MR

Blood Typing & Cardiac Physiology Notes

Blood Typing, Rh-Factor, and Immunological Implications

  • ABO surface proteins
    • A & B are glycoproteins embedded in the erythrocyte plasma membrane.
    • O type = absence of A & B antigens.
  • Additional erythrocyte antigens
    • 56+ minor proteins (c, e, f, etc.).
    • Clinically trivial for routine transfusion but crucial for tissue / organ transplants—greater antigen match ↓ rejection risk.
  • Rh (D) antigen
    • Rh⁺ = D glycoprotein present; Rh⁻ = absent.
  • Transfusion compatibility (simplified)
    • Rh⁺ individual can receive Rh⁺ or Rh⁻ of same ABO.
    • Rh⁻ individual safely receives Rh⁻; can receive Rh⁺ once (sensitisation risk).
    • Order matters: Rh⁻ → Rh⁺ exposure builds anti-D antibodies.
  • Pregnancy & erythroblastosis fetalis (Hemolytic Disease of the Newborn)
    • Scenario: Rh⁻ mother + Rh⁺ father ⇒ high probability Rh⁺ fetus.
    • 1st pregnancy: usually uneventful until delivery—placental separation lets fetal Rh⁺ cells mix with maternal blood.
    • Maternal immune system forms anti-D IgG; antibodies cross placenta.
    • Subsequent Rh⁺ pregnancy ⇒ fetal RBC destruction, anemia, possible fetal death.
  • Preventive pharmacology
    • RhoGam® (and newer analogues) = passive anti-D immunoglobulin.
    • Administered prophylactically throughout pregnancy ⇢ masks D antigen, blunts maternal B-cell activation.
    • Side-effect: general immunosuppression (↓ broader immune competence; liver/kidney stress).
  • Immunity parallels
    • Rh sensitisation mimics vaccination: exposure → memory B cells.
    • Contrast with ABO: natural, pre-formed anti-A/anti-B antibodies exist without prior exposure.

Macro-Organization of the Cardiovascular System

  • Heart + blood vessels = cardiovascular system; integrates tightly with respiratory (cardiorespiratory) & renal systems.
  • 4 pumps
    • 2 atria (volume boosters).
    • 2 ventricles (primary pressure generators).
  • Left ventricle (LV)
    • Vital: failure ⇒ death; systemic pressures high.
  • Right ventricle (RV)
    • Can be tolerably weak; pumps into low-pressure pulmonary circuit.

Circuits & Hemodynamics

  • Pulmonary circuit
    • Short path, minimal gravity opposition.
    • Peak systolic pressure ≈ 25\;\text{mmHg}.
    • Venous return pressure ≈ 2\;\text{mmHg}.
  • Systemic circuit
    • Vast: fingertips → toes → brain.
    • Requires high pressure to overcome length & gravity.
    • Peak systolic pressure ≈ 120\;\text{mmHg} ("120" of the classical 120/80).
    • Venous return again ≈ 2\;\text{mmHg}.
  • Universal rule: fluids move down a pressure gradient (high → low).
    "Water follows the path of least resistance." – fluid-dynamic aphorism.

Valvular Anatomy & One-Way Flow

Atrioventricular (AV) Valves

  • Right AV = Tricuspid (3 cusps).
  • Left AV = Bicuspid / Mitral (2 cusps; bishop’s mitre resemblance).
  • Structure
    • Cusps = dense irregular CT sheets ("floppy flags").
    • Chordae tendineae = collagen cords anchoring free cusp edges to papillary muscles.
    • Papillary muscles = ventricular myocardium projections; contract with ventricle.
  • Mechanics
    1. Ventricular diastole ↓ pressure → valve opens, blood falls Atria → Ventricles.
    2. Ventricular systole ↑ pressure → cusps pushed upward, valve closes.
    3. Papillary muscles tighten chordae to prevent prolapse/inversion; they do NOT initiate opening/closing.
    • Hot-air-balloon metaphor: chordae = tethering ropes; papillary = ground anchors.

Semilunar Valves

  • Pulmonary (right) & Aortic (left); each has 3 half-moon cusps ("semilunar").
  • No chordae / papillary muscles.
  • Cup-shaped cusps
    • Ventricle pressure > artery ⇒ cusps pushed aside → valve open.
    • Ventricle relaxes, artery pressure > ventricle ⇒ blood back-fills cups, cusps meet at center → valve closes.
  • AV and Semilunar valves are never open simultaneously—prevents ventricle-to-atrium/artery back-flow catastrophe.

Cardiac Skeleton (Fibrous Skeleton)

  • Interconnected CT rings anchoring all 4 valves.
  • Functions
    • Rigid support for valve cusps (origin point).
    • Electrical insulation—separates atrial myocardium from ventricular myocardium.
    • Only electrical bridge = AV node (hole through skeleton).
  • Dense nature → heart tissue feels "chewy" (culinary chicken-heart anecdote).

Valve Pathologies

  • Incompetent / Insufficient / Regurgitant valves
    • Valve fails to seal; blood leaks backward (e.g., Mitral valve prolapse, Aortic regurgitation).
    • Heart re-pumps leaked volume → chronic overload, eventual failure.
  • Stenotic valves
    • Valve fails to open fully (developmental defects, scarring e.g., rheumatic fever).
    • Ventricle/atria must generate abnormally high pressures ⇒ hypertrophy, heart failure.
  • One valve can be both stenotic and regurgitant.
  • Treatment
    • Surgical replacement: open-heart or minimally invasive (TAVR for aortic).
    • Mechanical (ceramic/metal click) or bioprosthetic (bovine/pig) valves.

Walls & Coverings of the Heart

  • Pericardium (double-layer serous membrane)
    • Visceral layer (epicardium) adheres to myocardium.
    • Parietal layer lines fibrous pericardial sac.
    • Space = pericardial cavity; hyaluronic-rich fluid ↓ friction during cyclical size change.
    • Pericarditis = inflamed pericardium; sharp chest pain, treat with antibiotics & rest.
  • Fibrous pericardium = outer tough CT coat.
  • Myocardium
    • Striated, branching cardiomyocytes arranged in spiraling bundles.
    • Generates chamber pressure; thicker in LV (systemic work-horse).
  • Endocardium
    • Simple squamous epithelium + CT (continuous with vascular endothelium).
    • Barrier: chamber blood does not directly nourish myocardium.

Cardiac Muscle Architecture & Septa

  • Cells short, branched, interconnected.
  • Bundles wrap in multiple orientations → wringing contraction.
  • Interventricular septum
    • Muscular wall (part of LV) separating RV & LV.
    • Evolutionary note: second ventricle arose in birds/mammals; amphibians possess 3-chamber heart.

Numerical / Equation Summary (LaTeX formatting)

  • Pulmonary systolic pressure: P_{pul\,sys} \approx 25\;\text{mmHg}
  • Systemic systolic pressure: P_{sys\,sys} \approx 120\;\text{mmHg}
  • Venous return pressure (both circuits): P_{v\,return} \approx 2\;\text{mmHg}
  • Flow rule: \Delta P = P{high} - P{low} \quad ; \quad Q \propto \Delta P

High-Yield Connections & Implications

  • Exercise physiology
    • "Central adaptations" (heart) vs "peripheral" (vessels, muscle).
    • LV hypertrophy in endurance athletes = physiologic, contrasted with pathologic hypertrophy in stenosis.
  • Autonomic dysfunction
    • Poor perfusion/"toe loss" from autonomic neuropathy; distinguishes vascular obstruction vs neural control failure.
  • Immunology parallel
    • Vaccines & Rh sensitisation both rely on antigen exposure → memory cell creation.
    • Rh prophylaxis essentially = temporary immune ignorance.
  • Historical etymology
    • Mitral valve named for a Catholic bishop’s mitre; terms precede Protestant Reformation.

Quick Mnemonics & Metaphors

  • Blood-flow order (per side): Vein → Atrium → Ventricle → Artery ("VAVA").
  • AV valves = "floppy doors with ropes"; semilunar = "three upside-down cups".
  • Chordae & Papillary ~ hot-air balloon tethers preventing balloon (cusp) flip.
  • Pressure opens/closes valves – think of "check-valves" in plumbing.