Red blood cells are the superheroes of our circulatory system. These tiny, disc-shaped cells pack a powerful punch, carrying oxygen to every corner of our body. Without a nucleus, they're streamlined for maximum efficiency, like a delivery truck with no driver's seat.

Hemoglobin is the secret weapon inside red blood cells. This protein is a master of disguise, changing shape to grab or release oxygen as needed. It's affected by factors like temperature and pH, adapting to our body's ever-changing needs like a chameleon.

Erythrocyte Structure and Function

Structure of erythrocytes

• Erythrocytes, also known as red blood cells (RBCs), are the most abundant cells in the blood with a biconcave disc shape that increases surface area for efficient gas exchange
• Erythrocytes lack a nucleus and most organelles which allows for more space to carry hemoglobin
• Erythrocyte plasma membrane is highly permeable to gases like O2 and CO2 and contains glycoproteins and glycolipids that determine blood type (A, B, AB, O)
• Erythrocytes are packed with hemoglobin, an oxygen-carrying protein, with each erythrocyte containing about 250 million hemoglobin molecules

Lifecycle of erythrocytes

• Erythropoiesis is the process of erythrocyte formation, which occurs primarily in the bone marrow
  • Hematopoietic stem cells differentiate into erythroid progenitor cells stimulated by erythropoietin (EPO), a hormone produced by the kidneys
• Erythrocyte maturation stages:
  1. Proerythroblast (earliest recognizable erythroid cell)
  2. Basophilic erythroblast
  3. Polychromatophilic erythroblast
  4. Orthochromatic erythroblast
  5. Reticulocyte (immature erythrocyte released into the bloodstream)
  6. Mature erythrocyte
• Mature erythrocytes circulate in the bloodstream for about 120 days before senescent or damaged erythrocytes are removed from circulation by macrophages in the liver and spleen
  • The iron from the degraded hemoglobin is recycled for the production of new erythrocytes

Hemoglobin and Oxygen Transport

Hemoglobin for oxygen transport

• Hemoglobin is a quaternary protein consisting of four polypeptide chains (two α and two β subunits), each containing a heme group which is an iron-containing porphyrin ring that binds reversibly with oxygen
• Hemoglobin can exist in two conformational states: tense (T) with lower affinity for oxygen and relaxed (R) with higher affinity
• Oxygen binding to hemoglobin is cooperative and allosteric, meaning when one subunit binds oxygen it increases the affinity of the other subunits resulting in a sigmoidal oxygen-hemoglobin dissociation curve
• Factors that affect hemoglobin's affinity for oxygen:
  • $P_{50}$ is the partial pressure of oxygen at which hemoglobin is 50% saturated
  • Increased $P_{50}$ (right-shift of the dissociation curve) decreases hemoglobin's affinity for oxygen, favoring oxygen release
    • Caused by increased temperature, decreased pH (Bohr effect), increased 2,3-bisphosphoglycerate (2,3-BPG), or increased $PCO_2$
  • Decreased $P_{50}$ (left-shift of the dissociation curve) increases hemoglobin's affinity for oxygen, favoring oxygen loading
    • Caused by decreased temperature, increased pH, decreased 2,3-BPG, or decreased $PCO_2$

Erythrocyte Assessment and Disorders

• Hematocrit is the percentage of blood volume occupied by erythrocytes, providing information about oxygen-carrying capacity
• Red blood cell count measures the number of erythrocytes per volume of blood, helping assess overall erythrocyte production and lifespan
• Anemia is a condition characterized by a decrease in the number of erythrocytes or hemoglobin concentration, leading to reduced oxygen-carrying capacity
• Sickle cell disease is a genetic disorder causing abnormal hemoglobin production, resulting in crescent-shaped erythrocytes that can obstruct blood flow and cause tissue damage