COMPLETE BLOOD DISORDER STUDY NOTES

1. CLASSIFICATION OF ANEMIA

• Excessive blood loss (hemorrhage - acute or chronic)
• Excessive destruction (hemolysis - intrinsic or extrinsic)
• Deficient production (nutritional deficiency, bone marrow failure)

• Microcytic (small RBCs) - seen in iron deficiency, thalassemia, chronic disease
• Normocytic (normal-sized RBCs) - seen in acute blood loss, aplastic anemia, hemolytic anemias
• Macrocytic (large RBCs) - seen in megaloblastic anemias (B12/folate deficiency)

• Hypochromic (pale cells, low hemoglobin) - iron deficiency
• Normochromic (normal color) - most other anemias

2. LABORATORY TESTS - RBC INDICES

• Total number of RBCs per microliter of blood.

• Hemoglobin content of blood in grams per deciliter (g/dL).
• Reduced in anemia; elevated in polycythemia.

• Measures the proportion of red cell mass in 100 mL of total plasma volume.
• Expressed as a percentage.
• Increased by: dehydration, polycythemia.
• Decreased by: blood loss, anemia, hemodilution (e.g., pregnancy).
• Clinical significance: used to diagnose and monitor anemia and polycythemia; a Hct below normal indicates the severity of anemia.

• Average volume (size) of a single red blood cell, measured in femtoliters (fL).
• Formula: MCV = 10 x (Hct / RBC count)
• Normal: approximately 80-100 fL.
• Low MCV = microcytic anemia (iron deficiency, thalassemia, chronic disease).
• High MCV = macrocytic anemia (B12 deficiency, folate deficiency).
• It tells you if cells are too small or too large.

• Average concentration of hemoglobin inside each red cell, expressed as g/dL.
• Formula: MCHC = Hgb / Hct
• Normal: approximately 32-36 g/dL.
• Low MCHC = hypochromic anemia (iron deficiency - cells are pale because they have little hemoglobin per unit volume).
• Normal MCHC = normochromic (most anemias except iron deficiency/thalassemia).
• Elevated MCHC = seen in hereditary spherocytosis (cells are packed with hemoglobin because they lost membrane surface area but kept the same volume of cytoplasm).

• Average mass of hemoglobin per red cell.
• Formula: MCH = (Hgb x 10) / RBC count
• Low in iron deficiency (hypochromic cells); high in megaloblastic anemia.

• Percentage of immature red cells (reticulocytes) in blood.
• Elevated reticulocyte count = bone marrow is producing more RBCs in response to loss or destruction (hemolytic anemia, acute blood loss).
• Low reticulocyte count = bone marrow is failing to compensate (aplastic anemia, nutritional deficiency with inadequate production).

3. HEREDITARY SPHEROCYTOSIS

• Red blood cells lose their normal biconcave disc shape and become small, round spheres (spherocytes).
• These spherocytes are stiff, cannot deform, and are trapped and destroyed in the spleen.
• The result is hemolytic anemia.

• It is usually transmitted as an autosomal dominant trait (a more severe autosomal recessive form also exists).
• It is caused by inherited mutations in proteins of the red cell membrane skeleton.

• The red cell membrane has a cytoskeleton made of proteins - the most important being spectrin, ankyrin, band 3, band 4.1, and band 4.2.
• Spectrin forms a flexible meshwork just under the lipid bilayer and is connected to integral membrane proteins (band 3, glycophorin) through linker proteins like ankyrin and band 4.1.
• Mutations in spectrin, ankyrin, band 3, or band 4.2 weaken the connections between the membrane skeleton and the lipid bilayer.
• Because the skeleton is not properly anchored, the lipid bilayer becomes unstable and continuously sheds small membrane vesicles into the circulation as the cell ages.
• The cell loses surface area (membrane) but retains its cytoplasm (volume), so the surface-area-to-volume ratio decreases progressively.
• As surface area shrinks relative to volume, the cell rounds up into a sphere - it cannot stay biconcave anymore.

• Normal biconcave red cells are very deformable - they can squeeze through tiny capillaries and splenic sinusoids.
• Spherocytes are rigid and non-deformable.
• They get stuck in the narrow, tortuous splenic cords (the red pulp of the spleen).
• Once trapped, they are recognized and phagocytosed by resident macrophages in the spleen.
• This is extravascular hemolysis.

• On blood smear: spherocytes appear as small, dark red cells with no central pallor (because the membrane is too tight).
• The MCHC is elevated (cells are packed with hemoglobin per unit volume).
• Splenomegaly is prominent - the spleen enlarges due to congestion and increased macrophage activity (weight can reach 500-1000 g; normal is 150-200 g).
• Compensatory hyperplasia of red cell progenitors in the bone marrow.
• Reticulocytosis (elevated reticulocyte count) because the marrow is working hard to replace destroyed cells.
• Cholelithiasis (pigment gallstones) occurs in 40-50% of patients - caused by excess bilirubin from RBC destruction being excreted into bile.
• Jaundice from elevated bilirubin (unconjugated).
• Splenectomy corrects the anemia even though spherocytes persist, because without the spleen there is no site of destruction.

4. ANEMIA DUE TO BLOOD LOSS

• Acute blood loss anemia
• Chronic blood loss anemia (which leads to iron deficiency anemia over time)

• Caused by sudden hemorrhage (trauma, ruptured aortic aneurysm, GI bleed).
• The primary problem is loss of intravascular volume (circulatory shock can develop).
• Initially, because whole blood is lost (RBCs and plasma together), the Hct and Hgb do not fall immediately.
• Within hours, fluid shifts from the interstitial space into the bloodstream to restore volume - this hemodilution causes the Hct, Hgb, and RBC count to fall.
• The anemia is normocytic and normochromic (cells are normal in size and color - it is simply fewer normal cells).
• The reticulocyte count rises within a few days as the marrow ramps up production.
• If bleeding is internal, iron is recycled and used to make new RBCs.

• Caused by slow, persistent bleeding - GI bleeding (ulcers, colon cancer, hemorrhoids), menstrual disorders.
• Often asymptomatic until Hgb falls below 8 g/dL.
• The continuous loss of blood means iron is continuously lost from the body (iron cannot be recovered from external bleeding).
• Iron stores are gradually depleted, impairing hemoglobin synthesis.
• The result is microcytic (small) and hypochromic (pale) anemia - iron deficiency anemia.
• Lab findings: low serum ferritin, low serum iron, elevated TIBC (transferrin is upregulated when iron is scarce), low MCV, low MCHC.

• Acute: normocytic, normochromic; problem is volume loss, shock risk.
• Chronic: microcytic, hypochromic; problem is iron depletion over time leading to iron deficiency anemia.

5. HEMOLYTIC ANEMIA

• Premature destruction of red blood cells before their normal 120-day lifespan.
• Iron and hemoglobin breakdown products are retained and recycled.
• Compensatory increase in erythropoiesis (bone marrow works harder).

• Normocytic, normochromic red cells (destruction is the problem, not defective production).
• Elevated reticulocyte count (compensatory marrow response).
• Elevated indirect (unconjugated) bilirubin from heme breakdown - causes jaundice.
• Pigment gallstones (bilirubin-rich) from chronic excess bilirubin in bile.
• Splenomegaly (the spleen is the main site of extravascular hemolysis).

• Intrinsic (hereditary) - defects within the RBC itself: hereditary spherocytosis, G6PD deficiency, sickle cell, thalassemia.
• Extrinsic (acquired) - factors outside the RBC cause destruction: immune-mediated, mechanical, infection.

• Occurs inside blood vessels.
• Caused by: mechanical injury from defective cardiac valves; transfusion reactions; exogenous toxins.
• Hemoglobin is released directly into the blood - leads to hemoglobinemia (Hgb in blood), hemoglobinuria (Hgb in urine), hemosiderinuria.

• Occurs in the spleen, liver, and bone marrow (phagocytic cells destroy abnormal RBCs).
• Caused by: extreme shape changes (spherocytes, sickle cells) that trigger phagocytosis.
• Produces jaundice and pigment gallstones.

6. SICKLE CELL ANEMIA

• Caused by a single point mutation in the beta-globin gene: glutamic acid (a negatively charged amino acid) is replaced by valine (a non-polar, hydrophobic amino acid) at the 6th position of the beta-globin chain.
• This produces abnormal hemoglobin S (HbS).
• When HbS is deoxygenated, it undergoes a conformational change that allows HbS molecules to form long, stiff polymers via intermolecular contacts involving the hydrophobic valine.
• These polymers distort the red cell into an elongated, crescent/sickle shape.
• Transmission: autosomal recessive. Homozygotes (2 HbS genes) have sickle cell disease; heterozygotes (1 HbS gene, 1 HbA gene) have sickle cell trait (mostly asymptomatic, protective against malaria).

• Level of HbS vs. other hemoglobins: HbA greatly retards HbS polymerization. HbF (fetal hemoglobin) also inhibits it - so newborns do not manifest disease until HbF falls at 5-6 months of age. This is why hydroxyurea works - it increases HbF production.
• Concentration of HbS inside the cell: dehydration raises intracellular HbS concentration and worsens sickling.
• Transit time through the microvasculature: slow blood flow (as in the spleen, bone marrow, or inflamed tissues) gives more time for sickling to occur.

• Chronic hemolytic anemia: repeated sickling damages the membrane (calcium influx, potassium/water loss, membrane skeleton damage), producing irreversibly sickled cells that are fragile and hemolyze. Mean RBC lifespan is only 20 days (normal 120 days).
• Vascular occlusion: sickled cells are rigid and sticky (abnormal membrane proteins adhere to endothelium). They block small blood vessels, causing ischemia and infarction in organs. This leads to pain crises and organ damage.

• Chronic hemolytic anemia: pallor, fatigue, jaundice, pigment gallstones.
• Vaso-occlusive pain crises: episodic, severe pain in abdomen, chest, bones, and joints due to tissue ischemia. Triggered by infection, dehydration, cold, acidosis, inflammation (anything that slows blood flow).
• Acute chest syndrome: the most dangerous complication. Caused by vascular occlusion in pulmonary vasculature (pulmonary infarction) or atypical pneumonia in the setting of sickling. Features: chest pain, fever, hypoxia, new pulmonary infiltrate.
• Autosplenectomy: repeated splenic infarcts in childhood eventually destroy the spleen, leaving a small fibrotic nubbin by adulthood. This means patients are functionally asplenic and highly susceptible to encapsulated bacteria (Streptococcus pneumoniae, H. influenzae) - they need prophylactic penicillin and full pneumococcal vaccination.
• Bone changes: marrow hyperplasia causes bone resorption and new bone formation - "crewcut" appearance of the skull on X-ray, prominent cheekbones.
• Organ damage: chronic ischemia affects liver, kidney (renal tubular defects), retina, brain (stroke), and skin (leg ulcers).
• Priapism: painful, prolonged erection from vascular congestion; can lead to penile fibrosis and erectile dysfunction.
• Splenomegaly in children (before autosplenectomy).
• Increased susceptibility to infection (especially encapsulated organisms after autosplenectomy).

7. ANEMIA OF CHRONIC INFLAMMATION (Anemia of Chronic Disease)

• AIDS, osteomyelitis, bacterial endocarditis, lung abscess (chronic infections)
• Rheumatoid arthritis, Crohn disease (chronic immune disorders)
• Cancers (Hodgkin lymphoma, carcinomas of lung and breast)
• Chronic kidney disease

• Hepcidin binds to ferroportin, the only known cellular iron export protein.
• When hepcidin binds ferroportin, it causes ferroportin to be internalized and degraded.
• Without ferroportin on the cell surface, iron cannot exit cells (cannot be exported from enterocytes in the gut, or from macrophages in the bone marrow/liver/spleen).
• In chronic inflammation, inflammatory cytokines - especially IL-6 - are chronically elevated.
• IL-6 signals the liver to produce large amounts of hepcidin.
• Elevated hepcidin downregulates ferroportin in marrow macrophages (which store recycled iron from destroyed RBCs) and in gut enterocytes.
• Iron becomes trapped inside macrophages and gut cells - it cannot be transferred to developing red blood cells in the bone marrow.
• Erythroid precursors are iron-starved even though total body iron stores are normal or elevated - this is called functional iron deficiency.
• In addition, chronic inflammation directly blunts erythropoietin (EPO) synthesis by the kidney - reducing the signal to the marrow to produce more red cells.
• The result: low red cell production, shorter red cell lifespan, iron-restricted erythropoiesis.

• Normocytic, normochromic anemia (can be mildly microcytic/hypochromic in severe cases).
• Low reticulocyte count (marrow is suppressed).
• Low serum iron (iron trapped in macrophages, not in circulation).
• Normal to elevated serum ferritin (stores are full - opposite of iron deficiency).
• Normal to decreased TIBC (transferrin is a negative acute-phase reactant - it decreases with inflammation; the opposite of iron deficiency, where TIBC is elevated).
• Increased bone marrow iron stores (stainable iron in macrophages).

• In iron deficiency: low ferritin, elevated TIBC, no iron in marrow macrophages.
• In anemia of chronic disease: elevated/normal ferritin, decreased TIBC, plenty of iron in marrow macrophages - it just cannot be released.

8. CHRONIC INFLAMMATION AND THE KIDNEY - ERYTHROPOIETIN

• Erythropoietin (EPO) - produced by peritubular interstitial cells of the kidney (specialized fibroblast-like cells in the renal cortex).

• EPO is the primary growth factor that drives red blood cell production.
• It is produced by the kidney in response to a decrease in oxygen levels (tissue hypoxia).
• EPO acts on erythroid progenitor cells (CFU-E and BFU-E) in the bone marrow.
• It prevents their apoptosis and promotes their proliferation and differentiation into mature red blood cells.
• EPO also stimulates reticulocyte release from the marrow into circulation.
• The cycle: low O2 -> kidney senses hypoxia -> produces EPO -> marrow makes more RBCs -> O2 delivery improves -> EPO production falls (negative feedback).

• Chronic inflammation (cytokines like TNF, IL-1, IL-6) directly suppresses EPO gene expression in renal peritubular cells.
• Chronic kidney disease (e.g., from repeated infections, glomerulonephritis, diabetic nephropathy) destroys the renal parenchyma, reducing the number of EPO-producing cells.
• Less EPO means the bone marrow receives less stimulation to produce red cells.
• This contributes to the anemia of chronic disease AND to the separate anemia of chronic kidney disease (renal anemia).
• Both hepcidin elevation and EPO deficiency together make the anemia worse.

9. MEGALOBLASTIC ANEMIA

• Both folate and vitamin B12 are required for the synthesis of thymidine, a building block of DNA.
• Specifically: folate (in the form of tetrahydrofolate, THF) provides a methyl group for the conversion of deoxyuridine monophosphate (dUMP) to thymidine monophosphate (dTMP).
• Vitamin B12 is needed to regenerate THF from methyltetrahydrofolate. Without B12, folate gets "trapped" as methylTHF (the methyl trap hypothesis) and cannot be used for thymidine synthesis.
• When thymidine (and therefore DNA) synthesis is impaired, rapidly dividing cells cannot complete DNA replication fast enough to keep up with cell growth.
• Nuclear maturation is delayed, but cytoplasmic growth (RNA synthesis, protein synthesis) continues at a normal rate.
• The result: cells grow large in cytoplasm (because RNA and protein synthesis are normal) but have immature, dysfunctional nuclei that cannot divide properly.
• These large cells with immature nuclei are called megaloblasts (in the marrow) or macrocytes (in the blood).
• The most severely affected cells are those that divide the fastest: hematopoietic marrow cells, and mucosal cells of the GI tract (which is why sore tongue/glossitis and diarrhea occur).
• Because they cannot divide normally, many of these cells die within the marrow before reaching the blood - this is called ineffective erythropoiesis. The patient is anemic despite a hyperactive marrow.
• On blood smear: large oval red cells (macro-ovalocytes), and hypersegmented neutrophils (neutrophils with 5 or more nuclear lobes) - a hallmark finding.
• MCV is elevated; MCHC is normal.

• Folate is found in green leafy vegetables, fruits, cereals, and meats. It is absorbed in the small intestine.
• Lost during cooking.
• Causes: malnutrition/dietary lack (alcoholism, elderly), malabsorption (celiac disease, GI disorders), drugs (methotrexate - blocks folate metabolism; triamterene; anticonvulsants), pregnancy (requirement increases 5-10 fold; deficiency linked to neural tube defects).
• No neurological symptoms (unlike B12).
• Diagnosis: low serum and red cell folate levels, elevated MCV.

• Present in all animal-based foods; virtually never from dietary lack except strict vegans.
• Absorption: B12 must bind intrinsic factor (IF) - a protein secreted by parietal cells of the gastric fundus. The B12-IF complex is absorbed in the terminal ileum via cubilin receptors.
• Stored in the liver; stores last 5-20 years - so deficiency develops slowly after years of malabsorption.
• Causes:
  • Pernicious anemia (most common): autoimmune destruction of gastric parietal cells. Autoantibodies block IF or prevent the B12-IF complex from binding cubilin. Histology shows chronic atrophic gastritis with loss of parietal cells and lymphocytic infiltrate.
  • Gastrectomy: removes parietal cells (no IF production).
  • Ileal resection/Crohn disease: removes the absorption site.
  • Gastric atrophy/achlorhydria in elderly: pepsin is needed to release B12 from food.
• Extra feature specific to B12 deficiency (not folate): neurological damage.
  • B12 is also needed for the synthesis of myelin.
  • Deficiency causes demyelination of the posterior and lateral columns of the spinal cord - called subacute combined degeneration.
  • Symptoms: symmetric tingling/numbness/burning in hands and feet, unsteady gait, loss of position and vibration sense, spastic weakness.
  • These neurological symptoms may occur even without anemia, and may not reverse even after B12 replacement.
• Diagnosis: low serum B12, normal/elevated folate, elevated MCV, hypersegmented neutrophils, anti-intrinsic factor antibodies (in pernicious anemia), anti-parietal cell antibodies.

10. IRON DEFICIENCY ANEMIA - PATHOGENESIS

• Chronic blood loss (most common in developed countries): GI bleeding (ulcers, colon cancer, hemorrhoids), menorrhagia, endometrial cancer.
• Dietary deficiency (most common in developing countries): vegetarian/vegan diets (heme iron in meat is more bioavailable than non-heme iron in plants); exclusively milk-fed infants.
• Increased demand not met by diet: pregnancy, infancy, adolescence.
• Malabsorption: celiac disease, gastritis, gastrectomy (iron is absorbed mainly in the duodenum).

• Stage 1 (Iron depletion): Iron stores are depleted first. Serum ferritin falls (ferritin reflects storage iron). Bone marrow shows no stainable iron in macrophages.
• Stage 2 (Iron-deficient erythropoiesis): Serum iron falls, transferrin (TIBC) rises (the liver makes more transferrin to capture every available iron molecule). Transferrin saturation decreases. Hemoglobin synthesis begins to be impaired. Red cells start to become smaller and paler.
• Stage 3 (Iron deficiency anemia): Hemoglobin synthesis is clearly insufficient. Microcytic, hypochromic anemia is fully established. MCV, MCHC, and MCH are all low.

• Dietary iron (mainly ferric Fe3+) is reduced to ferrous Fe2+ by duodenal cytochrome B.
• Fe2+ enters the enterocyte via DMT-1 (divalent metal transporter 1).
• Inside the cell, iron is either stored as ferritin or exported to the blood via ferroportin.
• Exported iron is oxidized back to Fe3+ by hephaestin/ceruloplasmin and binds to transferrin for transport.
• Hepcidin (from the liver) controls this by degrading ferroportin - when hepcidin is low (iron deficient state), ferroportin is upregulated, absorbing more iron.

• Low serum ferritin (first to fall - best indicator of iron stores).
• Low serum iron.
• High TIBC (transferrin is upregulated).
• Low transferrin saturation (< 20%).
• Low MCV (microcytic cells).
• Low MCHC (hypochromic cells).
• Blood smear: microcytic, hypochromic red cells with anisocytosis (variable size) and poikilocytosis (variable shape).
• Platelet count is often elevated (thrombocytosis) for unclear reasons.
• EPO is elevated, but the marrow response is blunted because there is no iron to make hemoglobin.

11. APLASTIC ANEMIA

• Failure of multipotent myeloid stem cells in the bone marrow, leading to pancytopenia (low RBCs, low WBCs, low platelets).
• The marrow becomes replaced by fat and is virtually devoid of hematopoietic cells.

• High-dose radiation exposure.
• Chemical and toxic agents: benzene, chloramphenicol, cytotoxic drugs, certain pesticides.
• Viral infections: hepatitis (most common viral cause), Epstein-Barr virus (mononucleosis), HIV.
• Idiopathic (most common, ~65% of cases).
• Inherited: Fanconi anemia, telomerase mutations.

• Most cases are immune-mediated: stem cells are antigenically altered (by drugs, viruses, or unknown insults), provoking an autoimmune T-cell response. Activated Th1 cells release cytokines (IFN-gamma, TNF) that kill hematopoietic stem cells.
• Evidence: 60-70% of patients respond to immunosuppressive therapy targeting T cells.
• A minority have intrinsic stem cell defects (telomerase mutations) causing premature senescence.

• Insidious or acute onset.
• Anemia symptoms: weakness, fatigue, pallor, dyspnea.
• Thrombocytopenia: petechiae, ecchymoses, bleeding from nose, gums, GI tract, vagina.
• Neutropenia: serious, life-threatening infections.
• No splenomegaly (important - helps distinguish from other causes of pancytopenia).

12. THROMBOCYTOPENIA

• Aplastic anemia (general marrow failure)
• Drug-induced suppression: alcohol, thiazides, cytotoxic chemotherapy drugs
• Infections that infect megakaryocytes: measles, HIV
• Nutritional deficiency: B12 or folate deficiency (megaloblastic process affects megakaryocytes too)
• Bone marrow replacement: leukemia, metastatic cancer, granulomatous disease (marrow is physically crowded out)
• Myelodysplastic syndromes

• Immune thrombocytopenic purpura (ITP): autoantibodies (usually IgG) coat platelet surface antigens; coated platelets are phagocytosed by macrophages in the spleen.
  • Acute ITP: typically in children, follows viral infection (immune complexes), self-limited.
  • Chronic ITP: typically in adults, especially women; requires treatment.
• Drug-induced immune thrombocytopenia: quinidine, heparin (heparin-induced thrombocytopenia - HIT), sulfa compounds - drug acts as a hapten or forms complexes with platelet proteins.
• Systemic lupus erythematosus, B-cell lymphomas.
• Alloimmune: post-transfusion purpura, neonatal alloimmune thrombocytopenia.
• Infections: HIV, dengue fever, infectious mononucleosis.

• Disseminated intravascular coagulation (DIC): widespread clotting consumes platelets.
• Thrombotic microangiopathies: TTP (thrombotic thrombocytopenic purpura), HUS (hemolytic uremic syndrome) - abnormal vWF multimers or endothelial injury causes platelet aggregation and destruction in the microvasculature.
• Giant hemangiomas.

• Hypersplenism: an enlarged spleen traps an excess proportion of circulating platelets (normally the spleen holds about one-third of the platelet pool; an enlarged spleen can trap up to 90%).

• Petechiae: tiny (1-2 mm), flat, red or purple dots from minor capillary bleeding. Pinprick-size. Do not blanch on pressure. Characteristic of thrombocytopenia and platelet disorders.
• Purpura: larger (> 3 mm), flat hemorrhagic patches in the skin from bleeding into the skin, still not raised.
• Ecchymoses: bruises - large areas of hemorrhage into the skin and subcutaneous tissue.
• These skin findings reflect platelet-type bleeding (mucosal surfaces, small vessel bleeding) - the hallmark of primary hemostasis defects (as opposed to deep bleeding into joints, which is the hallmark of coagulation factor defects).

13. HOW ARE PLATELETS ACTIVATED (Primary Hemostasis)

• Vascular injury exposes subendothelial collagen and von Willebrand factor (vWF).
• Platelet adhesion: platelets adhere to exposed vWF (and collagen) via glycoprotein Ib (GpIb) receptors on their surface. This tethers platelets to the damaged wall.
• Platelet activation: adhesion triggers a conformational change in the platelet. The activated platelet:
  • Changes shape (from disc to spiky sphere, increasing surface area for interactions).
  • Releases the contents of its alpha-granules (contain vWF, fibrinogen, fibronectin, factor V, platelet factor 4) and dense granules (contain ADP, serotonin, calcium).
  • Released ADP and thromboxane A2 (TXA2) recruit more platelets from the blood - amplifying the response.
  • The GpIIb/IIIa receptor undergoes a conformational change, increasing its affinity for fibrinogen.
• Platelet aggregation: fibrinogen bridges bind GpIIb/IIIa receptors on adjacent platelets, linking them together. This forms the primary hemostatic plug - a loose platelet aggregate that physically seals the vessel injury. This is the end of primary hemostasis.

14. PRIMARY VS. SECONDARY HEMOSTASIS

• Involves platelets and vWF.
• Occurs rapidly (seconds to minutes) at the site of injury.
• Steps: vascular spasm -> platelet adhesion (via GpIb-vWF) -> platelet activation -> platelet aggregation (via GpIIb/IIIa-fibrinogen) -> formation of the primary platelet plug.
• The plug is loose and not reinforced.
• Defects cause: mucosal-type bleeding - petechiae, purpura, ecchymoses, nosebleeds, gum bleeding, heavy menstrual periods, GI bleeding.

• Involves coagulation factors (the clotting cascade).
• Occurs slower (minutes) and consolidates the platelet plug.
• Steps: tissue factor (TF) is exposed at the injury site and activates factor VII -> this initiates the extrinsic coagulation cascade -> sequential activation of factors X, V, II (prothrombin converted to thrombin) -> thrombin converts fibrinogen to fibrin -> fibrin strands are woven through the platelet plug -> factor XIII crosslinks fibrin, making the clot solid and stable. This is the secondary (definitive) hemostatic plug.
• Defects cause: deep tissue bleeding - hemarthrosis (bleeding into joints), deep muscle hematomas, delayed bleeding (bleeding stops initially because the primary platelet plug forms, but then resumes when it dissolves without adequate fibrin reinforcement).

15. VON WILLEBRAND FACTOR (vWF) - WHICH CELL PRODUCES IT

• vWF is produced by endothelial cells (the main source - stored in Weibel-Palade bodies and released on demand) and by megakaryocytes (which release it into platelet alpha-granules).
• It is a large multimeric glycoprotein that circulates in plasma.
• Functions: (1) mediates platelet adhesion to exposed subendothelial collagen at injury sites (via GpIb receptor); (2) acts as a carrier protein for factor VIII, protecting it from premature degradation in the blood.

16. BILIRUBIN - TYPES, CAUSES OF JAUNDICE, CONJUGATED vs. UNCONJUGATED

• When RBCs are destroyed (either normally after 120 days, or prematurely in hemolysis), the hemoglobin is broken down.
• Heme is cleaved from globin and converted first to biliverdin and then to bilirubin by macrophages in the spleen, liver, and bone marrow.
• This initial bilirubin is unconjugated (indirect) bilirubin.

• Water-insoluble (lipid-soluble) - cannot be excreted in urine.
• Transported in blood bound to albumin.
• Crosses the blood-brain barrier - toxic to brain tissue (can cause kernicterus in neonates).
• Cannot be excreted in urine (does NOT make urine dark).
• Elevated in: hemolytic anemias (excess RBC destruction), neonatal jaundice (immature liver enzyme), Gilbert syndrome, Crigler-Najjar syndrome (defective UDP-glucuronosyltransferase).

• Inside hepatocytes, unconjugated bilirubin is conjugated (made water-soluble) by the enzyme UDP-glucuronosyltransferase (UGT1), which attaches glucuronic acid molecules to it.
• Now it is conjugated (direct) bilirubin - water-soluble.
• Secreted into bile via bile canaliculi (requires MRP2 transport protein).
• Passes into the intestines where gut bacteria convert it to urobilinogen (which gives stool its brown color) and urobilin.
• Because it is water-soluble, it can appear in urine - so when conjugated bilirubin is elevated, the urine turns dark (bilirubinuria) - a key clinical sign.
• Elevated in: hepatocellular disease (hepatitis, cirrhosis - impaired secretion into bile), obstructive jaundice (gallstones, pancreatic cancer blocking bile ducts), Dubin-Johnson syndrome, Rotor syndrome.

• Jaundice (icterus) is yellow discoloration of the skin, sclera, and mucous membranes due to deposition of bilirubin when serum bilirubin exceeds approximately 2-3 mg/dL.
• Pre-hepatic (hemolytic) jaundice: excess RBC destruction produces more unconjugated bilirubin than the liver can conjugate. The liver is normal but overwhelmed. Unconjugated bilirubin rises. Urine is not dark (bilirubin not in urine), but urobilinogen in urine is increased. Stool is normal/dark.
• Hepatic jaundice: liver cell damage (hepatitis, cirrhosis) impairs both uptake/conjugation AND secretion of bilirubin. Both conjugated and unconjugated bilirubin rise. Dark urine (conjugated bilirubin in urine), pale stool possible.
• Post-hepatic (obstructive) jaundice: blockage of bile ducts prevents conjugated bilirubin from entering the intestine. Conjugated bilirubin backs up into the blood. Dark urine (bilirubinuria), pale/clay-colored stool (no bile reaching intestines), no urobilinogen in stool or urine. Itching (pruritus) from bile salt deposition in skin.