrombin (Factor IIa) which catalyses Fibrinogen (Factor I) → Fibrin (Factor Ia)
• After the vessel wall is repaired, fibrinolysis dissolves the clot
• Key Steps in Fibrinolysis:

  • Tissue plasminogen activator (t-PA) is released from the repaired vessel wall
  • t-PA converts plasminogen (a plasma protein) to plasmin
  • Plasmin degrades fibrin
• RBCs come from myeloid cells via a series of distinct stages, stimulated by erythropoietin (EPO) secreted by the kidneys in response to hypoxia
• Loss of the nucleus in RBCs allows more hemoglobin to fit in the cell
• RBC maturation is completed after reticulocytes enter the bloodstream
• The lifespan of a typical RBC is ~4 months, and RBC contents are recycled into new RBCs or excreted
• Blood is the central component of the cardiovascular system, containing plasma and formed elements (RBCs, WBCs, and platelets) participating in gas transport, immune defenses, and clotting
• Formed elements are produced by red bone marrow, and different formed elements are made via different lineages under different stimulating factors
• Blood types come from antigens present (or absent) on the surface of RBCs, with antibodies automatically produced against any RBC antigen not found on an individual's own cells
• Mismatch between foetal and parental Rh blood type can lead to hemolytic disease of the newborn (HDN), which can be avoided by injecting the Rh- person with anti-Rh antibodies at the end of their first pregnancy