Blood Composition and Extracellular Matrix

• Blood is a specialized connective tissue.
• Blood consists of cells suspended in plasma

Composition

• Cells: Includes red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes).
• Plasma: The fluid portion that carries the formed elements
• Blood when separated can be found in test tubes and the extracellular matrix

Blood Outside the Circulatory System

• The extracellular matrix (ECM) surrounds blood vessels.
• Plasma proteins interact to form a clot, which includes formed elements and a yellowish serum liquid.
• Serum includes growth factors and proteins, making its biological properties distinct from plasma.

Separation of Blood Components

• Blood collected with anticoagulants can be separated via centrifugation into three layers:
• Bottom Layer: Red blood cells (erythrocytes); Also know as the hematocrit layer
• Middle Layer: Buffy coat that includes white blood cells and platelets.
• Top Layer: Consists of plasma.
• Plasma volume accounts for 55% of blood.
• Formed elements include red blood cells, white blood cells, and platelets.

Plasma

• Plasma is the liquid component of blood, making up about 55% of its total volume.

Main Components

Plasma Proteins

• Albumin: Produced in the liver and maintains osmotic pressure.
• Globulins: Involved in transport and immunity.
• Fibrinogen: Participates in blood clotting.

Other Dissolved Substances

• Nutrients: Includes glucose and amino acids.
• Respiratory Gases: Oxygen (O2) and carbon dioxide (CO2).
• Nitrogenous Wastes: Includes urea.
• Inorganic Ions: Electrolytes such as sodium and potassium.

BLOOD CELLS

• Blood cells can be studied histologically using blood smears.
• Blood smears are prepared by spreading a thin layer of blood onto microscope slides.
• Acidic dyes(eosin) and basic dyes (methylene blue) are used to stain the smears.
• These stains highlight cellular structures such as nuclei, cytoplasm, and specific granules.

ERYTHROCYTES (RED BLOOD CELLS, RBCS)

Structure

• Biconcave, flexible discs, approximately 7.5 µm in diameter.
• They lack nuclei and organelles but are filled with hemoglobin.
• Plasmalemmas contain glycoproteins, which determine blood group antigens (e.g., ABO system).
• Supported by a submembranous cytoskeletal network of spectrin and actin.

Functions

• Transport oxygen (O2) via hemoglobin, forming oxyhemoglobin.
• Transport carbon dioxide (CO2) by forming carbaminohemoglobin.
• The biconcave shape enhances gas exchange efficiency.

Lifespan

• Approximately 120 days in circulation.
• Old or damaged (senescent) RBCs are phagocytosed by macrophages in the spleen, liver, and bone marrow.

Medical Applications

• Anemia: A decrease in RBC count or hemoglobin concentration, leading to fatigue, pallor, and shortness of breath.
• Sickle Cell Anemia: A genetic disorder causing RBC deformation, leading to capillary blockage and reduced oxygen delivery.
• Polycythemia: an increase in RBC count, which can occur as an adaptation to high altitudes but may also increase blood viscosity.

LEUKOCYTES (WHITE BLOOD CELLS, WBCS)

• Leukocytes are classified into granulocytes and agranulocytes based on the presence of cytoplasmic granules.

Granulocytes

• Neutrophils
• Eosinophils
• Basophils

Agranulocytes

• Lymphocytes
• Monocytes

A. Granulocytes (Neutrophils)

Proportion

• 50-70% of circulating WBCs.

Structure

• Multi-lobed nucleus with cytoplasm containing azurophilic and specific granules.

Functions

• First responders to bacterial infections.
• Phagocytose bacteria, releasing the following to fuse their granules
• Myeloperoxidase
• Lysozyme
• Defensins
• Release cytokines and chemokines to recruit other immune cells.

Lifespan

• 6-8 hours in the blood and 1-4 days in tissues.

Medical Note

• Neutrophil deficiencies result in recurrent bacterial infections.

Eosinophils

Proportion

• 1-4% of WBCs.

Structure

• Consists of a bilobed nucleus.
• Large, acidophilic granules containing major basic protein (MBP) and eosinophilic peroxidase.

Functions

• Combat parasitic infections by releasing toxic proteins.
• Modulate allergic responses by degrading histamine and leukotrienes.
• Phagocytose antigen-antibody complexes.

Medical Note

• Eosinophilia (increase in eosinophils) is associated with allergies (e.g., asthma) and parasitic infections.

Basophils

Proportion

• Less than 1% of WBCs.

Structure

• Consists of Irregularly lobed nucleus, often obscured by large basophilic granules containing histamine, heparin, and leukotrienes.

Functions

• Participate in allergic reactions by releasing inflammatory mediators.
• Have surface IgE receptors that trigger degranulation upon allergen binding.

B. Agranulocytes (Lymphocytes)

Proportion

• Consists of 20-40% of WBCs.

Types of Lymphocytes

• Cells produce antibodies.
• Cells mediate cellular immunity.
• Natural Killer (NK) Cells: Kill virus-infected and tumor cells.

Functions

• Recognize antigens.
• Produce cytokines to regulate immune responses.

Medical Note

• Lymphomas involve abnormal lymphocyte proliferation.

Monocytes

Proportion

• Consists of 2-8% of WBCs.

Structure

• Consists of Large cells (12-20 μm).
• Kidney-shaped nucleus with basophilic cytoplasm containing lysosomal granules.

Functions

• Precursors to macrophages, osteoclasts, and dendritic cells.
• Phagocytose pathogens and cellular debris.
• Present antigens to lymphocytes to activate adaptive immunity.

Medical Note

• Chronic inflammation involves continuous monocyte recruitment, leading to tissue damage.

Platelets (Thrombocytes)

Structure

• Consists of Small, discoid, anucleate fragments (2-4 μm).
• Derived from megakaryocytes. Contain cytoskeleton and central granules with clotting factors.

Functions

1. Primary Aggregation

• Adhere to damaged endothelium, forming a platelet plug.

2. Secondary Aggregation

• Release ADP and glycoproteins to recruit additional platelets.