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Red Blood Cells (RBCs): Structure, Hemoglobin, Life Cycle, Functions and Immunity

What are Red Blood Cells?

Red blood cells or RBCs are also called erythrocytes. They serve as functional components in blood responsible for transportation of gases and nutrients across entirety of human body.

They play a crucial role in oxygen transport as they contain hemoglobin.

  • Hemoglobin is a protein that attaches to oxygen and transports it from lungs to body tissues.
  • Normal erythrocyte concentration is 3.9-5.5 million per microliter in women and 4.1-6 million per microliter in men.
  • They appear red due to presence of iron-containing pigment called hemoglobin.
  • Oxygen is absorbed by hemoglobin and supplied to various parts of body.
  • RBCs are formed in bone marrow located in ends of long, flat, and irregular bones.

Structure of Red Blood Cells

RBCs are unique in that they lack a nucleus and mitochondria, allowing them to contain more hemoglobin and perform primary function efficiently.

  • Red-colored cells having biconcave shape.
  • Flattened discs lacking nuclei and cytoplasmic organelles.
  • Contain red protein hemoglobin for gas transport, primarily oxygen and some carbon dioxide.
  • Measures 7.5 µm in diameter with 2.6 µm thick at rim and 0.8 µm thick at center.

Detailed Structural Components

Biconcave Shape

  • Small, round and biconcave.
  • Enables them to move through tiny blood vessels and reach body's tissue.

Membrane

  • Cell membrane composed of proteins and lipids.
  • Provides structural integrity and malleability to cell.

Cytoskeleton

  • Cytoskeleton composed of proteins such as actin, spectrin, band 3, protein 4.1, ankyrin.
  • Helps to maintain cellular structural integrity and flexibility.

Hemoglobin Content

  • Each RBC contains approximately 270 million hemoglobin molecules - original text mentions 270 molecules, scientifically around 270 million.
  • Responsible for oxygen transport.

Structure of Hemoglobin

Formation of Hemoglobin

  • Hemoglobin is complex protein comprising globin and iron containing haem.
  • Produced within developing erythrocytes in red bone marrow.
  • In mature erythrocytes, hemoglobin binds with oxygen to create oxyhemoglobin, imparting characteristic red color to blood.
  • Hemoglobin plays crucial role in excretion. It helps transport CO2 from body cells to lungs.
  • Each hemoglobin molecule contains four iron atoms, and each atom can carry one molecule of oxygen.
  • Consequently, single hemoglobin molecule can transport up to four oxygen molecules.

Life Cycle of Red Blood Cells - Erythropoiesis

Lifespan of RBCs in circulation is approximately 120 days.

Erythrocytes originate in red bone marrow before entering bloodstream and undergo multiple developmental stages.

Erythropoiesis is process of RBC development from stem cells that takes around seven days.

Three Phases of Life Cycle

Life cycle comprises three phases:

  • 1. Production
  • 2. Maturation
  • 3. Destruction

Production

  • Erythropoiesis primarily occurs in bone marrow.
  • Begins with hematopoietic stem cells (HSCs), giving rise to multipotent progenitors.
  • Progenitors develop into erythroid-committed precursors and mature RBCs.
  • Erythropoietin is primary regulator of erythropoiesis, driving RBC precursor proliferation and differentiation and preventing erythroblast apoptosis.

Maturation

  • Maturation involves several morphological changes to produce highly functional specialized cells.
  • Includes expulsion of nucleus and loss of organelles, such as:
    • Smooth and rough endoplasmic reticulum
    • Golgi apparatus
    • Ribosomes
  • Leading to formation of mature biconcave RBCs.

Destruction

  • Mature RBCs have lifespan of about 120 days.
  • At end of lifespan, removed from circulation and destroyed.
  • Primarily destroyed by macrophages in spleen and liver.

Functions of Red Blood Cells

Primary role is transporting respiratory gases.

  • Oxygen transport:

    • Hemoglobin in RBCs combines with oxygen, forming oxyhemoglobin.
    • Approximately 97% of oxygen transports as oxyhemoglobin.

  • Carbon dioxide transport:

    • Hemoglobin combines with carbon dioxide forming carboxyhemoglobin and approximately 30% of CO2 is transported in this form.
    • RBC contains carbonic anhydrase, crucial for converting water and carbon dioxide to bicarbonate.
    • Facilitates carbon dioxide transport, about 63% in bicarbonate form.

  • Buffering action:

    • Hemoglobin serves as effective buffer, regulating Hydrogen ion concentration.
    • Contributes to maintaining acid-base balance.

  • Blood group determination:

    • RBCs carry blood group antigens A, B, and Rh factor.
    • Aids in blood group determination and prevents reactions in incompatible blood transfusion.

Roles of Red Blood Cells in Immunity

RBCs play crucial role in immune response against pathogens besides oxygen transport. They contribute to innate immune response:

  • Binding and scavenging:

    • Can bind and scavenge chemokines, nucleic acids, and pathogens in circulation.
    • Helps in clearance of these substances and reduces potential harm to host.

  • Phagocytosis:

    • Can be susceptible to phagocytosis in certain settings, leading to innate immune activation.
    • For example, when plasma CpG DNA levels increase beyond homeostatic norms, such as during sepsis or infection, TLR9-dependent binding to RBCs results in causing anemia and innate immune activation.

  • Reactive oxygen species (ROS) production:

    • In erythrocytes, hemoglobin and heme stimulate ROS to eliminate and destroy hemolytic pathogens.

  • Immune recognition:

    • Certain blood group antigens on red blood cells are present on molecules with receptors for specific microbes, resulting in hemolysis.
    • Interaction can happen when:
      • Hidden antigens become exposed.
      • Microbial toxins change characteristics of antigens.
      • Microbes directly affect formation of new red blood cells.

Summary

Red blood cells or erythrocytes are biconcave, nucleus-free cells 7.5 µm in diameter containing approximately 270 million hemoglobin molecules, each with four iron atoms carrying four oxygen molecules. Formed in bone marrow at ends of long, flat and irregular bones with normal counts of 3.9-5.5 million per microliter in women and 4.1-6 million in men, they appear red due to hemoglobin. Structure includes protein-lipid membrane and cytoskeleton of actin, spectrin, band 3, protein 4.1 and ankyrin providing flexibility. Life cycle or erythropoiesis takes seven days from hematopoietic stem cells regulated by erythropoietin, with maturation involving loss of nucleus and organelles, and destruction after 120 days by spleen and liver macrophages. Functions include 97% oxygen transport as oxyhemoglobin, 30% CO2 as carboxyhemoglobin and 63% as bicarbonate via carbonic anhydrase, buffering of hydrogen ions and blood group determination with A, B and Rh antigens. Immune roles include scavenging chemokines and pathogens, TLR9-mediated activation during sepsis, ROS production and immune recognition via blood group antigens.

References

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