22. Erythrocytes (RBC) — Number and Functions, ESR, Hemoglobin, Erythrocyte Indices, Iron Metabolism, Hemolysis, Control of Erythropoiesis and RBC Count

1. Erythrocytes (RBC) — Number and Functions

Characteristics of RBCs:

• Most abundant blood cells

• Biconcave disks (~7.5 microns diameter)

• Shape allows easy passage through capillaries and efficient oxygen diffusion

Number of Erythrocytes:

• Female: 3.9 - 5.3 × 10¹² / litre

• Male: 4.5 - 6.0 × 10¹² / litre (higher due to testosterone stimulation)

Functions of Erythrocytes:

1. Transport of Oxygen: Hemoglobin in RBCs binds oxygen in the lungs (forming oxyhemoglobin) and releases it to tissues.

2. Transport of Carbon Dioxide: Hemoglobin binds CO₂ (carbaminohemoglobin). About 63% of CO₂ is transported as bicarbonate, catalyzed by carbonic anhydrase inside RBCs.

3. Acid-Base Balance: RBCs regulate hydrogen ion concentration helping maintain blood pH.

2. Erythrocyte Sedimentation Rate (ESR)

• Measures how fast RBCs settle in anticoagulated blood in 1 hour.

• Determined by plasma protein ratio (albumin/globulins) and RBC number.

• Normal albumin:globulin ratio = 2.5:1.5

• Increased globulins reduce negative charge on RBCs → increased ESR (due to agglutination).

Normal ESR values:

• Men ≤ 50 years: ≤ 15 mm/h

• Men > 50 years: ≤ 20 mm/h

• Women ≤ 50 years: ≤ 22 mm/h

• Women > 50 years: ≤ 28 mm/h

Clinical significance:

• ESR ↑ in infections, inflammation, malignancies, myocardial infarction, pregnancy, age > 50

• ESR ↓ in polycythemia, sickle cell anemia, chronic fatigue syndrome

3. Hemoglobin

• Protein in RBCs made of 4 subunits, each with a heme group (contains Fe²⁺) binding 1 O₂ molecule → 4 O₂ per hemoglobin molecule.

• Hemoglobin changes shape depending on oxygen binding (affinity changes).

• : 4 polypeptide (globin) chains (α, β, γ, δ).

Types of hemoglobin:

• Normal: HbA (2α, 2β) - adult, HbF (2α, 2γ) - fetal, HbA2 (minor adult form)

• Abnormal: HbS (sickle cell disease), HbC (poor oxygen carrier)

Normal hemoglobin levels:

• Men: 160 ± 20 g/l

• Women: 140 ± 20 g/l

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4. Erythrocyte Indices

Used to classify types of anemia by RBC size and hemoglobin content:

Index	Formula	Normal Range	Meaning
MCH (Mean Cell Hemoglobin)	Hemoglobin (g/l) / RBC number (×10¹²/l)	28–33 pg	Average hemoglobin per RBC
MCHC (Mean Cell Hemoglobin Concentration)	Hemoglobin (g/l) / Hematocrit (l/l)	300–360 g/l	Average concentration of hemoglobin in RBCs
MCV (Mean Cell Volume)	Hematocrit (l/l) / RBC number (×10¹²/l)	82–98 fL	Average volume of RBC

ERYTHROCYTE IS A CELL SO INDICES ALWAYS STARTS WITH MEAN CELL … smth smth

5. Iron Metabolism

• Total body iron ≈ 4 grams

• Essential for hemoglobin, myoglobin, cytochromes, enzymes (peroxidase, catalase)

• Iron forms: heme (Fe²⁺ in hemoglobin) and non-heme iron

• Fe²⁺ absorbed in duodenum → transported by ferroportin → oxidized to Fe³⁺ → binds transferrin → stored in liver as ferritin

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6. Hemolysis

• Premature destruction of RBCs

• Types:

  • Alpha: Partial destruction (hemoglobin reduced but RBC intact)

  • Beta: Complete destruction (RBC ruptures)

  • Gamma: No destruction

Osmotic resistance: Measure of RBC’s resistance to hemolysis in varying saline dilutions

• Minimal hemolysis: 0.42–0.44% NaCl

• Maximal hemolysis: 0.32–0.34% NaCl

Sites and effects:

• Intravascular hemolysis: Release of hemoglobin into plasma (hemoglobinemia)

• Extravascular hemolysis: Damaged RBCs removed by spleen; hemoglobin processed by liver, bone marrow, macrophages

7. Control of Erythropoiesis and Erythrocyte Count

• Erythropoiesis = formation of mature RBCs from hematopoietic stem cells

• Stimulated by: Erythropoietin (EPO), secreted by kidneys in response to low oxygen levels (hypoxia) → stimulates red cell precursors → increases RBC production (negative feedback loop)

• Inhibited by: Estrogens

• Requires: Vitamin B12 and folic acid (B9) for DNA synthesis and RBC maturation

Factors affecting RBC count:

• Increase: stress, high altitude, after meals

• Decrease: pregnancy, early neonatal period (physiological jaundice)