18.1 Gas Exchange in the Lungs and Tissues

Overview of Gas Exchange

Gas exchange is a vital physiological process that occurs in the lungs and tissues, enabling the transfer of oxygen (O2) into the blood and the removal of carbon dioxide (CO2) from the body. This process is essential for cellular respiration and maintaining homeostasis.

• Components of Gas Exchange: Involves the movement of O2 from alveoli to blood and CO2 from blood to alveoli.

• Hypoxia: A condition where tissues are deprived of adequate oxygen supply.

• Hypercapnia: An excess of CO2 in the bloodstream.

Partial Pressures and Gas Movement

Partial pressure refers to the pressure exerted by a single gas in a mixture of gases. The movement of O2 and CO2 is driven by differences in their partial pressures between alveoli, blood, and tissues.

• Physiological Partial Pressures:

  • Alveoli: High O2, low CO2

  • Arterial blood: High O2, low CO2

  • Venous blood: Low O2, high CO2

  • Tissues: Low O2, high CO2

• Diagram: Illustrates the movement of gases and relative partial pressures (see Fig 18.2).

Factors Affecting Gas Exchange

Several factors influence the efficiency of gas exchange between the lungs and tissues:

• Composition of Inspired Air: The concentration of O2 in the air affects alveolar O2 levels.

• Alveolar Ventilation: The rate and depth of breathing impact gas exchange.

• Surface Area: Greater alveolar surface area enhances gas exchange.

• Diffusion Barrier Permeability: Thicker barriers slow diffusion.

• Diffusion Distance: Increased distance reduces efficiency.

Pathologies Affecting Gas Exchange

Various diseases can impair gas exchange by affecting diffusion or ventilation:

• Emphysema: Destruction of alveolar walls reduces surface area.

• Fibrotic Lung Disease: Thickening of alveolar membrane impairs diffusion.

• Pneumonia: Fluid in alveoli increases diffusion distance.

• Asthma: Bronchoconstriction limits airflow.

Oxygen and Carbon Dioxide Gradients

The difference in concentration and partial pressure of gases drives their movement:

• Example: O2 moves from alveoli (high PO2) to blood (low PO2); CO2 moves from blood (high PCO2) to alveoli (low PCO2).

18.2 Gas Transport in the Blood

Fick Equation and Oxygen Consumption

The Fick equation relates oxygen consumption to cardiac output and arterial-venous oxygen difference.

• Fick Equation:

• Applications: Used to assess tissue oxygenation and metabolic rate.

Oxygen Transport and Hemoglobin

Oxygen is transported in the blood primarily bound to hemoglobin, with a small amount dissolved in plasma.

• Oxygen Solubility: O2 has low solubility in plasma; hemoglobin increases carrying capacity.

• Oxyhemoglobin Dissociation Curve: Shows the relationship between PO2 and hemoglobin saturation (see Fig 18.8).

• Factors Affecting Curve: pH, temperature, and 2,3-BPG shift the curve, affecting O2 release (see Fig 18.9).

Carbon Dioxide Transport

CO2 is transported in the blood in three forms:

• Dissolved in Plasma: ~7% of CO2

• Bound to Hemoglobin: ~23% of CO2

• As Bicarbonate Ion (HCO3-): ~70% of CO2

The conversion of CO2 to bicarbonate is catalyzed by the enzyme carbonic anhydrase (CA):

• Chloride Shift: Exchange of Cl- and HCO3- across the red blood cell membrane maintains ionic balance.

• Bicarbonate as Buffer: Helps regulate blood pH.

18.3 Regulation of Ventilation

Neural Control of Breathing

Breathing is regulated by neural centers in the brainstem that coordinate the rate and depth of respiration.

• Key Centers:

  • Medullary Respiratory Centers: Dorsal respiratory group (DRG) and ventral respiratory group (VRG)

  • Pontine Centers: Pneumotaxic center and apneustic center

  • Pre-Bötzinger Complex: Generates respiratory rhythm

• Peripheral Chemoreceptors: Located in carotid and aortic bodies; respond to changes in blood O2, CO2, and pH.

• Central Chemoreceptors: Located in the medulla; sensitive to changes in CO2 and pH in cerebrospinal fluid.

Feedback Mechanisms

Ventilation is adjusted in response to changes in blood gas levels and pH to maintain homeostasis.

• Increased CO2 or decreased pH: Stimulates increased ventilation.

• Decreased O2: Stimulates peripheral chemoreceptors to increase ventilation.

Summary Table: Forms of Gas Transport in Blood

Example: Oxygen-Hemoglobin Dissociation Curve

The curve demonstrates how hemoglobin saturation changes with varying PO2 levels. A rightward shift (due to increased temperature, decreased pH, or increased 2,3-BPG) facilitates O2 release to tissues.

Additional info: Academic context and definitions have been expanded for clarity and completeness. Table entries and explanations are inferred from standard physiology knowledge and the provided outline.