Erythrocyte Series PDF

PAGE 1 UNIVERSITÄTSMEDIZIN NEUMARKT A. M. https://edu.umch.de www.umfst.ro CAMPUS HAMBURG Prof.univ.dr. Anca Bacârea 2024 May RED BLOD CELLS Asist.univ.dr. Andreea Tinca Red blood cels (RBC)...

PAGE 1 UNIVERSITÄTSMEDIZIN NEUMARKT A. M. https://edu.umch.de www.umfst.ro CAMPUS HAMBURG Prof.univ.dr. Anca Bacârea 2024 May RED BLOD CELLS Asist.univ.dr. Andreea Tinca Red blood cels (RBC) PAGE 2 The major function of RBCs, also known as erythrocytes, is to transport hemoglobin, which, in turn, carries oxygen from the lungs to the tissues. Other functions The hemoglobin in the cells is an excellent acid-base buffer (as RBC they contain a large quantity of carbonic anhydrase, an is true of most proteins), so the RBCs are responsible for most enzyme that catalyzes the reversible reaction between carbon of the acid-base buffering power of whole blood. dioxide (CO2) and water to form carbonic acid (H2CO3). RBC PAGE 3 Normal RBCs, shown are biconcave disks having a mean diameter of about 7.8 micrometers and a thickness of 2.5 micrometers at the thickest point and 1 micrometer or less in the center. The shape of RBCs can change remarkably as the cells squeeze through capillaries. In healthy men, the average number of RBCs per cubic millimeter is 5,200,000 (±300,000); in women, it is 4,700,000 (±300,000). People living at high altitudes have greater numbers of RBCs (adaptative phenomenon). RBC PAGE 4 Personal collection of prof Anca Bacarea RBC membrane deformability PAGE 5 RBCs are biconcave and average 90 fL in volume. Their average surface area is 140 mm2, a 40% excess of surface area compared with a 90-fL sphere. This excess surface-to-volume ratio enables RBCs to stretch undamaged as they pass through narrow capillaries and through splenic pores 2 mm in diameter; this property is called RBC deformability. The RBC plasma membrane, which is 5 mm thick, is 100 times more elastic than a comparable latex membrane. The deformable RBC membrane provides the broad surface area and close tissue contact necessary to support the delivery of O2 from lungs to body tissue and CO2 from body tissue to lungs. RBC membrane deformability PAGE 6 RBC deformability depends also on relative cytoplasmic (hemoglobin) viscosity. The normal mean cell hemoglobin concentration (MCHC) ranges from 32% to 36% and as MCHC rises, internal viscosity rises. MCHCs above 36% compromise deformability and shorten the RBC life span because viscous cells become damaged as they stretch to pass through narrow capillaries or splenic pores. As RBCs age, they lose membrane surface area, while retaining hemoglobin. As the MCHC rises, the RBC, unable to pass through the splenic pores, is destroyed by splenic macrophages. RBC membrane deformability PAGE 7 The RBC membrane consists of approximately 8% carbohydrates, 52% proteins, and 40% lipids. The lipid portion, equal parts of cholesterol and phospholipids, forms a bilayer universal to all animal cells. – Membrane enzymes maintain the cholesterol concentration by regularly exchanging membrane and plasma cholesterol. – Phospholipids form an impenetrable fluid barrier (hydrophilic polar head groups are arrayed upon the membrane’s surfaces, oriented toward both the plasma and the cytoplasm and their hydrophobic nonpolar acyl tails arrange themselves to form a central layer hidden the aqueous plasma and cytoplasm). The membrane maintains extreme differences in osmotic pressure, cation concentrations, and gas concentrations between external plasma and the cytoplasm. RBC membrane deformability PAGE 8 Phosphatidylcholine and sphingomyelin predominate in the outer layer; phosphatidylserine (PS) and phosphatidylethanolamine form most of the inner layer. If phospholipid distribution is disrupted -> sickle cell anemia, thalassemia senescent RBCs Splenic macrophages possess receptors that bind phostatidilserine and destroy senescent and damaged RBCs. RBC membrane deformability PAGE 9 RBC membrane proteins – Although cholesterol and phospholipids constitute RBC membrane structure, transmembrane (integral) and cytoskeletal (skeletal, peripheral) proteins make up 52% of the membrane structure by mass. – The transmembrane proteins serve a number of RBC functions: Through glycosylation they support surface carbohydrates, which join with glycolipids to make up the protective glycocalyx. Transport and adhesion sites and signaling receptors. Any disruption in transport protein function leads to a rise in viscosity and loss of deformability. RBC cytoskeleton PAGE 10 Under the cell membrane there is a two-dimensional network of fibers = cytoskeleton: – anchors the membrane proteins, allowing a uniform distribution - this increases the efficiency of membrane transport mechanisms – provides elasticity, deformability to the red blood cells – connects to transmembrane proteins through actin and ankyrin molecules Osmotic balance and permeability PAGE 11 The RBC membrane is impermeable to cations Na+, K+, and Ca2+. It is permeable to water and the anions bicarbonate (HCO3-) and chloride (Cl-), which freely exchange between plasma and RBC cytoplasm. Aquaporin 1 is a transmembrane protein which allows inward water flow in response to internal osmotic changes. The ATP–dependent cation pumps Na1-ATPase and K1-ATPase regulate the concentrations of Na+ and K+ Ca2+-ATPase extrudes calcium, maintaining low intracellular levels of 5 to 10 mmol/L. Calmodulin, a cytoplasmic Ca2+-binding protein, controls the function of Ca2+-ATPase. The cation pumps consume 15% of RBC ATP production. ATP loss or pump damage permits Ca2+ and Na+ influx, with water following osmotically. The cell swells, becomes spheroid, and eventually ruptures. This phenomenon is called colloid osmotic hemolysis. RBC production PAGE 12 The bone marrow of essentially all bones produces RBCs until a person is 5 years old. The marrow of the long bones, except for the proximal portions of the humeri and tibiae, becomes quite fatty and produces no more RBCs after about age 20 years. Beyond this age, most RBCs continue to be produced in the marrow of the membranous bones, such as the vertebrae, sternum, ribs, and ilia. Even in these bones, the marrow becomes less productive as age increases. RBC production PAGE 13 Hematopoiesis is the process by which mature blood cells are generated and functional Bone marrow (BM) is the central component generating blood cells: red cells, granulocytes, monocytes, lymphocytes, platelets and hematopoietic functions are proliferation, differentiation and cell release into circulation. BM consists of: – reticulovascular stroma (with supporting role, nutrition and movement of hematopoietic cells); – medullar parenchyma - composed of active cells forming islands of hematopoiesis, usually arranged around a trophic cell - "nurse cell." Nurse cells are involved in erythropoiesis (iron stores) in myelopoesis and megacaryopoesis.  PAGE 14 Erythrocyte sequence Guyton and Hall, Textbook of Medical Physiology, 2016 Erythropoiesis PAGE 15 Erythropoiesis typically occurs in erythroid islands. These are macrophages surrounded by erythroid precursors in various stages of development. Macrophages eliberate cytokines that are vital to the maturation process and survival of the RBCs. They promote erythroid maturation by producing trophic cytokines, providing iron for hemoglobin synthesis, and removing extruded nuclei. Erythropoiesis PAGE 16 Although movement of cells through the marrow cords is sluggish, developing cells would exit the marrow prematurely in the outflow whithout an anchoring system within the marrow that holds them there until development is complete. There are three components to the anchoring system: – a stable matrix of accessory and stromal cells to which normoblasts can attach – bridging (adhesive) molecules for that attachment – receptors on the erythrocyte membrane Erythropoiesis PAGE 17 The major cellular anchor for the RBCs is the macrophage. RBCs are also anchored to the extracellular matrix of the bone marrow, chiefly by fibronectin (glycoprotein). When it comes time for the RBCs to leave the marrow, they cease production of the receptors for the adhesive molecules. Without the receptor, the cells are free to move from the marrow into the venous sinus and to traverse the barrier created by the adventitial cells on the cord side, the basement membrane, and the endothelial cells lining the sinus. RBC production PAGE 18 The different committed stem cells will produce colonies of specific types of blood cells. A committed stem cell that produces erythrocytes is called a colony-forming unit–erythrocyte (CFU-E). Likewise, colony-forming units that form granulocytes and monocytes have the designation CFU-GM. Growth and reproduction are controlled by proteins called growth inducers. At least four have been described. One of these, interleukin-3 (IL3), promotes growth and reproduction of virtually all the types of committed stem cells, whereas the others induce growth of only specific types of cells. The growth inducers promote growth but not differentiation of the cells, which is done by proteins called differentiation inducers. RBC production PAGE 19 As RBCs mature, several general trends affect their appearance: – Diameter of the cell decreases. – The diameter of the nucleus decreases, n:c ratio also decreases. – The nuclear chromatin pattern becomes coarser, clumped, and condensed, the nucleus is pyknotic. – Nucleoli disappear. Nucleoli represent areas where the ribosomes are formed and are seen early in cell development as cells begin actively synthesizing proteins – The cytoplasm changes from blue to gray-blue to salmon pink. Erythroid precursors PAGE 20 Rodak's Hematology, 2016 Production of RBC PAGE 21 The total mass of RBCs in the circulatory system is regulated within narrow limits. Conditions required for a normal RBC count: – Properly functioning bone marrow – Iron needed for hemoglobin production – Vitamin B 12 cobalamin, is a water soluble vitamin, a cofactor in DNA synthesis, in both fatty acid and amino acid metabolism and role in the proliferation of developing red blood cells in the bone marrow – Folate (folic acid) is essential for the synthesis of DNA – Proper protein synthesis - available amino acids Composition of RBC PAGE 22 Mature red blood cells lack all internal cell structures and consist of cytoplasm within a plasma membrane envelope. Specific proteins: – Hemoglobin ~ 96% of the red blood cells' dry content (by weight), and around 35% of the total weight – Proteins preventing the oxidation of hemoglobin hemoglobin→methemoglobin (cannot carry oxygen) – Proteins required for energy production (glycolysis) – Carbonic anhydrase (CA) – Membrane proteins – Structural proteins Tissue oxygenation is the PAGE 23 most essential regulator of red blood cell production Guyton and Hall, Textbook of Medical Physiology, 2016  PAGE 24 Tissue oxygenation is the most essential regulator of red blood cell production At very high altitudes, where the quantity of oxygen in the air is greatly decreased, insufficient oxygen is transported to the tissues and RBC production is greatly increased. In this case, it is not the concentration of RBCs in the blood that controls RBC production but the amount of oxygen transported to the tissues in relation to tissue demand for oxygen. Regulation of erythrocyte production PAGE 25 Erythropoietin (EPO) The principal stimulus for RBC production duirng hypoxia is a circulating hormone called erythropoietin. In the absence of erythropoietin, hypoxia has little or no effect to stimulate RBC production. In the absence of erythropoietin, few RBCs are formed by the bone marrow. When large quantities of erythropoietin are formed and if nutrients are available, the rate of RBC production can rise to perhaps 10 or more times normal. The erythropoietin mechanism for controlling RBC production is a powerful one. Regulation of erythrocyte production PAGE 26 Erythropoietin (EPO) EPO is a glycoprotein hormone with a molecular weight of 34 kD. It consists of a carbohydrate unit that reacts specifically with RBC receptors and a terminal sialic acid unit, which is necessary for biological activity EPO is a true hormone, produced mainly by the kidneys (90%), and in smaller amount (10%) by the liver and acting at a distant location (the bone marrow). It is a growth factor (or cytokine) that initiates an intracellular message to the developing RBCs by binding to its receptor; this process is called signal transduction. Regulation of erythrocyte production PAGE 27 Erythropoietin (EPO) Renal tissue hypoxia leads to increased tissue levels of hypoxia-inducible factor–1 (HIF-1), which stimulates the production of erythropoietin. HIF-1 functions as a regulator of adaptive responses induced by hypoxia. At times, hypoxia in other parts of the body, but not in the kidneys, stimulates kidney erythropoietin secretion, which suggests that there might be some non-renal sensor that sends an additional signal to the kidneys to produce this hormone. In particular, both norepinephrine and epinephrine and several of the prostaglandins stimulate erythropoietin production. Regulation of erythrocyte production PAGE 28 Erythropoietin (EPO) EPO has three major effects: 1. Allow early release of reticulocytes from the bone marrow; 2. Prevent apoptotic cell death; 3. Reduce the time needed for cells to mature in the bone marrow. Regulation of erythrocyte production PAGE 29 Erythropoietin (EPO) EPO promotes early release of developing erythroid precursors from the marrow by two mechanisms: – EPO induces changes in the adventitial cell layer of the marrow/sinus barrier that increase the width of the spaces for RBC egress into the sinus. – EPO downregulates the expression of adhesion molecule receptors so that cells can exit the marrow earlier than they normally would. Regulation of erythrocyte production PAGE 30 Erythropoietin (EPO) EPO increases the number of cells that will be able to mature into circulating erythrocytes. It does this by decreasing apoptosis. Under normal circumstances, many red cell progenitors will undergo apoptosis. When increased numbers of red cells are needed, apoptosis can be avoided. Additionally, EPO is able to stimulate production of various anti-apoptotic molecules, which allows the cell to survive and mature. Regulation of erythrocyte production PAGE 31 Erythropoietin (EPO) EPO binds on receptors located on erythrocyte progenitors. Tthis phenomenom activates JAK2 protein associated with its cytoplasmic domain. Activated JAK2 then phosphorylates (activates) the signal transduction and activator of transcription pathway, leading to the production of the anti-apoptotic molecule Bcl-XL (now called Bcl-2 like protein 1). EPO-stimulated cells develop this molecule on their mitochondrial membranes, preventing release of cytochrome c, an apoptosis initiator. Regulation of erythrocyte production PAGE 32 Erythropoietin (EPO) Another effect of EPO is to increase the rate at which the surviving precursors can enter the circulation. This is accomplished by two means - increased rate of cellular processes and decreased cell cycle times. EPO also can reduce the time it takes for cells to mature in the marrow by reducing individual cell cycle time, specifically the length of time that cells spend between mitoses. The normal transit time in the marrow of approximately 6 days from pronormoblast to erythrocyte can be shortened by only about 1 day by this effect. Hemoglobin PAGE 33 Synthesis of hemoglobin begins in the proerythroblasts and continues even into the reticulocyte stage of the RBCs. Therefore, when reticulocytes leave the bone marrow and pass into the blood stream, they continue to form hemoglobin for another day or so until they become mature erythrocytes. https://alevelbiolo gy.co.uk/notes/he moglobin/ Hemoglobin PAGE 34 Molecular weight: 64.5 kDa Structure: – 4 globular protein subunits – Each subunit: a protein chain tightly associated with a non protein prosthetic heme group – heme: ferrous ion (Fe2 ++) + porphyrin ring; each Fe2+ ion can reversibly bind one O2 molecule. There are four iron atoms in each hemoglobin molecule; each of these can bind with one molecule of oxygen, making a total of four molecules of oxygen (or eight oxygen atoms) that can be transported by each hemoglobin molecule. – if the iron is oxidized to Fe3 ++(ferric ion) → methemoglobin (cannot bind oxygen)  Hemoglobin PAGE 35 There are several slight variations in the different subunit hemoglobin chains The different types of chains are designated alpha chains, beta chains, gamma chains, and delta chains. The most common form of hemoglobin in the adults, hemoglobin A, is a combination of two alpha chains and two beta chains. Common types of hemoglobin: – In adults: hemoglobin A (α 2 β 2 ) most common, over 95 % of hemoglobin hemoglobin A 2 (α 2 δ 2 ) about 3 % of hemoglobin, unknown biological relevance – In fetus: hemoglobin F (α2γ2) disappears until the 6 th month of life; higher affinity for oxygen excess O2 binding capacity after birth – hemolysis (jaundice of the newborn) Iron metabolism PAGE 36 Iron is important for the formation of hemoglobin and other essential elements. The total quantity of iron in the body averages 4 to 5 grams of which: – 65% is in the form of Hb – 4% is in the form of myoglobin – 1% is in the form of the various heme compounds – 0.1% is combined with the protein transferrin in the blood plasma – 15 to 30% is stored for later use, mainly in the reticuloendothelial system and liver parenchymal cells, principally in the form Guyton and Hall, Textbook of of ferritin Medical Physiology, 2016 Iron metabolism PAGE 37 When iron is absorbed from the small intestine, it combines with apotransferrin, to form transferrin, which is then transported in the plasma. The iron can be released to any tissue cell at any point in the body. In the plasma iron circulates: – bound to proteins (mainly transferrin) - protein binding prevents renal loss transferrin: bounds 2 ferric (Fe 3+) ions normally 1/3 of the binding sites are saturated – unbound iron ions are highly toxic and catalyse the production of reactive oxygen species Fenton reaction - is a catalytic process that converts hydrogen peroxide, a product of mitochondrial oxidative respiration, into a highly toxic hydroxyl free radical Iron metabolism PAGE 38 Excess iron in the blood is deposited especially in the hepatocytes and less in the reticuloendothelial cells of the bone marrow. Smaller quantities of the iron in the storage pool are in an extremely insoluble form called hemosiderin. A man excretes about 0.6 mg of iron each day, mainly into the feces. Additional quantities of iron are lost when bleeding occurs. For a woman, additional menstrual loss of blood brings long-term iron loss. Iron metabolism PAGE 39 When RBCs have lived their life span of about 120 days and are destroyed, the hemoglobin released from the cells is ingested by monocyte-macrophage cells. There, iron is liberated and is stored mainly in the ferritin pool to be used as needed for the formation of new hemoglobin. Short life span of RBCs → ~ 1% destroyed daily → ~ 25 mg iron released used for the production of new RBCs. Regulation of iron metabolism PAGE 40 Regulation of iron metabolism is made by a peptide hormone produced by the liver – hepcidin. Hepcidin is produced when the plasma iron levels increase. Hepcidin inhibits ferroportin, thus iron remains in enterocytes and macrophages. Cytokines released during erythropoiesis inhibit hepcidin production. Inflammatory cytokines stimulate hepcidin production → iron cannot be released from enterocytes and macrophages. Role of hemoglobin in oxygen transport PAGE 41 About 97% of the oxygen transported from the lungs to the tissues is carried in hemoglobin. 3% is transported in the dissolved state in the water of the plasma and blood cells. When PO2 is high, as in the pulmonary capillaries, O2 binds with the hemoglobin, but when PO2 is low, as in the tissue capillaries, O2 is released from the hemoglobin. This is the basis for almost all O2 transport from the lungs to the tissues. Role of hemoglobin in oxygen transport PAGE 42 There is a progressive increase in the percentage of hemoglobin bound with O2 as blood PO2 increases, which is called the percent saturation of hemoglobin. Because the blood leaving the lungs has a PO2 of about 95 mm Hg, the usual O2 saturation of systemic arterial blood averages 97%. In normal venous blood returning from the peripheral tissues, the PO2 is about 40 mm Hg, and the saturation of hemoglobin averages 75%. The percentage of the blood that gives up its O2 as it passes through the tissue capillaries is called the utilization coefficient. The normal value is about 25%, that is, 25% of the oxygenated hemoglobin gives its O2 to the tissues. Hemoglobin “buffers” tissue pO2 PAGE 43 Under basal conditions, the tissues require about 5 ml of O2 from each 100 ml of blood passing through the tissue capillaries. Referring to the O2-hemoglobin dissociation curve, for the O2 to be released, the PO2 must fall to about 40 mm Hg. The tissue PO2 normally cannot rise above this 40 mm Hg level because, if it did, the amount of O2 needed by the tissues would not be released from the hemoglobin. In this way, the hemoglobin normally sets an upper limit on the PO2 in the tissues at about 40 mm Hg. Factors that shift the oxygen-hemoglobin PAGE 44 dissociation curve Shift to the right: – Decreased pH – increased CO2 concentration – increased blood temperature – increased 2,3-biphosphoglycerate (BPG) Guyton and Hall, Textbook of Medical Physiology, 2016 The Bohr effect PAGE 45 As the blood passes through the tissues, CO2 diffuses from the tissue Bohr effect = increase of O2 release in into the blood increasing PCO2, which the tissues and enhanced oxygenation in turn raises the blood H2CO3 in the lungs which occurs when blood (carbonic acid) and the H+ CO2 and H+ increase and shift of the concentration. These effects force O2 oxygen-hemoglobin dissociation curve away from the hemoglobin and to the right. therefore delivering increased amounts of O2 to the tissues. The Bohr effect PAGE 46 The opposite effects occur in the lungs, where CO2 diffuses from the Therefore, the quantity of O2 that blood into the alveoli, reducing blood binds with the hemoglobin at any PCO2 and H+ concentration, shifting given alveolar PO2 becomes the O2-hemoglobin dissociation curve increased, allowing greater O2 to the left and upward. transport to the tissues.  PAGE 47 Effect of BPG to cause rightward shift of the oxygen-hemoglobin dissociation curve 2,3-bisphosphoglyceric acid is an important regulator of hemoglobin’s affinity for O2. It binds with greater affinity to deoxygenated hemoglobin than to oxygenated hemoglobin It interacts with deoxygenated hemoglobin and it decreases the affinity for oxygen-> promotes the release of oxygen molecules bound to the hemoglobin and enhances the ability of RBCs to release oxygen near tissues that need it most - 2,3-BPG is an allosteric effector.  PAGE 48 Effect of BPG to cause rightward shift of the oxygen-hemoglobin dissociation curve In hypoxic conditions that last longer than a few hours, the quantity of BPG in the blood increases considerably, thus shifting the O2-hemoglobin dissociation curve even farther to the right. This shift causes O2 to be released to the tissues. Therefore, under some conditions, the BPG mechanism can be important for adaptation to hypoxia, especially to hypoxia caused by poor tissue blood flow. Effect of exercise PAGE 49 During exercise, the dissociation curve shifts to the right, thus delivering extra amounts of O2 to the active, exercising muscle fibers. The exercising muscles, in turn, release large quantities of CO2; this and other acids released by the muscles increase the H+ concentration in the capillary blood. The temperature of the muscle rises 2° to 3°C, which can increase O2 delivery to the muscle fibers even more. This rightward shift of the curve forces O2 to be released from the blood hemoglobin to the muscle at PO2 levels as great as 40 mm Hg, even when 70 percent of the O2 has already been removed from the hemoglobin.  PAGE 50 Effect of blood flow on metabolic use of oxygen The total amount of O2 available each minute for use in any given tissue is determined by the quantity of O2 that can be transported to the tissue in each 100 milliliters of blood and the rate of blood flow. If the rate of blood flow falls to zero, the amount of available O2 also falls to zero. Thus, there are times when the rate of blood flow through a tissue can be so low that tissue PO2 falls below the critical 1 mm Hg required for intracellular metabolism. Under these conditions, the rate of tissue usage of O2 is blood flow limited. Combination of hemoglobin with carbon PAGE 51 monoxide-displacement of O2 Carbon monoxide (CO) combines with hemoglobin at the same point as O2; it can therefore displace O2 from the hemoglobin, decreasing the O2-carrying capacity of blood. It can bind with about 250 times as much tenacity as O2 A CO partial pressure of only 0.4 mmHg in the alveoli, can to compete equally with the O2 for combination with the hemoglobin (O2 has 100 mmHg) and causes half the hemoglobin in the blood to become bound with CO instead of with O2. A CO pressure of only 0.6 mm Hg can be lethal.  PAGE 52 Transport of carbon dioxide in the blood Transport of CO2 by the blood is not nearly as problematical as transport of O2 is because even in the most abnormal conditions, CO2 can usually be transported in far greater quantities than can O2. However, the amount of CO2 in the blood is related with the acid-base balance of the body fluids. Under normal resting conditions, an average of 4 milliliters of CO2 are transported from the tissues to the lungs in each 100 milliliters of blood. Guyton and Hall, Textbook of Medical Physiology, 2016 Transport of carbon dioxide in the blood PAGE 53 Transport of carbon dioxide in the dissolved state (7%) The dissolved CO2 in the blood reacts with water to form carbonic acid. Transport of carbon dioxide in the form of bicarbonate ion (70%) Inside the red blood cells is a protein enzyme called carbonic anhydrase, which catalyzes the reaction between CO2 and water Transport of carbon dioxide in the blood PAGE 54 H2CO3 dissociates into H+ and HCO3−. Most of the H+ then combine with the Hb in the red blood cells because the hemoglobin protein is a powerful acid-base buffer. In turn, HCO3− diffuse from the red blood cells into the plasma, while chloride ions diffuse into the red blood cells to take their place. (Hamburger phenomenon). Transport of carbon dioxide in the blood PAGE 55 Transport of carbon dioxide in combination with hemoglobin and plasma proteins— carbaminohemoglobin (23%) – CO2 reacts directly with amine radicals of the hemoglobin molecule to form the compound carbaminohemoglobin (CO2Hgb). This combination is a reversible reaction that occurs with a loose bond, so the CO2 is easily released into the alveoli, where the PCO2 is lower than in the pulmonary capillaries. – A small amount of CO2 also reacts in the same way with the plasma proteins in the tissue capillaries. This reaction is much less significant for the transport of CO2. The Haldane effect PAGE 56 Binding of O2 with hemoglobin tends to displace CO2 from the blood. This effect, is called the Haldane effect. First, the more acidic hemoglobin has less tendency to combine with CO2 to form carbaminohemoglobin The Haldane effect results from the fact that the combination of O2 with Hb in the lungs causes the Hb to become a stronger acid. This displaces CO2 from the blood and into the Second, Hb releases more H+ into the alveoli in two ways: surroundings, and these ions bind with bicarbonate ions to form carbonic acid, which then dissociates into H20 and CO2, which is later released from the blood into the alveoli. The destruction of RBC PAGE 57 The average RBC has sufficient enzyme function to live 120 days. Because RBCs lack mitochondria, they rely on glycolysis for production of adenosine triphosphate (ATP). The loss of glycolytic enzymes is central to this process of cellular aging, called senescence, which culminates in phagocytosis by macrophages. This is the major way in which RBCs die normally. The destruction of RBC PAGE 58 Macrophage-Mediated Hemolysis (Extravascular Hemolysis) In the spleen, a significant amount of blood is found, and cells experience stress due to slow movement through the red pulp. This sluggish flow leads to a depletion of glucose in the surrounding plasma, causing glycolysis to slow down. Additionally, the low pH in this environment promotes iron oxidation. Reduced iron levels are critical for cellular function and require energy to maintain. As ATP- dependent processes become compromised, membrane systems relying on ATP begin to fail. Enzymes responsible for maintaining membrane phospholipid location and reduction are affected, leading to lipid and protein oxidation. The failure of ATP-dependent processes also impacts the Na⁺/K⁺ pump. As this pump fails, intracellular Na⁺ levels rise while K⁺ levels decrease, causing a loss of selective membrane permeability. Consequently, water enters the cell, causing it to lose its discoid shape and become spherical. The destruction of RBC PAGE 59 Macrophage-Mediated Hemolysis (Extravascular Hemolysis) RBCs must remain highly flexible to exit the spleen by squeezing through the so-called splenic sieve formed by the endothelial cells lining the venous sinuses and the basement membrane. Spherical RBCs are rigid and are not able to squeeze through the narrow spaces and they become trapped. In this situation, they are readily ingested by macrophages that patrol along the sinusoidal lining. When an RBC lyses within a macrophage, the major components are catabolized: – The iron is removed from the heme. It can be stored in the macrophage as ferritin until transported out. – The globin of hemoglobin is degraded – The protoporphyrin component of heme is degraded through several intermediaries to bilirubin, which is released into the plasma and ultimately excreted by the liver in bile. The destruction of RBC PAGE 60 Macrophage-Mediated Hemolysis (Extravascular Hemolysis) Some researchers view erythrocyte death as a nonnucleated cell version of apoptosis, termed eryptosis, that is precipitated by oxidative stress, energy depletion, and other mechanisms that create membrane signals that stimulate phagocytosis. The destruction of RBC PAGE 61 Mechanical Hemolysis (Fragmentation or Intravascular Hemolysis) Although most natural RBC deaths occur in the spleen, a small portion of RBCs rupture intravascularly (within the lumen of blood vessels). The vascular system can be traumatic to RBCs, with turbulence occurring in the chambers of the heart or at points of bifurcation of vessels. Small breaks in blood vessels and resulting clots can also trap and rupture cells. The intravascular rupture of RBCs from purely mechanical or traumatic stress results in fragmentation and release of the cell contents into the plasma; this is called fragmentation or intravascular hemolysis. When the membrane of the RBC has been breached, regardless of where the cell is located when it happens, the cell contents enter the surrounding plasma.  PAGE 62 Hemoglobin degradation Guyton and Hall, Textbook of Medical Physiology, 2016 Bilirubin PAGE 63 Bilirubin is transported into the liver bound to a protein (serum albumin) = unconjugated bilirubin = indirect bilirubin (~ 0.5mg/dl). Hepatocytes bind the bilirubin albumin complex, this dissociates and bilirubin is transported in the cell by a Na+ bilirubin cotransporter. In the liver bilirubin is conjugated with glucuronic acid to become more water soluble = conjugated bilirubin = direct bilirubin. Conjugated bilirubin is excreted from the liver in the bile; the excretion of bilirubin from liver to biliary canaliculi is an active, energy dependent and rate limiting process. The conjugated bilirubin normally cannot pass back into the blood - very low concentration in the blood (0.1 mg/dl). Bilirubin PAGE 64 The intestinal bacteria convert bilirubin to urobilinogen. Urobilinogen is absorbed from the gastrointestinal tract (mainly the large intestine) into the portal system. Most of the urobilinogen absorbed is uptaken by hepatocytes and resecreted in the bile-enterohepatic circulation. Some urobilinogen is absorbed into the systemic circulation and excreted by the kidneys with urine. The remainder travels down the digestive tract and is converted to stercobilinogen, this is oxidized to stercobilin, which is excreted and is responsible for the color of feces (stool). References PAGE 65 Guyton and Hall, Textbook of Medical Physiology, 2016 Rodak's Hematology, fifth edition, 2016