Red blood cell

Red blood cells (RBCs), also known as erythrocytes, are the predominant type of blood cell in vertebrates, primarily responsible for oxygen (O2) transport to body tissues via the circulatory system. In academic and medical contexts, these cells are often termed erythrocytes, a name derived from Ancient Greek words: erythros, meaning 'red', and kytos, meaning 'hollow vessel', with the suffix -cyte now signifying 'cell'. Other designations include red cells, erythroid cells, and occasionally haematids. Erythrocytes absorb oxygen in the lungs (or gills in fish) and subsequently release it into tissues as they navigate the body's capillary networks.

Red blood cells (RBCs), referred to as erythrocytes (from Ancient Greek erythros 'red' and kytos 'hollow vessel', with -cyte translated as 'cell' in modern usage) in academia and medical publishing, also known as red cells, erythroid cells, and rarely haematids, are the most common type of blood cell and the vertebrate's principal means of delivering oxygen (O§3132§) to the body tissues—via blood flow through the circulatory system. Erythrocytes take up oxygen in the lungs, or in fish the gills, and release it into tissues while squeezing through the body's capillaries.

The cytoplasm of red blood cells is densely packed with hemoglobin (Hb), an iron-rich biomolecule crucial for oxygen binding and imparting the characteristic red color to both the cells and blood. Each human red blood cell typically contains around 270 million hemoglobin molecules. The cell's membrane, composed of proteins and lipids, confers vital physiological properties such as deformability and structural stability, enabling the cell to traverse the circulatory system, particularly the narrow capillary network.

Mature human red blood cells are characterized by their flexible, biconcave disc shape. They are anucleated and lack organelles, a structural adaptation that maximizes internal volume for hemoglobin storage, effectively functioning as hemoglobin-filled sacs enclosed by a plasma membrane. In adult humans, approximately 2.4 million new erythrocytes are generated every second. These cells originate in the bone marrow and circulate for an average of 100–120 days before their constituents are recycled by macrophages. A complete circulation throughout the body takes approximately 60 seconds. Red blood cells constitute a significant proportion of the human body's cellular mass, accounting for 20–30 trillion cells, or about 84% of all cells, and represent nearly half (40% to 45%) of the total blood volume.

Packed red blood cells refer to erythrocytes that have been collected, processed, and preserved in a blood bank for the purpose of blood transfusion.

Structure

Vertebrates

Red blood cells are present in the vast majority of vertebrates, including mammals and humans, where they serve as the primary cellular component for oxygen transport within the blood. A notable exception is the crocodile icefish (family Channichthyidae), which lacks red blood cells. These fish thrive in highly oxygenated, cold waters, transporting oxygen dissolved directly in their blood plasma. Although they do not utilize hemoglobin, genetic remnants of hemoglobin genes are still detectable in their genome.

Vertebrate red blood cells are predominantly composed of hemoglobin, a complex metalloprotein. This protein contains heme groups, whose iron atoms reversibly bind to oxygen molecules (_O2§) in the lungs or gills, facilitating their release throughout the body. Oxygen readily diffuses across the erythrocyte's cell membrane. Beyond oxygen transport, hemoglobin within red blood cells also carries a portion of the metabolic waste product carbon dioxide from tissues. However, the majority of waste carbon dioxide is transported to the pulmonary capillaries of the lungs as bicarbonate (HCO₃⁻) dissolved in the blood plasma. Myoglobin, a molecule structurally related to hemoglobin, functions as an oxygen storage protein within muscle cells.

The characteristic color of red blood cells originates from the heme group within hemoglobin. While blood plasma is typically straw-colored, the erythrocytes themselves exhibit color variations based on hemoglobin's oxygenation state: oxyhemoglobin, formed when oxygen is bound, appears scarlet, whereas deoxyhemoglobin, after oxygen release, displays a dark red or burgundy hue. Despite these internal colors, blood can appear bluish when viewed through the vessel walls and skin. Pulse oximetry leverages these hemoglobin color changes to non-invasively determine arterial blood oxygen saturation through colorimetric analysis. Hemoglobin also possesses a significantly high affinity for carbon monoxide, forming carboxyhemoglobin, which is notably bright red. Clinically, patients presenting with flushing, confusion, and a seemingly normal 100% saturation reading on pulse oximetry may, in fact, be experiencing carbon monoxide poisoning.

The presence of oxygen-carrying proteins within specialized cells, rather than dissolved in body fluid, represented a significant evolutionary advancement in vertebrates. This adaptation facilitated reduced blood viscosity, elevated oxygen concentrations, and enhanced oxygen diffusion from the bloodstream into tissues. Vertebrate species exhibit considerable variation in red blood cell size; on average, the width of red blood cells is approximately 25% greater than the diameter of capillaries, a characteristic hypothesized to optimize oxygen transfer from these cells to surrounding tissues.

Mammals

Mammalian red blood cells typically exhibit a biconcave disc morphology, characterized by a flattened, centrally depressed form, a dumbbell-shaped cross-section, and a toroidal rim along the periphery. This specific configuration provides a high surface-area-to-volume (SA/V) ratio, which is crucial for efficient gas diffusion. Nevertheless, notable morphological exceptions exist within the order Artiodactyla (even-toed ungulates, such as cattle, deer, and their kin), which present a diverse array of unusual red blood cell forms. These include small, highly ovaloid cells in llamas and camels (family Camelidae), minute spherical cells in mouse deer (family Tragulidae), and fusiform, lanceolate, crescentic, irregularly polygonal, and other angular shapes observed in red deer and wapiti (family Cervidae). The evolutionary trajectory of red blood cell development in members of this order has diverged significantly from the typical mammalian pattern. Generally, mammalian red blood cells possess remarkable flexibility and deformability, enabling them to navigate through narrow capillaries and to maximize their surface contact by adopting a cigar-like shape, thereby efficiently releasing their oxygen payload.

Mammalian red blood cells are distinctive among vertebrates because, in most species, they lack a nucleus upon maturation. While nuclei are present during the early stages of erythropoiesis, they are extruded during development as the cells mature, a process that creates additional space for hemoglobin. These anucleated red blood cells, known as reticulocytes, subsequently shed all other cellular organelles, including mitochondria, Golgi apparatus, and endoplasmic reticulum.

The spleen functions as a reservoir for red blood cells, although this capacity is relatively restricted in humans. Conversely, in certain other mammals, such as dogs and horses, the spleen sequesters substantial quantities of red blood cells, which are then released into the bloodstream during periods of exertional stress, thereby augmenting oxygen transport capacity.

Human

A typical human red blood cell exhibits a disc diameter ranging from approximately 6.2 to 8.2 μm, with a maximum thickness of 2–2.5 μm and a central minimum thickness of 0.8–1 μm, making it considerably smaller than the majority of other human cell types. These cells possess an average volume of approximately 90 fL and a surface area of about 136 μm². They are capable of swelling into a spherical shape, accommodating up to 150 fL, without experiencing membrane distension.

Adult humans typically possess an estimated 20–30 trillion red blood cells at any given moment, which accounts for approximately 70% of all cells by numerical count. Women generally have about 4–5 million red blood cells per microliter (cubic millimeter) of blood, while men typically have 5–6 million; individuals residing at high altitudes, where oxygen tension is lower, will exhibit higher counts. Consequently, red blood cells are significantly more abundant than other blood components, with approximately 4,000–11,000 white blood cells and 150,000–400,000 platelets present per microliter.

On average, human red blood cells complete a full circulatory cycle in approximately 60 seconds.

The characteristic red coloration of blood is attributable to the spectral properties of the heme iron ions contained within hemoglobin. Each hemoglobin molecule binds four heme groups, and hemoglobin itself comprises approximately one-third of the total cellular volume. Hemoglobin facilitates the transport of over 98% of the body's oxygen, with the residual oxygen being dissolved in the blood plasma. Collectively, the red blood cells of an average adult human male store approximately 2.5 grams of iron, accounting for about 65% of the body's total iron content.

Microstructure

Nucleus

Mature mammalian red blood cells are anucleate, signifying the absence of a cell nucleus. In contrast, red blood cells in other vertebrates typically retain their nuclei; the sole documented exceptions include salamanders from the family Plethodontidae, where five distinct clades have independently evolved varying degrees of enucleated red blood cells (with the most pronounced evolution observed in certain species of the genus Batrachoseps), and fish belonging to the genus Maurolicus.

The elimination of the nucleus in vertebrate red blood cells has been proposed as an explanation for the subsequent accumulation of non-coding DNA within the genome. This hypothesis posits that efficient gas transport necessitates red blood cells to traverse extremely narrow capillaries, thereby imposing a size constraint. In the absence of nuclear elimination, the accumulation of repeat sequences would be limited by the volume occupied by the nucleus, which expands proportionally with genome size.

Mammalian nucleated red blood cells manifest in two distinct forms: normoblasts, which represent normal erythropoietic precursors to mature erythrocytes, and megaloblasts, which are abnormally large precursors characteristic of megaloblastic anemias.

Membrane Composition

Erythrocytes exhibit deformability, flexibility, the capacity for intercellular adhesion, and the ability to interact with immune cells. These functions are highly contingent upon the specific composition of the erythrocyte membrane, which plays a crucial role in these processes. The red blood cell membrane comprises three distinct layers: the exterior glycocalyx, rich in carbohydrates; the lipid bilayer, containing numerous transmembrane proteins in addition to its primary lipid constituents; and the membrane skeleton, a structural protein network situated on the inner surface of the lipid bilayer. In human and most mammalian red blood cells, proteins constitute half of the membrane's mass, with the remaining half composed of lipids, specifically phospholipids and cholesterol.

Membrane Lipids

The erythrocyte membrane features a characteristic lipid bilayer, analogous to those found in nearly all human cells. This lipid bilayer is constituted by cholesterol and phospholipids in approximately equal proportions by weight. The specific lipid composition is critical, as it dictates numerous physical properties, including membrane permeability and fluidity. Furthermore, the activity of many membrane proteins is modulated through interactions with lipids within the bilayer.

In contrast to cholesterol, which is uniformly distributed between the inner and outer leaflets, the five primary phospholipids exhibit an asymmetric disposition.

Outer Monolayer

• Phosphatidylcholine (PC);
• Sphingomyelin (SM).

Inner Monolayer

• Phosphatidylethanolamine (PE);
• Phosphoinositol (PI) (small amounts).
• Phosphatidylserine (PS);

The asymmetric distribution of phospholipids within the bilayer results from the activity of various energy-dependent and energy-independent phospholipid transport proteins. Proteins termed "flippases" translocate phospholipids from the outer to the inner monolayer, whereas "floppases" perform the inverse operation, moving phospholipids against a concentration gradient in an energy-dependent fashion. Furthermore, "scramblase" proteins facilitate the bidirectional movement of phospholipids simultaneously, down their concentration gradients, in an energy-independent manner. Significant scholarly debate persists concerning the precise identity of these membrane maintenance proteins within the erythrocyte membrane.

Maintaining an asymmetric phospholipid distribution within the bilayer, exemplified by the exclusive localization of phosphatidylserine (PS) and phosphoinositols (PIs) in the inner monolayer, is crucial for cellular integrity and function for several reasons:

• Macrophages identify and phagocytose erythrocytes that expose phosphatidylserine (PS) on their outer surface. Consequently, the sequestration of PS within the inner monolayer is imperative for cellular survival during frequent interactions with reticuloendothelial system macrophages, particularly those in the spleen.
• The premature destruction of thalassemic and sickle erythrocytes has been correlated with disruptions in lipid asymmetry, resulting in the exposure of phosphatidylserine (PS) on the outer monolayer.
• The exposure of phosphatidylserine (PS) can enhance the adhesion of erythrocytes to vascular endothelial cells, thereby impeding normal transit through the microvasculature. Therefore, the exclusive maintenance of PS within the inner leaflet of the bilayer is essential to ensure unimpeded blood flow in the microcirculation.
• Both phosphatidylserine (PS) and phosphatidylinositol 4,5-bisphosphate (PIP2) are capable of regulating membrane mechanical function through their interactions with skeletal proteins, including spectrin and protein 4.1R. Recent investigations indicate that spectrin binding to PS contributes to enhanced membrane mechanical stability. PIP2 augments the binding of protein band 4.1R to glycophorin C while diminishing its interaction with protein band 3, potentially modulating the linkage between the bilayer and the membrane skeleton.

Recent studies have elucidated the presence of specialized structures, termed "lipid rafts," within the red blood cell membrane. These structures are characterized by their enrichment in cholesterol and sphingolipids, and their association with particular membrane proteins, including flotillins, STOMatins (band 7), G-proteins, and β-adrenergic receptors. While recognized for their role in cell signaling within non-erythroid cells, lipid rafts in erythroid cells have been demonstrated to mediate β2-adrenergic receptor signaling and elevate cyclic AMP (cAMP) levels, thereby influencing the entry of malarial parasites into healthy red blood cells.

Membrane Proteins

The proteins comprising the membrane skeleton confer the red blood cell's deformability, flexibility, and durability. This enables the erythrocyte to traverse capillaries significantly narrower than its own diameter (7–8 μm) and to rapidly regain its discoid morphology upon the cessation of compressive forces, akin to the elastic recovery of a rubber object.

Over 50 distinct membrane proteins have been identified, with copy numbers ranging from several hundred to a million per erythrocyte. Approximately 25 of these proteins bear the diverse blood group antigens, including A, B, and Rh, among numerous others. These membrane proteins execute a broad spectrum of functions, encompassing the transport of ions and molecules across the erythrocyte membrane, mediating adhesion and interaction with other cells like endothelial cells, acting as signaling receptors, and fulfilling other as-yet-undetermined roles. Human blood types are determined by variations in the surface glycoproteins of red blood cells. Dysfunctions of these membrane proteins are linked to numerous pathologies, including hereditary spherocytosis, hereditary elliptocytosis, hereditary stomatocytosis, and paroxysmal nocturnal hemoglobinuria.

Erythrocyte Membrane Proteins: Functional Classification

Transport

• Band 3: Functions as an anion transporter and constitutes a critical structural element of the erythrocyte membrane, comprising up to 25% of its surface area, with approximately one million copies per cell. It also defines the Diego Blood Group.
• Aquaporin 1: A water transporter that defines the Colton Blood Group.
• Glut1: A transporter for glucose and L-dehydroascorbic acid.
• MCT1: A monocarboxylate transporter responsible for exporting lactic acid to the liver.
• Kidd antigen protein: Functions as a urea transporter.
• RHAG: A gas transporter, likely for carbon dioxide, which defines the Rh Blood Group and the associated unusual Rh_null blood group phenotype.
• Na⁺/K⁺-ATPase.
• Ca²⁺-ATPase.
• Na⁺/K⁺/2Cl⁻-cotransporter.
• Na⁺/Cl⁻-cotransporter.
• Na-H exchanger.
• K-Cl cotransporter.
• Gardos Channel.

Cell Adhesion

• ICAM-4: Interacts with integrins.
• BCAM: A glycoprotein that defines the Lutheran blood group, also known as Lu or laminin-binding protein.

Structural Role: These membrane proteins form linkages with skeletal proteins, potentially playing a crucial role in regulating cohesion between the lipid bilayer and the membrane skeleton. This function likely enables the erythrocyte to preserve its optimal membrane surface area by inhibiting membrane collapse (vesiculation).

• Ankyrin-based macromolecular complex: Comprises proteins that link the lipid bilayer to the membrane skeleton via interactions between their cytoplasmic domains and Ankyrin.
  • Band 3: Additionally assembles various glycolytic enzymes, the putative CO₂ transporter, and carbonic anhydrase into a macromolecular complex known as a "metabolon," which may be pivotal in regulating erythrocyte metabolism and its ion and gas transport functions.
  • RHAG: Also participates in transport and defines the associated unusual blood group phenotype Rh_mod.
• Protein 4.1R-based macromolecular complex: Consists of proteins that interact with Protein 4.1R.
  • Protein 4.1R: Characterized by weak expression of Gerbich antigens.
  • Glycophorin C and D: Glycoproteins that define the Gerbich Blood Group.
  • XK: Defines the Kell Blood Group and the McLeod unusual phenotype, characterized by the absence of Kx antigen and significantly reduced expression of Kell antigens.
  • RhD/RhCE: Defines the Rh Blood Group and the associated unusual Rh_null blood group phenotype.
  • Duffy protein: Hypothesized to be involved in chemokine clearance.
  • Adducin: Facilitates interaction with Band 3.
  • The interaction of dematin with the Glut1 glucose transporter.

Surface Electrostatic Potential

Surface electrostatic potential

The zeta potential, an electrochemical characteristic of cell surfaces, is determined by the net electrical charge of molecules exposed on the cell membrane. Erythrocytes typically exhibit a zeta potential of −15.7 millivolts (mV). A significant portion of this potential is attributed to the exposed sialic acid residues within the membrane; their removal reduces the zeta potential to −6.06 mV.

Function

Role in CO₂ Transport

Respiration, schematically represented here with a carbohydrate unit, generates approximately equivalent quantities of carbon dioxide (CO₂) and consumes oxygen (O₂).

${\displaystyle {\ce {HCOH + O2 -> CO2 + H2O}}}$

Consequently, the circulatory system's function encompasses the transport of carbon dioxide to the same extent as oxygen. The majority of carbon dioxide in the blood exists as bicarbonate ions, which serve as a crucial pH buffer. Therefore, unlike hemoglobin's role in O₂ transport, a physiological advantage exists in the absence of a specific CO₂ transporter molecule.

Nonetheless, red blood cells play a pivotal role in the CO₂ transport process due to two primary factors. Firstly, beyond hemoglobin, they possess numerous copies of the enzyme carbonic anhydrase within their cell membrane. Carbonic anhydrase, as implied by its nomenclature, catalyzes the interconversion of carbonic acid and carbon dioxide (the anhydride of carbonic acid). As a catalyst, it can process a multitude of CO₂ molecules, thereby fulfilling its essential function without requiring the high copy numbers necessary for O§4_{5§ transport by hemoglobin. In the presence of this catalyst, carbon dioxide and carbonic acid rapidly attain equilibrium while erythrocytes traverse the capillaries. Consequently, erythrocytes are instrumental in ensuring that the majority of CO§67§ is transported in the form of bicarbonate. Under physiological pH conditions, the equilibrium significantly favors carbonic acid, which predominantly dissociates into bicarbonate ions.}

${\displaystyle {\ce {CO2 + H2O <=>> H2CO3 <=>> HCO3- + H+}}}$

The hydrogen ions (H+) liberated by this swift reaction within red blood cells (RBCs), while still situated in the capillary, diminish hemoglobin's oxygen-binding affinity, a phenomenon known as the Bohr effect.

Red blood cells (RBCs) also significantly contribute to carbon dioxide transport through a second mechanism: carbon dioxide directly interacts with the globin protein constituents of hemoglobin, leading to the formation of carbaminohemoglobin compounds. The Haldane effect describes how, as oxygen is released in the tissues, an increased quantity of CO₂ binds to hemoglobin; conversely, when oxygen binds in the lungs, it displaces the CO₂ previously bound to hemoglobin. Although only a minor fraction of the CO§4_{5§ in venous blood is bound to hemoglobin, a substantial portion of the variation in CO§67§ content between venous and arterial blood is attributable to alterations in this bound CO§89§. This is because bicarbonate remains consistently abundant in both venous and arterial blood, fulfilling its crucial function as a pH buffer.}

In summary, carbon dioxide generated through cellular respiration swiftly diffuses into regions of lower concentration, particularly into adjacent capillaries. Upon entering a red blood cell (RBC), CO₂ undergoes rapid conversion into bicarbonate ion by carbonic anhydrase, an enzyme located on the inner surface of the RBC membrane. Subsequently, these bicarbonate ions exit the RBC, exchanging with chloride ions from the plasma, a process mediated by the band 3 anion transport protein situated within the RBC membrane. The bicarbonate ion does not re-diffuse from the capillary but is instead transported to the lungs. Within the pulmonary system, the reduced partial pressure of carbon dioxide in the alveoli prompts its rapid diffusion from the capillaries into the alveolar spaces. Carbonic anhydrase within the red cells maintains the bicarbonate ion in equilibrium with carbon dioxide. Consequently, as carbon dioxide departs the capillary and CO₂ is displaced by O§4_{5§ on hemoglobin, an adequate quantity of bicarbonate ion rapidly transforms into carbon dioxide to sustain this equilibrium.}

Secondary functions

Under conditions of shear stress within constricted vessels, erythrocytes release adenosine triphosphate (ATP), which induces vasodilation and relaxation of vessel walls, thereby facilitating unimpeded blood flow.

Deoxygenated hemoglobin within erythrocytes triggers the release of S-Nitrosothiols, compounds that also contribute to vasodilation, consequently enhancing blood perfusion to hypoxic regions of the body.

Erythrocytes are capable of enzymatically synthesizing nitric oxide, utilizing L-arginine as a substrate, a process also observed in endothelial cells. Physiological shear stress exposure activates nitric oxide synthase in red blood cells, leading to nitric oxide export, which potentially influences vascular tone regulation.

Furthermore, erythrocytes can generate hydrogen sulfide, a gaseous signaling molecule known to induce relaxation of vascular walls. The cardioprotective properties attributed to garlic are hypothesized to stem from the conversion of its sulfur compounds into hydrogen sulfide by red blood cells.

Erythrocytes also contribute to the host's immune defense; upon lysis by bacterial pathogens, their hemoglobin releases free radicals that disrupt the pathogen's cell wall and membrane, leading to its demise.

Cellular Processes

Lacking mitochondria, erythrocytes do not consume the oxygen they transport; instead, they generate adenosine triphosphate (ATP) through glucose glycolysis and subsequent lactic acid fermentation of pyruvate. Additionally, the pentose phosphate pathway is crucial for red blood cell function.

Given the anucleated nature of mature erythrocytes, protein biosynthesis is presumed to be absent within these cells.

The absence of nuclei and organelles in mature erythrocytes precludes the presence of DNA and the synthesis of new RNA (though existing RNAs are present), thereby limiting their repair capabilities and preventing cell division. This incapacity for protein synthesis implies that no virus can evolve to specifically target mature mammalian red blood cells. Nevertheless, parvovirus infections, such as human parvovirus B19, can impact erythroid precursors that still possess DNA, evidenced by the observation of giant pronormoblasts containing viral particles and inclusion bodies, which leads to a transient depletion of reticulocytes and subsequent anemia.

Life Cycle

Human erythrocytes are generated via erythropoiesis, a process spanning approximately 7 days, during which committed stem cells differentiate into mature red blood cells. In healthy adults, these mature cells circulate for approximately 100 to 120 days (or 80 to 90 days in full-term infants) before being cleared from circulation at the culmination of their lifespan. The erythrocyte lifespan is often diminished in various chronic pathologies.

Genesis

Erythropoiesis, the approximately 7-day process of new erythrocyte production, occurs continuously within the red bone marrow of large bones. (During embryonic development, the liver serves as the primary site of red blood cell genesis.) This production can be stimulated by erythropoietin (EPO), a hormone synthesized by the kidneys. Developing erythrocytes are termed reticulocytes just prior to and immediately following their egress from the bone marrow, typically comprising about 1% of the circulating red blood cell population.

Functional Longevity

The functional lifespan of an erythrocyte extends for approximately 100–120 days, throughout which these cells continuously traverse the circulatory system.

Senescence

As red blood cells age, their plasma membranes undergo modifications, rendering them vulnerable to selective recognition and subsequent phagocytosis by macrophages within the mononuclear phagocyte system, which encompasses the spleen, liver, and lymph nodes. This mechanism effectively eliminates senescent and defective cells, thereby continuously purifying the bloodstream. This phenomenon is designated eryptosis, or programmed red blood cell death. Typically, eryptosis proceeds at a rate equivalent to erythropoiesis, maintaining a stable total circulating red blood cell count. Elevated eryptosis levels are associated with numerous pathological conditions, such as sepsis, hemolytic uremic syndrome, malaria, sickle cell anemia, beta-thalassemia, glucose-6-phosphate dehydrogenase deficiency, phosphate depletion, iron deficiency, and Wilson's disease. Osmotic shock, oxidative stress, and energy depletion can induce eryptosis, as can a diverse array of endogenous mediators and xenobiotics. Furthermore, erythrocytes deficient in cGMP-dependent protein kinase type I or AMP-activated protein kinase (AMPK) exhibit heightened eryptosis. Key inhibitors of eryptosis comprise erythropoietin, nitric oxide, catecholamines, and elevated concentrations of urea.

A significant portion of the degradation products generated from this process is subsequently recirculated within the organism. The heme component of hemoglobin undergoes catabolism into iron (Fe³⁺) and biliverdin. Biliverdin is then reduced to bilirubin, which enters the plasma and is transported to the liver while bound to albumin. The liberated iron is released into the plasma for recirculation, facilitated by the carrier protein transferrin. Virtually all erythrocytes are cleared from circulation through this mechanism prior to reaching an age where hemolysis would naturally occur. Hemolyzed hemoglobin, when present, binds to haptoglobin, a plasma protein that prevents its renal excretion.

Clinical Significance

Pathological Conditions

Disorders involving red blood cells include:

• Anemias, also spelled anaemias, constitute a group of disorders defined by a diminished oxygen-carrying capacity of the blood, resulting from either a reduced erythrocyte count or structural abnormalities in red blood cells or hemoglobin.

• Iron deficiency anemia represents the most prevalent form of anemia, arising from inadequate dietary iron intake or impaired iron absorption, which subsequently impedes the formation of iron-containing hemoglobin.

• Pernicious anemia is an autoimmune condition characterized by the body's inability to produce intrinsic factor, a protein essential for the absorption of dietary vitamin B₁₂. Vitamin B₁₂ is crucial for the synthesis of both erythrocytes and hemoglobin.

• Sickle-cell disease is a hereditary disorder characterized by the presence of aberrant hemoglobin molecules. Upon deoxygenation in the tissues, these abnormal hemoglobin molecules polymerize, causing erythrocytes to adopt a characteristic sickle shape. These rigid, sickle-shaped erythrocytes exhibit reduced deformability and viscoelasticity, which can precipitate vascular occlusions, severe pain, cerebrovascular accidents, and other forms of tissue injury.

• Thalassemia refers to a group of genetic disorders marked by an imbalanced production ratio of hemoglobin subunits.

• Hereditary spherocytosis syndromes comprise a collection of inherited conditions defined by intrinsic defects in the erythrocyte membrane, leading to the formation of small, spherical, and fragile cells, in contrast to the typical biconcave, flexible morphology. These aberrant erythrocytes are subsequently sequestered and destroyed by the spleen. Additionally, various other hereditary disorders affecting the red blood cell membrane have been identified.

• Aplastic anemia results from the bone marrow's impaired capacity to generate all types of blood cells.

• Pure red cell aplasia is a condition characterized by the bone marrow's selective failure to produce erythrocytes.

• Hemolysis denotes the pathological process of excessive erythrocyte destruction. This phenomenon can arise from various etiologies and may culminate in hemolytic anemia.

• The malaria parasite undergoes a portion of its life cycle within erythrocytes, consuming their hemoglobin and subsequently lysing the cells, which contributes to febrile episodes. Both sickle-cell disease and thalassemia exhibit a higher prevalence in regions endemic for malaria, as these genetic mutations confer a degree of protective immunity against the parasite.

• Polycythemias, also known as erythrocytoses, are pathological conditions defined by an excessive proliferation of red blood cells. The resultant elevation in blood viscosity can manifest in a variety of clinical symptoms.

• In polycythemia vera, the elevated erythrocyte count originates from an intrinsic abnormality within the bone marrow.

• Microangiopathic diseases, such as disseminated intravascular coagulation and thrombotic microangiopathies, are characterized by the presence of diagnostic red blood cell fragments known as schistocytes. These pathologies induce the formation of fibrin strands that mechanically sever erythrocytes as they attempt to navigate past a thrombus.

Transfusion

Erythrocytes are administered during blood transfusions, with the blood sourced either from an allogeneic donor or autologously stored by the recipient. Donated blood necessitates rigorous screening to mitigate risks of transmitting blood-borne pathogens and to ensure donor safety. Typically, collected blood undergoes testing for prevalent and severe blood-borne pathogens, including Hepatitis B, Hepatitis C, and HIV. Blood products are meticulously typed (A, B, AB, or O) and cross-matched with the recipient's blood to minimize the risk of acute hemolytic transfusion reactions, which are mediated by surface antigens on the erythrocytes. Following these procedures, the blood is stored for subsequent administration. Transfusions can involve whole blood or, more commonly, packed red blood cells, where erythrocytes are separated from plasma.

Blood is frequently transfused in cases of diagnosed anemia, active hemorrhage, or anticipated significant blood loss, such as before surgical procedures. Prior to administration, a small sample of the recipient's blood undergoes cross-matching with the transfusion product.

In 2008, research indicated that human embryonic stem cells had been successfully differentiated into erythrocytes in vitro. A critical challenge involved inducing enucleation, which was overcome by culturing the cells on bone marrow-derived stromal cells. These artificially generated erythrocytes hold potential for future therapeutic blood transfusions.

A human trial was conducted in 2022, utilizing erythrocytes cultured from stem cells derived from donor blood.

Diagnostic Tests

Erythrocytes are central to several diagnostic blood tests. These encompass the RBC count (quantifying erythrocytes per unit volume of blood), hematocrit determination (measuring the percentage of blood volume attributed to red blood cells), and the erythrocyte sedimentation rate. Blood typing is essential for preparing for blood transfusions or organ transplantations.

Numerous erythrocyte-related pathologies are diagnosed via a blood film (or peripheral blood smear), which involves examining a thin layer of blood on a microscope slide. Such examination can reveal poikilocytosis, characterized by variations in erythrocyte morphology. Erythrocytes may also aggregate into stacks, known as rouleaux formation, a phenomenon more prevalent with elevated levels of specific serum proteins, such as during inflammatory states.

Erythrocyte Separation and Blood Doping

Erythrocytes are isolated from whole blood through centrifugation, a process of blood fractionation that separates cellular components from plasma. Packed red blood cells, prepared by removing plasma from whole blood, are routinely employed in transfusion medicine. In plasma donation procedures, erythrocytes are immediately returned to the donor, with only the plasma retained.

Certain athletes have engaged in blood doping to enhance performance. This typically involves extracting approximately 1 liter of blood, isolating, freezing, and storing the erythrocytes (which can be preserved for five weeks at −79 °C or −110 °F, or over ten years with cryoprotectants), and then re-injecting them shortly before a competition. While challenging to detect, this practice poses significant risks to the cardiovascular system, which is not adapted to manage the resultant increased blood viscosity. An alternative blood doping strategy involves erythropoietin injections to stimulate endogenous erythrocyte production. Both methods are prohibited by the World Anti-Doping Agency.

Historical Context

The initial description of erythrocytes is attributed to the Dutch biologist Jan Swammerdam, who utilized an early microscope in 1658 to examine frog blood. Subsequently, in 1674, Anton van Leeuwenhoek, independently, offered a more precise microscopic account of red blood cells, even estimating their size as "25,000 times smaller than a fine grain of sand."

During the 1740s, Vincenzo Menghini, working in Bologna, successfully demonstrated the presence of iron by applying magnets to the powdered residue or ash derived from heated red blood cells.

In 1901, Karl Landsteiner published his seminal discovery of the three primary blood groups: A, B, and C (subsequently reclassified as O). Landsteiner meticulously detailed the consistent reaction patterns observed when serum was combined with red blood cells, thereby establishing compatible and incompatible combinations among these blood types. A year subsequent to this, Alfred von Decastello and Adriano Sturli, both associates of Landsteiner, identified a fourth blood group, designated AB.

Utilizing X-ray crystallography in 1959, Max Perutz successfully elucidated the intricate structure of hemoglobin, the crucial protein within red blood cells responsible for oxygen transport.

The most ancient intact red blood cells ever identified were located within Ötzi the Iceman, a naturally preserved mummy of an individual who perished approximately 3255 BCE. This discovery of the cells occurred in May 2012.

• List of distinct cell types in the adult human body
• Blood substitute
• Erythrocrine
• Serum (blood)

References

Blood Groups and Red Cell Antigens by Laura Dean. This online textbook is searchable, downloadable, and available in the public domain.

• Blood Groups and Red Cell Antigens by Laura Dean. Searchable and downloadable online textbook in the public domain.
• Database of vertebrate erythrocyte sizes. Archived on 5 July 2008, via the Wayback Machine.
• Red Gold, a PBS website providing factual information and historical context.