Blood: Functions and Composition

Connective Tissue

• Connective tissue is formed by two components: cells and extracellular matrix (fibers and ground substance)
• Classified into four categories: connective tissue proper, cartilage, bone, and blood

Blood

• Volume: 5 L in an adult
• Location: within the closed circulatory system
• Functions:
  • Distributing vehicle
  • Transporting O2, CO2, metabolites, and hormones
  • Participates in the regulation of body temperature and acid-base and osmotic balance
  • Chief defenses against infection
• Components: formed elements and plasma

Plasma and Serum

• Plasma:
  • Aqueous solution
  • Translucent, yellowish, somewhat viscous
  • Components: 90% water and 10% of various substances
• Serum:
  • Clear yellow liquid
  • Forms when blood clots and is centrifuged

Blood Cells

Erythrocyte (Red Blood Cell, RBC)

• Structure:
  • Mean diameter: 7.5 μm
  • Biconcave discs in shape
  • Lack nucleus and organelles
  • Packed with O2-carrying protein hemoglobin (HB)
• Function: Gas transporting capability
• Quantity:
  • Normal concentration: 3.9-5.5 million/μL (female), 4.1-6.0 million/μL (male)
  • Normal concentration of HB: 10.5-13.5g/100ml (female), 12-15g/100ml (male)
• Life cycle:
  • Mean life: 120 days
  • Mature RBCs are worn out and removed by macrophages
  • Newborn RBCs: reticulocytes
  • Origin: bone marrow

Leukocyte (White Blood Cell, WBC)

• Classification:
  • Granular leukocytes
  • Agranular leukocytes
• Quantity:
  • Total count: 6000-10,000/μl
  • Ratios:
    • Neutrophils: 57-67%
    • Basophils: 0-0.75%
    • Eosinophils: 1-3%
    • Monocytes: 3-7%
    • Lymphocytes: 25-33%

Neutrophils

• Structure under LM:
  • 10-12μm in diameter
  • Nucleus consisting of 2-5 lobes
  • Cytoplasm contains abundant neutral granules
• Structure under EM:
  • Specific granules:
    • Neutral granules containing lysozyme, phagocytin, and alkaline phosphatase (ACP)
    • Azurophilic granules containing lysosomes
• Functions:
  • Kill and phagocytose bacteria
  • The first leukocytes to arrive at sites of infection
  • Actively pursue bacteria cells using chemotaxis
  • Release chemokines to attract other leukocytes
• Life span:
  • Short-lived cells with a half-life of 6-8 hours in blood
  • A life span of 1-4 days in connective tissue

Eosinophils

• Structure under LM:
  • 10-15μm in diameter
  • Nucleus consisting of 2-3 lobes
  • Cytoplasm contains large, distinctive, and evenly distributed acidic granules
• Structure under EM:
  • Specific granules containing major basic proteins (MBP), 50%
• Functions:
  • Anti-anaphylaxis and anti-parasitic infections
  • Killing parasitic worms
  • Moderating inflammatory and immune reactions
  • Phagocytising antigen-antibody complexes
  • Inactivating leukotrienes

Basophils

• Structure under LM:
  • 12-15μm in diameter
  • Nucleus divided into irregular lobes
  • Cytoplasm contains purple staining basic granules, which are not uniform in size and unevenly distributed
• Structure under EM:
  • Specific granules containing heparin, sulfated GAGs, and histamine
• Functions:
  • Immediate hypersensitivity reactions, such as allergic reactions to wasp stings
  • Delayed hypersensitivity reactions, similar to mast cells

Monocytes

• Structure under LM:
  • The largest of the WBCs, 12-15μm in diameter
  • Oval, horseshoe, or kidney-shaped nucleus
  • Cytoplasm is basophilic and contains very fine azurophilic granules (lysosomes)
• Function:
  • Precursors of the mononuclear phagocytic system
  • All monocyte-derived cells are antigen-presenting cells
  • Play a role in immune defense of tissue

Lymphocytes

• Classification:
  • According to size:
    • Small lymphocytes (6-8 μm)
    • Medium lymphocytes (9-12μm)
    • Large lymphocytes (13-20μm)
  • According to surface molecules:
    • T cells (thymus-dependent lymphocytes) for cell immunity
    • B cells (bone marrow-dependent lymphocytes) for humoral immunity
    • NK cells (natural killer cells) for killing tumors and cells infected by viruses
• Location:
  • Blood, lymphatic vessels, lymphoid tissues, and organs
  • Lymphocytes in the blood are small (90%), in a resting state
• Life span:
  • A few days
  • Many years
• Structure:
  • Nucleus is round and big, with condensed chromatin
  • Cytoplasm is scanty, slightly basophilic, and contains azurophilic granules
• Functions:
  • Participate in immune reactions
  • Lymphocytes are the main functional cells of the immune system

Platelets (Thrombocytes)

• Origin:
  • Fragments of cytoplasm detached from megakaryocytes in the bone marrow
• Quantity:
  • 150,000 to 400,000/μL
• Life span:
  • 10 days
• Structure under LM:
  • Non-nucleated, disk-like cell fragment, 2-4μm in diameter
  • Peripheral light blue-stained zone (hyalomere)
  • Central zone containing purple granules (granulomere)
• Function:
  • Promote blood clotting and help repair gaps in the walls of blood vessels by releasing blood-clotting-related factors