Blood and Plasma Basics

Blood Composition – Centrifuged Overview

• Interpretation of a spun centrifuge tube:
  • Bottom dark-red layer = hematocrit (packed red blood cells, RBCs).
  • Definition: % volume of blood occupied by erythrocytes.
  • Typical value ≈ 45\% (varies with individual & sex).
  • Middle thin whitish layer = buffy coat.
  • Thickness < 1\% of total volume.
  • Components: leukocytes (white blood cells, WBCs) + thrombocytes (platelets).
  • Top straw-yellow layer = plasma.
  • Accounts for ≈ 55\% of total blood volume.

Plasma – Composition & Functional Significance

• Water: \approx 92\% of plasma.
  • Solvent for transport; medium for heat distribution.
• Plasma proteins: \approx 7\%.
  • Create & maintain colloid osmotic (oncotic) pressure → essential for capillary fluid balance (referenced from previous lecture).
  • Major classes & roles:
  • Albumin (most abundant) → universal carrier for hydrophobic molecules (e.g.
    steroid hormones, some drugs) & osmotic regulator.
  • Immunoglobulins (antibodies) → adaptive immunity.
  • Fibrinogen → soluble precursor of fibrin; key to clot formation.
  • Plasminogen → precursor of plasmin; enables clot dissolution.
• Dissolved biomolecules & small solutes (≈ 1\%):
  • Amino acids, glucose, lipids, vitamins.
  • Electrolytes/ions (Na$^+$, K$^+$, Ca$^{2+}$, Cl$^-$, HCO$_3^-$, etc.).

Red Blood Cells (Erythrocytes) – Structure

• Shape: biconcave disc; resembles a donut with a thin central region.
  • Enlarged surface-area-to-volume ratio → optimizes gas diffusion.
• Organelles: absent (anucleated, no mitochondria, no ribosomes).
  • Described colloquially as "bags of hemoglobin".
• Cytoskeleton: flexible; enables passage through micro-capillaries.

Hemoglobin – Architecture & Gas Binding

• Globular tetrameric protein; each subunit contains one heme group with a central Fe (iron) atom.
  • Total iron atoms per Hb molecule = 4 → hence 4 potential O$_2$ binding sites.
• Abundance: ≈ 250\,\text{million} Hb molecules per erythrocyte.
  • O$2$-carrying capacity per RBC = 250\,\text{million} \times 4 = 1\,\text{billion} O$2$ molecules.
• Secondary role: minor acid-base buffering (binds H$^+$ partly as a by-product of CO$_2$ transport).

ATP Production Inside RBCs – Metabolic Constraint

• Energy needs fulfilled exclusively via anaerobic glycolysis (substrate-level phosphorylation).
  • Rationale: absence of mitochondria prevents consumption of transported O$_2$.
  • Rich in glycolytic enzymes.

Directionality of Gas Transport

• Oxygen pathway: lungs → heart → systemic tissues.
• Carbon-dioxide pathway: tissues → heart → lungs (exhalation).
  • CO$_2$ transport includes Hb buffering & conversion to bicarbonate (to be covered in later lectures).

Erythropoiesis – Formation of Red Blood Cells

• Trigger: tissue hypoxia (low O$_2$ sensed in kidney cortex).
• Hormone: erythropoietin (EPO) released from kidneys.
  • Functions as the primary hormonal stimulus for erythropoiesis in red bone marrow.
  • Clinical & sporting relevance (doping) noted though not discussed in depth here.

Buffy Coat – Preview of Later Topics

• Leukocytes (WBCs):
  • Integral to innate & adaptive immunity (detailed in forthcoming immune-system lecture).
• Platelets (thrombocytes):
  • Membrane-bound cell fragments; core actors in hemostasis & clotting cascade (future lecture).

Key Numerical & Statistical References

• Plasma volume fraction: 55\%.
• Buffy coat volume fraction: <1\%.
• Hematocrit / RBC volume fraction: \approx45\%.
• Water in plasma: \approx92\%.
• Plasma proteins: \approx7\%.
• Hemoglobin per RBC: 2.5 \times 10^8 molecules.
• O$_2$ binding capacity per RBC: \sim 1 \times 10^9 molecules.
• O$_2$ binding sites per Hb: 4.

Connections to Previous & Upcoming Content

• Colloid osmotic pressure (Starling forces) revisited here via plasma proteins.
• Detailed immune functions of leukocytes & the hemostatic role of platelets reserved for separate videos.
• Will later delve into CO$_2$ chemistry, bicarbonate buffering, and full cardiovascular system dynamics.