15.3 Gas Exchange, Transport & Respiratory Control

15.3 Gas Exchange, Transport & Respiratory Control

The ultimate physiological objective of the human respiratory system is to ensure the continuous delivery of molecular oxygen from ambient atmospheric air to the mitochondria of metabolically active cells while efficiently clearing the acidic byproduct carbon dioxide. Achieving this goal requires a seamless sequence of biophysical steps: bulk ventilation, microscopic gas diffusion across the alveolar-capillary barrier (external respiration), specialized gas transport within circulating blood, and microscopic exchange between systemic capillaries and peripheral tissues (internal respiration). Finally, the entire system must be governed by sensitive neural feedback loops that continuously adjust ventilation to maintain arterial blood gases and systemic pH within strict homeostatic limits.

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Basic Gas Laws Governing Respiration: Dalton's & Henry's Laws

The diffusion and transport of respiratory gases are governed by two foundational principles of physics:

1. Dalton's Law of Partial Pressures

Dalton's law states that the total pressure exerted by a mixture of non-reactive gases is equal to the sum of the partial pressures exerted independently by each individual gas in the mixture:

$P_{\text{total}} = P_{N_2} + P_{O_2} + P_{CO_2} + P_{H_2O}$

The partial pressure ($P_x$) of any individual gas is directly proportional to its percentage concentration in the mixture:

$P_x = P_{\text{total}} \times \% \text{ concentration of gas } x$

At sea level, atmospheric pressure is $760\text{ mmHg}$. Atmospheric air consists of:

• Nitrogen ($N_2$): ~78.6% $\rightarrow P_{N_2} = 760\text{ mmHg} \times 0.786 = 597.4\text{ mmHg}$
• Oxygen ($O_2$): ~20.9% $\rightarrow P_{O_2} = 760\text{ mmHg} \times 0.209 = 158.8\text{ mmHg} \approx 159\text{ mmHg}$
• Water Vapor ($H_2O$): ~0.46% $\rightarrow P_{H_2O} = 760\text{ mmHg} \times 0.0046 = 3.5\text{ mmHg}$
• Carbon Dioxide ($CO_2$): ~0.04% $\rightarrow P_{CO_2} = 760\text{ mmHg} \times 0.0004 = 0.3\text{ mmHg}$

As atmospheric air travels through the conducting airways, it is humidified ($P_{H_2O}$ reaches $47\text{ mmHg}$ at $37^\circ\text{C}$) and mixes continuously with residual alveolar gas. Consequently, alveolar partial pressures differ markedly from atmospheric air: alveolar $P_{O_2}$ is approximately $104\text{ mmHg}$, while alveolar $P_{CO_2}$ is $40\text{ mmHg}$.

2. Henry's Law

Henry's law states that when a mixture of gases is in contact with a liquid, each gas will dissolve in the liquid in direct proportion to its partial pressure and its intrinsic solubility coefficient:

• The greater the partial pressure of a gas above a liquid, the more of that gas will be driven into solution.
• However, the actual quantity of dissolved gas also depends heavily on its chemical solubility in water/plasma:
  • Carbon Dioxide ($CO_2$) is ~20 times more soluble in blood plasma than oxygen ($O_2$).
  • Oxygen ($O_2$) has relatively poor aqueous solubility.
  • Nitrogen ($N_2$) is practically insoluble in biological liquids at atmospheric pressure (though under hyperbaric scuba diving conditions, high $P_{N_2}$ forces nitrogen into lipid-rich nervous tissues, producing nitrogen narcosis, and rapid ascent causes decompression sickness / the "bends").

The Crucial Physiological Consequence: Because $CO_2$ is 20 times more soluble than $O_2$, an alveolar-capillary partial pressure gradient of only $5\text{ mmHg}$ for $CO_2$ achieves the exact same volume of gas exchange as the steep $64\text{ mmHg}$ partial pressure gradient for $O_2$.

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External vs. Internal Respiration

Respiration encompasses two distinct microscopic exchange events: external respiration in the pulmonary capillary beds and internal respiration in systemic tissue capillary beds.

Partial Pressure Cascades Across the Human Body

      Atmosphere (P_O2 = 159 mmHg, P_CO2 = 0.3 mmHg)
                           │
                           ▼
        Alveoli (P_O2 = 104 mmHg, P_CO2 = 40 mmHg)
               ▲                       │
  CO2 Diffuses │  EXTERNAL RESPIRATION │ O2 Diffuses
  (45 -> 40)   │                       ▼ (104 -> 40)
    Pulmonary Capillary Blood (Entering: P_O2 = 40, P_CO2 = 45)
                               (Leaving:  P_O2 = 104, P_CO2 = 40)
                                       │
                                       ▼
    Systemic Arterial Blood (P_O2 = 100 mmHg, P_CO2 = 40 mmHg)
                                       │
  O2 Diffuses  │  INTERNAL RESPIRATION │ CO2 Diffuses
  (100 -> 40)  │                       ▼ (45 -> 40)
               ▼                       ▲
   Metabolizing Tissue Cells (P_O2 <= 40 mmHg, P_CO2 >= 45 mmHg)

1. External Respiration (Pulmonary Gas Exchange)

External respiration occurs across the 0.5 $\mu\text{m}$ respiratory membrane between pulmonary alveoli and pulmonary capillary blood:

• Oxygen Exchange: Deoxygenated venous blood returning from systemic tissues enters pulmonary capillaries with a $P_{O_2}$ of approximately $40\text{ mmHg}$. Alveolar gas has a $P_{O_2}$ of $104\text{ mmHg}$. This steep $64\text{ mmHg}$ pressure gradient drives rapid diffusion of $O_2$ across the respiratory membrane into blood plasma and erythrocytes. Because the membrane is ultra-thin, capillary blood oxygen tension reaches complete equilibrium ($P_{O_2} = 104\text{ mmHg}$) within the first 0.25 seconds of transit—roughly one-third of the total 0.75-second erythrocyte transit time through the pulmonary capillary. This 3-fold safety margin ensures full blood oxygenation even during strenuous exercise when cardiac output accelerates.
• Carbon Dioxide Exchange: Deoxygenated capillary blood enters the lungs with a $P_{CO_2}$ of $45\text{ mmHg}$, while alveolar gas has a $P_{CO_2}$ of $40\text{ mmHg}$. Despite this gentle $5\text{ mmHg}$ gradient, $CO_2$ diffuses rapidly into the alveoli because of its 20-fold greater solubility. Capillary blood reaches equilibrium at $P_{CO_2} = 40\text{ mmHg}$ before exiting into the pulmonary veins.
• Ventilation-Perfusion ($V/Q$) Coupling: Efficient external respiration requires matching alveolar airflow (ventilation, $V$) with capillary blood flow (perfusion, $Q$):
  • Poorly Ventilated Alveoli (Low $P_{O_2}$): Local pulmonary arterioles constrict, shunting blood away from hypoventilated zones toward well-ventilated alveoli. (Note: This is the exact opposite of systemic arterioles, which dilate in response to tissue hypoxia!).
  • Well-Ventilated Alveoli (High $P_{O_2}$): Local pulmonary arterioles dilate, increasing perfusion to maximize $O_2$ loading.
  • High Alveolar $P_{CO_2}$: Causes local bronchioles to dilate, accelerating $CO_2$ elimination.

2. Internal Respiration (Systemic Gas Exchange)

Internal respiration occurs in peripheral capillary beds between systemic arterial blood and metabolically active tissue cells:

• Oxygen Exchange: Oxygenated blood leaves the left ventricle and enters systemic capillaries with a $P_{O_2}$ of approximately $100\text{ mmHg}$ (slightly lower than alveolar $104\text{ mmHg}$ due to physiological bronchopulmonary shunts). Resting tissue cells continuously consume oxygen during mitochondrial oxidative phosphorylation, maintaining intracellular $P_{O_2} \le 40\text{ mmHg}$ (and $< 20\text{ mmHg}$ in active muscle). $O_2$ diffuses rapidly down its concentration gradient from capillary erythrocytes into interstitial fluid and cells.
• Carbon Dioxide Exchange: Cellular catabolism continuously generates $CO_2$, raising intracellular $P_{CO_2} \ge 45\text{ mmHg}$ (and $> 50\text{ mmHg}$ in exercising muscle). Systemic arterial blood arrives with $P_{CO_2} \approx 40\text{ mmHg}$. $CO_2$ diffuses down its gradient from tissue cells into capillary blood until venous blood reaches $P_{CO_2} = 45\text{ mmHg}$.

Summary of Partial Pressure Gradients

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Gas Transport in Blood

Oxygen Transport

Molecular oxygen is transported in the blood in two distinct physical compartments:

1. Dissolved in Blood Plasma (1.5%): Due to its poor aqueous solubility (Henry's law), only about 0.3 mL of $O_2$ dissolves in each 100 mL of systemic arterial blood. This tiny fraction is biologically crucial because it exerts the arterial partial pressure ($PaO_2$) that drives diffusion into erythrocytes and tissues.
2. Bound to Hemoglobin (98.5%): The overwhelming majority of oxygen is chemically bound to hemoglobin (Hb) inside erythrocytes, forming oxyhemoglobin ($Hb\text{-}O_2$): $\text{Hb} + 4\,O_2 \rightleftharpoons \text{Hb}(O_2)_4$ Each hemoglobin molecule is a tetrameric protein containing four iron-bearing heme groups, allowing each molecule to bind up to four molecules of $O_2$. Binding displays positive cooperativity: binding of the first $O_2$ molecule induces a conformational change that increases the affinity of the remaining heme groups for subsequent $O_2$ molecules.

The Oxygen-Hemoglobin Dissociation Curve

The relationship between partial pressure of oxygen ($P_{O_2}$) and hemoglobin saturation (% $HbO_2$) is represented graphically by an S-shaped (sigmoidal) curve:

• Arterial Blood Plateau ($P_{O_2} = 100\text{ mmHg}$): Hemoglobin is approximately 98% saturated. At this high pressure, the curve is flat; a moderate drop in alveolar $P_{O_2}$ (e.g., to $80\text{ mmHg}$ due to high altitude or mild lung disease) barely reduces saturation (which remains ~95%), providing a vital physiological safety buffer.
• Venous Blood Steep Portion ($P_{O_2} = 40\text{ mmHg}$): In resting venous blood returning to the heart, hemoglobin is still approximately 75% saturated.
  • The Venous Reserve: Under resting conditions, blood unloads only about 25% of its carried oxygen to the tissues. The remaining 75% constitutes a massive venous reserve that can be immediately mobilized during periods of physical exertion or hypoxic stress.

The Oxygen-Hemoglobin Dissociation Curve & Shifts

 % Hb Saturation
 100 ┌────────────────────══════════  (Arterial: P_O2 = 100, Sat = 98%)
     │               ╭───╯
  75 │             ╭─╯                (Venous: P_O2 = 40, Sat = 75%)
     │           ╭─╯
  50 │         ╭─╯   <-- P_50 (~26.6 mmHg)
     │       ╭─╯
  25 │     ╭─╯       [LEFT SHIFT: Higher Affinity / Holds O2]
     │   ╭─╯         [RIGHT SHIFT: Lower Affinity / Drops O2 - Bohr Effect]
   0 └─┬─┴───┬───┬───┬───┬───┬───┬───┬───┬───
       0    20  40  60  80 100 120 (P_O2 in mmHg)

Factors Shifting the Curve: The Bohr Effect

The affinity of hemoglobin for oxygen is not fixed; it is dynamically modulated by the local chemical microenvironment of peripheral tissues:

• Right Shift (Decreased Affinity $\rightarrow$ Promotes $O_2$ UNLOADING):
  • Triggers: Increased carbon dioxide ($\uparrow P_{CO_2}$), increased hydrogen ion concentration / decreased pH ($\downarrow pH$, acidosis), elevated temperature ($\uparrow \text{Temp}$), and increased 2,3-Bisphosphoglycerate (2,3-DPG) (a byproduct of anaerobic erythrocyte glycolysis).
  • Physiological Meaning: Metabolically active tissues (such as contracting skeletal muscles during exercise) generate heat, $CO_2$, and lactic acid ($H^+$). These molecules bind to hemoglobin's globin chains, altering its quaternary structure and reducing its affinity for oxygen. Consequently, hemoglobin releases substantially more $O_2$ precisely where metabolic demand is greatest! This acid- and $CO_2$-mediated reduction in oxygen affinity is the Bohr Effect.
  • Mnemonic: "CADET, face Right!" ($CO_2$, Acid, 2,3-DPG, Exercise, Temperature).
• Left Shift (Increased Affinity $\rightarrow$ Tighter $O_2$ BINDING):
  • Triggers: Decreased carbon dioxide ($\downarrow P_{CO_2}$), increased pH ($\uparrow pH$, alkalosis), decreased temperature ($\downarrow \text{Temp}$), decreased 2,3-DPG, and Fetal Hemoglobin (HbF).
  • Fetal Hemoglobin Adaptation: Fetal hemoglobin consists of two alpha and two gamma ($\gamma$) chains, which cannot bind 2,3-DPG as effectively as adult hemoglobin (HbA). Consequently, HbF possesses a much higher oxygen affinity, shifting its curve far to the left. This allows the developing fetus to extract oxygen from maternal blood across the placental barrier.

Factors Influencing the Oxygen-Hemoglobin Dissociation Curve

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Carbon Dioxide Transport in Blood

Carbon dioxide produced by cellular metabolism is transported from systemic tissues to pulmonary alveoli via three distinct mechanisms:

Carbon Dioxide Transport Mechanisms in Blood

1. Dissolved in Plasma (7% - 10%)
   └── Molecular CO2 in aqueous solution; exerts arterial P_CO2

2. Carbaminohemoglobin (20% - 23%)
   └── Bound to globin amino terminals (Hb-CO2); modulated by Haldane Effect

3. Bicarbonate Ions in Plasma (70%)
   └── Catalyzed inside RBC by Carbonic Anhydrase (CA):
       CO2 + H2O <──[CA]──> H2CO3 <────> H+ + HCO3-
                                                │
                 Chloride Shift (Cl- Influx) <──┘ (HCO3- exits into plasma)

1. Dissolved in Plasma (7% to 10%)

Carbon dioxide is significantly more soluble in plasma than oxygen, allowing 7% to 10% of total $CO_2$ to dissolve directly in physical solution as molecular $CO_2$. This dissolved fraction exerts the arterial partial pressure ($PaCO_2 = 40\text{ mmHg}$).

2. Bound to Hemoglobin as Carbaminohemoglobin (20% to 23%)

Approximately 20% to 23% of carbon dioxide binds directly to the amino acids of hemoglobin, forming carbaminohemoglobin ($Hb\text{-}CO_2$): $\text{Hb} + CO_2 \rightleftharpoons \text{Hb-CO}_2$

• Important Exam Distinction: $CO_2$ does not bind to the iron-heme groups (where $O_2$ binds); instead, it binds to the free terminal amine groups ($-NH_2$) of the globin polypeptide chains. Thus, $CO_2$ and $O_2$ do not directly compete for the same binding locus.
• The Haldane Effect: The amount of $CO_2$ transported in blood is strongly influenced by the degree of blood oxygenation. Deoxygenation of hemoglobin increases its affinity for $CO_2$. In systemic capillaries, as hemoglobin unloads $O_2$ to tissues, its capacity to bind $CO_2$ increases dramatically. In the lungs, as hemoglobin binds $O_2$, it releases $CO_2$ into the alveoli.

3. As Bicarbonate Ions in Plasma (70%): The Chloride Shift

The vast majority (~70%) of all carbon dioxide is converted into bicarbonate ions ($HCO_3^-$) and transported in blood plasma.

The Carbonic Anhydrase Reaction Sequence:

1. As $CO_2$ diffuses from metabolizing tissue cells into systemic capillaries, it enters erythrocytes.
2. Inside erythrocytes, $CO_2$ combines with water to form carbonic acid ($H_2CO_3$). Although this hydration reaction is sluggish in plasma, erythrocytes contain high concentrations of the enzyme Carbonic Anhydrase (CA), which accelerates the reaction more than 5,000-fold: $CO_2 + H_2O \stackrel{\text{Carbonic Anhydrase}}{\rightleftharpoons} H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$
3. Carbonic acid is unstable and rapidly dissociates into hydrogen ions ($H^+$) and bicarbonate ions ($HCO_3^-$).
4. Buffering of $H^+$: The generated $H^+$ ions bind to deoxyhemoglobin ($Hb\text{-}H$), which acts as an effective intracellular buffer. This prevents dangerous acidification of the erythrocyte and promotes further oxygen unloading (the Bohr effect).
5. The Chloride Shift: As bicarbonate accumulates inside the erythrocyte, it diffuses out down its concentration gradient into the blood plasma via an anion exchanger protein (Band 3 / AE1). To preserve electrical neutrality across the erythrocyte membrane as negative bicarbonate ions exit, chloride ions ($Cl^-$) move from blood plasma into the erythrocyte. This ionic exchange is the Chloride Shift (Hamburger phenomenon).

Reversal in Pulmonary Capillaries (Reverse Chloride Shift): When deoxygenated blood reaches the pulmonary capillaries, all chemical processes reverse:

1. Bicarbonate ions ($HCO_3^-$) diffuse from blood plasma back into erythrocytes.
2. Chloride ions ($Cl^-$) diffuse out of the erythrocyte back into plasma (the Reverse Chloride Shift).
3. $H^+$ detaches from hemoglobin and recombines with $HCO_3^-$ to form carbonic acid ($H_2CO_3$).
4. Carbonic anhydrase converts $H_2CO_3$ back into $CO_2$ and $H_2O$.
5. Free molecular $CO_2$ diffuses across the respiratory membrane into the alveoli and is exhaled into the atmosphere.

Carbon Dioxide Transport Modalities Reference Table

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Neural and Chemical Control of Respiration

Ventilation is an involuntary, rhythmic process coordinated by brainstem neural networks and continuously modulated by chemoreceptor and mechanoreceptor inputs.

Neural Control Hierarchy of Respiration

PONS:
├── Pontine Respiratory Group (PRG / Pneumotaxic Center)
│   └── Fine-tunes rhythm; smooths transition between inspiration & expiration
MEDULLA OBLONGATA:
├── Dorsal Respiratory Group (DRG)
│   └── Sensory integration (chemoreceptors CN IX/X, stretch receptors)
└── Ventral Respiratory Group (VRG)
    └── Rhythm Generator (Eupnea: 12-15 bpm)
         ├── Inspiratory Neurons (2 sec) ──> Phrenic & Intercostal Nerves
         └── Expiratory Neurons (3 sec)  ──> Allows passive recoil

1. Medullary Respiratory Centers

The medulla oblongata contains two primary functional clusters of respiratory neurons:

• Ventral Respiratory Group (VRG): The primary rhythm-generating and integrative center. It contains alternating networks of inspiratory and expiratory neurons:
  • Inspiratory Neurons: Fire rhythmic bursts of action potentials for approximately 2 seconds, which travel down the spinal cord to excite the phrenic nerves (C3-C5) supplying the diaphragm and the intercostal nerves supplying the external intercostal muscles, initiating inspiration.
  • Expiratory Neurons: Fire for approximately 3 seconds, sending inhibitory impulses to the inspiratory neurons. The inspiratory muscles relax, allowing passive elastic recoil to produce expiration.
  • This cyclical firing establishes the normal resting ventilatory rate and rhythm: Eupnea (12 to 15 breaths per minute).
• Dorsal Respiratory Group (DRG): Located dorsally near the root of Cranial Nerve IX (near the nucleus tractus solitarius). The DRG functions as a sensory integration center, receiving afferent inputs from peripheral chemoreceptors and pulmonary mechanoreceptors and communicating this information to the VRG to adjust breathing rhythm.

2. Pontine Respiratory Group (PRG)

Located in the pons (formerly divided into the pneumotaxic and apneustic centers). The PRG modifies and fine-tunes the basic rhythm established by the VRG. It transmits continuous inhibitory impulses to medullary inspiratory neurons, smoothing the transition between inspiration and expiration. It prevents prolonged inspiratory spasms (apneusis) and coordinates ventilatory patterns during vocalization, swallowing, sleeping, and exercise.

3. Chemical Regulation of Respiration: Chemoreceptor Reflexes

While higher brain centers (hypothalamus, cerebral cortex) can exert voluntary control, resting respiration is regulated primarily by chemical stimuli acting on specialized chemoreceptors:

The Central Chemoreceptor Acid-Base Feedback Loop

[Elevated Arterial P_CO2 (Hypercapnia > 40 mmHg)]
               │
               ▼
[CO2 rapidly crosses Blood-Brain Barrier into CSF]
               │
               ▼
[CO2 + H2O <──> H2CO3 <──> H+ + HCO3- in CSF]
               │
               ▼
[CSF pH Drops (Acidosis: High H+ Concentration)]
               │
               ▼
[Direct Stimulation of Central Chemoreceptors in Medulla]
               │
               ▼
[Medullary Inspiratory Neurons stimulated: Hyperventilation!]
               │
               ▼
[Excess CO2 blown off; CSF pH & Arterial P_CO2 return to normal]

• Central Chemoreceptors:
  • Location: Situated bilaterally on the ventrolateral surface of the medulla oblongata, directly bathed in cerebrospinal fluid (CSF).
  • The Primary Drive for Breathing: Central chemoreceptors are the single most powerful regulators of resting ventilation. They account for approximately 70% to 80% of the ventilatory response to carbon dioxide.
  • Mechanism: Central chemoreceptors respond directly to hydrogen ion concentration ($pH$) in the CSF. Although hydrogen ions ($H^+$) in blood cannot cross the lipid blood-brain barrier, lipid-soluble carbon dioxide ($CO_2$) diffuses freely and rapidly from cerebral capillaries into the CSF. In the virtually unbuffered CSF, $CO_2$ hydrates to form carbonic acid, which dissociates into $H^+$ and $HCO_3^-$. The resulting drop in CSF pH directly excites the central chemoreceptors, stimulating the medullary inspiratory center.
  • Hypercapnia: An elevation in arterial $P_{CO_2} > 40\text{ mmHg}$ immediately triggers hyperventilation (increased rate and depth of breathing) to blow off excess $CO_2$ and restore systemic pH.
  • Hypocapnia: A drop in arterial $P_{CO_2} < 40\text{ mmHg}$ (e.g., from deliberate overbreathing) diminishes chemoreceptor stimulation, resulting in hypoventilation or temporary apnea (cessation of breathing) until metabolic $CO_2$ accumulates back to baseline.
• Peripheral Chemoreceptors:
  • Location: Situated in the Carotid Bodies (located at the bifurcation of the common carotid arteries, innervated by the glossopharyngeal nerve, CN IX) and Aortic Bodies (located along the aortic arch, innervated by the vagus nerve, CN X).
  • Primary Stimuli: Monitor arterial blood directly for fluctuations in $P_{O_2}$, $P_{CO_2}$, and $pH$ (hydrogen ions).
  • Sensitivity to Oxygen: Under normal physiological conditions, arterial $P_{O_2}$ exerts minimal control over resting ventilation because hemoglobin remains nearly 90% saturated even when $P_{O_2}$ drops to $60\text{ mmHg}$. However, if arterial $P_{O_2}$ drops below a critical threshold of $60\text{ mmHg}$ (severe hypoxemia), peripheral chemoreceptors fire rapidly, stimulating medullary centers to dramatically increase ventilation.
  • Clinical Pearl: Hypoxic Drive in Chronic COPD: Patients with end-stage chronic bronchitis or severe emphysema suffer chronic hypercapnia ($PaCO_2 > 50-60\text{ mmHg}$). Over weeks to months, renal retention of bicarbonate buffers CSF pH, desensitizing central chemoreceptors to carbon dioxide. Consequently, these patients come to rely primarily on low arterial $P_{O_2}$ detected by peripheral chemoreceptors as their primary stimulus to breathe (the "hypoxic drive"). Uncontrolled high-concentration oxygen can worsen carbon dioxide retention in these patients. This was traditionally attributed to loss of the hypoxic drive; worsened ventilation-perfusion matching and the Haldane effect also contribute. Oxygen is therefore titrated to a target saturation (often 88% to 92%) rather than withheld.

4. Pulmonary Reflexes: The Hering-Breuer Reflex

The Hering-Breuer Reflex (Inflation Reflex) is a protective mechanical feedback loop:

• Pulmonary stretch receptors (mechanoreceptors) embedded within the visceral pleura and bronchiole smooth muscle become excited when the lungs overinflate (tidal volume $> 1,000\text{ mL}$, as during heavy exercise).
• Afferent nerve impulses travel via the vagus nerves (CN X) to the medullary DRG/VRG, terminating inspiratory firing and allowing expiration to begin.
• This reflex protects the delicate alveolar parenchyma from mechanical barotrauma and overdistension.

Central vs. Peripheral Chemoreceptors Comparison