Gas Exchange & Partial Pressures: Comprehensive Study Notes

Definition & Directionality of Gas Exchange

• Gas exchange = bidirectional diffusion of $O<em>2$ and $CO</em>2$ along their partial-pressure (P) gradients.

  • Pulmonary interface (alveolus ↔ blood):

  • $O<em>2$: P{O2\,\text{alv}} > P{O_2\,\text{blood}} \Rightarrow diffusion into blood.

  • $CO<em>2$: P{CO2\,\text{blood}} > P{CO_2\,\text{alv}} \Rightarrow diffusion into alveolus.

  • Systemic interface (blood ↔ tissue): reverse directions.

• Both gases are non-polar → highly lipid-soluble → move by simple diffusion through any cell membrane.

Partial Pressure Fundamentals

• Gas pressure = force of random molecular impacts on container walls.

• Partial pressure ($P_X$) = share of total pressure contributed by gas species X.

• Total pressure ($P<em>T$) of a mixture = sum of all $P</em>X$.

• Fractional composition of dry atmospheric air (memorize):

  • $79\%$ $N<em>2$ | $21\%$ $O</em>2$ | Trace $\approx 0\%$ physiologically.

Example: Dry-air partial pressures at sea level (Chicago)

$(P<em>T = 760\,\text{mmHg})$ $(P</em>{N<em>2}=0.79\times760\,=600\,\text{mmHg})$ $(P</em>{O_2}=0.21\times760\,=160\,\text{mmHg})$

Humidification in Conducting Zone

• Inhaled air is warmed to core body T and becomes 100 % humidified by the airway water film.

• Water-vapor partial pressure at 37 °C and 100 % RH is constant:
  $P<em>{H</em>2O}=47\,\text{mmHg}$ (commit to memory).

• Humidification displaces other gases:
  $P<em>{O</em>2\,(humid)} = 0.21 \times (P_T - 47)$
  $=0.21 \times (760-47)=0.21\times 713 \approx 150\,\text{mmHg}$

• Consequences

  • Slight “dilution” of $O<em>2$ and $N</em>2$.

  • Unavoidable body-water loss with every breath.

Equilibrium vs. Solubility

• At partial-pressure equilibrium: $P<em>{O</em>2\,\text{gas}} = P<em>{O</em>2\,\text{liquid}}$ (similarly for $CO_2$).

• BUT concentrations differ because gases are poorly water-soluble.

  • Plasma holds few $O<em>2$ molecules even at $P</em>{O_2}=100\,\text{mmHg}$.

  • $CO<em>2$ is ≈ 20× more soluble than $O</em>2$, yet still on hydrophobic side.

• Necessitates auxiliary carriers (e.g.
  Hb in RBCs) for adequate transport — preview of next lecture.

Typical Resting Partial-Pressure Values

• Exercise ↓ tissue $P<em>{O</em>2}$ (e.g. 20 mmHg) & ↑ tissue $P<em>{CO</em>2}$ → steeper gradients → faster diffusion.

Factors Determining Rate of Diffusion (Fick-like)

1. Diffusible Surface Area (A)

   • More A → higher flux.

   • Physiological maximization: each alveolar sac is engulfed by capillary network.

   • Pathology – Emphysema

     • Cigarette smoke → inflammatory destruction of shared alveolar walls → coalesced large “blebs”.

     • ↓ capillary contacts → ↓A → impaired gas exchange → hypoxemia, hypercapnia.

2. Functional Membrane Thickness (Δx)

   • Thicker barrier → slower diffusion.

   • Pneumonia: exudate/water in alveoli lengthens path; gases are hydrophobic so extra fluid is major obstacle.

   • Similar in pulmonary edema, fibrosis.

3. Partial-Pressure Gradient (ΔP)

   • Determined by alveolar vs. blood values; modulated by ventilation, metabolism, altitude.

4. Diffusion Coefficient (D)

   • Encodes solubility & molecular weight; D{CO2} >> D{O2} → CO₂ diffuses 20× faster.

Alveolar Partial Pressures & Their Modifiers

1. Inspired Partial Pressure ($P<em>{I\,O</em>2}$, $P<em>{I\,CO</em>2}$)

   • Governed by altitude.

   • At higher elevation: $P<em>T$ falls ⇒ despite 21 % fraction, $P</em>{I\,O_2}$ drops ("thin air").

   • Clinical relevance: hypoxic challenge on Everest; worksheet explores compensation mechanisms.

2. Minute Alveolar Ventilation ($\dot V_A$)

   • Definition: volume reaching alveoli per minute.
     $\dot V<em>A = \dot V</em>E - \dot V<em>D$ where $\dot V</em>E$ = minute ventilation, $\dot V_D$ = dead-space ventilation.

   • Rule of thumb:

     • ↑$\dot V<em>A$ ⇒ alveolar values trend toward humidified atmospheric values. • $P</em>{A\,O<em>2}$ rises toward 150 mmHg (ceiling). • $P</em>{A\,CO_2}$ falls toward ≈0 mmHg (cannot reach 0 because blood keeps adding CO₂).

     • ↓$\dot V_A$ (hypoventilation) does opposite (risk of respiratory acidosis).

   • Graphically: curves asymptotically approach limits; plateau reflects water-vapor ceiling for $O_2$ & constant CO₂ addition.

3. Metabolic Rate (VO₂ & VCO₂)

   • Brainstem reflexes normally match ventilation to demand.

   • ↑ metabolic rate (exercise, fever) →
     • faster $O<em>2$ extraction, $CO</em>2$ production → steeper gradients.
     • Reflex ↑$\dot V_A$ (hyperpnea) restores alveolar pressures.

   • Failure to match (e.g.
     ventilatory muscle fatigue) leads to hypoxemia/hypercapnia.

Numerical / Formula Summary to Memorize

• Fractional dry-air composition: $F<em>{O</em>2}=0.21$, $F<em>{N</em>2}=0.79$.

• Water-vapor pressure at 37 °C: $47\,\text{mmHg}$.

• Humidified inspired $P<em>{O</em>2}$ (sea level): $\approx150\,\text{mmHg}$.

• Resting alveolar: $P<em>{A\,O</em>2}\approx100\,\text{mmHg}$, $P<em>{A\,CO</em>2}\approx40\,\text{mmHg}$.

• Typical tissue at rest: $P<em>{t\,O</em>2}\approx30\,\text{mmHg}$.

• Diffusion relationship (simplified Fick):
  $\text{Flux} = D\,A\,\dfrac{\Delta P}{\Delta x}$.

Clinical & Practical Implications

• Emphysema: chronic smoking → ↓A; patients present with dyspnea, need supplemental $O_2$.

• Pneumonia/Pulmonary edema: ↑Δx; risk of hypoxemia; positive-pressure ventilation or diuretics may help.

• High-altitude exposure: ↓$P<em>{I\,O</em>2}$; acclimatization involves hyperventilation, polycythemia, 2,3-BPG rise.

• Everyday water loss: humidification of each breath obligates insensible water loss → underscores importance of hydration.

• Ethical/philosophical: Public-health need to reduce smoking due to preventable emphysema; awareness of altitude sickness for climbers.

Study Suggestions / Worksheet Connections

• Practice converting total pressure to partial pressures (worksheet #1).

• Include humidification step (worksheet #2).

• Predict changes with altered $\dot V_A$, metabolism, or altitude (remaining worksheet problems).

• Re-draw diffusion pathways & label numerical values for both rest and exercise.