Gas Exchange and Transport - Chapter 18

The process of gas exchange is vital for supplying oxygen (O₂) to the body and removing carbon dioxide (CO₂).
Hypoxia: A condition characterized by reduced O₂ levels.
Hypercapnia: A condition characterized by elevated CO₂ levels.
To avoid hypoxia and hypercapnia, the body regulates three key arterial blood parameters:
- O₂
- CO₂
- pH

LO 18.1.1: Identify three arterial blood parameters that influence ventilation.
LO 18.1.2: Diagram the normal partial pressures of O₂ and CO₂ in the atmosphere, alveoli, arterial blood, resting cells, and venous blood.
LO 18.1.3: Discuss factors affecting gas exchange between the atmosphere and arterial blood.
LO 18.1.4: Differentiate between the concentration of a gas in solution and its partial pressure with O₂ and CO₂ as examples.

LO 18.2.1: Explain the Fick equation relating cardiac output to cellular oxygen consumption via mass flow and mass balance.
LO 18.2.2: Detail the role of hemoglobin in O₂ transport at the molecular and systemic levels.
LO 18.2.3: Describe the relationship between plasma O₂ partial pressures and O₂ transport.
LO 18.2.4: Draw and analyze the oxyhemoglobin saturation curve, including physiological significance and shifts due to pH, temperature, and 2,3-bisphosphoglycerate (2,3-BPG).
LO 18.2.5: Compare O₂ transport mechanisms in fetal versus adult hemoglobin.
LO 18.2.6: Write out the chemical reaction for converting CO₂ to bicarbonate (HCO₃⁻) including the catalyzing enzyme.
LO 18.2.7: Map CO₂ transport in arterial and venous blood including exchanges between blood and the alveoli or cells.

Figure 18.1 illustrates the need for O₂ uptake and CO₂ removal within the body and identifies the parameters measured in blood:

Arterial O₂ Pressure (PaO₂): 95 mm Hg (normal range 85–100)
Arterial CO₂ Pressure (PaCO₂): 40 mm Hg (normal range 35–45)
pH Levels: Arterial - 7.4 (normal range 7.38–7.42)
Venous gives similar parameters:
- Venous O₂: 40 mm Hg
- Venous CO₂: 46 mm Hg
- Venous pH: 7.37

Table 18.1 details the types of hypoxia and their definitions along with typical causes:

Hypoxic Hypoxia: Low arterial O₂.
- Causes: High altitude, alveolar hypoventilation, decreased lung diffusion capacity, abnormal ventilation-perfusion ratio.
Anemic Hypoxia: Decreased total O₂ bound to hemoglobin.
- Causes: Blood loss, anemia, carbon monoxide poisoning.
Ischemic Hypoxia: Reduced blood flow.
- Causes: Heart failure, shock, thrombosis.
Histotoxic Hypoxia: Failure of cells to utilize O₂ due to poisoning.
- Causes: Cyanide and other metabolic poisons.

Breathing: Involves bulk flow of air entering and exiting the lungs.
Diffusion: Individual gases move according to partial pressure gradients until equilibrium is achieved.
Gas Pressure: The total pressure of a mixed gas equals the sum of the partial pressures of each gas.

Atmospheric Composition: Low alveolar O₂ may arise from influenced inspired air (e.g., higher altitudes decrease O₂ levels) and inadequate alveolar ventilation (hypoventilation).
Variation in Alveolar PO₂: If ventilation is reduced due to factors like decreased lung compliance or increased airway resistance, it can lower alveolar PO₂.

Equation for Diffusion Rate:
ext{Diffusion Rate} ext{ } ext{= Surface Area} imes ext{Concentration Gradient} imes ext{Barrier Permeability}
Influencing Factors:
- Surface area: Increased surface area = greater diffusion potential.
- Barrier Permeability: Cell layers impact how easily gases pass between lung and blood.
- Diffusion Distance: Distance affects the rate of gas exchange.

Emphysema: Deterioration of alveolar structure reduces available surface area for diffusion.
Fibrotic Lung Disease: Thickened alveolar membrane impedes gas exchange.
Pulmonary Edema: Accumulation of fluid increases diffusion distance while still enabling normal arterial PCO₂ due to increased solubility of CO₂ in water.
Asthma: Elevated resistance of airways leads to reduced ventilation.

Gas solubility in liquids influences diffusion rates, relying on:
- Pressure Gradient: Gases move faster when the pressure difference is greater.
- Gas Solubility: Different gases exhibit varying solubility in liquids.
- Temperature: Typically affects the kinetic energy of molecules.

The primary transport of O₂ occurs in the blood, with the following:
- A negligible portion (<2%) exists dissolved in plasma.
- Hemoglobin (Hb) binds essentially all of the remaining O₂.
- Hemoglobin Structure: Contains 4 hemes (one for each O₂ molecule) where the binding operates under the principles of mass action, influencing reaction dynamics upon O₂ loading and unloading.
Percent Saturation of Hemoglobin is calculated by:
ext{Percent Saturation} = rac{ ext{Amount of O}2 ext{ bound}}{ ext{Max O}2 ext{ that could be bound}} imes 100
Oxyhemoglobin saturation curves illustrate the relationship between saturation and PO₂, showcasing the cooperative binding effect.

The shape of the hemoglobin O₂ saturation curve can shift due to changes in:
- Metabolic Activity: Increased activity (right shift) leads to decreased affinity and greater O₂ release, whereas decreased activity (left shift) increases affinity and reduces O₂ release.
- Bohr Effect: Describes the curve shift in response to pH changes.
- 2,3-BPG: Increased production during chronic hypoxia shifts the curve to the right, affecting O₂ binding.
- Fetal Hemoglobin (HbF): Contains 2 gamma chains enhancing its affinity for O₂, critical for fetal survival in low O₂ placental environments.

CO₂ is transported in three forms:
1. Dissolved in plasma (7% of total CO₂).
2. Bound to hemoglobin (carbaminohemoglobin) (23%).
3. Converted to bicarbonate ions (HCO₃⁻) (70%), facilitated by the enzyme carbonic anhydrase (CA).
Chemical Reaction: Represented as:
ext{CO}2 + ext{H}2 ext{O}
ightleftharpoons ext{H}^+ + ext{HCO}_3^-

This mechanism exchanges HCO₃⁻ out of the red blood cells for Cl⁻ ions to maintain electrical neutrality.

Hemoglobin buffers H⁺ ions, influencing blood pH levels and helping to mitigate respiratory acidosis when PCO₂ levels rise.

Keep in mind that total arterial O₂ content depends profoundly on the interplay between dissolved O₂ in plasma and the fractional O₂ bound to hemoglobin, influenced by multiple physiological factors.*
Arterial Measures:
- Dry air: 760 mm Hg with PO₂ = 160 mm Hg and PCO₂ = 0.25 mm Hg.
- Alveoli PO₂ = 100 mm Hg, PCO₂ = 40 mm Hg.
- Blood transition shows:
- Venous blood PO₂ ≈ 40 mm Hg, PCO₂ ≈ 46 mm Hg.
- Arterial blood PO₂ = 100 mm Hg, PCO₂ = 40 mm Hg.

Gas exchange is a complex process influenced by various pulmonary and systemic factors.
Understanding the mechanics of gas transport both in terms of chemistry and physiology is crucial for comprehending broader human physiological functions.