Red Blood Cells Syllabus PDF 2024

Red Blood Cells Dr. Aaron W. Young 27 September 2024 I. Learning Objectives At the end of this section, the student will be able to… 1. Describe the composition and function of blood and the basis of the hem- atocrit measurement. 2. D...

Red Blood Cells Dr. Aaron W. Young 27 September 2024 I. Learning Objectives At the end of this section, the student will be able to… 1. Describe the composition and function of blood and the basis of the hem- atocrit measurement. 2. Describe the unique structure and shape of the red cell. 3. Describe the metabolism of the red cell and the processes acting to pro- tect the cell and its contents from oxidative damage. 4. Explain the regulation of red cell number and formation, including as- pects of hemoglobin synthesis. 5. Describe the intra and extravascular breakdown of the red cell and the fate of the breakdown products. 6. Identify causes of anemia. 7. Explain the basis of blood cell typing. II. References Syllabus III. Vocabulary Understand the physiological processes underlying the following: oxygen transport in blood red blood cell modifications: lysis, crenation, echinocytes, spherocytes, sickle- shape Rapoport-Leubering shunt role of NADH and NADPH in the red blood cell phosphogluconate pathway erythropoiesis bilirubin bone marrow anemia erythropoietin red cell loss kidney premature breakdown, hemoglobinization hypoproliferative normoblast cell typing reticulocyte ABO normal red cell death rhesus extravascular and intravascular- transfusion reaction hemolylysis erythroblastosis fetalis haptoglobin rhogam RBCs - 1 IV. Lecture Guide INTRODUCTION The prime function of blood is to act as a transport medium. Transported fac- tors include: Nutrients, e.g. glucose, amino acids, fatty acids, O2, salts Waste products, e.g. CO2, urea, uric acid Messengers, e.g. hormones Defense components, e.g. antibodies, leukocytes, platelets, clot- ting factors Carrier proteins, e.g. albumin, transferrin, lipoproteins Heat H+ H2O Some of these materials are transported from one tissue to another (e.g. urea from liver to kidneys, O2 from lungs to all tissues). Some of these materials circulate in blood for special purposes, e.g. platelets, clotting factors, antibodies, complement. Still other materials circulate in blood simply because they are present in our environment, and although they have no physiological function, they can have profound effects under certain conditions (e.g. dissolved nitrogen at high pressure can cause nitrogen narcosis; diving to depths greater than 90 feet and rapid de- compression can cause dissolved nitrogen to bubble out of the blood, a situation that can lead to decompression sickness ["the bends"]). If a substance is freely soluble in water or is only required in small amounts, it is frequently transported as a simple solutions in plasma (e.g. most ions, glucose, amino acids, some hormones). However, many materials are poorly soluble or are required in amounts greater than can be carried as simple solutions. Some are also toxic in free solu- tion. For transport of these items special mechanisms have developed. As described previously, the transport of CO2 is achieved by reversibly con- verting much of it into freely soluble HCO3- using the erythrocyte enzyme car- bonic anhydrase. Most other poorly soluble or potentially toxic materials are transported bound to carrier proteins. Some examples follow: Substance Carrier protein Triglycerides lipoproteins Fatty acids serum albumin Iron transferrin Bilirubin serum albumin Thyroxine (thyroid hormone) thyroid binding globulin, prealbumin Blood is conveniently divided into its component parts when measuring the hematocrit. The hematocrit is the proportion of the whole blood composed of red blood cells. It is measured by centrifuging an anticoagulated blood sample and measuring the proportion of packed red cells. In addition to separating out the red cells from the remaining fluid of the blood (plasma) the centrifugation also results in the layering of platelets and white blood cells above the red cells. All of these sedimenting components (rbcs, platelets and wbcs) are referred to as the formed elements of the blood. Platelet function will be discussed further in the session on Hemostasis. RBCs - 2 MAJOR COMPONENTS OF HUMAN BLOOD Do not memorize these numbers – approximate relationships are fine: The transport of oxygen is principally achieved through use of the carrier protein hemoglobin. However, such large amounts of oxygen transport are re- quired that 15g of hemoglobin are needed per 100mL of blood. This is more than three times the amount of all the other plasma proteins combined (see below). If hemoglobin were transported in free solution in plasma at this concentration, it would drastically alter the osmotic properties of the blood. It would also be vul- nerable to a range of oxidative and proteolytic mechanisms. Instead, hemoglobin is packaged up in highly specialized structures, the erythrocytes. The erythrocyte is produced from precursor cells located in the marrow of the bones of the axial skeleton and in the proximal ends of the long bones. The mature erythrocyte has no nucleus, no ribosomes and no mitochondria. It is essentially a biconcave sac filled with a 30% solution of hemoglobin plus other enzymes and smaller molecules. It is incapable of synthesizing proteins or lipids or of carrying out oxidative metabolism. Nevertheless, the red cell survives for about 120 days in the circulation despite repeated deformations during its passage through capil- laries and despite repeated exposure to turbulence in the large vessels. The ability to withstand deformation is the secret of the cell's long life; and it is the loss of deformability upon aging that eventually contributes to the death of the cell. The shape of the red cell, as a flattened “biconcave disc”, enables it to deform without stretching the cell membrane. This is important because cell membranes are flexible but do not stretch readily. If they are forced to stretch, there is a ten- dency for tearing, which results in cell lysis. Being flattened also gives short dis- tances for oxygen to diffuse from the cell membrane to even the deepest hemo- globin molecules in the cell (~1 μm). The cell shape is maintained by many factors. Changes in some of these can result in abnormal appearances in the cells. Some examples include: ATP is required for various purposes, and when it is depleted, the cells take on a “crenated” appearance, with a ruffled membrane. Entry of large amounts of calcium ions into the cell results in the appearance of spikes on the cell surface, making the cells resemble sea urchins, and hence called echinocytes. Spectrin is a long rod-like molecule that forms a scaffold associated with the RBCs - 3  inner surface of the cell membrane. When cells have a deficiency in spec- trin, they can take on a spherical appearance and are called spherocytes. Hemoglobin itself can also profoundly affect cell shape. The classic example is the single amino acid change resulting in sickle cell disease. In the pres- ence of reduced oxygen or acidic pH, HbS crystallizes out of solution into long rigid rods, deforming the cell into the typical sickle shape. In the final analysis, the function of the erythrocyte is to protect hemoglobin from denaturation and degradation. It is no coincidence that the "lifespan" of hemoglobin in the red cell (intracorpuscular hemoglobin) is the same as the lifespan of the red cell itself, 120 days, whereas in the absence of a protective envelope, hemoglobin disappears rapidly from the plasma. I. METABOLISM OF THE ERYTHROCYTE Almost all of the red cell's metabolic energy is derived from anaerobic gly- colysis (the conversion of glucose to lactic acid). The principal task of the erythrocyte --- the transport of oxygen and carbon dioxide --- does not require energy, but relies instead on simple passive diffusion processes. Therefore, energy is utilized primarily in maintenance functions in the mature erythrocyte. There is a strong correlation between drop in ATP concen- tration and impaired survival in older red cells. A major use of the ATP derived from glycolysis is to power the sodium- potassium ATPase that helps to maintain sodium and potassium gradients across the red cell membrane. As much as 30% of the red cell's energy resources may be devoted to this purpose. ATP also fuels the active removal of calcium ions from the cell. Buildup of calcium ions is believed to cause cross linking of red cell membrane proteins and decreased deformability of the cell. Other metabolic processes in the red cell are essential for the specific function of the red cell and its survival. Three important processes are described below. An offshoot of glycolysis, the Rapoport-Leubering shunt produces 2,3 DPG (or BPG) which influences the affinity of hemoglobin for oxygen (Fig 1). Fig 1. The binding of oxygen to hemoglobin is influenced by the concentration of 2,3 diphosphoglycerate (2,3 DPG, or BPG) in the erythrocyte. Control of the con- centration of 2,3 DPG is therefore important in regulating oxygen transport. RBCs - 4  Another important role of the metabolism of the red cell is generation of reducing power to prevent oxidation of the cell membrane and the hemoglobin. The two most important agents in this regard are NADH, levels of which are maintained by glycolysis (Fig. 2), and NADPH which is generated as part of the hexose monophosphate shunt (Fig 3.) NADH is important to keep the iron in hemoglobin in the ferrous form. The fer- rous iron in hemoglobin is prone to oxidation to the fer- ric form. Restoring the non- functional ferric hemoglobin (methemoglobin) to its re- duced form requires the red cell enzyme methemoglobin reductase (aka diaphorase) using NADH as a cofactor (Fig 2). Fig 2. The reducing agent, NADH, derived from glycolysis is utilized to maintain hemoglobin iron in the Fe2+ state. Hemoglobin in the Fe3+ state (methemoglobin) cannot bind oxygen and is unstable. Denatured methemoglobin forms aggregates (Heinz bodies) that precipitate and shorten the life of the cell. Fig 3. The hexose monophosphate shunt (or phosphogluconate pathway) produces NADPH that is used to reduce glutathione. RBCs - 5  NADPH derived from the phosphogluconate pathway (Fig. 3) drives the re- duction of glutathione (GSSG). Reduced glutathione (GSH) protects the cell membrane and hemoglobin against oxidants, in particular hydrogen peroxide. Ox- idation of intracellular proteins, including hemoglobin, is associated with in- creased stiffness and fragility of older cells. Deficiency of the enzyme glucose 6- phosphate dehydrogenase (G6PD) blocks the pathway, and hence NADPH for- mation, and can lead to hemolytic anemia. This defect is prevalent in some ethnic subgroups, particularly those of Mediterranean and African descent. According to the World Health Organization, it is the most common human enzyme defect. II. ERYTHROPOIESIS (production of red cells) During fetal development red cells are produced in various locations including liver, spleen and bone marrow. By about the time of birth erythropoiesis is limited primarily to the bone marrow. On reaching adulthood, it is normally restricted primarily to the bone marrow of the proximal ends of the long bones and the axial skeleton. Red cells derive from pluripotential stem cells in the marrow which undergo progressive differentiation until they become committed to the red cell line. (For now, we will refer to nucleated erythrocyte precursor cells as normoblasts). The committed erythrocyte precursors give rise to normoblasts, which undergo three to five divisions while becoming increasingly differentiated. Hemoglobin synthe- sis occurs in normoblasts until its concentration reaches near mature cell levels. At this point cell division stops, the cell nucleus is extruded, and the reticulocyte is produced. Mitochondria also begin to break down and/or be extruded at this stage. The reticulocytes leave the bone marrow and enter the circulation. The reticulocyte, which contains residual RNA and a stainable "reticulum", gives rise to the mature erythrocyte without further cell division. The residual mRNA, ribosomes and reticulum in the reticulocyte provide for some continued Hb synthesis until these organelles also break down. One to 2% of all red cells in the circulation are reticulocytes. This number increases when the production of erythrocytes is accelerated in response to situa- tions that compromise oxygen delivery to the tissues, such as premature loss of mature cells. The reticulocyte index, a corrected value for the percentage of cir- culating reticulocytes, is used clinically in the diagnosis of anemia. The regulation of the number of circulating red cells is accomplished by controlling their rate of production rather than their rate of destruction. The rate of production from committed stem cells is primarily regulated by erythropoi- etin, a protein hormone produced mainly by cells in the kidney in response to low rates of oxygen delivery. Any factor that tends to lower tissue PO2 will stim- ulate renal erythropoietin production. Examples are high altitude, hemorrhage, and increased destruction of red blood cells. Erythropoietin stimulates erythro- poiesis not only in response to a hypoxic crisis, but also during day-to-day regu- lation of the output of red cells. III. HEMOGLOBIN SYNTHESIS AND IRON METABOLISM. Hemoglobin consists of two components: a) A tetrameric protein consisting of two each of two types of globin molecules, in the adult these are α and β –globins (in the fetus β is replaced by a γ form); b) heme - a porphyrin ring structure containing iron which binds oxygen. There are four heme groups, four iron molecules, and four oxygen-binding sites per hemoglobin molecule. α and β-globin are synthesized on ribosomes in RBCs - 6 the cytosol of the normoblast. The porphyrin group of heme is synthesized in the mitochondria of this cell (Fig 4). Fig 4. Synthesis of hemoglobin in normoblast (fig. by JFH) The iron necessary for hemoglobin synthesis comes from three sources that are listed in decreasing order of importance: (see also Fig 5.) a) iron salvaged from degraded red blood cells; b) iron mobilized from body stores; c) dietary iron absorbed from the GI tract. Iron is carried in the blood as part of a complex with the protein transferrin. The iron-transferrin complex enters the marrow and binds to a specific receptor on the normoblast. The iron then enters the cell where it is incorporated into he- moglobin, while the transferrin is returned to the plasma. The storage form of iron consists of the metal bound to one of two other structurally related proteins, ferritin or hemosiderin. The control of iron levels in the body is exerted by regu- lation of the rate of intestinal absorption of iron by the mucosal cells (enterocytes) that line the GI tract. This regulation will be discussed in greater detail during the GI section. Fig 5. Path of iron and other constituents during formation and breakdown of red cells. (Based on Ganong, Review of Medical Physiology, Lange/McGraw Hill, 2010) RBCs - 7 IV. THE DEATH OF THE RED CELLS Conditions that lead to destruction of freely circulating blood cells are com- paratively rare. This type of destruction termed intravascular hemolysis usually occurs as a result of transfusion reactions or Rh-incompatibility (see below). Cells may also lyse in the circulation in response to some toxins. Much more common is the destruction of red cells in the spleen and other regions of the reticulo-endothelial system. This is how most normal aged red cells are removed at the end of their approximately 120 day lifespan. The walls of the splenic sinusoids act as a filter with a pore size smaller than the red cells themselves. In order to pass through this filter, the red cells must be fluid and deformable. Red cells with hereditary defects and normal cells that have aged are not sufficiently deformable to pass and may be trapped, removed by survey- ing phagocytes (neutrophils and macrophages) or may fragment. Since most such cells lyse within the phagocytes this represents extravascular hemolysis, although that term is generally used for premature cell destruction. The hemoglobin and other red cell proteins are degraded to amino acids, which are reutilized for protein synthesis. The iron liberated from hemoglobin may be temporarily stored within the macrophage but is ultimately released to be bound by plasma transferrin and returned to the marrow or to other stores. The porphyrin group formerly bound to hemoglobin is also degraded (see below). Hemoglobin liberated after lysis of red cells in free circulation dissociates into dimers which bind to the serum protein haptoglobin. The hemoglobin-hap- toglobin complex is delivered to reticulo-endothelial cells especially in the liver where it is degraded, as above. During severe intravascular hemolysis, the capac- ity of haptoglobin to bind hemoglobin dimers is exceeded and the plasma hapto- globin becomes depleted. Hemoglobin dimers filtered in the kidney are be taken up by the cells of the renal tubule and if that capacity is exceeded some of the hemoglobin may appear in the urine, a condition known as hemoglobinuria. V. THE METABOLISM OF HEME Within reticuloendothelial cells the porphyrin group undergoes a ring open- ing reaction with the liberation of carbon monoxide. The other product of the reaction, a straight-chained porphyrin derivative, passes through intermediates to become bilirubin, which enters the plasma and binds to albumin. This complex is carried to the liver where the bilirubin is conjugated by the addition of glucu- ronic acid. The bilirubin glucuronide is ultimately secreted into the bile and then into the GI tract. Here it is converted by the action of intestinal bacteria to uro- bilinogen. The measurement of unconjugated and conjugated bilirubin, as well as uro- bilinogen, is clinically important, since these hemoglobin breakdown products increase in concentration whenever there is increased destruction of red blood cells, whether in the circulation, in the reticuloendothelial cells or in the marrow. Measurement of carbon monoxide in expired air can also provide a sensitive in- dex of elevated hemolysis. RBCs - 8 Fig 6. Fate of hemoglobin components after the red cell is taken up by macro- phage. (fig. by JFH) VI. THE ANEMIAS Anemia is defined in two ways: a) an insufficient number of circulating red blood cells. b) an insufficient amount of circulating hemoglobin. The second definition of anemia is physiolog- ically the most important because insufficient hemoglobin results in insufficient oxygen carrying capacity of the blood. In order that the number of circulating red blood cells remains at a constant, normal level, the rate of production of red cells must equal their rate of destruction. An anemia can, therefore, develop if the rate of produc- tion of red cells is abnormally low, if the rate of destruction of red cells is abnormally high, or if both conditions are present. Anemia also results from acute loss of blood (Fig. 7). Fig 7. Incidence of anemias (fig. by JFH) 1. Abnormal rate of production of red cells and hemoglobin. Inadequate erythropoiesis may result from several sources: a.) iron deficiency b.) Vitamin B12 deficiency c.) folic acid deficiency d.) diseases of the bone marrow e.) defective hemoglobin synthesis f.) inadequate erythropoietin production RBCs - 9  Iron deficiency anemia results from inadequate availability of iron for he- moglobin synthesis. This rarely occurs in males except after bleeding episodes. However, the condition is fairly common in females due to loss of iron in the menstrual flow and, during pregnancy, because of the relative iron deficiency re- sulting from the demands of fetal erythropoiesis. Iron deficiency only develops when the demand for iron exceeds the amount that is stored and the amount taken in through the diet. It is treated by oral iron supplements. Vitamin B12 deficiency leads to pernicious anemia, a condition that results from the inability to absorb vitamin B12 from the GI tract. This usually results from the failure of parietal cells in the stomach to produce intrinsic factor, a protein that is necessary for intestinal absorption of the vitamin. The condition is treated by giving vitamin B12 parenterally. Both vitamin B12 and folic acid are necessary for polynucleotide biosynthesis and, therefore, for proliferation of erythrocyte precursors in the marrow. Lack of these vitamins leads to large-scale destruction of erythrocyte precursors before they can complete their maturational sequence. Failure of the marrow to produce erythrocytes and other blood cells often results from unknown causes. Among the factors that can be identified are chemo- therapeutic agents and radiation. These agents destroy the stem cells and deplete the marrow of its primary erythrocyte precursor cells. An example of deficient erythropoiesis due to abnormal hemoglobin synthe- sis is thalassemia. In thalassemia, α and β globin chains are not synthesized in equal amounts. Either the α or the β chain is deficient. As with vitamin deficien- cies, this condition leads to destruction of erythrocyte precursors in the marrow. Inadequate erythropoietin production may result from renal disease. 2. Abnormal rate of destruction of red cells. Abnormally rapid destruction of red cells is called hemolysis; the anemia that can result from this is called hemolytic anemia. The marrow has the capacity to expand its rate of production of red cells by a factor of six. With normal marrow function, increased rates of erythropoiesis can, therefore, partly compensate for a hemolytic condition in which the life span of circulating erythrocytes is decreased considerably. Some factors that decrease the lifespan of red cells include: a.) genetic abnormalities of the red cell membrane leading to abnormally rigid or fragile cells; b.) genetic abnormalities of hemoglobin; c.) genetic abnormalities of red cell metabolism; d.) the presence of antibodies against red cell surface antigens which coat the cells and cause them to lyse (break open) e.) lytic agents released by certain bacteria. Examples: a.) A membrane abnormality: In hereditary spherocytosis, the red cells are spherical and the membranes fragile. They suffer damage while passing through the spleen. The disease is often treated by removing the spleen. b.) A hemoglobin abnormality: In sickle cell anemia a single glutamic acid resi- due in the β-chain of hemoglobin is replaced by a valine. In response to low RBCs - 10  PO2 and low pH, the hemoglobin polymerizes and causes the red cell to as- sume a rigid, sickle shape. Sickled cells are trapped in the microvasculature and the spleen. c.) A metabolic abnormality: In hereditary glucose-6-phosphate deficiency the activity of the hexose monophosphate shunt is abnormally low and leads to abnormally low production of NADPH and reduced glutathione. As a result, the red cell is sensitive to oxidizing agents. Oxidizing drugs can precipitate hemolytic crises in people with this genetic defect. Material below is a MANDATORY Reading Assignment (Not covered in lecture, but will be covered on assessments) VII. THE BLOOD GROUPS The blood group antigens are molecules associated with the external surface of the cell membrane. These antigens fall into genetically determined groups. The classic blood groups that are clinically important are the ABO group and the Rh group (although there are now numerous antigens identified, which are be- yond the scope of this session). 1. The ABO blood group (Table I) This group consists of different carbohydrate antigens termed A&B. Hu- man red cells carry A antigen (type A cells), or B antigen (type B cells), or both antigens (type AB cells), or neither antigen (type O cells). In the serum of each individual are antibodies directed against the antigen that does not appear on his or her own cells. People with type A cells have antibodies that attack B anti- gens; those with type B cells have antibodies that attack type A antigen; type AB blood contains neither antibody; type O blood contains both antibodies. During blood transfusion, if the donor's cells are transfused into incompati- ble plasma, that is, plasma that contains antibodies directed against the donor's cells, then these cells will be agglutinated and destroyed. It is necessary, there- fore, to match the donor and recipient by a process known as blood typing. Notice that since type O cells contain neither antigen, theoretically at least, type O individuals can serve as donor for any of these recipients. It is true that type O blood contains antibodies directed against cells of types A, B, and AB, but RBCs - 11 it is agglutination of the cells of the donor that is of concern. The antibodies pre- sent in donor blood are diluted during transfusion and usually have no effect. No- tice also that type AB blood should contain neither type of antibody. Individuals with this blood type should, therefore, theoretically be able to receive blood from any donor. It should be noted that the classical terms “universal donor” and “universal acceptor” are no longer appropriate, in light of the identification of numerous important RBC antigens, beyond the classic ABO and Rh groups, which can be of clinical significance when considering transfusion, etc. The anti-ABO group antibodies arise spontaneously, possibly as a result of early exposure to intestinal flora bearing similar antigens. When they encounter an incompatible cell, antibodies coat the cells and cause them to agglutinate and lyse. In autoimmune hemolytic anemia antibodies that attack the patient’s own cells develop with the same result. 2. The Rh blood group. This group contains numerous cell surface protein antigens, but the most widely identified clinically is the D antigen. Cells which bear D antigen are termed Rh-positive; cells without D antigen are termed Rh-negative. Problems with Rh-incompatibility arise during pregnancy and threaten the life of the fetus as a result of the following sequence of events: a.) An Rh-positive father and an Rh-negative mother conceive an Rh-posi- tive fetus. b.) Red cells from the baby introduce D antigen into the maternal circulation, generally at birth when there may be mixing with the fetal blood. c.) Antibodies against the D antigen form in the mother. This would have no effect on the first baby since this baby is born before the maternal anti-D antibodies develop. d.) During second and subsequent pregnancies with an Rh+ fetus, the mater- nal IgG antibodies to the D antigen, generated in response to the first baby, can be taken up by the placenta, attack the fetal red blood cells and cause hemolysis. This type of hemolysis is called erythroblastosis fetalis or hemolytic dis- tress of the newborn. The condition can be treated by an exchange transfusion whereby the Rh-positive fetal cells are exchanged for Rh-negative cells in utero and after birth, and/or by exchanging the mother’s plasma to reduce anti D anti- bodies. However, the effects of erythroblastosis fetalis are not always reversible, since the high bilirubin level resulting from red cell lysis and hemoglobin degra- dation can produce fetal brain damage. To prevent the above sequence of events, an Rh- mother can now be treated at the time of birth of an Rh+ baby by injection of antibodies to D antigen (Rhogam). These injected antibodies rapidly remove any D antigen that may have entered the maternal circulation. They therefore prevent the mother's im- mune surveillance system from detecting the antigen and prevent the maternal antibody production, which would otherwise follow. The injected antibodies sub- sequently degrade and have no effect on later pregnancies. (ABO incompatibilities can also give rise to forms of erythroblastosis fetalis (mother O, fetus A, B or AB), but these are usually less severe, in part because the fetal red cells express less of the A and B antigens than the adult, and also because they are expressed on other cell types, reducing the chances of the anti A and B antibodies attacking the red cells. A vs B incompatibilities rarely occur since most of the antibodies in a type A or B mother are IgM and not taken up by the placenta, unlike in O mothers where they are usually IgG.) RBCs - 12  V. Study Questions The blood of an anemic patient is found to contain many spherical red cells (sphe- rocytosis). Which of the following might result in this condition? A. Sickle cell disease B. G-6-PD deficiency C. A defect in spectrin D. Thalassemia E. ATP depletion Damage by elevated levels of hydrogen peroxide inside the red cell can dramati- cally shorten the life span of the cell. Which of the following has an important role in protecting the red cell from this oxidant? A. methemoglobin reductase B. 2,3 diphosphoglycerate (also known as 2,3 bisphosphoglycerate) C. glutathione reductase D. transferrin E. carbonic anhydrase An anemic patient is found to have a hematocrit of 20%, indicating that red cells represent 20% of the blood volume. Which of the following is true of the red cell? A Hemoglobin comprises almost half of the total protein in the RBC. B. Blood type is determined by the type of hemoglobin in the red cells of a given individual. C. RBCs have a normal lifespan of about 1 month. D. The RBC normally spends about 1-2 days in the circulation as a re- ticulocyte E. Most of the ATP in the RBC is produced by oxidative phosphoryla- tion. A 38 year old individual suffers a disorder that leads to the prompt hemolysis of many red cells in their circulation. Which of the following would be expected to occur during the following days? A. Abnormally high levels of free hemopexin B. Decreased levels of erythropoietin C. Increased levels of circulating bilirubin D. Decreased proportion of circulating red cells that are reticulocytes E. Abnormally high levels of free haptoglobin RBCs - 13 An anemic patient has a low reticulocyte count, indicative of a hypoproliferative anemia. Which of the following is most likely to cause a hypoproliferative ane- mia? A. Renal failure B. Sickle cell disease C. G-6-PD deficiency D. Removal of the spleen E. Hereditary spherocytosis A couple have already had one child who was born healthy. The mother is blood type O-. Which of the following blood types in the father would be of most con- cern for the second fetus suffering erythroblastosis fetalis? (+ indicates rhesus positive, - indicates rhesus negative) A. A- B. AB- C. O- D. B- E. AB+ (Answers: C, C, D, C, A, E) RBCs - 14