Blood Composition and Erythrocytes

Blood Composition

• Blood is a connective tissue, mesenchymal in origin.
• It's stained using a neutral stain like Leishman's stain (eosin + methylene blue dissolved in methyl alcohol).
• Blood cells comprise 45% of blood.
• Plasma makes up the remaining 55% of blood.
• The major component of plasma is water (90%).

Erythrocytes (Red Blood Cells)

• Originate in red bone marrow.
• Non-nucleated.
• Biconcave disc-shaped in side view; rounded in top view.
• Thickness: 2.2 µm at periphery, 0.8 µm at center.
• RBCs adhere to each other in slow circulation, forming stacks known as rouleaux. This is a temporary event.
• Surface tension causes these RBC stacks to sediment more easily, increasing Erythrocyte Sedimentation Rate (ESR).
• ESR increases with inflammation.
• Abnormal shapes include spherocytes (spherical), ovalocytes (oval), sickle cells (crescent-shaped), and poikilocytes (pear-shaped).
• These abnormal shapes are due to changes in the cell membrane or hemoglobin (Hb).
• Abnormal shapes are more fragile and prone to hemolysis, which leads to anemia.

Blood Cell Size and Color

• Normal RBC diameter: 6-9 µm, mean 7.5 µm.
• Abnormal sizes include microcytes (less than 6 µm) and macrocytes (more than 9 µm).
• Anisocytosis refers to different sizes of RBCs.
• RBCs appear colored due to hemoglobin (Hb).
• With Leishman's staining, RBCs appear acidophilic.
• Mature RBCs lack a nucleus and organelles.

Blood Cell Count

• Normal erythrocyte count in males: 5-5.5 million/mm³.
• Normal erythrocyte count in females: 4.5-5 million/mm³.
• Newborn babies have the highest RBC count, which then decreases over time.
• Anemia is characterized by a reduced number of RBCs below 4 million/mm³, or decreased hemoglobin concentration.

Polycythemia

• Increase in RBC count above 6 million/mm³ is called polycythemia.
• Polycythemia can be physiological (e.g., high altitudes, exercise, newborns) or pathological (e.g., chronic lung or heart diseases).

Erythrocyte Lifespan and Fate

• RBCs have a lifespan of about 120 days.
• Old RBCs are phagocytosed by macrophages in the liver, bone marrow, and spleen.
• Iron from broken-down RBCs is reused for new RBC production.
• Bile pigments are excreted as waste.

Erythrocyte Structure-Function Adaptations

• Plasticity (flexible shape) allows RBCs to squeeze through narrow vessels.
• Lipoproteins in RBC membranes are selective for gas exchange.
• Biconcave shape increases surface area for gas exchange.
• Rounded edges aid in easy flow through branched blood vessels (capillaries).
• Lack of nucleus/organelles frees up space for hemoglobin.
• Encased enzymes such as Hb reductase and carbonic anhydrase to bind/carry CO₂.

Leucocytes or White Blood Cells (WBCs)

• True cells having nuclei and organelles.
• Total WBC count: 4,000-11,000 cells/mm³.
• Differential count: percentage of each type of WBC.
• Abnormalities in total count include leukocytosis (> 11,000/mm³) due to physiological (pregnancy, exercise) or pathological (infection) reasons.
• Leukopenia (< 4,000/mm³) can occur due to x-rays, some antibiotics (chloramphenicol), and diseases like typhoid fever or influenza.

Granular Leucocytes

• Neutrophils (60-70%): Multilobed nucleus; numerous, fine, pale granules. First line of defense against bacteria via phagocytosis. Contain specific granules like phagocytin, lysozyme, and collagenase.
• Eosinophils (1-4%): Bilobed or horse-shoe-shaped nucleus; large, acidophilic granules. Related to parasitic infections and allergic reactions.
• Basophils (0-1%): Irregular/segmented nucleus; large basophilic granules; release histamine and heparin.

Non-Granular Leucocytes

• Lymphocytes (20-30%): Small to large; large nucleus with thin rim of cytoplasm. Important role in immunity. Three subtypes: cytotoxic (CD8), helper (CD4), regulatory/suppressor cells. Also includes NK cells.
• Monocytes (3-8%): Large, eccentric, kidney-shaped nucleus; pale basophilic cytoplasm with lysosomes. Differentiate into macrophages in tissues, involved in phagocytosis.

Comparison of Basophils and Mast Cells

• Basophils have a shorter lifespan and nucleus that are segmented or irregularly shaped.
• Mast cells have a longer lifespan, and a more rounder nucleus.
• Basophils granules are metachromatic.
• Mast cells stain with antibodies for relevant receptors.