General Histology of Blood
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Direct answer
Seven to eight micrometres across, biconcave and anucleate, the erythrocyte is the reference cell of the entire body — the yardstick by which histologists size everything else on a smear. Blood is plasma (about 55 per cent, water plus albumin, globulins and fibrinogen) and formed elements (about 45 per cent, the haematocrit): red cells at roughly 4.5 to 5.5 million per cubic millimetre with a 120-day lifespan, leukocytes at 4,000 to 11,000 per cubic millimetre, and platelets at 150,000 to 400,000 per cubic millimetre. The leukocyte differential runs neutrophils (55-70 per cent, the dominant granulocyte with three to five nuclear lobes), lymphocytes (25-33 per cent), monocytes (3-8 per cent, the largest leukocyte at 15-18 micrometres with a kidney-shaped nucleus), eosinophils (1-6 per cent, bilobed) and basophils (under 1 per cent, heparin- and histamine-packed). Serum is simply plasma minus fibrinogen and clotting factors. For dental practice this is the pre-operative page: bleeding history, platelet counts and anaemia assessment all begin here.
What you must remember
- Erythrocyte payload: 7-8 micrometres diameter, biconcave disc for gas exchange surface, anucleate, about 4.5-5.5 million per cubic millimetre, lifespan 120 days, removed by splenic macrophages; reticulocytes are the marrow-release index.
- Differential count with personalities: neutrophil 55-70 per cent (lobed nucleus, azurophilic and specific granules, first responder, pus); lymphocyte 25-33 per cent (small round cell, dense nucleus; B and T families); monocyte 3-8 per cent (largest, 15-18 micrometres, kidney/horseshoe nucleus, steel-blue cytoplasm, becomes tissue macrophage); eosinophil 1-6 per cent (bilobed, red-orange granules of major basic protein, allergies and parasites); basophil under 1 per cent (large purple metachromatic granules, heparin and histamine).
- Platelet facts: 2-4 micrometre cell fragments from megakaryocytes, 150,000-400,000 per cubic millimetre, open canalicular system plus alpha granules and dense granules (ADP, serotonin) — primary haemostasis in one line.
- Plasma versus serum: serum is plasma after fibrinogen is consumed in clotting — the distinction behind every "clotted sample" laboratory rejection.
- Dental bleeding thresholds commonly taught: platelet counts below about 100,000 per cubic millimetre (and certainly below 50,000) prolong surgical bleeding; abnormal counts, leukaemia and anticoagulant therapy require investigation before extractions.
- Oral mucosa as a blood-health window: pallor of lips and mucosa suggests anaemia; gingival hypertrophy with bleeding suggests leukaemic infiltration; spontaneous gingival bleeding suggests thrombocytopenia or coagulation defect — the classic triad of haematology presentations in the mouth.
- Granulocyte-oral link: qualitative neutrophil defects (such as leucocyte adhesion deficiency) present with rapidly destructive periodontitis in children — histology explaining aggressive periodontitis syndromes.
Reading a peripheral smear systematically
Train the eye with a grid. Scan at low power for the evenness of the film, then move to the monolayer where red cells just touch. Use the erythrocyte as your ruler: any cell the size of one red cell is a small lymphocyte; one and a half times is a neutrophil; twice is a monocyte. Read the nucleus next — segmented lobes joined by filaments make a neutrophil, two smooth lobes make an eosinophil, an S-shaped or bilobed dark mass buried under purple granules makes a basophil, a kidney bean makes a monocyte, and a single dense round nucleus with a sliver of cytoplasm makes a lymphocyte. Then read the granule colour: lilac-pink neutrophil, brick-red eosinophil, deep purple basophil. Platelets appear as tiny anucleate blue specks between cells, and their honest scarcity on a smear is the first hint of thrombocytopenia when a patient reports gum bleeding. This drill matters in dental training because the oral physician, not the haematologist, is usually the first to see the signs: a pale, sallow patient with smooth sore tongue and angular cheilitis points toward iron-deficiency or megaloblastic anaemia, and punch-out gingival bleeding with petechiae points toward a platelet problem that must be investigated before any extraction.
Dental relevance examiners probe
Two questions recur. First, "why ask about bleeding history before extraction" — the credited answer walks the haemostasis ladder: platelet number and function (primary plug), coagulation factors (fibrin clot, measured as prothrombin time and activated partial thromboplastin time), and vessel integrity; each rung has a dental flag, from prolonged oozing in thrombocytopenia to inherited haemophilia unmasked by a simple extraction. Second, "which blood diseases show in the mouth first" — anaemia (pallor, glossitis, angular cheilitis), leukaemia (hypertrophic, boggy, bleeding gingiva with infiltration), thrombocytopenia (spontaneous gingival bleeding, petechiae), and the neutrophil-defect syndromes of childhood aggressive periodontitis. A neat closing flourish for vivas: the basophil and the connective-tissue mast cell carry the same granule cargo but differ in lineage and location, which links this page back to general connective tissue histology in a single comparative sentence.
Frequently asked questions
Which is the largest and which the most numerous leukocyte?
The monocyte is largest (15-18 micrometres with a kidney-shaped nucleus), while the neutrophil is the most numerous (55-70 per cent of the differential in health).
What distinguishes eosinophils and basophils on a smear?
Eosinophils have bilobed nuclei and brick-red granules containing major basic protein (allergy, parasitic response); basophils have obscured bilobed nuclei and large purple granules of heparin and histamine.
What is the difference between plasma and serum?
Plasma is the liquid portion of unclotted blood containing fibrinogen; serum is what remains after clotting has consumed fibrinogen and other clotting factors.
Why are platelets essential before dental extraction?
They initiate primary haemostasis by adhesion, aggregation (ADP from dense granules) and providing phospholipid surfaces for coagulation; counts below roughly 100,000 per cubic millimetre risk prolonged post-operative bleeding.
How does anaemia present in the oral cavity?
As mucosal and lip pallor, atrophic glossitis with a smooth sore tongue, and angular cheilitis — findings that often prompt the haemogram in dental practice.