Hemoglobin, Jaundice & RBCs: A Study Guide (PDF)

Dr Sanad Ahmed MBBS, MSc., MD  Describes the chemical structure & synthesis of Hb.  Understands haemoglobin abnormalities and the different reactions of Hb.  List factors that affect the binding of O2 to Hb.  Define jaundice & compare between different types.  Heme i...

Dr Sanad Ahmed MBBS, MSc., MD  Describes the chemical structure & synthesis of Hb.  Understands haemoglobin abnormalities and the different reactions of Hb.  List factors that affect the binding of O2 to Hb.  Define jaundice & compare between different types.  Heme is an iron-containing porphyrin derivative. In a 70-kg man, there are about 900 g of hemoglobin.  Hb synthesis begins in the proerythroblast & continues up to the reticulocyte stage.  Reticulocytes leave the bone marrow and pass into the blood stream, they continue to form minute quantities of hemoglobin until they become mature erythrocytes. 1. Reaction of succinyl CoA and the amino acid glycine form a pyrrole molecule. Then, four pyrroles combine to form protoporphyrin. 2. Ferrous (Fe2+) binds to the ring making heme molecule.  Heme + polypeptide chain = Haemoglobin Chain.  4 haemoglobin Chains (2 pairs) = Hb molecule.  Abnormalities of the chains can alter the physical characteristics of the hemoglobin molecule.  One Hb molecule can bind 4 molecules of O2  It is the O₂ binding pigment found in red muscles.  It contains only one polypeptide chain.  It contain Heme molecule.  Heme is found in other compounds as well (cytochrome oxidase enzymes).  Results from gene mutation.  Different amino acids in the polypeptide chain.  Mostly change the O2 affinity.  May cause anaemia.  An example is Hb S = α2 β2 (sickle cell Hb).  Hb S = α 2 β2 (sickle cell Hb):  In the β chain a glutamate in position 6 has been replaced by valine.  Hb S have lower affinity to O2.  Sickle cell anaemia provides resistance to malaria.  Itpolymerizes (Crystalize) at low PaO₂ giving the RBC a sickle shape and the tendency to aggregate and block blood vessels → Ischemia (no blood flow) & pain.  Sickle cell anaemia occur in homozygous individuals ( inherited two abnormal β chain from the parents).  Heterozygous individuals =  One normal gene.  50% Hb A and 50% Hb S.  Sickle cell trait  Hb binds to O₂ reversibly by the ferrous (Fe++) in heme.  The binding is termed Oxygenation, not oxidation.  Hb bound to O₂ = oxygenated Hb= Oxy Hb  Hb not bound to O₂ = Deoxygenated Hb = Deoxy Hb.  The affinity of Hb to O2 is affected by many factors:  Type of polypeptide chain:  Hb F = higher O₂ affinity than Hb A, this will help the fetus in getting O₂ from maternal blood.  Hb S = lower O₂ affinity than Hb A, this helps in O₂ delivery to the tissue.  Other factors that affect the affinity of Hb to O₂ are: 1. CO₂ in blood (decreases the affinity to O₂). 2. H⁺ concentration (PH) (decreases the affinity to O₂). 3. Temperature (↑T decreases the affinity to O₂). 4. Concentration of 2,3–DPG (diphosphoglycerate).↑2,3–DPG →↓ affinity to O₂.  Carbamino-haemoglobin =  Hb bind reversibly to CO₂ at the amino terminal of the polypeptide chain.  5% of CO₂ transported in blood is bound to Hb.  Reaction with H⁺:  Hb in the RBCs is an important buffer H⁺ + Hb → HHb  Met-haemoglobin = Met Hb The ferrous ion is oxidized to Ferric ion (Fe3+) by oxidizing agents and stays with the Hb.  Carboxy-haemoglobin =Carboxy Hb  Hb bound to Carbon monoxide (CO)  CO displaces O₂ from Hb because of the higher affinity of Hb to CO than to O₂ (200-300 times).  In CO poisoning Hb concentration in blood is normal but it can not carry O₂.  Red cells are destroyed in the tissues especially the spleen.  Then taken up by the macrophages: 1. The Polypeptide portion is hydrolysed to amino acids and released to the circulation. 2. Iron is removed from heme and released back to the plasma bound to transferrin to be reused in RBC production. 3. Porphyrin ring is converted to biliverdin. 4. Biliverdin is converted to bilirubin.  Bilirubin is a yellow pigment which is insoluble in water.  Released from the macrophages into plasma then carried in the plasma bound to albumin.  Bilirubin bound to albumin can not be excreted in the urine but it is transported to the liver.  In the liver bilirubin is conjugated to glucouronic acid.  Conjugated bilirubin is soluble in water and is secreted into the bile to be excreted in faeces.  Conjugated bilirubin in bile is converted by intestinal bacteria into stercobilinogen and then urobilinogen.  Stercobilinogen is excreted in faeces and gives it the brown colour.  Urobilinogen have no colour (colourless) and is soluble in plasma.  Urobilinogen is absorbed back to the portal blood & most of the reabsorbed urobilinogen reaches the liver and is re-excreted in bile (entero-hepatic circulation).  Urobilinogen can be secreted in urine (only 5%).  Definition:  Yellowdiscoloration of the skin, sclera and mucous membranes.  when bilirubin level in the blood is more than 2 mg/dL.  Analytic methods allow determination of:  Total bilirubin  Conjugated bilirubin.  Unconjugated fraction.  Urine bilirubin and urobilinogen is determined in urine specimens  Normal ranges are:  Totalbilirubin → < 1.4 mg/dl  Urine bilirubin → undetectable  Urine urobilinogen → undetectable  Jaundice is classified based on the cause into: 1. Hemolytic jaundice. 2. Hepatic jaundice. 3. Obstructive jaundice.  Increased destruction of RBC results in increased level of unconjugated bilirubin in the blood, which can not be secreted in urine.  Liver is adding an increased amount of conjugated bilirubin to the intestine.  The production of urobilinogens is increased and so is its excretion in urine.  Laboratory findings in hemolytic jaundice: 1. Low Hb and high Reticulocyte count. 2. Elevated total bilirubin and unconjugated bilirubin. 3. No bilirubin in urine. 4. Elevated urobilinogen in urine.  Can be due to failure of the liver to conjugate the bilirubin= increases unconjugated bilirubin in blood and bilirubin is not secreted in urine.  Or failure of secretion of bilirubin into bile = increases conjugated bilirubin in blood and urine.  In hepatocellular liver disease the conjugated fraction predominates because the active biliary secretion is the process that is most affected by cell injury.  Due to obstruction of the flow of bile by a tumour or a gall stone.  The liver conjugates bilirubin but because of the obstruction conjugated bilirubin leaks back to the blood.  This increases the conjugated bilirubin in the blood and urine.  Because bile can not reach the intestine:  stercobilinogen is absent from stool = pale stool.  Fat is not absorbed =increased fat in stool & vitamin K deficiency (bleeding).  No urobilinogen is detected in urine.  Laboratory findings in obstructive jaundice: 1. Normal Hb and Reticulocyte count. 2. Elevated total bilirubin and conjugated bilirubin. 3. Elevated bilirubin in urine 4. Absent urobilinogen in urine. 5. Pale and fatty stool (steatorrhea).  Physiological hyper-bilirubinaemia and jaundice:  During intrauterine life, fetal bilirubin crosses the placenta and is excreted by the liver of the mother.  For the first few days, the conjugation of bilirubin with glucouronic acid by the fetal liver is poor.  This leads to accumulation of unconjugated bilirubin. But it is usually mild.  Hemolytic disease of the new borne:  Pathological jaundice in neonates  Hemolysis of the neonate’s RBC is caused by an immune attack by antibodies from the mother.  Is a bilirubin induced brain dysfunction.  Bilirubin is a highly neurotoxic substance. It my accumulates in the central nervous system causing irreversible neurological damage and even death.  Neonates are especially vulnerable to hyper-bilirubinemia-induced neurological damage.  Treatment is to lower the serum bilirubin levels either by phototherapy or exchange transfusion.  Phototherapy converts unconjugated bilirubin (isomerisation )into a soluble form that can be excreted in urine.  Describes the chemical structure & synthesis of Hb.  Understands haemoglobin abnormalities and the different reactions of Hb.  List factors that affect the binding of O2 to Hb and describes the steps involved in catabolism of Hb.  Define jaundice & compare between different types of jaundice regarding the cause and the laboratory findings