RBC Physiology

Oxygen delivery from lungs to tissues

One true function of RBCs

Only cell in mature form that contains no nucleus

Unique characteristic of RBCs

Erythron

All stages of erythrocytes (BM & PB)

RBC Mass

Cells in circulation / peripheral blood

120 days

Average life span of RBC

Normoblastic, Rubriblastic, Erythroblastic

Erythroid precursor nomenclature systems

Erythroid islands

Specialized functional anatomical unit of RBC maturation within the bone marrow

Central macrophage / Nurse cell

Provides iron storage (ferritin / hemosiderin) to erythroblasts

Performs nuclear phagocytosis of orthochromatic normoblast

Secretes cytokines and provides anchorage for erythroblasts

Erythroid progenitors

Includes CFU-GEMM, BFU-E, and CFU-E

Burst-Forming Unit-Erythroid (BFU-E)

Earliest committed progenitor

Multi-subunit colonies (“Bursts”)

Not actively proliferating (G0/G1 phase)

Not sensitive to EPO, responds to intermediate acting growth factors

Colony-Forming Unit-Erythroid (CFU-E)

Actively proliferating (S Phase); low capacity to divide compared to BFU-E

Influenced by EPO (Late-acting growth factor)

Pronormoblast / Rubriblast

Largest cell in the RBC maturation series

Purple red chromatin with 1 or 2 nucleoli; Dark blue cytoplasm

8:1 N:C ratio

Beginning of globin production (Undetectable Hb)

Basophilic normoblast / Prorubricyte

Deep purple red nucleus (condensation of chromatin); Parachromatin is larger and sharper

Deeper richer blue cytoplasm

6:1 N:C ratio

Detectable Hb synthesis

Polychromatic (Polychromatophilic) Normoblast / Rubricyte

Chromatin condensation; no nucleoli present

Accumulation of Hb pigmentation → murky gray-blue cytoplasm

4:1 to 1:1 N:C ratio

Last stage capable of mitosis

Increased Hb synthesis

Orthochromic Normoblast / Metarubricyte

Pyknotic nucleus

Increased salmon-pink color cytoplasm

1:2 N:C ratio

Pyrenocyte (Nucleus Ejection) & Howell-Jolly bodies (from RNA remnants)

In the spleen by splenic macrophages

Where pitting and membrane polishing occur for Reticulocytes

Polychromatic (Polychromatophilic) Erythrocyte / Reticulocyte

No nucleus; salmon-pink cytoplasm

Circulates in BM (1-2 days) and Peripheral Blood (1 day)

Pitting and Membrane Polishing

Completes the Hb production from the remaining ribosomes

Mature erythrocyte

No nucleus, salmon-pink with central pallor (1/3 of the cell)

Circulates in the peripheral blood

O2 and CO2 delivery

Hypoxia

Decrease in oxygen content within the tissues

Impaired oxygen transport to the tissues (i.e., anemia)

Low oxygen tension (i.e., high altitude)

Peritubular fibroblasts

Primary oxygen-sensing system

Decreased O2 levels → Kidneys release EPO

2,3-diphosphoglycerate (2,3-DPG)

Modulates oxygen affinity by the concentration of phosphates

Shift to the right

Increased 2,3-DPG concentration

Decreased O2 affinity

Increased O2 delivery

Shift to the left

Decrease 2,3-DPG

Increased O2 affinity

Decreased O2 delivery

Erythropoietin

“True hormone” — produced in the kidney, affects the bone marrow

Mediated by GATA1

Thermostable, nondialyzable, glycoprotein hormone — carbohydrate unit + terminal sialic acid unit

GATA1 gene

Mediates ability of cell to respond to EPO

Absence of this gene would mean lack of cell response to EPO, leading to apoptosis

JAK2 gene

Activates signal transduction pathways

STAT5 pathway

Promotes transcription of specific genes in RBC nucleus

Prolyl Hydrolase (PHD)

Enzyme that is highly active and bind to available O2

Involved in hydroxylation of HIF-1 which serves as a tag for VHL protein

Von Hippen-Lindau (VHL) protein

Recognized the tag of HIF-1 and binds to it, destroying the transcription factor protein → EPO turned off

No activation during state of hypoxia

HIF1-HIF2 complex

Binds to hypoxia response element, which activates the EPO gene

→ EPO is released

Early release of retics, inhibition of apoptosis, reduced BM transit time

Major effects of EPO

Early release of reticulocytes

Increase width of the spaces — changes in the adventitial layer

Decreased expression of fibronectin receptor

“Shift / Stress reticulocytes”

Inhibition of apoptosis

“Direct EPO rescue from apoptosis”

Reduced production of Fas Ligand

Increased production of antiapoptotic molecule — Bcl-XL (Bcl-2-like protein 1)

Reduced marrow transit time

Increased rate of cellular processes; decreased cell cycle times — early cell cycle arrest (cessation of cell dviision)

Secretion of Erythroferrone — decrease hepcidin production

“True shift reticulocytes”

Normoblastic hyperplasia

Indirect stimulation

Thyroid hormones increase Basal Metabolic Rate (BMR) of all cells

Increased BMR and O2 consumption → Hypoxia → EPO release

Direct stimulation

Thyroid hormones enter BM and binds to thyroid hormone receptors on erythroid progenitor cells (BFU-E & CFU-E)

Acts with EPO to promote proliferation and maturation of RBCs

Pituitary hormones

Releases TSH, which controls release of T3 & T4

Increased demand for O2 → hypoxia

Thyroid hormones

Essential for maintaining RBC mass

Macrophage-mediated / Extravascular hemolysis

Major physiological pathway for senescent or abnormal removal — splenic (“culling”) or hepatic macrophages

Lack of ATP → oxidation of membrane lipids and proteins (globin)

Lost of discoid shape (spheroid) → difficulty squeezing through the splenic sieve

Erythrophagocytosis - mononuclear phagocyte system

Eryptosis - nonnucleated cell death

Mechanical / Fragmentation / Intravascular Hemolysis

Rupture within the lumen of blood vessels

Purely mechanical / traumatic stress — “fragmentation”

Small contributor of RBC destruction under normal circumstances

Hereditary / Chuvash polycythemia

Disorder of EPO

Autosomal recessive

Increased cellular expression of HIF-1a due to 598C->T mutation (VHL gene)

Familial polycythemia

Autosomal dominant

Defect in the regulation of EPO

Decreased plasma volume

Effect on plasma when there is an increased in packed cell volume (hematocrit) or total erythrocyte count

Megaloblastic anemia

Defective nuclear maturation

Nuclear maturation lags behind cytoplasmic maturation (Vitamin B12 / folate deficiencies)

Impaired ability of cells to synthesize DNA

Cholesterol

tensile strength (“ratio of cholesterol: phospholipids remain constant” = balance of deformability & strength)

Phospholipids

Outer layer: phosphatidylcholine & sphingomyelin

Inner layer: phosphatidylserine & phosphatidylethanolamine

Glycolipids

Clumps/rafts, support carbohydrate side chains that anchor the glycocalyx, bear copies of carbohydrate-based blood group antigens (i.e., ABH, Lewis)

Transmembrane proteins

Functions as transport sites, adhesion sites, and signaling receptors

Ankyrin Complex & Actin Junction Complex (Protein 4.1 complex): 2 major macromolecular complexes; provides membrane structural integrity and vertical membrane structure

Ankyrin complex

Primary anchor which attaches spectrin to Band3 protein in the lipid bilayer, holding the membrane and cytoskeleton together

Vertical anchorage

Actin junctional complex

Actin and Band4.1 will link spectrin to glycophorin C for secondary stabilization

Horizontal anchorage

Cytoskeleton / Peripheral proteins

Assemble to form antiparallel heterodimer (Filamentous a-spectrin & B-spectrin)

Provides lateral / horizontal stability

Spectrin dimer bonds disassociate and reassociate during deformation

Permeable substances

Water and anions (chloride & bicarbonate)

Regulated by Aquaporin 1 (formation of pores/channels to create inward flow of water)

Band 3 (Anion exchanger 1)

Allows transport of bicarbonate to replace fluoride

Bicarbonate out, fluoride in

Impermeable substances

Cations (sodium, potassium, calcium)

Involves ATP-dependent cation pumps (3 Na out, 2 K in)

Calmodulin: Controls the Ca2+ ATPase pump