Red cells and erythropoiesis

Where blood is made

Extramedullary haematopoiesis returns in the liver and spleen when the marrow cannot cope, as in thalassaemia major (frontal bossing, splenomegaly) and myelofibrosis.

Erythropoiesis

• Sequence: stem cell, myeloid progenitor, BFU-E, CFU-E, proerythroblast, basophilic, polychromatic and orthochromatic normoblast, reticulocyte, red cell. The process takes about 7 days.
• As cells mature they shrink, haemoglobin accumulates and the nucleus condenses; the nucleus is extruded at the orthochromatic normoblast stage, producing a reticulocyte. Early basophilia comes from ribosomal RNA, not granules.
• Reticulocytes still contain RNA, circulate 1–2 days, and show marrow response: high after bleeding or haemolysis, low in marrow failure or untreated deficiency.
• Erythropoietin (EPO) is made mainly by peritubular interstitial cells of the renal cortex (about 90%; liver about 10%) when oxygen delivery to the kidney falls (anaemia, hypoxia, altitude), sensed by hypoxia-inducible factor. It prevents apoptosis of CFU-E and drives proliferation.
• Chronic kidney disease causes normocytic anaemia from EPO deficiency. Inappropriate EPO causes secondary polycythaemia (renal cell carcinoma, hepatoma, cerebellar haemangioblastoma, fibroids). Polycythaemia vera (JAK2 mutation) has low EPO, itching after a hot bath, erythromelalgia and splenomegaly.
• Requirements: iron, B12 and folate (DNA synthesis), vitamin B6 (haem synthesis), vitamin C, copper, protein, thyroid hormone and androgens.

The red cell

• Biconcave disc about 7.5 µm across, without nucleus or mitochondria. The spectrin–ankyrin–band 3 cytoskeleton maintains shape and flexibility; defects cause hereditary spherocytosis.
• Energy comes only from glycolysis, which supplies ATP for the Na⁺/K⁺ pump and, via the Rapoport–Luebering shunt, 2,3-BPG. The hexose monophosphate shunt produces NADPH to keep glutathione reduced; G6PD deficiency causes haemolysis with primaquine, dapsone, sulphonamides or fava beans.
• Osmotic behaviour: red cells swell in hypotonic fluid. Urea crosses the membrane freely, so a urea solution is an ineffective osmole: urea enters, water follows, and cells swell and burst even if the solution is hyperosmolar to plasma. Isosmotic NaCl does not change cell volume.
• Lifespan about 120 days; old cells are removed by macrophages in the red pulp of the spleen.

Haemoglobin and oxygen carriage

• Each Hb carries four O₂; 1 g of Hb binds about 1.34 mL O₂. Iron must be ferrous (Fe²⁺); ferric iron forms methaemoglobin.
• Oxygen diffusing from alveolus to haemoglobin crosses five membranes: two of the alveolar epithelial cell, two of the capillary endothelial cell, and the red cell membrane.
• The sigmoid dissociation curve reflects cooperative binding. The P50 (PO₂ at 50% saturation) is normally about 27 mmHg (3.6 kPa).

• A COPD patient whose Hb is 50% saturated at 37 mmHg has a right-shifted curve, mainly from raised 2,3-BPG and acidosis, which aids tissue unloading. Saturation increases (left shift) with alkalosis, cold and HbF.
• Carbon monoxide binds Hb about 240 times more avidly than O₂, lowers O₂ content and shifts the remaining curve left. PaO₂ and pulse oximetry can look normal. Half-life of carboxyhaemoglobin: about 4–6 hours on room air, about 1 hour on 100% oxygen, and shorter with hyperbaric oxygen.

Haemorrhage and the haemoglobin

• Immediately after bleeding, whole blood is lost, so the Hb concentration is normal; a pale tachycardic man 20 minutes after a stab wound has not yet haemodiluted.
• Over 12–72 hours, interstitial fluid moves into vessels (transcapillary refill) and plasma volume recovers; Hb then falls. Plasma proteins return within days, reticulocytes rise from about day 3, and red cell mass is the last value to return to normal (4–8 weeks).

Erythrocyte sedimentation rate

Red cells stack into rouleaux and settle faster when plasma fibrinogen and immunoglobulins neutralise their negative surface charge. ESR rises with raised fibrinogen and globulins (infection, myeloma, giant cell arteritis and polymyalgia rheumatica, often above 100 mm/h), anaemia and macrocytes, and with age and pregnancy. It falls with polycythaemia, sickle cells and spherocytes (which cannot stack) and low fibrinogen.

Breakdown and haemolysis

• Macrophages release iron (recycled by transferrin), CO and biliverdin, which becomes unconjugated bilirubin. Free haemoglobin from intravascular haemolysis binds haptoglobin (which falls) and may appear in urine.
• Haemolysis markers: raised unconjugated bilirubin, LDH and reticulocytes; low haptoglobin. Film clues: spherocytes (hereditary spherocytosis or autoimmune; the direct Coombs test separates them), schistocytes (HUS, TTP, DIC, valves), bite cells and Heinz bodies (G6PD), sickle cells, target cells.
• Haemolytic disease of the newborn (erythroblastosis fetalis) is a type II hypersensitivity: maternal IgG anti-D crosses the placenta and coats fetal red cells.

Red cell indices and anaemia

• Gastrectomy: loss of acid (iron) and intrinsic factor (B12): early microcytic, later megaloblastic; a total gastrectomy guarantees B12 deficiency. Low ferritin with high MCV suggests mixed deficiency. Macrocytosis with ataxia in an elderly woman suggests B12 deficiency.

• Mentzer index (MCV divided by red cell count) below 13 favours thalassaemia trait; raised HbA₂ confirms β-thalassaemia trait.

Remember Low oxygen delivery to the kidney raises EPO; reticulocytes show the marrow has answered.

Exam trap A normal haemoglobin straight after injury does not exclude major haemorrhage.

How FCPS asks about this

• Site of EPO production: peritubular interstitial cells of the renal cortex, not juxtaglomerular cells.
• Stage at which the nucleus is extruded: orthochromatic normoblast.
• Last value to normalise after haemorrhage: red cell mass; Hb is normal in the first minutes.
• Red cells in hyperosmolar urea: swell and lyse, because urea is permeant.
• Factor raising ESR: fibrinogen.
• Haemoglobin 50% saturated at 37 mmHg in COPD: right shift from raised 2,3-BPG.
• Red cell energy and oxidant defence: glycolysis only; NADPH from G6PD.