Red blood cells (RBCs), or erythrocytes, are the most numerous formed elements of blood. Their red color is due to hemoglobin, the protein responsible for carrying oxygen and contributing to carbon dioxide transport. Mature human RBCs are specialized cells designed mainly for the transport of respiratory gases.
Normal RBC Count
The normal RBC count varies with age, sex, altitude and physiological conditions. In healthy adults, it is approximately 4.0–5.5 million cells/µL of blood, with the average count generally higher in males than females.
Morphology of Red Blood Cells
Shape
A normal RBC is a biconcave disc. The center is thinner than the peripheral region. This shape increases the surface area available for gas exchange, allows rapid diffusion of oxygen and carbon dioxide, and provides flexibility when RBCs pass through narrow capillaries.
Size
| Feature | Approximate value |
|---|---|
| Diameter | About 7–8 µm |
| Central thickness | About 1 µm |
| Peripheral thickness | About 2 µm |
| Mean corpuscular volume (MCV) | About 80–100 fL |
Structure
Mature human RBCs are anucleate and lack mitochondria, Golgi apparatus and other major organelles. Therefore, they obtain energy mainly through anaerobic glycolysis. Their membrane is supported by a cytoskeleton containing proteins such as spectrin and actin, which help maintain the characteristic biconcave shape and flexibility.
Properties of RBCs
- Rouleaux formation: RBCs can stack like coins, particularly when plasma protein concentrations are increased.
- Suspension stability: Under normal conditions, RBCs remain suspended in circulating blood.
- Specific gravity: RBCs have a higher specific gravity than plasma.
- Hematocrit: RBCs normally occupy roughly 40–45% of blood volume in adults, although values vary with sex and physiological state.
Lifespan of RBCs
The average lifespan of a normal RBC is approximately 120 days. With aging, RBC membranes become less flexible and the cells are progressively removed from circulation.
Most aged RBCs are removed by macrophages of the spleen, liver and bone marrow. For this reason, the spleen is traditionally described as an important site of RBC destruction. Hemoglobin is broken down into globin, iron and heme-derived pigments. Iron is conserved and reused, while heme is ultimately converted to bilirubin and transported to the liver for further processing and excretion.
Functions of Red Blood Cells
- Oxygen transport: Hemoglobin binds oxygen in the lungs and carries it to tissues.
- Carbon dioxide transport: RBCs participate in CO₂ transport, including conversion of CO₂ to bicarbonate through carbonic anhydrase.
- Acid-base regulation: Hemoglobin acts as an important buffer in maintaining blood pH.
- Blood group determination: RBC membranes carry important antigens, including ABO and Rh antigens.
Variations in RBC Number
An increased RBC count is called polycythemia, while a reduced RBC mass is commonly associated with anemia. RBC production is strongly influenced by erythropoietin released mainly by the kidneys in response to reduced tissue oxygen availability.
Physiological increases may occur with exposure to high altitude because of reduced oxygen availability. Exercise and other conditions can also produce temporary changes in circulating RBC concentration. During pregnancy, plasma volume increases considerably, producing hemodilution and a relative fall in RBC concentration.
Variations in RBC Size
- Microcytes: RBCs smaller than normal, commonly seen in iron deficiency.
- Macrocytes: RBCs larger than normal, commonly associated with megaloblastic conditions.
- Anisocytosis: Marked variation in RBC size within the same blood sample.
Variations in RBC Shape
- Spherocytes: Spherical RBCs with loss of the normal central pallor.
- Sickle cells: Crescent-shaped RBCs seen in sickle cell disease.
- Elliptocytes: Oval or elongated RBCs.
- Crenated cells: Shrinkage and irregular projections, often associated with hypertonic conditions.
- Poikilocytosis: Increased variation in RBC shape.
Structural Abnormalities
Abnormal inclusions may occasionally be seen within RBCs. Important examples include Howell–Jolly bodies, which represent nuclear DNA remnants, and basophilic stippling, which appears as fine basophilic granules within RBCs and may occur in several hematological conditions, including lead toxicity.
High-Yield MBBS Revision
- Normal RBC shape: Biconcave disc
- Average diameter: About 7–8 µm
- RBC lifespan: About 120 days
- Main site of removal: Spleen and other reticuloendothelial tissues
- Main function: Transport of respiratory gases
- Important enzyme in RBCs: Carbonic anhydrase
- Important membrane proteins: Spectrin and actin
- Increased RBC count: Polycythemia
- Decreased RBC mass: Anemia
- Variation in size: Anisocytosis
- Variation in shape: Poikilocytosis
Medical Disclaimer: The information provided in this article is strictly for educational, study, and exam-preparation purposes. It does not constitute professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider for clinical decisions.