Red Blood Cells: Structure and Function (I)

Vocabulary practice flashcards covering red blood cell structure, hemoglobin function, oxygen affinity regulation, erythrocyte metabolic pathways, heme and iron metabolism, and erythrocytic laboratory indices.

Primary Function of Erythrocytes

To transport hemoglobin, which carries oxygen ($\text{O}_2$) to the tissues, while concentrating hemoglobin, protecting it from oxidation and degradation, and protecting body tissues from free hemoglobin and free oxygen.

Hemoglobin Structure

A tetrameric respiratory protein consisting of four globin polypeptide chains (such as two $\alpha$ and two $\beta$ chains) and four iron-containing heme moieties.

Heme

A tetrapyrrole porphyrin ring complexed with a central ferrous iron ($\text{Fe}^{2+}$) atom that reversibly binds oxygen for transport to tissues.

Ferrous Iron ($\text{Fe}^{2+}$)

The reduced oxidation state of heme iron capable of binding and releasing oxygen; iron can only bind $\text{O}_2$ when maintained in this $+2$ state.

Bohr Effect

The physiological adjustment of hemoglobin's oxygen affinity to the local tissue environment, where lower $\text{pH}$ and lower $\text{O}_2$ promote oxygen unloading in metabolically active tissues, and higher $\text{pH}$ promotes oxygen loading in the alveoli.

2,3-Diphosphoglycerate (2,3-DPG)

An intra-erythrocytic glycolytic metabolite produced via the Rapaport-Lubering pathway that binds hemoglobin and decreases its oxygen affinity, facilitating $\text{O}_2$ unloading to tissues.

Right Shift (Oxygen-Hemoglobin Dissociation Curve)

A decrease in hemoglobin's affinity for oxygen that optimizes tissue $\text{O}_2$ unloading; caused by decreased $\text{pH}$, increased $\text{2,3-DPG}$, increased $\text{CO}_2$, and increased temperature.

Left Shift (Oxygen-Hemoglobin Dissociation Curve)

An increase in hemoglobin's affinity for oxygen that optimizes alveolar $\text{O}_2$ loading; caused by increased $\text{pH}$, decreased $\text{2,3-DPG}$, decreased $\text{CO}_2$, and decreased temperature.

High Altitude Adaptation (Immediate vs. Acclimation)

Immediate responses (minutes to hours) include hyperventilation causing a transient left shift to enhance alveolar loading and splenic contraction; acclimation (days to weeks) involves increased 2,3-DPG causing a right shift and an eventual increase in red cell mass driven by erythropoietin (EPO).

Methemoglobin

An abnormal, oxidized form of hemoglobin in which heme iron is in the ferric ($\text{Fe}^{3+}$ / $\text{Fe}^{\text{III}}$) state and cannot bind $\text{O}_2$, resulting in clinical cyanosis and muddy brown blood.

Methemoglobin Reductase Pathway

An erythrocyte antioxidant pathway that utilizes $\text{NADH}$ derived from glycolysis to reduce ferric iron ($\text{Fe}^{3+}$) back to the functional ferrous state ($\text{Fe}^{2+}$).

Rapaport-Lubering Pathway

A shunt branching from glycolysis at 1,3-DPG that synthesizes 2,3-DPG via DPG synthetase to modulate hemoglobin's oxygen binding affinity.

Pentose Phosphate Shunt

A metabolic pathway producing $\text{NADPH}$ via glucose-6-phosphate dehydrogenase (G6PD) to maintain reduced glutathione (GSH) through glutathione reductase, protecting red cells against hydrogen peroxide ($\text{H}_2\text{O}_2$) toxicity.

Feline Hemoglobin Sulfhydryl Groups

Feline hemoglobin contains 8 oxidant-sensitive sulfhydryl ($\text{-SH}$) groups on its globin chains, which is more than most other mammals and makes feline erythrocytes exceptionally vulnerable to oxidative injury.

Porphyrias

Disorders resulting from inherited enzyme deficiencies in heme synthesis or liver disease-induced accumulation of plant porphyrins, manifesting clinically as teeth and bone discoloration, dental fluorescence under UV light, and dermal photosensitization burns.

Basophilic Stippling

The abnormal microscopic aggregation of ribosomal machinery within erythrocytes on a Wright-stained blood film, characteristically seen in dyserythropoiesis secondary to lead toxicosis.

Lead Toxicosis (Dyserythropoiesis)

Toxic inhibition of critical enzymes in heme synthesis—predominantly $\delta$-aminolevulinic acid (ALA) dehydratase and heme synthetase—resulting in nucleated red blood cells, impaired hemoglobin production, and basophilic stippling.

Transferrin

The primary circulating serum glycoprotein that binds and transports ferric iron through the bloodstream to developing erythroid cells and storage tissues.

Ferritin

An intracellular, short-term, water-soluble iron storage protein complex found in enterocytes and macrophages that allows rapid mobilization of iron.

Hemosiderin

An insoluble, long-term intracellular iron storage aggregate located primarily within bone marrow and splenic macrophages that stains visibly bright blue with Prussian blue stain.

Ferroportin

The primary transmembrane iron export portal protein expressed on duodenal enterocytes and macrophages that transports iron into the circulation and is targeted for degradation by hepcidin.

Hepcidin

A peptide hormone synthesized in the liver that serves as the master negative regulator of iron homeostasis by binding ferroportin, causing its internalization and degradation to reduce intestinal iron absorption and macrophage iron release.

Erythroblastic Island

A functional multicellular unit in the bone marrow composed of a central nurse macrophage surrounded by developing erythroid precursor cells receiving iron for hemoglobin synthesis.

Hematocrit (HCT)

The calculated metric most representative of RBC mass, expressed as the percentage of whole blood volume occupied by erythrocytes, determined as $\text{RBC count} \times \text{MCV}$.

Packed Cell Volume (PCV)

A direct physical correlate of hematocrit measured by mechanically centrifuging anticoagulated whole blood in a microhematocrit tube to determine the percentage of packed red cells.

Mean Corpuscular Volume (MCV)

An erythrocyte index indicating the average individual volume of a red blood cell expressed in femtoliters ($\text{fl}$, classifying cells as normocytic, microcytic, or macrocytic).

Mean Corpuscular Hemoglobin (MCH)

An erythrocyte index representing the average absolute mass of hemoglobin within a single red blood cell, expressed in picograms ($\text{pg}$, calculated as $\text{Hgb} \div \text{RBC count}$).

Mean Corpuscular Hemoglobin Concentration (MCHC)

An erythrocyte index indicating the average concentration of hemoglobin packed within a given volume of red blood cells, expressed in $\text{g/dl}$ (calculated as $\text{Hgb} \div \text{HCT}$, classifying cells as normochromic or hypochromic).

Breed Differences in RBC Mass

Normal physiological variations where Sighthounds, 'hot-blooded' horses, and beef cattle maintain significantly higher baseline RBC mass (HCT/PCV) than other breeds.

Breed and Age Differences in MCV

Akita, Shiba Inu, Shar-Pei, and Husky breeds naturally possess microcytic (smaller) red cells, whereas Sighthounds can be macrocytic; additionally, young animals (including foals and calves up to 1 year of age) have lower MCV and lower overall RBC mass compared to adults.

Erythrocyte

A red blood cell whose primary function is to transport hemoglobin carrying oxygen to tissues, protecting hemoglobin and tissues from oxidation and degradation, and optimizing oxygen delivery.

Hemoglobin (Hgb)

An iron-containing protein consisting of four globin chains ($2\beta$ and $2\text{a}$ chains) and four heme groups with central ferrous ($Fe^{2+}$) iron that reversibly binds and transports oxygen ($O_2$).

Ferrous Iron ($Fe^{2+}$)

The reduced state of iron in heme that is capable of binding and releasing oxygen to tissues.

Bohr Effect

The adaptation of hemoglobin's oxygen affinity to the local tissue environment, where lower pH and lower $O_2$ promote unloading in metabolically active tissues, while higher pH and higher $O_2$ facilitate oxygen loading in the alveoli.

2,3-Diphosphoglycerate (2,3-DPG)

An organic phosphate metabolite produced in red blood cells that regulates hemoglobin oxygen affinity; elevated levels decrease affinity to promote oxygen transfer to tissues.

Shift to the Right

A change in the oxygen-hemoglobin dissociation curve representing decreased oxygen affinity and optimized $O_2$ unloading, triggered by decreased pH, increased 2,3-DPG, increased $CO_2$, or increased temperature.

Shift to the Left

A change in the oxygen-hemoglobin dissociation curve representing increased oxygen affinity and optimized $O_2$ loading, triggered by increased pH, decreased 2,3-DPG, decreased $CO_2$, or decreased temperature.

Splenic Contraction

An immediate physiological response to high-altitude hypoxia occurring within minutes to hours that expels stored red blood cells into the circulation.

Embden-Meyerhof Pathway

The primary anaerobic glycolytic pathway in erythrocytes converting glucose to pyruvate or lactate, producing ATP and NADH essential for cell metabolism.

Rapaport-Lubering Pathway

An erythrocyte metabolic pathway branching off glycolysis from 1,3-DPG via DPG synthetase to synthesize 2,3-DPG, which modulates hemoglobin oxygen affinity.

Pentose Phosphate Shunt

A metabolic pathway in the red cell that utilizes glucose-6-phosphate dehydrogenase (G6PD) to generate NADPH, providing reducing power for glutathione reductase to convert GSSG to GSH and clear $H_2O_2$.

Methemoglobin (MetHgb)

An oxidized form of hemoglobin containing ferric ($Fe^{3+}$) iron that is unable to bind oxygen and gives whole blood a muddy brown coloration.

Methemoglobin Reductase Pathway

An antioxidant red cell pathway that uses NADH generated by glycolysis and the enzyme methemoglobin reductase to continuously reduce ferric iron ($Fe^{3+}$) back to ferrous iron ($Fe^{2+}$).

Feline Hemoglobin

A species form of hemoglobin characterized by containing 8 oxidant-sensitive sulfhydryl (SH) groups, making feline erythrocytes uniquely susceptible to oxidative injury.

Porphyria

A disorder caused by inherited enzyme defects in porphyrin synthesis or hepatic accumulation of plant porphyrins, resulting in teeth and bone discoloration, dental fluorescence, and dermal burns from photosensitization.

Basophilic Stippling

The microscopic aggregation of ribosomal machinery within erythrocytes seen in lead toxicosis due to impaired heme synthesis and dyserythropoiesis.

Lead Toxicosis

Heavy metal poisoning that inhibits ALA dehydratase and heme synthetase in the heme synthesis pathway, resulting in dyserythropoiesis, increased nucleated RBCs, and basophilic stippling.

Transferrin

The circulating serum carrier protein responsible for transporting iron ($Fe^{2+}$) throughout the body to developing erythroid cells and storage tissues.

Ferritin

An intracellular protein complex that provides short-term, readily mobilizable iron storage inside enterocytes and macrophages.

Hemosiderin

An insoluble, long-term iron storage complex within bone marrow and splenic macrophages that appears as bright blue granular pigment when stained with Prussian blue.

Ferroportin

The transmembrane iron portal channel on duodenal enterocytes and macrophages that mediates the export of iron into circulation, regulated by hepcidin.

Hepcidin

A liver-synthesized regulatory hormone that binds ferroportin causing its internalization and degradation, thereby decreasing intestinal iron absorption and macrophage iron release.

Erythroblastic Island

A specialized bone marrow microenvironmental unit containing a central nurse macrophage surrounded by developing erythroid precursor cells.

Hematocrit (HCT)

A calculated percentage of blood volume composed of red blood cells, determined mathematically by multiplying red cell count by mean corpuscular volume ($\text{RBC count} \times \text{MCV}$).

Packed Cell Volume (PCV)

A direct physical metric of red blood cell mass determined by centrifuging whole blood in a microhematocrit tube, closely correlating with calculated hematocrit.

Mean Corpuscular Volume (MCV)

A red blood cell index indicating the average volume of a single erythrocyte expressed in femtoliters ($\text{fl}$), used to classify cell populations as normocytic, microcytic, or macrocytic.

Mean Corpuscular Hemoglobin (MCH)

A red blood cell index reflecting the average mass of hemoglobin inside an individual erythrocyte in picograms ($\text{pg}$), calculated by dividing hemoglobin by total red cell count ($\text{Hgb} \text{ ÷ } \text{RBC count}$).

Mean Corpuscular Hemoglobin Concentration (MCHC)

A red blood cell index reflecting the average concentration of hemoglobin within a volume of packed erythrocytes in grams per deciliter ($\text{g/dl}$), calculated by dividing hemoglobin by hematocrit ($\text{Hgb} \text{ ÷ } \text{HCT}$).

Physiologic Microcytosis

A normal occurrence of smaller red blood cells (low MCV) seen in young animals such as calves and foals up to one year of age, as well as specific canine breeds including Akita, Shiba Inu, Shar-Pei, and Husky.