Characteristics and Functions of RBCs, WBCs, and platelets

Think of blood as a busy city. Red blood cells are the delivery trucks carrying oxygen, white blood cells are patrol teams defending against invaders, and platelets are the road-repair crew rushing to patch leaks. Here’s a clear, exam-ready guide.

Red Blood Cells (RBCs, Erythrocytes)

Key characteristics

Shape and size: Biconcave discs, 7–8 µm in diameter; the shape increases surface area for gas exchange and allows flexibility in narrow capillaries.
Contents: Packed with hemoglobin (∼33% of cell weight); about 280 million Hb molecules per RBC.
Organelles: No nucleus or mitochondria; ATP is made anaerobically, so RBCs don’t consume the oxygen they carry.
Membrane markers: Glycolipid antigens (ABO, Rh) on the membrane determine blood groups.
Normal counts:
- Men ≈ 5.4 million/µL
- Women ≈ 4.8 million/µL

Functions

Oxygen transport: Each Hb has 4 heme groups; the Fe2+ in each heme binds 1 O2 molecule reversibly. O2 is loaded in the lungs and released in tissues.
Carbon dioxide transport: About 23% of CO2 binds to globin chains and is carried to the lungs for exhalation.
Vascular regulation: Hemoglobin can bind and release nitric oxide (NO). Released NO causes vasodilation, improving blood flow and O2 delivery.

Life cycle and regulation (high‑yield)

Lifespan: ~120 days. Aging RBCs become fragile and are removed by macrophages in spleen and liver.
Recycling:
- Globin → amino acids (reused).
- Iron (as Fe3+) → transported by transferrin, stored as ferritin, then reused in marrow.
- Heme (without iron) → biliverdin → bilirubin → bile → intestine → urobilinogen → urobilin (urine, yellow) and stercobilin (feces, brown).
Erythropoiesis: Proerythroblast → reticulocyte (nucleus ejected) → mature RBC (1–2 days after entering blood).
Control by EPO: Hypoxia (e.g., anemia, high altitude) stimulates kidneys to release erythropoietin, speeding maturation and increasing RBC count.

Fig: Formation and destruction of red blood cells, and the recycling of hemoglobin components.

Quick clinical connects

Low EPO in renal failure → anemia.
Reticulocyte count reflects marrow response; high in blood loss/hemolysis, low in production problems.
Jaundice arises when bilirubin handling is impaired

White Blood Cells (WBCs, Leukocytes)

General characteristics

Have nuclei and lack hemoglobin.
Normal count: 5,000–10,000/µL.
- Leukocytosis >10,000/µL: often infection, stress, surgery.
- Leukopenia <5,000/µL: radiation, shock, some drugs.
Movement: Leave blood via emigration (diapedesis) using adhesion molecules (selectins, integrins); follow chemical trails (chemotaxis).
MHC markers: Present on WBCs and most nucleated cells (not on RBCs); important for self-recognition.

Types

Granulocytes (“BEN are GRAN”):
- Neutrophils: Pale lilac granules; 2–5 lobed nucleus; “polymorphs/PMNs.” First responders.
- Eosinophils: Red‑orange granules; bilobed nucleus.
- Basophils: Blue‑purple granules often obscure nucleus.
Agranulocytes:
- Lymphocytes: Round nucleus with thin blue rim of cytoplasm; include B cells, T cells, and NK cells.
- Monocytes: Largest WBC; kidney/horse‑shoe nucleus; become macrophages in tissues.

Functions by cell type

Neutrophils: Rapid phagocytosis of bacteria; kill with lysozyme, oxidants (O2−, H2O2, OCl−), and defensins that punch holes in microbes.
Monocytes → macrophages: Arrive later, in larger numbers; vigorous phagocytosis; clean up debris.
Eosinophils: Dampen allergies (histaminase), eat antigen‑antibody complexes, attack parasitic worms.
Basophils: Release histamine, heparin, serotonin in inflammation and hypersensitivity; similar to mast cells in tissues.
Lymphocytes:

B cells: Make antibodies; great against bacteria and their toxins.
T cells: Kill virus‑infected cells, fungi, cancer cells; mediate transplant rejection and some allergies.
NK cells: Destroy a broad range of infected or tumor cells without prior sensitization.

Table: Significance of High and Low White Blood Cell Counts

WBC TYPE	HIGH COUNT MAY INDICATE	LOW COUNT MAY INDICATE
Neutrophils	Bacterial infection, burns, stress, inflammation.	Radiation exposure, drug toxicity, vitamin B12 deficiency, or systemic lupus erythematosus (SLE).
Lymphocytes	Viral infections, some leukemias.	Prolonged illness, immunosuppression, or treatment with cortisol.
Monocytes	Viral or fungal infections, tuberculosis, some leukemias, other chronic diseases.	Bone marrow suppression, treatment with cortisol.
Eosinophils	Allergic reactions, parasitic infections, autoimmune diseases.	Drug toxicity, stress.
Basophils	Allergic reactions, leukemias, cancers, hypothyroidism.	Pregnancy, ovulation, stress, or hyperthyroidism.

Practical note

Differential WBC count helps pinpoint causes: bacterial vs viral infection, allergy, parasitic disease, drug effects, or hematologic disorders.

Mnemonic for relative abundance in blood: “Never Let Monkeys Eat Bananas” = Neutrophils > Lymphocytes > Monocytes > Eosinophils > Basophils.

Platelets (Thrombocytes)