Ch18 ) Blood

Hematocrit and Centrifuged Blood Layer Analysis

• Microhematocrit Determination Methodology:

  • Centrifugation separates whole blood into distinct liquid and cellular components based on density.

  • Mathematical calculation of component percentages:

$\text{Component Percentage} = \frac{\text{Component Volume}}{\text{Total Volume}} \times 100$

• Patient Blood Sample Centrifugation Data:

  • Patient Sample 1:

  • Total Volume: $100\,\text{units}$

  • Red Blood Cell (RBC) Volume: $41\,\text{units}$ ($41\%$ hematocrit)

  • Buffy Coat (WBC / Platelet) Volume: $2\,\text{units}$ ($2\%$)

  • Plasma Volume: $100 - (41 + 2) = 57\,\text{units}$ ($57\%$)

  • Patient Sample 2:

  • Total Volume: $100\,\text{units}$

  • Red Blood Cell Volume: $28\,\text{units}$ ($28\%$ hematocrit)

  • Plasma Volume: $70\,\text{units}$ ($70\%$)

  • Buffy Coat Volume: $100 - (70 + 28) = 2\,\text{units}$ ($2\%$)

  • Patient Sample 3:

  • Total Volume: $100\,\text{units}$

  • Red Blood Cell Volume: $70\,\text{units}$ ($70\%$ hematocrit)

  • Plasma Volume: $28\,\text{units}$ ($28\%$)

  • Buffy Coat Volume: $100 - (70 + 28) = 2\,\text{units}$ ($2\%$)

  • Patient Sample 4:

  • Total Volume: $90\,\text{units}$

  • Red Blood Cell Volume: $50\,\text{units}$

$\text{RBC Percentage} = \frac{50}{90} \times 100 = 55.5\%$

- Buffy Coat Volume: $3\,\text{units}$ (slightly enlarged buffy layer relative to standard samples)
- Plasma Volume: $47\,\text{units}$

Microscopic Layers of Centrifuged Blood

• Buffy Coat (White Layer):

  • Situated directly between the lower, dense red blood cell pellet and the upper, clear yellow fluid plasma.

  • Contains white blood cells (leukocytes) and platelets (thrombocytes).

  • Consistently accounts for the smallest percentage of total blood volume (typically $1\%$ to $3\%$).

  • Derivation formula when determining white blood cell percentage:

$\text{WBC \%} = 100\% - (\text{RBC \%} + \text{Plasma \%})$

• Example derivation: Given $70\%$ plasma and $28\%$ RBC, $70\% + 28\% = 98\%$, yielding $100\% - 98\% = 2\%$ buffy coat.

Histology of Blood as Fluid Connective Tissue

• Connective Tissue Classification:

  • Blood is categorized as a fluid connective tissue.

  • Lacks rigid structural anchoring fibers (such as collagen or reticular fibers) embedded in the extracellular matrix to hold components in fixed places.

  • Contains a liquid extracellular matrix composed of blood plasma containing dissolved, circulating colloid proteins rather than fixed structural fiber networks.

• Circulating Plasma Proteins:

  • Albumin: A key colloid osmotic protein circulating freely in plasma.

  • Fibrinogen: A soluble plasma protein essential for blood coagulation.

  • Immunoglobulins: Circulating antibodies and autoantibodies involved in immune defense.

Formed Elements of Blood

• Erythrocytes (Red Blood Cells):

  • Etymology: Erythro- denotes red pigment.

  • Primary formed element responsible for respiratory gas transport.

  • Most abundant formed element in healthy blood.

• Leukocytes (White Blood Cells):

  • Etymology: Leuko- denotes pale or white.

  • Biological example of prefix: The bald eagle scientific name includes Leukocephalus, meaning "white head" (leuko = white, cephalus = head).

  • Identification tool note: The Merlin Bird App (m e r l i n) identifies avian species by photo or recorded audio song.

  • Physical size: Leukocytes represent the largest individual cell types found within a standard blood smear.

• Platelets (Thrombocytes):

  • Cellular fragments rather than whole cells.

  • Functional analogy: Comparable to tearing off a small piece of a paper towel to apply to a small razor nick or shaving bleed.

  • Cell of Origin: Derived from precursor cells known as megakaryocytes.

  • Anatomic Mechanism: Megakaryocytes reside adjacent to blood capillaries and extend cytoplasmic processes (analogous to unrolling toilet paper rolls) directly into the vascular lumen, allowing circulating blood flow to shear off small membrane-bound fragments as platelets.

Chemical and Physical Properties of Blood Plasma

• Plasma Composition:

  • Water: Constitutes approximately $92\%$ of total plasma volume.

  • Plasma Proteins: Albumin, fibrinogen, and immunoglobulins.

  • Electrolytes: Dissolved charged ions carrying positive or negative charges ($+$ or $-$).

  • Hormones: Chemical messengers secreted by endocrine glands.

  • Dissolved Gases: Oxygen ($O_2$), Nitrogen ($N_2$), and Carbon Dioxide ($CO_2$).

  • Nutrients: Essential organic compounds transported to peripheral tissues.

  • Waste Products: Metabolic bi-products carried to excretory organs.

  • Physical Model: Functions dynamically as a circulating river carrying dissolved gases, nutrients, and waste products.

• Physical Characteristics of Blood:

  • Color Saturation:

  • Bright Red: High oxygen content (arterial blood).

  • Dark Red: Low oxygen content (deoxygenated venous blood). Textbook diagrams and anatomical models represent deoxygenated blood as blue or purple.

  • Volume:

  • Average adult human blood volume: Approximately $5\,\text{L}$.

  • Sex differences: Biological males generally exhibit slightly higher total blood volumes than biological females due to greater muscle mass regulated by testosterone.

  • Viscosity:

  • Represents the fluid thickness, internal friction, and resistance to flow.

  • Directly influenced by biological sex, circulating erythrocyte counts, and testosterone concentrations.

Histological Examination and Microscopy Protocols

• Microscope Magnification Parameters:

  • Blood smear evaluation utilizes low to medium power objectives (scanning red objective lens and low/medium yellow objective lens).

  • High-power objective ($400\times$) requiring oil immersion is avoided when slides are prepared without oil immersion media.

• Cellular Quantification Protocol:

  • Field-of-view counts are estimated proportionally across fields rather than counted manually as precise individual integers.

• Comparative Tissue Analysis:

  • Histological comparison: Blood plasma exhibits structural fluid similarities when compared against thyroid follicle colloid (stained red/purple in endocrine histology slides).

Pathology: Sickle Cell Anemia

• Microscopic Characteristics on Blood Smear:

  • Erythrocyte Morphology: Presence of abnormally shaped RBCs appearing as elongated, crescent-shaped slivers instead of normal biconcave discs.

  • Erythrocyte Abundance: Substantially reduced RBC density (sparse distribution), defining anemia.

  • Platelet Concentration: Markedly increased platelet counts (reactive thrombocytosis).

  • Leukocyte Distribution: White blood cells remain distinguishable as the largest individual cells in the visual field.

• Pathophysiology and Clinical Consequences:

  • Deformed, sliver-like sickle cells fail to roll smoothly through microvasculature compared to rounded erythrocytes.

  • Crescent cells physically jam and obstruct small capillaries, causing microvascular occlusion and blood clotting.

  • Secondary clinical manifestations: Severe painful crises, severe ischemic tissue damage, and systemic symptoms including vomiting.

  • Medical Intervention: Frequent blood transfusions are administered to replace defective RBCs, increase oxygenation, and reduce vaso-occlusive events.

Diagnostic Protocols

• Accurate clinical diagnosis requires evaluation across multiple independent blood samples to establish meaningful comparative baselines.

• Identification practice relies on structural labeling, cell differential analysis, and slide-based comparative histology.