Blood

Blood

1. Blood Cells

   • float in the plasma
   • Leucocytes
     • nucleate cells are part of the body's defence system
     • capable of amoeboid movement
     • 1% of blood cells = 7,000 /mL^3 in human blood
     • formed in the red bone marrow
       • after birth, one type proliferates in lymphoid tissue
     • agranulocytes
       • lymphocytes
         • smallest of the leucocytes but second most commonest 30%
         • produced in lymphoid tissue of lymph nodes, spleen, thymus, GIT
         • ultimately derived from lymphocytic stem cells in the bone marrow
           • migrate to the lymphoid tissue before and after birth
         • 2 major types
           • T lymphocytes - responsible for cellular immunity
           • B lymphocytes - responsible for humoral immunity (antibodies)
       • monocytes
         • largest of the leucocytes, moderately common 5%
         • ability to phagocytose invading organisms
         • leave circulation, enlarge and transform in the tissues into macrophages
         • widely distributed in lymph nodes, lung, alveoli, liver, spleen, etc.
     • granulocytes (polymorphonuclear cells)
       • neutrophils
         • commonest leucocyte 62%
         • characteristic lobed nucleus
         • purplish granules in cytoplasm which appear to be lysosomes
         • where tissue is damaged, they leave the capillaries and commence the phagocytosis of damaged cells and invading pathogens
           • pus in wounds is largely composed of dead neutrophils
       • eosinophils
         • acidiphilic granules are considered to be lysosomes
         • only weakly phagocytotic, increase in parasitic infections
         • 1-3%
       • basophils
         • rarest of the leuocytes 0.4%
         • contain basophilic granules
           • function unclear but contain quantities of heparin and histamine - may have a relationship with tissue mast cells involved in allergic reactions
   • Erythrocytes
     • red blood cell corpuscles - highly specialised cells.
       • corpuscle refers to any cell floating freely in a liquid.
     • produced by red bone marrow
       • they are round, anucleate bags of haemoglobin in mammals and nucleate and oval in vertebrates
     • primary function
       • transport oxygen to body tissues and carbon dioxide away from the tissues.
         • survive for ~120 days in the bloodstream before they are broken down and removed by the liver and spleen (2.5 million/second)
           • product of breakdown = bilirubin, eliminated via the bile duct causing the brown colouration of faeces
     • structure
       • biconcave disc shape gives large surface area to volume ratio for gaseous exchange
       • 8.5 μm diameter, 2.5 μm thickness
       • 5.2 mil/mL in adult males; 4.7 mil/mL in adult females
   • Thrombocytes
     • platelets
     • associated with blood clotting
       • function - involved in haemostasis via clot formation
         • complex process brought about by 2 processes. most clotting caused by plasma protein fibrinogen
     • anucleate disc-like cell fragments 2 – 3 μm diameter
     • originate from magakaryoblasts in red bone marrow. on entering the blood stream, they have a life span of ~8 days

2. Haemopoiesis

   • process by which erythrocytes are replaced
   • in the embryo and foetus, there are a number of sites
     • liver, spleen, lymph nodes = early sites
     • as bones develop in the 4/5 month of foetal life, haemopoiesis begins in the red bone marrow cavities
       • almost all blood cells and platelets are produced after birth in the red bone marrow
         • exception of lymphocytes which proliferate after birth in various lymphoid tissues throughout the body
       • red bone marrow found in the ends of long bones (femur) and in flatter bones (ribs, sternum, skull and pelvis)
   • all blood cells are derived from a common pleuripotent stem cell - haemocytoblast
     • give rise to 5 types of unipotential stem cells
       • magakaryoblast
         • platelets
       • proerythroblast
         • erythrocytes
         • haemopoiesis is controlled by erythropoietin released by the kidney in response to hypoxia
           • hormone lasts 1-2 days in circulation and max production of erythrocytes occurs at 5-7 days
           • can be induced by high altitudes or severe blood loss or anaemia
       • lymphoblast
         • lymph nodes
           • lymphocytes
       • myeloblast
         • neutrophils
         • basophils
         • eosinophils
       • monoblast
         • monocytes
           • macrophage

3. Blood Clotting

   • intrinsic - begins with the blood itself and involves platelets. extrinsic - begins with the trauma or damage to the tissues outside of the blood vessels
     • in practice both operate at wound sites or where tissue damage/trauma has occurred
   • a series of plasma proteins (β globulins) called blood clotting factors are activated in a cascade sequence of reactions where the activation of one catalyses the activation of the next.
     • presence of Ca ions - both pathways produce active catalyst, thrombokinase or prothrombin activator
       • in turn converts prothrombin (formed in liver) to thrombin
         • thrombin acts as the enzyme converting soluble fibrinogen to insoluble fibrin
           • fibrin attaches to the damage site and creates a growing matrix of fibrin platelets and trapped blood cells
             • haemostatic plug or blood clot
               • clot formation can occur after 15 sec (extrinsic), 1 - 3 minutes (intrinsic)
             • fibrin polymerises into long threads, contracts after a few minutes and clear fluid leaks from clot - serum
   • inhibition of clotting
     • heparin (anticoagulant) produced by basophils in circulation and by mast cells found in connective tissue especially in lung and liver
     • blood banks remove free Ca ions to prevent clotting (sodium citrate or sodium oxalate compounds). this blood can be stored for weeks in the refrigerator.