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This journey is built for self-paced study. There are five stops — one per gland — and they’re designed to take about 30 minutes each. The recommended pace is one stop per day, Monday through Friday, but you can move faster if you’d like.
Every stop has the same rhythm: a short reading, a clickable interaction, and a few self-check questions. There’s no submission — this is for you, before the practical.
Whole Blood
FORGOTTEN BY MOST
Before we get to individual cellsThe basic structural and functional units of life., hold this factA statement based on direct observation that is repeatedly confirmed. in mind: blood is a connective tissue. That sounds weird because it does not connect anything in the usual structural sense, but the histology classification is based on the cell-plus-matrix architecture. Connective tissues are cells embedded in a non-cellular matrix. In dense connective tissue (tendon), the matrix is collagenA structural protein in the dermis that provides strength and elasticity. fibers. In bone, the matrix is mineralized collagen. In blood, the matrix is plasmaThe liquid component of blood. – a mostly-aqueous fluid full of dissolved proteinsLarge molecules made of amino acids with various functions in the body., electrolytes, and nutrients.
The cellular portion (called the “formed elements”) makes up about 45% of blood volume in adults. The rest is plasma. If you spin a blood sample in a centrifuge, the heavier cells pack down at the bottom; this packed cell column as a percentage of total volume is called the hematocritThe percentage of blood volume occupied by red blood cells.. A normal hematocrit is roughly 40-52% in men and 36-46% in women. The yellowish liquid floating on top is plasma, which is about 92% waterThe universal solvent essential for life. and 8% dissolved solutes (proteins, salts, glucoseA simple sugar that is the main source of energy for cells., hormones, waste products).
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Erythrocytes
Oxygen Backbacks
Red blood cells are the salmon-pink majority of every blood smear. They are anucleate – they ejected the nucleusThe control center of the cell that contains DNA and directs cellular activities. AND degraded the mitochondria during developmentThe process of growth and differentiation.. The cost is steep: with no nucleus, the cell cannot synthesize new proteins or divide. With no mitochondria, it cannot use oxygen for its own energyThe capacity to do work or cause change. needs (it runs entirely on anaerobicprocess that does not use oxygen glycolysisprocess of breaking glucose into two three-carbon molecules with the production of ATP and NADH, ironic for a cell whose whole job is carrying oxygen). The benefit is even steeper: each cell now has room for about 280 million hemoglobinThe oxygen-carrying protein in red blood cells that gives blood its red color. moleculesGroups of atoms bonded together., packing oxygen-carrying capacity to its physical limit.
The biconcave disc shape is doing two jobs simultaneously. First, it maximizes surface area for gas exchange – a biconcave disc has roughly 30% more surface area than a sphere of the same volume. Second, the cell is remarkably flexible – it folds, bends, and squeezes through capillariesThe smallest blood vessels where gas, nutrient, and waste exchange occurs between blood and tissues. that are only 3-4 micrometers wide, narrower than the cell at rest (7-8 micrometers). The membrane skeleton, built from spectrin protein, gives this elasticity. Mutations in spectrin produce hereditary spherocytosis, where the RBC cannot maintain its biconcave shape and gets sequestered in the spleen prematurely.
HINT: Salmon-pink Cheerio with central pallorAn unusual paleness of the skin due to reduced blood flow.An unusual paleness of the skin due to red about 1/3 of cell diameter. Use as size reference for everything else on a smear.
On a smear, the biconcave shape shows up as a cell with a darker rim and a paler center – the “central pallor” is the biconcave dimple seen from above. The pallor should occupy about a third of the cell’s diameter on a normal smear. If it is larger, the cell is hypochromic (low hemoglobin content). If RBCs vary noticeably in size on a single field, that is anisocytosis. If they have abnormal shapes (sickle, target, teardrop), that is poikilocytosis.
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MICROGRAPH:
Anemia
All the different anemias… or is it anemiae?
Anemia means the blood cannot carry enough oxygen. It can happen because there are too few RBCs, because each RBC has too little hemoglobin, or because the RBCs have an abnormal shape. The MCV (Mean Corpuscular Volume) is the deciding lab value – it names the pattern.
Microcytic (MCV < 80 fL)
RBCs smaller than normal, often hypochromic (larger central pallor). Classic cause: iron deficiency anemiaA condition characterized by a deficiency of red blood cells or hemoglobin, leading to reduced oxyge. Other causes: thalassemia, chronic disease anemia. The marrow knows it does not have enough iron to fill cells, so it makes them smaller.
Normocytic (MCV 80-100 fL)
RBCs the right size but too few. Can be acute blood loss (haven’t had time to compensate), chronic disease, hemolytic anemiaAnemia caused by the destruction of red blood cells before their normal lifespan ends. (cells destroyed in the bloodstream), or early kidney failure (low EPO).
Macrocytic (MCV > 100 fL)
RBCs larger than normal, often with no central pallor (cytoplasmThe gel-like substance within a cell that contains organelles and cytosol. uniformly stained). Classic causes: vitamin B12 deficiency, folate deficiency. Pernicious anemia is the autoimmune version where the stomach cannot make intrinsic factorA stomach-secreted protein necessary for vitamin B12 absorption and B12 cannot be absorbed.
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MICROGRAPH:
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Leukocytes
White blood cells are the immune system’s mobile force. There are five types, and they are most usefully learned in order of abundance because shifts in that order (relative neutrophilia, lymphocytosis, eosinophilia) are diagnostic. Memory device:
Never Let Monkeys Eat Bananas =
Neutrophil >
Lymphocyte >
Monocyte >
Eosinophil >
Basophilcells that stain with basic dyes Anterior pituitary that secrete FSH, LH, TSH, and ACTH..
Functionally the five split between two layers of immunity. The innate immune system is fast (minutes to hours), generic (responds the same to broad classes of threat), and includes the neutrophil, eosinophil, basophil, and monocyte-macrophage line. The adaptive immune system is slow on first exposure (days), specific to particular antigensMolecules on the surface of cells that trigger an immune response., and remembers – it is the lymphocyte’s domain. Both layers always run in parallel; the innate response slows things down enough for the adaptive response to learn.
Structurally the five split between granulocytes (visible cytoplasmic granules, multi-lobed nucleiClusters of neurons in the CNS responsible for processing information.) and agranulocytes (no visible granules, single nuclei). Granulocytes: neutrophil, eosinophil, basophil. Agranulocytes: lymphocyte, monocyte. The structural difference is the easiest cue on a slide; the functional difference is the most clinically useful.
MICROGRAPH: BASOPHILS
SOURCE:
ID BASOPHIL
Dark blue-purple granules so dense they OBSCURE the nucleus. The cell looks like a ball of purple. 8-10 micrometers. <1% of WBCs – genuinely rare. If you find one, confirm the granules are unmistakable.
MICROGRAPH: NEUTROPHILS
This is from this website.
ID NEUTROPHIL
Multi-lobed nucleus (string of dark beads), faint pink granules. 10-12 micrometers. Most abundant WBC. Watch out: don’t confuse with eosinophil – both are multi-lobed, but eosinophil granules are bright orange.
MICROGRAPH: EOSINOPHILS
This is from this website.
ID EOSINOPHIL
Bilobed nucleus (sunglasses), BRIGHT ORANGE granules. 10-12 micrometers. The orange jumps off the slide – if you have to squint, it’s not an eosinophil. ~3% of WBCs.
MICROGRAPH: LYMPHOCYTE
This is from this website.
ID LYMPHOCYTE
Huge round dark nucleus fills almost the entire cell. Thin rim of pale cytoplasm. ~ size of an RBC. ~30% of WBCs. Three subtypes (T, B, NK) look identical on a smear.
MICROGRAPH: Monocyte
This is from this website.
ID MONOCYTE
LARGEST blood cell (2x RBC or more). Kidney/horseshoe nucleus does NOT fill the cell. Pale gray-blue cytoplasm visible all around. ~5% of WBCs. Future macrophage.
Platelets
Platelets are not whole cells. They are fragments pinched off from giant bone marrow cells called megakaryocytes. One megakaryocyte produces about 5,000 to 10,000 platelets by extending long projections into the marrow sinusoids that fragment off into circulating bits. Each platelet is about 2-3 micrometers across – much smaller than any other formed element – and lives about 8-10 days. Normal count is 150,000 to 450,000 per microliter.
Despite being fragments, platelets contain plenty of internal machinery. They have alpha granules (containing fibrinogen plasma protein converted into fibrin during blood clotting., growthAn increase in size and number of cells. factors, von Willebrand factor) and dense granules (containing ADPA molecule produced when ATP releases energy., ATPThe energy currency of cells used for muscle contraction., serotoninA monoamine neurotransmitter involved in mood, sleep, and sensory perception., calcium). Their surface has receptorsProteins located on the surface or inside cells that bind specific molecules (e.g., neurotransmitter that recognize collagen (signaling tissue injury) and fibrinogen (allowing platelets to bind to each other). The platelet membrane is unusually rich in phospholipids that help organize the coagulation cascade we will meet next.
Platelet count abnormalities are clinically important. Thrombocytopenia (low platelets) causes bleeding. Below 100,000 = mild bleeding risk; below 50,000 = significant risk during surgery; below 20,000 = high spontaneous bleeding risk; below 10,000 = imminent spontaneous bleeding (often the trigger for transfusion). Causes include immune destruction (ITP), decreased production (chemotherapy, marrow failure), and sequestration (large spleen). Thrombocytosis (high platelets) is less commonly bleeding-related and more often associated with chronic inflammation or hematologic disorders.
MICROGRAPH:
This is from this website.
Reading – how to read a blood smear systematically
When you sit down at the microscope with an unknown smear, here is a workflow that will not let you down:
Step 1 – Low power scan (10x). Get an overall impression. Is the field cellular? Are there obvious clumps or unusual structures? Find a region where RBCs are close to each other but not piled up; this is called the “monolayer” and is where you do detailed work.
Step 2 – RBC assessment (40x or 100x oil). Are they uniform in size? Do they have appropriate central pallor? Any abnormal shapes? This gives you size category (microcytic, normocytic, macrocytic), color category (hypochromic or normochromic), and any poikilocytosis hints.
Step 3 – WBC differential. Identify and count 100 consecutive WBCs, classifying each into the five types. The percentages give you the differential. While doing this, watch for any unusual morphology (left shift, atypical lymphocytes, hypersegmented neutrophils in B12 deficiency).
Step 4 – Platelet estimate. Are platelet clusters present? On a typical 100x oil field you should see 7-20 platelets per field; lower suggests thrombocytopenia, higher suggests thrombocytosis.
Step 5 – Synthesize. Combine everything with the lab values. Cells under the scope often tell a clearer story than the numbers alone. Hypersegmented neutrophils on smear + macrocytic anemia on CBC = B12 or folate deficiency, before any antibody tests come back.
Wrap-up – decision tree and at-a-glance
Use the decision tree to commit to an ID quickly on a practical exam. First question is binary: nucleus or not? Then split on granules and size from there.
Explore More on Blood Histology
Link to more on Blood Histology
Erythrocytes
Platelets
Basophils
Neutrophils
Eosinophils
Monocytes
Lymphocytes
List of terms
- cells
- fact
- collagen
- plasma
- proteins
- hematocrit
- water
- glucose
- nucleus
- development
- energy
- anaerobic
- glycolysis
- hemoglobin
- molecules
- capillaries
- pallor
- anemia
- hemolytic anemia
- cytoplasm
- intrinsic factor
- Basophil
- antigens
- nuclei
- fibrinogen
- growth
- ADP
- ATP
- serotonin
- receptors