Lesson 5.4 Gas Exchange in the Lungs/External Repiration

Introduction to Gas Exchange

• Definition of gas exchange as the process by which oxygen and carbon dioxide are exchanged between the lungs and the bloodstream.

• Location of gas exchange: respiratory zone, which includes respiratory bronchioles and alveoli.

• Primary function of the respiratory system: to supply oxygen to tissues and remove carbon dioxide to prevent accumulation.

Physiology of Gas Exchange

• Gas exchange occurs between alveoli and capillaries surrounding them:

  • Deoxygenated blood from the heart enters the lungs via the right side of the heart through the pulmonary artery.

  • This blood enters tiny capillaries within the lungs.

  • In the capillaries, blood releases carbon dioxide into the lungs to be exhaled and picks up inhaled oxygen.

  • The oxygenated blood returns to the left side of the heart, circulating through the body to deliver oxygen and nutrients.

Mechanism of Gas Movement

• Movement of gases occurs through diffusion:

  • Definition of diffusion: the movement of substances, including gases, from areas of higher concentration to areas of lower concentration.

  • Gas exchange involves partial pressure gradients.

Partial Pressure Gradient

• Partial Pressure: Refers to the pressure that a specific gas contributes to the total pressure of a mixture of gases.

  • Determined by the formula:
    $P<em>g = P</em>{atm} imes$
    where

  • $P_g$ is the partial pressure of gas,

  • $P_{atm}$ is atmospheric pressure,

  • %_g is the percentage of the specific gas in atmospheric air.

  • Two factors affecting partial pressure of a gas:

  1. Concentration: Higher gas concentration leads to higher partial pressure.

  2. Solubility: The influence of gas solubility in liquids on its ability to diffuse.

     • Example: Carbon dioxide is more soluble in water/blood than oxygen.

External Gas Exchange

• Transfer mechanisms:

  • Oxygen moves from alveoli to blood.

  • Carbon dioxide moves from blood to alveoli.

  • Both processes occur entirely by diffusion.

• Factors influencing gas diffusion:

  1. Partial pressure gradients: Between alveolar air and blood.

  2. Health of lung tissue: Impacts the efficiency of gas exchange.

Oxygen Transport

• Oxygen is not soluble in water; approximately 1% can dissolve in plasma.

• Majority of oxygen is carried bound to hemoglobin in red blood cells.

  • High partial pressure of oxygen in the lungs facilitates binding to hemoglobin.

Carbon Dioxide Transport

• Carbon dioxide concentration is higher in capillaries due to uptake from tissues.

• Transport mechanism:

  • About 80-90% of carbon dioxide is carried as bicarbonate (HCO3-).

  • Chemical Reaction:
    $CO<em>2 + H</em>2O <br>ightleftharpoons H<em>2CO</em>3 <br>ightleftharpoons H^+ + HCO_3^-$

  • Reaction indicates conversion of carbon dioxide to carbonic acid, then to hydrogen ions and bicarbonate.

  • About 5-10% is bound to hemoglobin, and another 5-10% is dissolved in blood.

Factors Affecting Gas Exchange Efficiency

• Membrane Thickness: Thinner membranes enhance diffusion rates.

  • Conditions affecting thickness:

  • Fluid accumulation (e.g., asthma, pulmonary edema).

  • Diseases like pulmonary fibrosis thickening lung tissue.

• Surface Area: The alveoli provide a vast surface area for gas exchange.

  • Example: A single lung expanded could cover the size of a tennis court.

  • Diseases like emphysema reduce surface area, impeding gas exchange.

• Pressure Differences: External factors, such as altitude, affect oxygen intake.

Conclusion

• Understanding gas exchange dynamics is fundamental to comprehending respiratory health and diseases.

• Importance of partial pressure, solubility, membrane thickness, surface area, and environmental factors in gas diffusion.

• Importance of recognizing how lung disease can impact the physiology described.