Blood Components
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Direct answer
Every drop of blood is 55 per cent plasma and 45 per cent formed elements — red cells, white cells and platelets — totalling about 5 litres, or 7-8 per cent of body weight, in an adult. The red cell count sits near 5 million per microlitre with a 120-day lifespan, white cells number 4000-11,000 with neutrophils the commonest, and platelets 150,000-450,000 surviving 8-10 days. Erythropoietin, some 90 per cent of it from peritubular interstitial cells of the kidney, drives red cell production on hypoxic demand. FMGE pulls its questions from the counts, the differential order, the plasma proteins and the site of haemopoiesis at each age.
What you must remember
- Haematocrit: 40-50 per cent in men, 37-47 per cent in women; plasma is 90-92 per cent water with 6-8 g/dL protein — albumin 3.5-5 g/dL (the oncotic workhorse), globulins, and fibrinogen (largest, first in the clotting cascade numbering).
- Red cells: 5-5.5 million per microlitre in men, 4.5-5 million in women; biconcave and anucleate; lifespan 120 days; old cells destroyed in the spleen; reticulocytes 0.5-1.5 per cent as the marrow-response index.
- Erythropoietin: 90 per cent kidney (peritubular interstitial cells, oxygen-sensing via HIF-1alpha), 10 per cent liver — hence the normocytic anaemia of chronic kidney disease.
- White cell differential in order: neutrophils 40-75 per cent, lymphocytes 20-40, monocytes 2-10, eosinophils 1-6, basophils under 1; monocyte is the largest leucocyte, small lymphocyte the smallest.
- Platelets: 150,000-450,000 per microlitre, non-nucleated megakaryocyte fragments, lifespan 8-10 days, removed by splenic macrophages — hence post-splenectomy thrombocytosis.
- Haemopoietic relay: yolk sac first, liver the chief fetal organ by the mid-trimester, bone marrow dominant from the seventh month; after puberty haemopoiesis survives only in flat bones.
- ESR by Westergren: under 15 mm/hour in men, under 20 in women; it rises with anaemia and with the fibrinogen of inflammation, and falls in polycythaemia and sickle cell disease.
Reading a haemogram the way the exam wants
Treat every complete blood count as a story in four lines. The first line is haemoglobin with indices — 13 g/dL in men and 12 in women are the usual Indian laboratory cut-offs, and an MCV below 80 or above 100 immediately splits the anaemias. The second line is the white cell count with differential: a count above 11,000 with neutrophilia and band forms suggests bacterial infection, while a relative lymphocytosis fits viral illness; an absolute eosinophilia in India should always prompt a search for parasites before anything exotic. The third line is the platelet count, read together with the bleeding pattern — petechiae and gum oozing point to platelets, deep haematomas to clotting factors. The fourth line is the reticulocyte count, which tells you whether the marrow is answering: it climbs within a week of correct iron therapy and stays low when the marrow is failing or the anaemia is of chronic disease.
Consider how this plays out in a classic stem: a pale woman with heavy periods shows haemoglobin 8 g/dL, MCV 70, and a reticulocyte count that rises from 1 to 5 per cent after two weeks of oral iron. Every value is doing physiological work — the low MCV narrows the cause, and the reticulocyte jump proves the marrow is intact and the diagnosis right. A pancytopenia instead redirects you to the marrow, which in adults you sample at the posterior iliac crest precisely because long-bone marrow has turned fatty.
How the exam frames it
Expect paired comparisons rather than isolated definitions. Which cell lives longest — the red cell at 120 days against the platelet's ten. Which is the largest leucocyte (monocyte) and which the smallest (small lymphocyte). Which cell is most numerous in blood (neutrophil) and which WBC becomes the tissue macrophage (monocyte). Which plasma protein is smallest (albumin, which is why it leaks first in nephrotic syndrome) and which is largest (fibrinogen). Why ESR rises — rouleaux formation under fibrinogen and globulins, not any change in red cell number alone — and why it stays normal in sickle disease, whose cells cannot stack.
Frequently asked questions
Where is erythropoietin produced and what stimulates it?
About 90 per cent in peritubular interstitial cells of the kidney, released when HIF-1alpha rises with cellular hypoxia; renal disease therefore brings a normocytic normochromic anaemia.
Which is the largest and which the smallest leucocyte?
The monocyte (12-20 micrometre) is largest; the small lymphocyte is smallest — a standard viva one-liner.
What is the platelet lifespan and what happens after splenectomy?
Platelets live 8-10 days and are cleared by splenic macrophages, so removing the spleen routinely raises the platelet count.
Why is bone marrow aspirated from the iliac crest in adults?
Because beyond puberty haemopoiesis persists only in flat and membranous bones — sternum, vertebrae, ribs and ilium.
Why does ESR rise in inflammation?
Acute-phase fibrinogen and globulins reduce the zeta potential so red cells form rouleaux and sediment faster; Westergren normals are under 15 mm/hour in men and under 20 in women.