Primary/Secondary Homeostasis

• haemostasis refers to the physiological mechanisms that halt blood loss from damaged blood vessels
• On average there are between 9-12 pints of blood in the body - women have 9 pints on average and men have 12 pints.
• ~60,000 miles of blood vessels in the human body, could wrap around the earth more than twice.
• The basic but vital functions of the blood are to transfer O2 and nutrients (e.g. glucose, amino acids etc.) to tissues, and transfer waste, CO2 and lactic acid away from the tissues - more complex functions/roles are communication (e.g. hormones), response to injury, wound healing and coagulation.
• there are 6 main stages of haemostasis:

  1. endothelium (non-thrombogenic surface)
  2. blood platelets (primary haemostatic response) - bleeding time
  3. blood clotting (secondary haemostatic response - fibrin-high tensile stregnth
  4. clot retraction (fully contracted - firm clot)
  5. tissue regeneration
  6. fibrinolysis
• the endothelium is 1 cell in width. and plays a central and vital role in haemostasis.
• Blood platelets (thrombocytes) can sense/detect damage to the endothelium - these are minuscule amounts roughly 2-3 uM in diameter
• Primary haemostatic response is when platelets can detect collagen in exposed sub-endothelium, and bind to the collagen which activates the platelets that induce aggregation and forms a primary plug.
• an increase in shear force can lead to the primary platelet plug travelling into a smaller vessel that can lead to an embolism, (e.g. a pulmonary embolism).
• The secondary haemostatic response is the blood clotting cascades (coagulation), where fibrin is deposited between the platelets and strengthens/reinforces the platelet primary plug - this is especially important in larger vessels which are under higher shear forces
• Bleeding time test - measures platelet function due to the formation of a platelet plug and is a reliable method to evaluate clinical bleeding in uremic patients.
• for a bleeding time test a small cut of a certain depth and size is made using an automatic device
• The tertiary haemostatic response is clot retraction where platelets can retract to condense the platelet plug and pull it into the blood vessel.
• during the tertiary haemostatic response, the clot is still leaky and serum can leak, however, the clot becomes denser and water-tight, preventing serum loss.
• the quaternary haemostatic response is tissue regeneration where the release of growth factors from platelet alpha granules occurs - PDGF (SMP), VEGF-A and VEGF-C (angiogenesis)
• Adventitia is the outermost layer of the blood vessel – composed of collagen and fibroblasts to shield and anchor the vessel to surrounding tissue, with a similar function to serosa (epithelial cells)
• Smooth muscle (media) mediates vasodilation and constriction in response to various mediators.
• Sub-endothelial matrix contains collagen a structural protein.
• Valves within veins are covered by endothelium.
• the endothelium has a central role in blood clotting.
• Structure of the Artery and Vein

  A) endothelium

  B) sub-endothelial matrix

  C) smooth muscle (media)

  D) adventitia

  E) serosa (endothelial cells)

  F) valve

  
• Normal healthy endothelium is a non-thrombogenic surface and limits the haemostatic response by:

  1. inhibiting platelet activation
  2. inhibiting coagulation
  3. promoting anticoagulation
  4. promoting fibrinolysis
• Under steady-state healthy conditions, the endothelium is a non-thrombogenic surface with eNOS (endothelial nitric oxide)and COX1 constitutively expressed by endothelium cells.
• both nitric oxide (NO) and prostacyclin (PGl2) are released from the endothelium cells ensuring blood flow continues
• nitric oxide is produced as a by-product of the conversion of L-arginine to citrulline, producing NO as a by-product
• prostacyclin synthesis starts with:

  • membrane phospholipids being acted upon by phospholipase (PLA2) to create arachidonic acid (AA)
  • which gets acted upon by cyclooxygenase (COX1) to create endoperoxides (PGH2/PGG2)
  • which is further acted upon by prostacyclin synthase to create prostacyclin (PGl2)
• PGI2 and NO inhibit platelet activation by causing an increase in cAMP/cGMP leading to a PKA and PKG activation and preventing degranulation.
• endoperoxides within the platelet are acted upon by thromboxane synthase to create TXA2, which acts with PGl2 upon adenylate synthase (AC) to convert ATP to cAMP
• cAMP in platelets are further acted upon by phosphodiesterase (PDE) to create AMP which causes a decrease in Ca2+
• platelet von Willebrand factor interaction is used to form the initial obstruction to blood loss (platelet adhesion)
• platelets enable platelet-platelet interactions (aggregation) to propagate the thrombus
• Platelets provide a negatively charged lipid surface to support coagulation
• platelets deliver various haemostatically active molecules to enhance their local concentration
• platelets localize thrombus formation
• platelets promote vasoconstriction and vessel repair
• platelets maintain the integrity of endothelial cell junctions
• Role of platelets in haemostasis:

  1. platelet adhesion
  2. aggregation
  3. negatively charged lipid surface
  4. localize thrombus formation
  5. promote vasoconstriction and vessel repair
  6. maintain integrity of EC junctions
• Megakaryocytes differentiate from hematopoietic stem and progenitor cells (HSPC) in the bone marrow
• As megakaryocytes mature in the bone marrow, the nucleus becomes multilobed via endomitotic synchronous replication (DNA replication in the absence of nuclear or cytoplasmic division) and the cytoplasm undergoes an increase in volume
• an important feature of the mature megakaryocyte is the formation of an elaborate demarcation membrane system (DMS)