Respiratory Physiology Comprehensive Notes

Pleural Anatomy, Mechanics, & the ‘Glass-Plate’ Analogy

• Ribs are wrapped externally by a tough sheet of connective tissue; internally the comparable sheet is the parietal pleura.

• Every rib movement therefore drags the parietal pleura, mechanically linking chest-wall motion to lung expansion.

• Pleural cavity facts

  • Extremely thin space; essentially a microscopic film of viscous “ground substance.”

  • Analogy: two glass panes with a drop of water— they cannot be pulled apart (surface tension) but can slide; visceral & parietal pleura behave the same.

  • Fluid permits friction-free sliding yet prevents separation, creating the negative intrapleural pressure that keeps lungs partially inflated at all times.

Pressures at Rest & the Origin of Residual Volume

• Atmospheric (barometric) pressure at sea level ≈ 760\,\text{mmHg}.

• End-expiration values (normalized by subtracting 760\,\text{mmHg}):

  • Intrapulmonary (alveolar) pressure = 0\,\text{mmHg} (no airflow).

  • Intrapleural pressure ≈ -4\,\text{mmHg}.

• Gradient: P{intrapulmonary} > P{intrapleural} ⇒ lungs are pushed outward, preventing total collapse → Residual Volume (RV)— the air that can never be exhaled.

Pneumothorax Examples

• Open (knife) wound perforates parietal pleura → intrapleural pressure equilibrates with atmosphere → lung collapses.

• Spontaneous (visceral tear) allows alveolar air to enter pleural space → same result.

• Until pressure is re-established, the affected lung cannot ventilate.

Boyle’s Law Applied Twice in Every Breath

P1V1 = P2V2

1. Muscle contraction (diaphragm, external intercostals) ↑ thoracic & intrapleural volume → ↓ P_{intrapleural} (≈ -4 \to -7 \text{mmHg}).

2. Lowered pleural pressure lets lungs expand → ↑ intrapulmonary volume → ↓ P_{intrapulmonary} (≈ 0 \to -1\,\text{mmHg}) ⇒ air rushes in (tidal inspiration).

• Exhalation = passive muscle relaxation; volumes reverse, pressures rise, air flows out.

Ventilation vs. Respiration

• Ventilation = bulk movement of air regardless of content.

• Respiration = gas exchange driven by partial-pressure gradients (PPGs) of O2 & CO2.

Dalton’s Law & Atmospheric Mathematics

• Each gas in a mixture exerts its own pressure independent of others.

• Atmospheric composition (sea level)

  • N2 ≈ 79\%, O2 ≈ 20.93\%, CO2 ≈ 0.03\%, H2O ≈ 0.5\%.

• Partial pressure calculation: P{O2}=0.2093\times760\approx159\,\text{mmHg} (memorize).

• Independence means O2 can diffuse into blood while CO2 simultaneously diffuses out—
  identical to glass beads rolling opposite on the same slope.

Typical O2 & CO2 Gradients (mmHg)

• O_2 moves 105 → 40 (lung to blood) & 100 → 40 (blood to tissue).

• CO_2 moves 46 → 40 (blood to lung) & 45 → 40 (tissue to blood) — opposite directions concurrently, illustrating Dalton’s independence.

Hemoglobin (Hb) Conformations & Cooperative Binding

• Two allosteric states:

  • Relaxed (R) = open, high-affinity; occurs when at least one O_2 is bound.

  • Taut (T) = closed, low-affinity; no O_2 bound.

• Cooperative sequence (toy-taking analogy):

  1. In pulmonary capillary a random O_2 binds a T-state Hb → converts to R.

  2. Remaining 3 heme sites load rapidly (truck fully loaded).

  3. Reaches tissue; one O2 leaves (low P{O_2}) → Hb flips back to T → dumps rest.

Saturation Concept

• Percent saturation = % of Hb molecules with all four sites filled.

  • Example with 5 Hb: 1 saturated ⇒ 20\%; 4 saturated ⇒ 80\%.

Oxyhemoglobin Dissociation Curve (ODC)

• X-axis: P{O2}; Y-axis: % saturation (Hb in R-state).

• Characteristics

  • Sigmoid shape: plateau above 60\,\text{mmHg} protects arterial O_2 content across altitudes.

  • Steep portion (20-60 mmHg) enhances unloading as tissue P{O2} falls.

• Altitude example: Denver P{atm}\approx620\,\text{mmHg} → inspired P{O_2}\approx130 → alveolar \approx90 → arterial sat still ≥96\% due to plateau.

Modifiers of the Curve

Capillary Transit Time

• RBC spends ≈ 0.75\,\text{s} in pulmonary capillary; Hb saturates within 0.25\,\text{s} → large safety margin.

• Diseases that thicken diffusion barrier (pneumonia, pulmonary edema) erode this margin.

CO_2 Transport & Bicarbonate Buffer

1. Dissolved in plasma: \sim10\%.

2. Bound to globin amino groups (carbamino-Hb): \sim20\%.

3. As bicarbonate (\sim70\%) CO2 + H2O \leftrightarrow H2CO3 \leftrightarrow H^+ + HCO_3^-

   • Enzyme: carbonic anhydrase inside RBCs.

   • HCO_3^- exits RBC → plasma, acting as major blood buffer, limiting pH swings.

   • In lungs reaction reverses; CO_2 re-forms & is exhaled (baking-soda analogy).

Neural Control of Breathing

• Ventral Respiratory Group (VRG, medulla)

  • Generates inspiratory rhythm (reverberating circuit).

  • Motor outflow via phrenic nerve (diaphragm) & intercostal nerves.

• Dorsal Respiratory Group (DRG, medulla)

  • Integrates peripheral stretch & chemoreceptor input; fine-tunes VRG output; protective cut-off when lungs over-stretch.

• Pontine Respiratory Centers (PRC)

  • ‘Apneustic’ & ‘pneumotaxic’ regions; smooth the VRG’s abrupt pattern → gentle sinusoidal breathing; adapt rate/depth to speech, exercise, etc.

• Automatic circuitry is distinct from (but communicates with) the reticular activating system; damage to reticular formation causes coma, not apnea per se.

Key Definitions & Values to Memorize

• Boyle’s Law relationship P \propto \dfrac{1}{V} (constant T).

• Atmospheric pressure 760\,\text{mmHg} at sea level; intrapleural pressure \approx -4\,\text{mmHg} (rest).

• Tidal Volume (TV): air in/out each quiet breath; Residual Volume (RV): air never exhaled.

• Typical partial pressures (mmHg): P{A\,O2}=105, arterial O2=100, venous O2=40; arterial CO2=40, venous CO2=45.

• Hb saturation at sea level ≈ 98\%; drops subtly with altitude, more with fever/acidosis.

Clinical & Real-World Connections

• Knife trauma or spontaneous bleb rupture → pneumothorax → urgent re-establish negative pleural pressure.

• Exercise: ↓ tissue P{O2}, ↑ temp & ↓ pH together triple-boost unloading.

• High altitude: ODC plateau preserves arterial O2 but lower alveolar O2 may still limit maximal exercise.

• Hyperventilation (blowing off CO_2) raises pH (alkalosis), shifts curve left, can precipitate cerebral vasoconstriction & dizziness.

• Bicarbonate therapy & kidney regulation hinge on same buffer system that transports CO_2.

These bullet-point notes encapsulate every major & minor concept, numerical value, analogy, and physiologic linkage discussed in the transcript, forming a self-contained study resource on respiratory mechanics, gas exchange, hemoglobin dynamics, CO_2 transport, and neural control of breathing.