Blood is composed of three major types of formed elements: Red Blood Cells (RBCs), White Blood Cells (WBCs), and Platelets. Each of these cell types has distinct characteristics and plays vital roles in maintaining the body’s physiological balance.

1. Red Blood Cells (RBCs)

Characteristics of RBCs

• Shape: Biconcave discs, which increases the surface area for gas exchange.
• Size: Typically about 7-8 micrometers in diameter.
• Nucleus: Anucleate, meaning they lack a nucleus.
• Life Span: Approximately 120 days.
• Hemoglobin Content: RBCs are filled with hemoglobin, a protein that binds oxygen.
• Color: Red due to the presence of hemoglobin.

Functions of RBCs

1. Oxygen Transport: The primary function of RBCs is to carry oxygen from the lungs to the tissues and organs, and return carbon dioxide (CO2) from the tissues to the lungs for exhalation.
   • Hemoglobin, contained within RBCs, binds oxygen in the lungs and releases it in tissues where oxygen concentration is lower.
2. Carbon Dioxide Removal: RBCs transport about 20-30% of carbon dioxide produced by cells back to the lungs, where it is exhaled. Most CO2 is carried as bicarbonate (HCO₃⁻) dissolved in plasma.
3. Buffering: Hemoglobin plays a role in maintaining the pH of the blood by binding to hydrogen ions, thereby buffering the blood.

2. White Blood Cells (WBCs)

Characteristics of WBCs

• Shape: Irregular or amoeboid in shape. They have a well-defined nucleus.
• Size: Larger than RBCs, typically ranging from 10-17 micrometers.
• Nucleus: Nucleated, varying in shape depending on the type of WBC.
• Life Span: Varies significantly depending on the type. Some WBCs live for days to weeks, while others (like memory cells) can live for years.
• Types: WBCs are divided into two main categories:
  • Granulocytes (have granules in their cytoplasm): Neutrophils, eosinophils, basophils.
  • Agranulocytes (lack cytoplasmic granules): Lymphocytes (T-cells, B-cells, NK cells) and monocytes (which differentiate into macrophages and dendritic cells).

Functions of WBCs

1. Immune Response: WBCs are integral to the body’s immune system, fighting off infections, pathogens, and foreign invaders.
   • Neutrophils: Engulf and digest bacteria and fungi (phagocytosis).
   • Eosinophils: Combat parasitic infections and modulate allergic reactions.
   • Basophils: Release histamine and other chemicals during allergic reactions and inflammation.
   • Lymphocytes: Involved in adaptive immunity. B-cells produce antibodies, T-cells destroy infected cells, and natural killer (NK) cells target tumor cells and infected cells.
   • Monocytes: Differentiate into macrophages and dendritic cells, which are key in antigen presentation and the clearance of pathogens and dead cells.
2. Phagocytosis: Some WBCs (like neutrophils and macrophages) engulf and digest pathogens, dead cells, and debris, playing a key role in cleaning up after infections and injuries.
3. Inflammation Response: WBCs are central to inflammation processes. They migrate to infection or injury sites, releasing signaling molecules that promote healing and fight infections.

3. Platelets (Thrombocytes)

Characteristics of Platelets

• Shape: Small, disk-shaped fragments, without a nucleus.
• Size: About 2-4 micrometers in diameter.
• Origin: Platelets are fragments of megakaryocytes, which are large cells in the bone marrow.
• Life Span: About 7-10 days.
• Appearance: Platelets are granular, with various proteins involved in clotting.

Functions of Platelets

1. Blood Clotting (Hemostasis): Platelets are essential for the process of hemostasis, which stops bleeding by forming blood clots.
   • Platelet Adhesion: Upon vessel injury, platelets adhere to the exposed collagen and other components of the damaged vessel wall.
   • Platelet Activation: When platelets adhere, they become activated and release chemical signals (such as ADP, serotonin, and thromboxane A2), which attract more platelets to the injury site.
   • Platelet Aggregation: Activated platelets stick together (aggregation) to form a platelet plug at the site of injury.
2. Formation of Fibrin Clot: Platelets release clotting factors (such as fibrinogen) that interact with other plasma proteins to form a fibrin mesh, which solidifies the clot and prevents further bleeding.
3. Wound Healing: Platelets release growth factors (e.g., platelet-derived growth factor, PDGF) that promote tissue repair and regeneration.

Comparison Summary: RBCs, WBCs, and Platelets

Conclusion

• RBCs are critical for oxygen and carbon dioxide transport, enabling cells throughout the body to perform cellular respiration and maintain homeostasis.
• WBCs defend the body against infections, regulate immune responses, and assist in the repair of tissue damage.
• Platelets are indispensable for blood clotting, preventing excessive blood loss when blood vessels are injured and playing a role in tissue healing.

These three blood components work together to ensure the body functions optimally, with RBCs carrying essential gases, WBCs defending against disease, and platelets facilitating wound healing and clotting.