Week 33 - The components, functions and coagulation of blood

• Main functions of the Blood

  • Transport of important blood components: Oxygen, nutrients, waste materials, hormones and cytokines
  • Protection from infection and blood loss: The immune response to infection, Haemostasis (blood clotting to stop blood loss) after injury
  • Homeostasis: This is a process designed to keep a constant internal environment, including thermoregulation, pH balance and water balance
• Major soluble components of blood plasma

  • Albumins
  • Alpha and beta globulins
  • Gamma globulins
  • Fibrinogen
• Formed elements/cells within the blood

  • Red blood cells (erythrocytes)
  • Platelets (thrombocytes)
  • White blood cells (N ever L et M onkeys E at B ananas!)
• Albumins


  Produced by the liver and provide the osmotic pressure needed to maintain blood volume and pressure, by drawing water from the tissues into the capillaries
• Alpha and beta globulins


  Transport lipids and fat-soluble vitamins
• Gamma globulins


  The antibodies produced by activated B lymphocytes
• Fibrinogen


  An important clotting factor produced by the liver that gets converted into Fibrin during the clotting process to reduce blood loss
• Red blood cells (erythrocytes)

  • Biconcave discs (7.5μm in diameter) and have no nucleus
  • Life span of 4 months; dead cells are removed by macrophages in the liver, spleen and bone marrow
  • New cells are made in the bone marrow under hormonal control (erythropoietin)
  • Packed with haemoglobin, which is composed of globin protein chains combined with non-protein haem groups
• Platelets (thrombocytes)


  • About 10^12 (1 trillion!) circulating in the bloodstream of an adult, and they survive for about 10 days
  • About one third of them are stored in the "red pulp" of the spleen where they can be released when demand for them is high e.g., after significant blood loss after injury
• Platelet formation (thrombopoiesis)


  1. Megakaryocytes are giant cells produced mainly in the bone marrow by undergoing mitosis (but no cell division) by up to seven times to form a polyploid cell

  2. Thrombopoietin regulates the growth and maturation of megakaryocytes

  3. Platelets are cell fragments which peel off the outer edges of these giant cells

  4. Platelets are smaller than red blood cells, are colourless and have no nucleus, but contain secretory granules
• Blood clotting (haemostasis)


  1. Vasoconstriction (narrowing) of blood vessel diameter

  2. Formation of a sticky platelet plug

  3. Production of a web of insoluble fibrin proteins around and within the platelet plug to cause blood coagulation
• Platelet aggregation and activation is normally prevented by an intact epithelial layer
• Activation does not happen normally because: (a) an undamaged layer of endothelial cells separates the blood from exposure to collagen, (b) the endothelium secretes nitric oxide (NO) and a prostaglandin (PGI2), both of which act as vasodilators and inhibitors of platelet aggregation, (c) CD39 is present, which is an enzyme found on endothelial cells that can break down ADP to AMP
• Formation of a platelet plug

  1. Von Willebrand's factor (VWF) binds to both collagen and platelets so anchoring them firmly to the injured site to form a platelet plug

  2. Collagen activates the platelets which means that the platelets degranulate to release secretory granules containing ADP and thromboxane A, both of which attract more platelets to the site and makes the platelets very sticky
• Coagulation of blood: intrinsic pathway

  1. Collagen (which is negatively charged) is exposed to blood plasma which then activates an enzyme protease within the plasma called factor XII

  2. This sets off a cascade of enzyme mediated conversions of inactive to active plasma proteins which feed into a common pathway which end with the production of fibrin
• Coagulation of blood: extrinsic pathway

  1. Damaged tissue releases tissue thromboplastin (not normally found in the blood) and activates factor VII

  2. The two pathways then unite to form a common pathway to create thrombin, an enzyme that converts soluble fibrinogen into insoluble fibrin polymers, which become part of the clot
• The fibrin polymers weave through the platelet plug to trap red blood cells within their mesh
• Recombinant tissue plasminogen activator (t-PA)

  Activates plasminogen which dissolves clots. This was made by inserting human t-PA genes into bacteria
• Sodium citrate or EDTA


  Chelate (bind to) calcium ions, which are needed by many blood clotting factors
• Heparin


  Prevents clotting and activates anti-thrombin III that in turn inactivates thrombin
• Alteplase


  A recombinant tissue plasminogen activator (rt-PA) used as a thrombolytic agent that cleaves plasminogen to form plasmin, an enzyme involved in the degradation of fibrin clots
• tPA (tissue plasminogen activator) can be used to rapidly dissolve a clot in a blocked blood vessel. This process is called fibrinolysis