📝 STANDARD DESCRIPTION:
Blood = specialized fluid CT. ~7-8% of body weight.
2 components:
• Plasma (~55%) — water + plasma proteins (albumin, globulins, fibrinogen) + electrolytes + nutrients + wastes + hormones.
• Formed elements (~45%) = cells:
1. Erythrocytes (Red Blood Cells, RBCs):
• Anucleate biconcave discs in MAMMALS (have nucleus in birds & reptiles!).
• Carry O₂ via haemoglobin. Counts and lifespans vary widely among domestic species and must be interpreted with species-specific laboratory intervals.
2. Leukocytes (White Blood Cells, WBCs):
• Granulocytes: Neutrophil, Eosinophil, Basophil.
• Agranulocytes: Lymphocyte, Monocyte.
3. Thrombocytes (Platelets):
• Cell fragments from megakaryocytes; clotting; concentration is species- and laboratory-dependent and should be reported using the relevant veterinary reference interval.
📚 DETAILED DESCRIPTION:
BLOOD UNDER THE MICROSCOPE
Blood is the only liquid connective tissue. The standard laboratory study uses Romanowsky-type stains (Giemsa, Wright's, Leishman's) on a thin air-dried smear.
I. PLASMA (~55% of blood volume):
• 91% water + 9% solutes.
• Plasma proteins: Albumin (most abundant, oncotic pressure), Globulins (α, β, γ — γ = antibodies), Fibrinogen (clotting).
• Plasma minus fibrinogen = SERUM.
II. ERYTHROCYTES (RBCs):
• Mammalian: anucleate, biconcave discs (~7 μm in dog/cat, ~5 μm in cattle, ~5.5 μm in horse).
• Avian/Reptilian: RETAIN nucleus and are oval — important histology distinction.
• Filled with haemoglobin → eosinophilic (pink-orange) on stain.
• Reticulocytes — young RBCs with residual ribosomes; counted as marrow regenerative response indicator (sparse in cat, abundant in dog with anaemia).
• Lifespan: ~120 d (dog), 70-90 d (cat), 110-130 d (cow), 140-150 d (horse).
• Removed by macrophages in spleen + liver; iron recycled.
III. LEUKOCYTES (WBCs):
A. Granulocytes (with cytoplasmic granules):
• Neutrophil: 3-5 lobed nucleus, fine pinkish granules; phagocyte; first responder to bacterial infection. ~25-75% of leukocytes (varies). In birds = heterophil (red rod-like granules).
• Eosinophil: Bilobed nucleus, large round bright RED-orange granules; combats parasites + modulates allergy. ~1-8%.
• Basophil: Bilobed nucleus often hidden by large dark BLUE-purple granules with histamine + heparin; allergic & anti-parasitic. <1%; rare in dog, more common in cattle/horse.
B. Agranulocytes:
• Lymphocyte: Round dense nucleus + thin rim of basophilic cytoplasm; ~12 μm. T- or B- subtypes (cannot distinguish on smear). 20-50% of WBCs in dogs; HIGHEST in cattle (~60%) — cattle are 'lymphocytic' species.
• Monocyte: Largest WBC (12-20 μm); kidney-shaped/bean-shaped nucleus; pale grey-blue cytoplasm. Becomes a tissue macrophage. ~3-10%.
IV. THROMBOCYTES (Platelets):
• Small (~2-4 μm) discoid cytoplasmic fragments; no nucleus.
• Made by megakaryocytes in bone marrow.
• Aggregate at vessel injury → primary haemostatic plug → activate clotting cascade.
• Avian/Reptilian: Have NUCLEATED thrombocytes (oval cells with nuclei).
V. SPECIES DIFFERENCES IN LEUKOGRAM:
• Dog/Cat: Neutrophils dominate (~60-75%).
• Cattle/Sheep: Lymphocytes dominate (~60%).
• Horse: Roughly 50:50 N:L.
• Pig: similar to horse.
• Bird: Heterophil (= neutrophil); higher lymphocytes.
VI. BLOOD GROUPING:
• Dog: DEA (Dog Erythrocyte Antigen) system; DEA 1 most important for transfusion.
• Cat: A/B/AB; major fatal mismatch in B-blood-type cats getting A blood.
• Cattle: 11 systems; J most important.
// COMPARATIVE ANALYSIS
| CATTLE |
Lymphocytes most numerous WBC (~60%). RBCs smallest of domestic mammals (~5 μm). |
| HORSE |
RBCs rouleaux (stacks of coins) on smear — normal! Neutrophils + lymphocytes nearly equal. |
| BIRD |
RBCs and platelets BOTH nucleated. Heterophils replace neutrophils. |
CLINICAL RELEVANCE
Anaemia: Low RBC count → pale mucous membranes. Leucocytosis: ↑ WBC → infection or stress. Thrombocytopenia: ↓ platelets → bleeding. Eosinophilia: Parasites or allergy. Lymphocytosis: Lymphoma or chronic infection.
