Hematology: Blood, Hemapoiesis, Hemoglobin, and Blood Volume — Comprehensive Study Notes

Blood: Characteristics and Plasma Composition

  • Blood is a connective tissue with a fluid matrix (plasma) and formed elements (erythrocytes, leukocytes, platelets).
  • Whole blood volume in adults: about 5 L (females: 4-5 L; males: 5-6 L).
  • Blood contributes ~7-8% of body weight.
  • Physical characteristics:
    • Color ranges from scarlet (oxygen-rich) to dark red (oxygen-poor).
    • Temperature: 38°C (100.4°F).
    • pH: 7.35-7.45.
    • Viscosity is about 4.5-5.5 relative to water (water = 1.0).
  • Plasma vs formed elements:
    • Plasma constitutes ~55% of whole blood.
    • Buffy coat <1% (leukocytes and platelets).
    • Erythrocytes ~44% of whole blood.
  • Blood is a colloid.
  • Plasma composition:
    • Water makes up ~92% of plasma.
    • Plasma proteins ~7% of plasma.
    • Other solutes ~1% of plasma.
    • Major plasma proteins: Albumins (~58% of plasma proteins), Globulins (~38%), Fibrinogen (~4%), Regulatory proteins (<1%).
    • Gamma globulins are antibodies (immunoglobulins).
  • Plasma solutes include electrolytes, nutrients, gases, and wastes.
  • Normal plasma electrolytes and key ranges (arterial plasma):
    • Sodium (Na⁺): 135-145 mEq/L
    • Potassium (K⁺): 3.5-5.0 mEq/L
    • Calcium (Ca²⁺): 8.4-10.2 mg/dL
    • Chloride (Cl⁻): 96-106 mEq/L
    • Bicarbonate (HCO₃⁻): 23.1-26.7 mEq/L
    • Phosphate (PO₄³⁻): 2.5-4.1 mEq/L
    • Hydrogen ion (pH): 7.35-7.45
  • Plasma functions:
    • Transports hormones, nutrients, and waste.
    • Regulates pH and osmolarity.
    • Maintains blood pressure and volume.
    • Plays a role in immunity and coagulation.
  • Major plasma solutes and molecules include:
    • Glucose, amino acids, lactate, lipids, cholesterol, triglycerides, phospholipids.
    • Respiratory gases (O₂, CO₂) dissolved or bound.
    • Electrolytes and wastes (urea, creatinine, bilirubin).
  • Blood plasma is 90-92% water and 6-8% proteins; 1% other solutes.
  • Serum vs plasma: (not explicitly in slides) note that clotting factors are in plasma; serum is plasma minus fibrinogen and clotting factors.

Perflourocarbons (PFC)

  • PFCs are synthetic blood substitutes that carry dissolved gases.
  • Mixed with an emulsifier to form a liquid suspension compatible with blood.
  • PFCs can carry about ~20% more gas than plasma.
  • Potential uses:
    • Restoring oxygen delivery
    • Treating traumatic brain injury
    • Treating anemia
    • Increasing chemotherapy effectiveness
    • Reducing the need for blood transfusion

Hemopoiesis: Key Concepts and Terminology

  • Hemo-: blood; poiesis: production or development.
  • Hemopoiesis (hematopoiesis): production and development of all blood cells.
    • Erythrocytes: Erythropoiesis
    • Leukocytes: Leucopoiesis
    • Thrombocytes (platelets): Thrombopoiesis
  • Initiation and sites:
    • Begins in the 2nd week of embryonic life.
    • Fetal sites: yolk sac, liver, spleen, and later bone marrow.
    • HbF (fetal hemoglobin) has higher O₂ affinity than HbA (adult).
  • Postnatal sites:
    • Red bone marrow becomes the primary hematopoietic site in adults, especially in flat bones (sternum, ribs, pelvis, vertebrae).
    • Active marrow is found in axial skeleton and proximal ends of appendicular skeleton in adults and children; red marrow gradually replaced by yellow marrow with age.
  • Blood cell lineages:
    • Erythropoiesis (RBCs)
    • Leukopoiesis (WBCs): granulopoiesis (neutrophils, eosinophils, basophils), monocytopoiesis (monocytes/macrophages), lymphopoiesis (B and T lymphocytes)
    • Thrombopoiesis (platelets) from megakaryocytes
  • Hematopoietic stem cells (HSCs):
    • Multipotent; reside in bone marrow.
    • Differentiate into two main progenitor lines:
    • Myeloid stem cells → RBCs, platelets, monocytes, granulocytes
    • Lymphoid stem cells → B cells, T cells, NK cells
  • Regulation:
    • Growth factors and cytokines control progenitor production.
    • Examples: Erythropoietin (EPO), Thrombopoietin (TPO), Colony-stimulating factors (CSFs), Interleukins.
    • EPO produced primarily by kidneys; TPO produced by liver; both have clinical uses (e.g., EPO to treat anemia in kidney disease; G-CSF/GM-CSF to treat neutropenia).
  • Fetal development and sites of hemopoiesis:
    • Yolk sac (3-8 weeks)
    • Liver (6-30 weeks)
    • Spleen (9-28 weeks)
    • Bone marrow (28 weeks to adult)
  • Metabolic and regulatory details:
    • Hematopoietic growth factors (HGFs) regulate progenitor cell production.
    • EPO, TPO are major hormonal regulators; CSFs and interleukins influence WBC production.
    • Renal failure can reduce EPO production, leading to anemia.

Hemopoietic Growth Factors and Regulation

  • Erythropoietin (EPO)
    • Produced by kidneys (and a small amount by liver).
    • Stimulates RBC production in bone marrow.
    • Clinical uses: treat anemia in end-stage kidney disease and chemotherapy-induced anemia; can be referred to as epoetin alfa.
    • Regulation: triggered by hypoxia (low O₂). Negative feedback once O₂-carrying capacity improves.
  • Thrombopoietin (TPO)
    • Produced by the liver.
    • Stimulates platelet formation from megakaryocytes.
  • Colony-stimulating factors (CSFs)
    • Stimulate specific leukocyte lineages:
    • G-CSF: granulocyte production (neutrophils).
    • GM-CSF: granulocytes and monocytes.
    • M-CSF: monocytes/macrophages.
    • Multi-CSF (GM-CSF family) promotes broader myeloid lineage.
  • Cytokines and interleukins
    • Act as autocrines/paracrines to promote progenitor cell proliferation and differentiation.
  • Important note on regulation
    • EPO drives RBC production; iron availability and B vitamins (B12, folate) are essential for erythropoiesis.
    • Iron deficiency or functional iron deficiency can limit RBC production even with adequate EPO signaling.

Erythropoiesis: From Stem Cell to Mature Erythrocyte

  • General pathway:
    • Hematopoietic stem cell → Myeloid lineage progenitors → (CFU-E, BFU-E) → Proerythroblast → Erythroblast stages → Normoblast → Reticulocyte → Erythrocyte.
  • Timeline:
    • From BFU-E to reticulocyte ~8 days; to mature RBC ~26 days.
  • Stages (key transitions):
    • Proerythroblast: large, nucleated cell; start of erythroid lineage.
    • Basophilic erythroblast: active Hb synthesis begins; ribosome-rich stage.
    • Polychromatic erythroblast: Hb accumulation; color changes.
    • Orthochromatic erythroblast (late normoblast): nucleus condenses and is ejected.
    • Reticulocyte: immature RBC, lacks organelles except ribosomes; matures to erythrocyte in circulation.
  • EPO responsiveness:
    • EPO acts on CFU-E to promote erythroid differentiation and maturation.
  • Roles of nutrients and factors:
    • Iron is essential for heme synthesis.
    • B vitamins (especially B12 and folic acid) are needed for DNA synthesis during rapid cell division.
  • Erythrocyte maturation and lifespan:
    • Mature RBCs lack nucleus and organelles; lifespan ~
      120 ext{ days}.
  • Clinical notes:
    • EPO production is stimulated by hypoxia and can be reduced in kidney disease. Testosterone can upregulate EPO production, contributing to higher male hematocrit.

Hemoglobin and Oxygen Transport

  • Hemoglobin (Hb) structure:
    • Tetrameric protein with 4 subunits; adult HbA is ext{HbA} = ext{α}2 ext{β}2, HbF is ext{HbF} = ext{α}2 ext{γ}2, HbA2 is ext{HbA2} = ext{α}2 ext{δ}2.
    • Each subunit contains a heme group with an iron (Fe²⁺) ion that binds one O₂ molecule.
    • One Hb molecule can carry up to 4 O₂ molecules: 4 ext{ O}_2 per Hb.
  • Normal Hb concentrations (adult):
    • Men: 13.5-16.5 ext{ g/dL}
    • Women: 12.1-15.1 ext{ g/dL}
    • Children: 11-16 ext{ g/dL}
    • Pregnant women: 11-12 ext{ g/dL}
  • Oxygen transport concepts:
    • Oxyhemoglobin: Hb bound to O₂ (loading in lungs).
    • Deoxyhemoglobin: Hb without O₂ (after delivering O₂ to tissues).
    • Carboxyhemoglobin and carbaminohemoglobin are other Hb-bound species (not detailed in slides, but relevant in practice).
  • Fetal Hb affinity:
    • HbF has higher O₂ affinity than HbA, facilitating maternal-fetal transfer.
  • Pathology overview:
    • Sickle-cell disease (HbS) arises from HBB gene mutations; variants include HbSS, HbSC, HbS-β-thalassemia, and other Hb-derived conditions.
  • Hemoglobin degradation products:
    • Heme is broken down to biliverdin and then bilirubin; bilirubin is excreted in bile from the liver.
    • Iron released from heme is transported by transferrin and stored as ferritin in liver or spleen.
  • Measurement of Hb-related parameters helps diagnose anemia and other disorders (CBC, Hb electrophoresis).

Iron Metabolism, Transferrin, and Vitamin B12/Folate

  • Iron distribution in the body (approximate):
    • Total iron in the body ~4 g.
    • Red cell mass (Hb) ~50% of total iron.
    • Storage iron in ferritin and hemosiderin ~30%.
    • Liver and spleen, bone marrow store significant iron (~25%).
    • Other heme proteins (cytochromes, myoglobin, etc.) ~5%.
    • About 0.1% of iron is in serum (transferrin-bound iron circulation is dynamic).
  • Iron absorption and transport:
    • Dietary iron is absorbed in the duodenum.
    • Heme iron (~10-15% absorption) is absorbed more efficiently than non-heme iron (~1-5% typical for non-heme source depending on enhancers/inhibitors).
    • Non-heme iron is reduced (Fe³⁺ to Fe²⁺) for uptake via divalent metal transporter 1 (DMT1).
    • Iron in plasma is transported bound to transferrin; each transferrin molecule can carry up to two Fe³⁺ ions and carries iron between gut, liver, bone marrow, and macrophages.
  • Transferrin and iron status tests:
    • Transferrin testing is used to assess iron status; often referred to as Total Iron Binding Capacity (TIBC).
    • Elevated TIBC indicates low body iron stores; low TIBC indicates high iron stores.
  • Iron storage proteins:
    • Ferritin stores iron intracellularly (primarily in liver, spleen, and bone marrow).
    • Hemosiderin stores iron when ferritin stores are exceeded.
  • Regulation of iron for erythropoiesis:
    • Iron is required for heme synthesis within erythroblasts; iron deficiency impairs Hb production.
    • EPO signaling promotes erythropoiesis, increasing demand for iron
  • Vitamin B12 (cobalamin) and folate in erythropoiesis:
    • Both are essential for DNA synthesis in rapidly dividing cells.
    • B12 and folate are required for proper RBC precursor maturation.
    • B12 is absorbed with intrinsic factor; pernicious anemia results from intrinsic factor deficiency, causing impaired B12 absorption.
    • Sources of B12 include meat and dairy products.

Regulation and Resources for Erythropoiesis

  • Key regulators:
    • Tissue oxygenation is the single most important regulator of erythropoiesis.
    • Erythropoietin (EPO) stimulates RBC production; iron availability and vitamins (B12, folate) are essential cofactors.
    • Cytokines (CSFs, interleukins) modulate the production of progenitor cells.
  • Hormonal control and feedback:
    • EPO release from kidneys is triggered by hypoxia and decreased O₂ availability.
    • Negative feedback occurs as circulating RBCs increase and tissue oxygenation improves.
    • Testosterone stimulates EPO production, contributing to higher male hematocrit; altitude and other hypoxic conditions raise EPO levels.

Erythrocyte Lifecycle and Destruction

  • Lifespan and turnover:
    • Erythrocyte lifespan is about 120 days.
    • Old erythrocytes are phagocytosed by macrophages in the liver and spleen.
  • Hemoglobin breakdown and iron recycling:
    • Globin proteins are broken down into amino acids.
    • Heme (minus iron) is converted to biliverdin, then bilirubin, which is excreted in bile.
    • Iron is salvaged and transported by transferrin to the bone marrow for reuse.
  • Bilirubin handling:
    • Unconjugated bilirubin binds albumin and is taken to the liver where it is conjugated and excreted into bile.
    • In the intestine, bilirubin is converted to urobilinogen; most is excreted as stercobilin in feces, with some reabsorbed and excreted in urine.

Red Blood Cell Membrane and Cytoskeleton

  • Erythrocyte membrane proteins:
    • Spectrin is a key cytoskeletal protein in the RBC membrane skeleton.
    • Provides flexibility and mechanical stability for deformability as RBCs traverse capillaries.
  • Importance of spectrin:
    • Maintains biconcave shape and membrane integrity under shear stress.
    • Mutations in spectrin can cause hereditary elliptocytosis or hereditary spherocytosis.

Formed Elements: Components and Differential

  • Formed elements include:
    • Erythrocytes (RBCs): biconcave discs, anucleate, ~7.5 μm in diameter, lifespan ~120 days; function: O₂ and CO₂ transport; 4 heme-bound O₂ per Hb molecule.
    • Leukocytes (WBCs): complete cells; include neutrophils, eosinophils, basophils, lymphocytes, monocytes.
    • Platelets (thrombocytes): cell fragments; essential for hemostasis.
  • Normal concentrations (approximate):
    • Leukocytes: 4.5-11.0 × 10³/μL
    • Platelets: 150-400 × 10³/μL
    • Erythrocytes: 4.2-6.2 × 10⁶/μL
  • Leukocyte differential (percent of WBCs):
    • Neutrophils: 50-70%
    • Lymphocytes: 20-40%
    • Monocytes: 2-8%
    • Eosinophils: 2-4%
    • Basophils: 0.5-1%
  • RBC morphology and indices:
    • Mean Corpuscular Volume (MCV): average volume of a single RBC
    • Normal range for adults: 80-100 fL (μm³)
    • Formula: ext{MCV} = rac{Hct imes 10}{ ext{RBC count (in millions/μL)}}
    • Mean Corpuscular Hemoglobin (MCH): average Hb content per RBC
    • Common range: ~27-34 pg
    • Formula: ext{MCH} = rac{Hb ext{ (g/dL)}}{ ext{RBC count (in millions/μL)}} imes 10
    • Mean Corpuscular Hemoglobin Concentration (MCHC): Hb per RBC volume
    • Common range: ~32-39 g/dL
    • Formula: ext{MCHC} = rac{Hb ext{ (g/dL)}}{Hct} imes 100
  • Hematocrit (Ht): proportion of blood volume occupied by RBCs
    • Typical adult ranges: Male 42-54%, Female 38-46%
    • Newborns can be higher: 55-68%
  • RBC size and life details from tables:
    • Erythrocyte diameter ~7.5 μm
    • WBCs are larger; neutrophils ~11.25-22.5 μm (depending on cell type)
    • Platelets ~2 μm diameter; lifespan ~8-10 days

Blood Volume and Body Fluid Compartments

  • Blood volume (BV):
    • BV ≈ 5 L in average adult; approximately 7% of body weight.
    • For a 70 kg person: BV \approx 0.07 \times 70~\text{kg} = 4.9~\text{L} \ (≈5~\text{L})
  • Hematocrit and plasma volume relationship:
    • Plasma Volume (PV) = BV × (1 − Hct)
    • Therefore, BV = PV / (1 − Hct)
    • Example: If Hct = 0.50 and PV = 3.0 L, then BV = \frac{3.0}{1-0.50} = 6.0~\text{L}
  • Total Body Water (TBW) distribution (typical for 70 kg adult):
    • TBW ≈ 60% of body weight → TBW \approx 0.60 \times 70\text{ kg} = 42\text{ L}
    • Intracellular fluid (ICF) ≈ 40% of body weight → ~28 L
    • Extracellular fluid (ECF) ≈ 20% of body weight → ~14 L
    • Major ECF subcompartments:
    • Interstitial fluid ≈ 75% of ECF (~10.5 L)
    • Plasma (intravascular) ≈ 25% of ECF (~3.5 L)
  • Plasma composition and plasma proteins:
    • Albumins (~58% of plasma proteins) maintain oncotic pressure
    • Globulins (~38%) contribute to transport and immune function
    • Fibrinogen (~4%) essential for coagulation
    • Regulatory proteins (<1%)
  • Plasma vs formed elements distribution:
    • Plasma ~55% of whole blood; formed elements ~37-54% depending on labeling; RBCs ~44% of whole blood.
  • Blood is a colloid; plasma is primarily water with dissolved proteins and solutes.

Regulation of Blood Volume and Capillary Exchange

  • Negative feedback regulation of blood volume involves several systems:
    • Renin-Angiotensin-Aldosterone System (RAAS): Triggered by low blood volume/pressure. Renin leads to angiotensin II production, causing vasoconstriction and aldosterone-mediated Na⁺ and water reabsorption in kidneys, expanding BV.
    • Antidiuretic Hormone (ADH, vasopressin): Triggered by increased plasma osmolality or decreased BV; promotes water reabsorption in kidneys, increasing BV.
    • Atrial Natriuretic Peptide (ANP) and Brain Natriuretic Peptide (BNP): Released by the heart in response to elevated BV/pressure; promote Na⁺ and water excretion, reducing BV.
  • Neural regulation:
    • Sympathetic nervous system increases heart rate and vasoconstriction during stress, exercise, or bleeding to maintain BP and redirect blood to vital organs.
  • Capillary exchange (Starling forces):
    • Hydrostatic pressure pushes fluid out of capillaries; oncotic pressure pulls fluid back in; balance maintains fluid exchange and BV.
  • Clinical relevance:
    • Hypovolemia: decreased BV due to hemorrhage, dehydration, burns; symptoms include hypotension, tachycardia, reduced organ perfusion; treatment is fluid resuscitation.
    • Hypervolemia: increased BV due to heart/kidney disease or excessive fluids; symptoms include hypertension, edema, dyspnea; treatment includes diuretics and treating underlying condition.
  • Blood volume assessment methods:
    • Indicator dilution (inject tracer, measure dilution in blood).
    • Hemoglobin and hematocrit as indirect estimates of BV.
  • Hematocrit and anemia:
    • Low hematocrit indicates fewer RBCs or dilutional effect; signals potential anemia or bleeding.
  • Sickle-cell disease (HbS):
    • Mutation in HBB gene leads to abnormal Hb that polymerizes under low O₂, causing sickling and vaso-occlusion.
  • Iron deficiency and anemia:
    • Iron deficiency leads to microcytic, hypochromic anemia; low MCV and MCH; Hb synthesis impaired.
  • Vitamin B12/folate deficiencies:
    • Impair DNA synthesis, leading to megaloblastic anemia; B12 absorption requires intrinsic factor.
  • Blood doping and EPO:
    • EPO stimulation increases RBC mass to boost oxygen-carrying capacity; raises blood viscosity and cardiovascular risk; banned in athletics.
  • Blood products:
    • Whole blood and various components (RBCs, platelets, plasma, cryoprecipitate) used for transfusion; selection depends on clinical need.

Key Tables and Formulas (LaTeX-ready)

  • Blood volume relationships:
    • BV = \frac{PV}{1 - Hct}
    • PV = BV \times (1 - Hct)
  • Erythropoiesis timeline:
    • From BFU-E to reticulocyte: ~8 days; to mature RBC: ~26 days.
  • Erythrocyte indices:
    • \text{MCV} = \frac{\text{Hct} \times 10}{\text{RBC (in millions/µL)}}
    • \text{MCH} = \frac{\text{Hb (g/dL)}}{\text{RBC (in millions/µL)}} \times 10
    • \text{MCHC} = \frac{\text{Hb (g/dL)}}{\text{Hct}} \times 100
  • Typical normal ranges (select):
    • Erythrocytes: 4.2-6.2 \times 10^6/\mu L
    • Leukocytes: 4.5-11.0 \times 10^3/\mu L
    • Platelets: 150-400 \times 10^3/\mu L
    • Hematocrit (adult male): 42\%-54\%; adult female: 38\%-46\%
    • Hb (adult male): 13.5-16.5\text{ g/dL}; Hb (adult female): 12.1-15.1\text{ g/dL}
  • Blood volume estimates:
    • BV ≈ 0.07 × body weight (kg) (for a 70 kg person ≈ 4.9 L)
  • TBW distribution for a typical 70 kg adult:
    • TBW ≈ 42 L; ICF ≈ 28 L; ECF ≈ 14 L; Interstitial ≈ 10.5 L; Plasma ≈ 3.5 L

Quick Connections to Foundational Principles

  • Structure-function relationship in RBCs:
    • Biconcave shape increases surface area-to-volume ratio for gas exchange; spectrin cytoskeleton provides flexibility and resilience during capillary passage.
  • Homeostatic regulation:
    • Oxygen delivery to tissues controls erythropoiesis via EPO; iron availability and vitamins supply the substrates for Hb synthesis.
  • Integration with organ systems:
    • Kidney plays a dual role by producing EPO and filtering wastes; liver produces plasma proteins and TPO; bone marrow produces blood cells; spleen/macrophages recycle aged RBC components.

Summary

  • Blood is a dynamic, regulated tissue with a plasma matrix and formed elements, essential for transport, regulation, and defense.
  • Hemopoiesis is a tightly controlled process involving multiple sites (fetal and adult) and lineages (erythroid, myeloid, lymphoid).
  • Erythropoiesis is regulated chiefly by EPO in response to tissue oxygen needs, with iron, B12, and folate as critical cofactors.
  • Hemoglobin structure enables oxygen transport; HbF and HbA2 variations reflect developmental and genetic differences.
  • Iron metabolism involves absorption, transport by transferrin, storage in ferritin, and recycling during RBC destruction; disorders of iron balance underpin common anemias.
  • Blood volume, plasma volume, and body-fluid distribution are interrelated via Starling forces and regulatory systems (RAAS, ADH, ANP).
  • Clinical relevance spans anemia, polycythemia, blood transfusion practices, and the physiological basis for common laboratory indices (MCV, MCH, MCHC).