HEMA 311: Erythrocytes

Erythrocyte Biconcavity

Contributes to transporting gasses, flexibility, and deformability of the RBC

Discocyte

Other term for biconcave RBC shape

Hemoglobin

A tetramer that contributes to the color and central pallor size of the RBC

Salmon Pink

Normal color of an RBC due to Hgb

5 Million

Average number of circulating RBCs (per uL) of blood

Complex of Heme

• 4 protoporphyrin IX molecules

• 4 ferrous iron

4

How many Oxygen molecules can a Hgb molecule carry?

ATP

Slows the oxidation of proteins and iron by environmental peroxides and superoxide anions

• Maintains Hgb function and membrane integrity

120 days (± 20 days)

Life span of RBC

Old / Senescent RBCs

Engulfed by MACROs in the spleen

Severely Damaged RBCs

Engulfed by Macros in the liver

Reusable Components of RBC

1. From Hemoglobin:

   • globin chains

   • iron

2. From Cell Membrane:

   • phospholipids

   • proteins

Protoporphyrin Ring

A component in Hgb that isn’t reusable and is excreted as bilirubin

Anaerobic Glycolysis

Due to the lack of mitochondria in an RBC, it relies on this process for energy production (ATP)

Hypoxia

Low levels of oxygen in tissues that stimulates EPO production

Embden-Meyerhof Pathway

Main metabolic pathway of RBCs that undergo anaerobic glycolysis to produce ATP

90 - 95%

Amount of ATP produced by EMP

2

How many net ATP is produced in EMP

ATP Deficiency

Results in

• premature cell death due to inherited defects in glycolysis

• loss of viability during storage of blood for transfusion

• loss of flexibility

• deformity

Pyruvate Kinase Deficiency

Results in the formation of echinocytes due to decreased ATP

Burr Cells

Other term for echinocytes; due to an accumulation of lipids in the outer half of the red blood cell's membrane

True

True/False: The iron in our blood needs to be in the ferrous state to be functional

False

True/False: The iron in our blood needs to be in the ferric state to be functional

Hexose Monophosphate Shunt Pathway (HMP)

An alternative pathway that undergoes aerobic / oxidative glycolysis

5 - 10%

Amount of ATP produced by HMP

Pentose Phosphate Shunt (PPS)

Other term for HMP

Glucose-6-Phosphate Dehydrogenase (G6PD)

The enzyme reactant of HMP to produce ATP

Ribulose-5-Phosphate

A product of HMP that is required for the synthesis of RNA

Reduced Nicotinamide Adenine Dinucleotide Phosphate (NADPH)

A product of HMP that reduces oxidized glutathione (GSSG) to produce reduced glutathione (GSH)

Reduced Glutathione (GSH)

A product of HMP that prevents the oxidation of Hgb to MetHgb by maintaining the RBCs ferrous state

Glutathione Reductase

Enzyme required to reduce GSSG

G6PD Deficiency

Most common inherited RBC enzyme deficiency worldwide, leads to deficiency in:

• NADPH

• GSH

Oxidative Damage

A deficiency in G6PD makes RBCs vulnerable to _____ and will cause Hgb oxidation (Hgb > MetHgb)

Oxidized / Denatured Hgb

Leads to formation RBC inclusions in the cytoplasm

Heinz Bodies

RBC inclusion that is formed due to the Hgb being oxidized

Degmacyte / Bite Cell / Bronze Cell

The result in the removal of Heinz bodies in the RBC cytoplasm, causing a pitted golf ball appearance

Methemoglobin (MetHgb)

The affected Hgb is called as ____ when the iron is in the ferric state

65%

Methemoglobin-reducing capacity of MRP

Cytochrome-b5-Reductase

Other term for MetHgb reductase

Methemoglobin Reductase Pathway

A EMP shunt that reduces ferric iron back to ferrous iron

Rapoport-Luebering Pathway

Produces 2,3-biphosphoglycerate to lower Hgb’s affinity to oxygen (tense state), helping RBCs release oxygen more easily to tissues that need it (in case of hypoxia)

Acidic

pH environment that inhibits the activity of biphosphoglycerate mutase

Right shift

The direction of the hemoglobin-oxygen dissociation curve when Hgb has low affinity to oxygen

8%

Percentage of carbohydrates in RBC membrane

52%

Percentage of proteins in RBC membrane

40%

Percentage of lipids in RBC membrane (equal parts of phospholipids and cholesterol, glycolipids)

RBC Membrane Cholesterol

Contributes to the tensile strength of the lipid bilayer of the RBC

Acanthocytosis

Indicates underlying lipid membrane defects in RBCs

RBC Membrane Phospholipids

Forms an impenetrable fluid barrier

1. Hydrophilic Polar Head Groups (Phosphatidyl-choline + Sphingomyelin)

   • arranged on membrane’s surface and oriented towards both the aqueous plasma and cytoplasm

2. Hydrophobic Nonpolar Acyl Tails (Phosphatidyl-serine + phosphatidyl-ethanol-amine)

   • sequestered/hidden from the aqueous plasma and cytoplasm to arrange themselves to form a central layer

Phosphatidyl-serine (PS)

The only negatively charged phospholipid that can redistribute to the outer layer of the RBC membrane and is responsible for apoptosis (splenic macrophages have receptors that bind to PS to destroy senescent and damaged RBCs)

Disruption of Phospholipid Distribution in RBC Membrane

Caused by sickle cell anemia and thalassemia (due to loss of phospholipid asymmetry “PS exposure”)

Phosphatidyl-inositol

Phospholipid that is present in both in both inner and outer layers of RBC membrane

Eryptosis

other term for RBC death

Factors that increase PS distribution to outer layer

1. CRP

2. Inflammatory conditions

Glycolipids

Sugar-bearing lipids that make up 5% of external half of RBC membrane; supports CARB side chains that extends to glycocalyx

Glycocalyx

Layer of CARBs that prevents microbial attack and protects RBCs from mechanical damage

Glycolipids.

Lipid portion of RBC membrane that carries carbohydrate-based blood group antigens (ABO and Lewis system).

Spectrins

Cytoskeletal proteins that form a hexagonal lattice, providing lateral or horizontal membrane stability

Transmembrane Proteins

Penetrates lipid bilayer and serve transport + adhesion sites and signaling receptors

Glycosylation

Through what process are glycolipids and transmembrane proteins joined together to make up the protective glycocalyx, supporting surface carbohydrates?

Cations that can’t permeate (are impermeable) into RBC membrane

1. Sodium (Na)

2. Potassium (K)

3. Calcium (Ca)

Anions that can permeate (are permeable) into RBC membrane

1. Bicarbonate (HCO3)

2. Chloride (Cl)

Cation Pumps

• Consumes 15% of RBC ATP (ATP-dependent)

• Sodium-ATPase: regulates Na intracellular-extracellular ratio (1:12)

• Potassium-ATPase: regulates K intracellular-extracellular ratio (25:1)

• Calcium-ATPase: extrudes/kicks out Ca to maintain low intracellular levels (5-10 umol per L)

Calmodulin

Cytoplasmic calcium-binding protein that controls Calcium-ATPase function

Aquaporin 1

Transmembrane protein that forms pores to create inward water flow (in response to internal osmotic changes)

Colloid Osmotic Hemolysis

Water enters RBC due to Na and Ca influx/accumulation, causing RBC to swell and eventually rupture (caused by ATP loss or pump damage)

• “Where salt goes, water follows”

• It’s why sodium and calcium have low intracellular-extracellular ratios

Senescence

Major way in which eryptosis occur; loss of glycolytic enzymes is central to this process of cellular aging

Extravascular Hemolysis

• “Macrophage-mediated”

• 90% of Eryptosis

• Senescent RBCs produce less ATP

• Oxidation of RBC membrane

• Disruption in cation pumps, increasing intracellular Na

• Water enters RBC > Swelling

• RBC loses flexibility and deformability > Discoid shape is lost > Spherocyte

• Can’t pass through splenic sieve to exit spleen

• Clinical Significance:

  1. Increased unconjugated bilirubin leading

  2. Increased urobilinogen + stercobilinogen (urine and fecal bilirubin breakdown products)

Ferritin

A protein-iron complex that is stored in the macrophage upon RBC lysis

Protoporphyrin

Component of heme in Hgb that is degraded into bilirubin

Globin

Component of Hgb that is degraded and returned to metabolic amino acid pool

Culling

Process of removing senescent RBCs

Intravascular Hemolysis

• “Fragmentation Hemolysis”, “Mechanical Hemolysis”

• 10% of Eryptosis

• Hemoglobin is broken down and released as free Hgb into the plasma

• Haptoglobin: transports α- and β-dimers of Hgb into the liver (by hepatocytes)

• Hemopexin: transports heme to liver (by LRP/CD91)