Haematology & Haemostasis

Introduction to Blood and Haematopoiesis

Blood is a specialised connective tissue consisting of a liquid plasma phase (~55% of volume) and formed elements — red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes). Plasma contains water (~91%), proteins (albumin, globulins, fibrinogen), electrolytes, hormones, nutrients, and waste products.

Haematopoiesis is the continuous production of blood cells from multipotent haematopoietic stem cells (HSCs) in the red bone marrow. In adults, active haematopoiesis occurs predominantly in the axial skeleton (vertebrae, sternum, ribs, pelvis, proximal long bones). HSCs are characterised by self-renewal and multipotency; they give rise to two major progenitor lines: the common myeloid progenitor (CMP) and the common lymphoid progenitor (CLP). The CMP produces erythrocytes, megakaryocytes/platelets, granulocytes (neutrophils, eosinophils, basophils), and monocytes/macrophages. The CLP produces B lymphocytes, T lymphocytes, and natural killer (NK) cells.

Erythropoiesis is the process of red blood cell production. Erythropoietin (EPO), a glycoprotein hormone produced primarily by peritubular interstitial cells in the renal cortex in response to hypoxia, is the principal regulator. EPO binds the EPO receptor on erythroid progenitors (BFU-E and CFU-E) → JAK2/STAT5 signalling → proliferation and differentiation → proerythroblast → basophilic erythroblast → polychromatophilic erythroblast → orthochromatic erythroblast → reticulocyte (released into blood, still contains RNA detected by supravital staining with new methylene blue) → mature erythrocyte (after ~24–48 hours in circulation, ribosomal RNA degraded). The full process takes ~7 days. Reticulocyte count (normal 0.5–1.5% of RBCs, or 25–75 × 10⁹/L) is a key marker of erythropoietic activity; elevated in haemolytic anaemia or haemorrhage (marrow responding), low in aplastic anaemia or iron deficiency. Key reference ranges: haemoglobin male 130–180 g/L, female 120–160 g/L; MCV 80–100 fL (microcytic <80, macrocytic >100); RBC lifespan ~120 days; spleen/liver remove senescent RBCs via phagocytosis.

Haemoglobin Structure and Oxygen Transport

Haemoglobin (Hb) is a tetrameric protein consisting of four globin chains, each bound to a haem group (iron-containing porphyrin ring). Adult haemoglobin HbA (α₂β₂) constitutes ~97% of adult Hb; HbA₂ (α₂δ₂) makes up ~2.5%; HbF (fetal, α₂γ₂) is the predominant Hb from ~10 weeks gestation until approximately 6 months postnatal, when β-chain synthesis replaces γ. HbF has higher O₂ affinity than HbA because the γ-chains bind 2,3-bisphosphoglycerate (2,3-BPG) less avidly than β-chains, facilitating O₂ transfer from maternal to fetal blood across the placenta.

The haemoglobin-oxygen dissociation curve is sigmoidal due to cooperative binding (positive cooperativity): binding of the first O₂ to one haem group causes a conformational shift from the T-state (tense, low affinity) to the R-state (relaxed, high affinity), making subsequent O₂ binding progressively easier. Each Hb molecule carries up to 4 O₂; normal O₂ carrying capacity is 1.34 mL O₂/g Hb. Rightward curve shifts (decreased O₂ affinity → enhanced O₂ delivery to tissues): increased CO₂, increased H⁺ (Bohr effect), increased temperature, increased 2,3-BPG, sickle haemoglobin (HbS). Leftward shifts (increased affinity): alkalosis, hypothermia, decreased 2,3-BPG, carbon monoxide (CO), HbF, methaemoglobin (Fe³⁺ form — cannot bind O₂; caused by dapsone, prilocaine, nitrites; treated with methylene blue).

HbS results from a single amino acid substitution (glutamic acid → valine at position 6 of the β-chain; GAG → GTG). Under low O₂ tension, HbS polymerises into rigid fibres causing RBC sickling → vaso-occlusion, haemolysis, and end-organ damage (sickle cell disease). HbS heterozygotes (sickle cell trait, HbAS) are largely asymptomatic but may have sickle crises under extreme hypoxia.

Primary Haemostasis: Platelet Plug Formation

Haemostasis is the process that stops bleeding after vascular injury, comprising three overlapping phases: primary haemostasis (platelet plug), secondary haemostasis (coagulation cascade), and fibrinolysis.

Vascular injury exposes subendothelial collagen and von Willebrand factor (vWF). vWF is a large multimeric glycoprotein synthesised by endothelial cells (stored in Weibel-Palade bodies) and megakaryocytes (stored in α-granules of platelets). vWF binds collagen and, via high shear stress, unfolds to expose the A1 domain, which binds GPIb-IX-V on the platelet surface → platelet adhesion (tethering). GPIb-mediated signalling activates platelets → shape change (disc → spiky sphere via actin cytoskeleton reorganisation) → secretion from α-granules (vWF, fibrinogen, P-selectin, factor V) and dense granules (ADP, serotonin, thromboxane A₂ precursors, calcium). ADP (P2Y₁/P2Y₁₂ receptors) and thromboxane A₂ (TXA₂, TP receptor) amplify platelet activation and recruit further platelets. Clopidogrel/ticagrelor block P2Y₁₂; aspirin irreversibly inhibits COX-1 → prevents TXA₂ synthesis. Activated platelets expose phosphatidylserine (normally on inner leaflet) → provides anionic surface for coagulation complex assembly. GPIIb/IIIa (integrin αIIbβ₃), the most abundant platelet surface receptor, undergoes conformational activation → binds fibrinogen and vWF → crosslinks platelets (platelet aggregation, the primary plug). Abciximab, eptifibatide, tirofiban are GPIIb/IIIa inhibitors used in acute coronary syndromes.

Secondary Haemostasis: Coagulation Cascade

The coagulation cascade amplifies the haemostatic signal through a series of enzymatic reactions, each activating the next zymogen (inactive precursor) by limited proteolysis. Coagulation factors are numbered I–XIII (no VI); most are serine proteases synthesised in the liver; factors II, VII, IX, X, and protein C/S require vitamin K for γ-carboxylation of glutamate residues (enabling Ca²⁺ binding and phospholipid surface attachment — mechanism of warfarin).

The modern cell-based model of coagulation describes three overlapping phases: initiation, amplification, and propagation. Initiation: tissue factor (TF, exposed on damaged sub-endothelium and monocytes) binds factor VIIa → TF-VIIa complex activates factor X → small amounts of thrombin (factor IIa) generated. Amplification: small thrombin amounts activate platelets (PAR-1/4), activate cofactors V and VIII, and activate factor XI → exponential thrombin generation. Propagation: on the platelet surface, large-scale factor Xa (+ factor Va = prothrombinase complex) and factor IXa (+ factor VIIIa = tenase complex) generate a thrombin burst → thrombin cleaves fibrinogen (factor I) → fibrin monomers → polymerisation → factor XIIIa (activated by thrombin) crosslinks fibrin → stable fibrin clot.

Thrombin is the pivotal enzyme: activates fibrinogen, factors V, VIII, XI, XIII; activates platelets; stimulates protein C (anticoagulant); and activates TAFI (thrombin-activatable fibrinolysis inhibitor). Natural anticoagulants: antithrombin III (inhibits thrombin, Xa, IXa — enhanced 1000× by heparin/heparan sulphate); protein C + protein S (inactivate Va and VIIIa; activated by thrombin-thrombomodulin complex on endothelium); tissue factor pathway inhibitor (TFPI, inhibits TF-VIIa-Xa complex).

Fibrinolysis

Fibrinolysis dissolves the clot after wound healing is complete. Tissue plasminogen activator (tPA), released from endothelial cells, converts plasminogen (bound to fibrin) → plasmin. Plasmin degrades fibrin → fibrin degradation products (FDPs), including D-dimer (crosslinked fibrin fragments detectable by ELISA — elevated in DVT/PE, DIC, pregnancy). Streptokinase and alteplase (recombinant tPA) are thrombolytics used in acute MI, ischaemic stroke, and massive PE. Tranexamic acid (TXA) and aminocaproic acid inhibit fibrinolysis by blocking the lysine-binding site on plasminogen, preventing fibrin binding.

Clinical Correlations

Anaemia classification: microcytic hypochromic (MCV <80, MCH <27) → iron deficiency (most common — serum ferritin ↓, TIBC ↑), thalassaemia, anaemia of chronic disease (late), sideroblastic; normocytic normochromic (MCV 80–100) → acute blood loss, haemolysis, aplastic anaemia, anaemia of chronic disease (early); macrocytic (MCV >100) → B12 deficiency (subacute combined degeneration of the spinal cord), folate deficiency (megaloblastic — hypersegmented neutrophils), hypothyroidism, alcohol, myelodysplasia.

Disseminated intravascular coagulation (DIC): pathological systemic activation of coagulation (triggers: sepsis, obstetric emergencies, malignancy, trauma) → consumption of clotting factors and platelets → paradoxical bleeding; elevated PT/APTT, ↓ fibrinogen, ↑ D-dimer, ↓ platelets; treat underlying cause + FFP/cryoprecipitate/platelets.

Haemophilia A (factor VIII deficiency, X-linked recessive): elevated APTT, normal PT; treat with factor VIII concentrate or emicizumab (bispecific antibody mimicking factor VIII). Haemophilia B (factor IX deficiency, Christmas disease): similar presentation; treat with factor IX concentrate. von Willebrand disease (most common inherited bleeding disorder): deficiency or dysfunction of vWF → impaired platelet adhesion + reduced factor VIII (vWF is carrier); prolonged bleeding time, normal PT/APTT or mildly elevated; treat with desmopressin (DDAVP, releases vWF from Weibel-Palade bodies) or vWF concentrate.