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Introduction

  • Blood is a specialised form of connective tissue
    • Derived from the mesoderm
      • Mesoderm –a germ layer that forms in the embryos of animals
  • Blood is made up of erythrocytes and leukocytes suspended in plasma
    • Also contains:
      • platelets (thrombocytes)
      • chylomicrons – small lipid droplets
      • immunoglobulins – specific proteins (antibodies)
  • the average adult has 5 litres of blood

Functions of blood

  • Transport of various things
    • Gases (oxygen/nitrogen) to the lung and body tissues
    • Nutrients, electrolytes
    • Wastes (uric acid)
    • Hormones (insulin)
    • Antibodies
  • Protection against pathogenic agents using the immune system
  • Control of body temperature by changes in circulation
    • Vasoconstriction, vasodilation
  • Maintenance of pH balance using buffers
    • Allows for enzyme activity
  • Maintenance of fluid balance

Contents of blood

  • if an anticoagulant such as heparin is added to prevent clotting and then blood centrifuged, a layers form
    • top layer is plasma – 55%
    • thin middle white layer is the buffy coat composed of leukocytes and platelets – 1%
    • lowermost layer is red and thick containing red blood cells – 45%
      • this percentage is important, known as the hematocrit
      • <30%, possible anemia
      • >60%, polycythemia

Plasma

  • A fluid that transports and serves as the solvent for the body’s nutrients
    • Slightly alkaline
    • Transports dissolved gases (eg CO2), electrolytes, waste, hormones, enzymes
  • Nutrients are mostly made up of proteins
    • Most common protein is albumin
      • Maintains the osmotic pressure of the blood on the blood vessel wall
    • Others are: globulins (alpha, beta, gamma and immuno), and fibrinogens
    • Immunoglobulins are antibodies that recognise foreign molecules (antigens) and prevent invasion of harmful organisms and substances into the body

Erythrocytes

  • Flexible membranous sacs of haemoglobin
    • Transport O2 from the lungs to the cells
    • Return CO2 to the lungs to be exhaled
    • Flexible so that it can travel through small capillaries
  • Has no nucleus or organelles to increase the amount of oxygen it can carry
    • Dense, amorphous appearance, its plasma membrane is a trilaminar (three-layered) unit membrane about 7nm thick
    • Glycocalyx (a polysaccharide layer) covers the external membrane surface
  • Contains haemoglobin which binds with oxygen, carbon dioxide and carbon monoxide allowing transportation
  • Approximately 8.5μm*2.5μm in a round biconcave disk shape
    • About 5 million RBCs per cubic millimetre
      • High altitudes and carbon monoxide increase the number of RBCs
    • Lifespan is about 120 days
      • Senile cells are destroyed by phagocytosis (the engulfing of particles) in the liver and bone marrow or the spleen
        • Haemoglobin is degraded and iron released into the bone marrow for reuse
        • Nonferrous components are converted in the liver into the bile pigment bilirubin which is excreted in bile

Reticulocytes

  • Using conventional dyes (Wright’s or Giemsa’s) 1-2% of larger cells show a slight basophilic (a basic stasin) tint while all RBCs stain a clear pink or copper colour
    • These are immature RBCs (reticulocytes) recently released into the blood from the red bone marrow

Leukocytes, ie WBCs

  • Colourless and contain no pigment in their fresh state
    • Have a white appearance when centrifuged
  • Have a nucleus, cytoplasmic organelles
  • Are made in the red bone marrow and lymphoid tissues
    • Use the blood for transport to specific areas
      • Pass through capillary walls by ameboid movement into tissue spaces
  • Functions:
    • Phagocytosis
    • Immune reactions
    • Wound repair
    • Control of infections
  • About 5000-10000 per cubic millimetre
    • Count rises in acute infections
    • >12000 leukocytosis
    • <5000 called leukopenia
  • 2 classifications
    • granular (more than one lobe per nucleus – polymorphonuclear)
    • nongranular (no lobulation – mononuclear)
  • several types: neutrophils, eosinophils, basophils, lymphocytes, monocytes

Neutrophils - granular

  • Most common WBCs – 60-70%
  • Can be found adhered to the walls of blood vessels
    • Others escape through the capillaries into the tissue spaces/internal cavities
      • Act here as phagocytes – ingesting bacteria and pyrogenic (pus forming) cocci
  • Are attracted to sites of infection/inflammation by chemotaxis and move by ameboid movement to required areas
    • Here, they engulf bacteria and release hydrolases that destroys surrounding cells
  • Shape/contents:
    • 10-14 μm in diameter with a multilobed nucleus
      • 3-5 ovoid lobes connected by strands of chromatin
    • the cytoplasm is filled with small, uniform granules that contain alkaline phosphatase and bactericidal proteins – phagocytins
      • these destroy bacteria ingulfed by the neutrophil
    • inside also are lysosomes containing peroxidase, acid phosphatase and other enzymes
    • cytoplasmic organelles are represented by a small golgi complex and a few mitochondria
    • Lifespan of 6hours – a few days

Eosinophils - granular

  • make up 2-4% of all leukocytes
  • Orange-red granules of about 1μm in diameter
    • Are lysosomes containing peroxidase, histaminase and hydrolytic enzymes
    • Also has a bilobed nucleus
  • 11-14μm, slightly larger than neutrophils
    • number increases during parasitic infestations, chronic infections and allergic reactions (esp. digestive, respiratory tracts)
  • have sluggish ameboid movement
    • however, can freely move through capillary walls into tissue spaces
      • accumulate in large numbers, esp in the intestinal mucosa
  • don’t actively destroy bacteria
    • actively phagocytise antigen antibody complexes
  • live 8-12 days

Basophils - granular

  • 0.5%-1% of WBCs
  • smaller than other leukocytes – 8-10μm
  • distinguished by:
    • have many round, deep basophilic granules of various sizes
      • granules are scattered all over the large bilobed nucleus
  • contain electron-dense particles containing: serotonin, histamine, heparin
  • precise function is unclear, but collect at inflammation sites
    • produce about 50% of histamine in the blood
    • play some role in allergy control
  • live a few hours to a few days

Lymphocytes – nongranular

  • most numerous of the nongranular leukocytes 20-30% of WBCs
  • 5-14μm
  • spherical cells containing:
    • a large, round, dark-staining nucleus surrounded with a blue cytoplasm
    • may have a few azurophilic (lysosomal) granules
    • ribosomes that cause a strong basophilia
    • some mitochondria are present
    • the golgi complex is small and inactive
  • when the lymphocyte is small, its basophilic nucleus almost fills the cell
    • thus only a thin rim of purplish blue cytoplasm is visible
  • lymphocytes circulate through the spleen, thymus, lymph nodes and lymphoid tissue
  • are used in an immune response to direct attack, and also can produce antibodies
  • live hours to years

Monocytes - nongranular

  • spherical, large agranulocytes
    • Kidney, horseshaped nucleus
      • 2-3 nucleoli present
    • 12-20μm diameter
    • bluish gray cytoplasm containing a few azurophilic granules
    • prominent nucleoli and heterochromatin clustered at the inner surface of the nuclear membrane
    • some elongated mitochondria
    • an active golgi complex
  • 2-6% WBCs
  • Macrophages – phagocytosis, engulf things
  • Distinguished by:
    • Large size
    • Indented and lighter staining of nucleus
    • Abundance of cytoplasm which is a blue-gray colour
    • More azurophilic granules
  • Live for months

Platelets

  • Also known as thrombocytes
    • Fragments of cytoplasm
    • Formed by the pseudopodia of megakaryocytes (giant cells in the red bone marrow)
    • Lack nuclei
    • Roughly biconvex disks about 2-4μm
    • 200 000-400 000/cubic millimetre
    • removed in the lungs and spleen by phagocytosis
    • have a central core (granulomere) which contains basophilic granules and a light blue peripheral zone (the hyalomere)
    • contain a few mitochondria
    • under electron microscope, electron dense round, membrane bound granules can be seen
  • function:
    • sealing small tears in blood vessels and are required for blood clotting
  • live 5-10 days