L5 - Erythrocyte structure & function pt. 1

What is the Primary Function of Erythrocytes?

Transport and protect hemoglobin, which carries oxygen to tissues for aerobic metabolism

Why do RBCs enclose Hemoglobin?

• To concentrate hemoglobin

• Protect it from degradation and oxidation

• Prevent free hemoglobin from damaging tissues

What Factors are required for Adequate Oxygen Delivery?

1. Functional RBCs and hemoglobin

2. Adequate RBC mass

3. Sufficient blood pressure

4. Appropriate vascular tone

What is the Difference between Anemia and Erythrocytosis?

• Anemia: decreased RBC mass

• Erythrocytosis: increased RBC mass

What gives Blood Its Red Color?

Iron-containing hemoglobin

• Oxygenated blood is bright red

• Deoxygenated blood is darker red

What is the General Structure of Adult Mammalian Hemoglobin?

• Four globin chains (2α, 2β), each containing one heme group

• Each heme binds one O₂ molecule

How does Hemoglobin Composition change during Development?

• Embryonic and fetal globins support oxygen acquisition before birth

• Adult globins predominate afterward

What is the Difference between Ferrous and Ferric Iron?

• Ferrous (Fe²⁺) binds oxygen

• Ferric (Fe³⁺) cannot bind oxygen and forms methemoglobin

Where do Oxygen Loading and Unloading occur?

• Loading occurs in the lungs

• Unloading occurs in Peripheral tissues

What is the Bohr Effect?

Changes in local pH and CO₂ alter hemoglobin's oxygen affinity → promoting unloading in active tissues

How do Low pH and High CO₂ affect Hemoglobin–Oxygen Affinity?

They decrease affinity → promoting oxygen unloading to metabolically active tissues

How do High pH and Low CO₂ affect Hemoglobin–Oxygen Affinity?

They increase affinity → promoting oxygen loading in the lungs

What is 2,3-DPG, and What is its Function?

An RBC metabolite that decreases hemoglobin's oxygen affinity → facilitating oxygen unloading

What does the Oxygen–Hemoglobin Dissociation Curve represent?

The relationship between oxygen partial pressure (PO₂) and the percentage of hemoglobin saturated with oxygen

What Conditions cause a Right Shift of the Oxygen Dissociation Curve?

1. Decreased pH

2. Increased CO₂

3. Increased temperature

4. Increased 2,3-DPG

What is the Effect of a Right Shift?

• Decreased oxygen affinity

• Increased oxygen unloading to tissues

What Conditions cause a Left Shift of the Oxygen Dissociation Curve?

1. Increased pH

2. Decreased CO₂

3. Decreased temperature

4. Decreased 2,3-DPG

What is the Effect of a Left Shift?

• Increased oxygen affinity

• Reduced oxygen unloading to tissues

What are the Immediate Adaptations to High Altitude?

Hyperventilation causes respiratory alkalosis and a LEFT shift → splenic contraction can temporarily increase RBC mass

What are the Long-Term Adaptations to High Altitude?

• Increased 2,3-DPG causes a RIGHT shift

• Increased EPO stimulates RBC production and raises RBC mass

QUESTION: For the following conditions, in which direction would you expect the oxygen-Hgb dissociation curve to shift (↑ O2 affinity)

• A horse finishing a 24-hr endurance ride

ANSWER → RIGHT

• Endurance exercise increases metabolic activity, CO₂ production, and temperature, promoting oxygen unloading

QUESTION: For the following conditions, in which direction would you expect the oxygen-Hgb dissociation curve to shift (↑ O2 affinity)

• Stored RBCs in the blood bank, with depletion of 2,3-DPG

ANSWER → LEFT

• 2,3-DPG decreases during storage, causing hemoglobin to retain oxygen more tightly

QUESTION: For the following conditions, in which direction would you expect the oxygen-Hgb dissociation curve to shift (↑ O2 affinity)

• A cow with metabolic alkalosis due to abomasal displacement

ANSWER → LEFT

• Increased blood pH increases hemoglobin's oxygen affinity

QUESTION: For the following conditions, in which direction would you expect the oxygen-Hgb dissociation curve to shift (↑ O2 affinity)

• An intrepid Poodle, climbing Mt. Everest

ANSWER → RIGHT

• Long-term acclimation increases 2,3-DPG, promoting oxygen unloading to tissues

Why do Mature RBCs rely on Anaerobic Glycolysis?

They lack mitochondria and therefore → generate ATP through anaerobic glycolysis

What are the Functions of the Embden–Meyerhof Pathway?

1. Produces ATP for membrane ion transport and osmotic balance

2. Plus NADH for methemoglobin reduction

What is the Function of the Pentose Phosphate Pathway?

1. Produces NADPH to support the glutathione antioxidant system

2. Protect RBCs from oxidative damage

What is the Function of the Methemoglobin Reductase Pathway?

1. Uses NADH to reduce Fe³⁺ to functional Fe²⁺ → maintaining hemoglobin's oxygen-binding ability

What is Methemoglobinemia, and What are Its Clinical Signs?

Excess oxidized hemoglobin (Fe³⁺), causing impaired oxygen transport, cyanosis, and muddy-brown blood

How can Methemoglobinemia occur despite Normal RBC Mass?

RBC numbers may be adequate, but oxidized hemoglobin cannot bind oxygen effectively

What is the Function of the Rapoport–Luebering Pathway?

Produces 2,3-DPG from glycolytic intermediates to regulate oxygen unloading

How does Hemoglobin Synthesis change during RBC Maturation?

Hemoglobin progressively accumulates while other cellular components decrease

• Synthesis ends with maturation

What are the Major Steps of Heme Synthesis?

1. Succinyl-CoA and glycine initiate synthesis

2. Reactions occur in mitochondria and cytosol

3. Iron is inserted into the porphyrin ring

What is the Function of Ferrochelatase (Heme Synthetase)?

Inserts iron into the porphyrin ring during the final step of heme synthesis

How are Globin Chains Synthesized, and What determines Hemoglobin Type?

• Globin genes direct protein synthesis

• Amino acid sequences determine embryonic, fetal, adult, and species variants

Why is Feline Hemoglobin particularly Sensitive to Oxidative Injury?

It contains eight oxidation-sensitive sulfhydryl groups, MORE than most other mammals.

What are Porphyrias, and What Clinical Signs can occur?

Heme synthesis disorders causing porphyrin accumulation, tooth/bone discoloration or fluorescence, and photosensitivity

How does Lead Toxicosis interfere with Hemoglobin Synthesis?

Inhibits ALA dehydratase and ferrochelatase → reducing heme synthesis and disrupting RBC maturation

What RBC Abnormalities can occur with Lead Toxicosis?

1. Increased circulating nucleated RBCs

2. Basophilic stippling from retained ribosomal material

Why are Adequate Iron Stores essential for Hemoglobin Synthesis?

Iron forms the oxygen-binding center of heme

• Deficiency limits hemoglobin synthesis and can cause anemia

What are the Functions of Transferrin, Ferritin, and Hemosiderin?

• Transferrin transports plasma iron

• Ferritin stores readily available iron

• Hemosiderin provides longer-term storage

How is Dietary Iron absorbed and transported?

1. Enterocytes absorb iron, store it or export it through ferroportin

2. Plasma transferrin transports exported iron.

How do Bone Marrow Macrophages support Erythropoiesis?

Nurse macrophages acquire, store, and release iron to developing erythroid cells for hemoglobin synthesis

How is Iron recycled from Senescent RBCs?

Macrophages remove old RBCs, recover hemoglobin iron, and store or export it for reuse in erythropoiesis

What is Hepcidin, and How does It regulate Iron Availability?

A liver-produced hormone that binds and degrades ferroportin, decreasing intestinal iron absorption and macrophage iron release

How do Blood Loss, Iron Deficiency, and Excess Iron affect Hepcidin?

• Blood loss and iron deficiency decrease hepcidin

• Abundant iron stores increase hepcidin

How does Inflammation affect Hepcidin and Erythropoiesis?

Inflammation increases hepcidin —> restricting iron availability and potentially causing anemia of inflammation

What is RBC Mass, and Which Measurements assess It?

The physical volume of circulating RBCs → assessed using HCT/PCV, RBC count, and hemoglobin concentration

How are Hematocrit and Packed Cell Volume determined?

Both reflect RBC mass.

• HCT is calculated from RBC count × MCV

• PCV is physically measured after centrifugation

What do MCV, MCH, and MCHC measure?

• MCV: average RBC volume

• MCH: hemoglobin amount per RBC

• MCHC: average hemoglobin concentration within RBCs

Table 1. Oxygen–Hemoglobin Dissociation Curve

Feature	Right Shift	Left Shift
O₂ affinity	Decreased	Increased
O₂ unloading	Increased	Decreased
pH	↓	↑
CO₂	↑	↓
Temperature	↑	↓
2,3-DPG	↑	↓
Primary effect	Delivers O₂ to tissues	Retains O₂ more tightly
Example	Exercise	Metabolic alkalosis

Remember:

• Right = Release O₂

• Left = Hold O₂

Table 2. Major RBC Metabolic Pathways

Pathway	Main Product	Primary Function
Embden–Meyerhof	ATP, NADH	Energy for ion transport and osmotic balance; NADH production
Pentose phosphate	NADPH	Antioxidant protection via glutathione
Methemoglobin reductase	Fe²⁺	Maintains functional iron for O₂ binding
Rapoport–Luebering	2,3-DPG	Promotes O₂ unloading

Remember: Mature RBCs lack mitochondria and rely on anaerobic glycolysis for ATP.

Table 3. Hemoglobin Synthesis Disorders

Disorder	Mechanism	Key Findings
Porphyria	Defective heme synthesis enzymes → porphyrin accumulation	Teeth/bone discoloration or fluorescence; photosensitivity
Lead toxicosis	Inhibits ALA dehydratase and ferrochelatase	Basophilic stippling; increased nucleated RBCs
Iron deficiency	Insufficient iron for heme synthesis	Decreased hemoglobin production; anemia

Table 4. Key Molecules in Iron Metabolism

Molecule	Primary Function
Transferrin	Transports iron in plasma
Ferritin	Readily available intracellular iron storage
Hemosiderin	Long-term intracellular iron storage
Ferroportin	Exports iron from enterocytes and macrophages
Hepcidin	Degrades ferroportin, decreasing iron availability

Key relationship: ↑ Hepcidin → ↓ Ferroportin → ↓ Iron availability

Table 5. Hepcidin Regulation in Clinical Conditions

Condition	Hepcidin	Iron Availability	Erythropoiesis
Hemorrhage	↓	↑	↑
Large iron stores	↑	↓	↓
Iron deficiency	↓	↑*	↑*
Inflammation	↑	↓	↓

*In iron deficiency, decreased hepcidin promotes iron availability and erythropoietic drive, but actual RBC production may remain limited by insufficient iron stores.