Clase 39 Fisiología Respiratoria - Intercambio Gaseoso Pulmonar (Hematosis) (IG:@doctor.paiva)

39th Class of Physiology: Gas Exchange

Introduction to Gas Exchange

• The class begins with an introduction by Eduardo Paiva, focusing on respiratory physiology and gas exchange.

• Key topics include general diffusion principles, partial pressures of gases, and the composition of atmospheric air.

Structure of the Respiratory System

• The respiratory system is divided into bronchi, terminal bronchioles, alveolar sacs, and alveoli; collectively known as the respiratory unit.

• This specialized structure facilitates gas diffusion from oxygen to blood and carbon dioxide to alveoli through a selective membrane.

Understanding Diffusion

• Diffusion is defined as the movement of solutes across a selectively permeable membrane from higher to lower concentration areas.

• Gas pressure results from particle impacts against surfaces; it correlates with molecular concentration in respiratory physiology.

Partial Pressures in Gases

• A mixture of gases (nitrogen, oxygen, carbon dioxide) influences diffusion speed based on their partial pressures.

• Atmospheric pressure at sea level is 760 mmHg; nitrogen constitutes 78%, exerting a pressure of 597 mmHg.

Composition and Behavior of Gases

• Oxygen (21%) exerts 159 mmHg while carbon dioxide (1%) exerts only 4 mmHg. These values sum up to total atmospheric pressure.

• Dalton's Law states that total pressure equals the sum of individual gas partial pressures.

Factors Affecting Partial Pressure

• The partial pressure depends on gas concentration and its solubility coefficient—how easily it passes through membranes.

• Henry's Law relates partial pressure to dissolved gas concentration over its solubility coefficient.

Solubility Comparison Between Gases

• Carbon dioxide diffuses more readily than oxygen due to its higher solubility—20 times greater than that of oxygen at body temperature.

Mechanism Behind Gas Exchange

• Oxygen enters capillaries while carbon dioxide exits due to differences in their partial pressures between alveoli and capillaries.

Factors Influencing Net Diffusion Rate

Understanding Gas Exchange in the Lungs

Diffusion Coefficient and Gas Solubility

• The diffusion coefficient is crucial for understanding how gases like oxygen diffuse compared to others, with oxygen's coefficient set as a baseline of 1.

• Carbon dioxide is noted to be 20 times more soluble than oxygen, highlighting its significant role in respiratory physiology.

Humidification and Partial Pressure

• When air is inhaled, it undergoes humidification in the airways, raising the water vapor pressure to 47 mmHg at body temperature.

• The composition of atmospheric air differs from that which reaches the alveoli; this transformation affects gas pressures significantly.

Alveolar Gas Composition

• The partial pressure of oxygen decreases from 159 mmHg in atmospheric air to only 104 mmHg in alveolar air due to ongoing gas exchange.

• In contrast, carbon dioxide levels rise from 0.03 mmHg in atmospheric air to 40 mmHg within the alveoli as CO2 is continuously expelled from the bloodstream.

Changes During Breathing

• Water vapor pressure increases from 37 mmHg in atmospheric conditions to 47 mmHg upon reaching the alveoli, affecting other gas concentrations.

• Alveolar air is partially replaced with fresh atmospheric air during breathing; however, some residual volume remains after exhalation.

Residual Volume and Gas Exchange Dynamics

• The presence of residual volume prevents complete collapse of alveoli and ensures continuous gas exchange even between breaths.

• Continuous diffusion occurs for both oxygen and carbon dioxide throughout respiration, maintaining necessary blood gas levels.

Slow Renewal of Alveolar Air

• Only one-seventh of alveolar air is replaced with new atmospheric air per breath, leading to slow renewal rates that stabilize blood gas concentrations.

• This slow replacement mechanism helps prevent drastic fluctuations in carbon dioxide levels and tissue pH during temporary interruptions in breathing.

Importance of Steady State Conditions

• Maintaining steady-state conditions through gradual changes allows for effective buffering against sudden shifts in blood chemistry during respiratory pauses.

Understanding Gas Concentrations in the Lungs

The Importance of Carbon Dioxide Concentration

• Higher ventilation (hyperventilation) leads to lower concentrations of gases, such as carbon dioxide, in the alveoli. Increased ventilation expels more CO2.

• The concentration and partial pressure of oxygen in the alveoli are influenced by two main factors: diffusion speed and renewal rate of oxygen entering the lungs.

Oxygen Diffusion Dynamics

• For example, if oxygen diffuses at 250 ml/min, a normal alveolar ventilation must provide 4.2 liters/min to maintain adequate oxygen pressure (104 mmHg).

• During moderate exercise, diffusion speed can increase to 1000 ml/min, necessitating an increase in alveolar ventilation to nearly 20 liters/min to meet higher oxygen demands.

Blood Oxygen Pressure Changes

• Venous blood has an initial oxygen pressure of 40 mmHg; after diffusion within 0.25 seconds, it rises to 104 mmHg upon reaching arterial blood.

• This demonstrates how venous blood transitions from low (40 mmHg) to high (104 mmHg) oxygen pressure through efficient gas exchange.

Carbon Dioxide Dynamics

• In contrast, carbon dioxide has a diffusion rate of about 200 ml/min; normal ventilation should also be around 4.2 liters/min for stable CO2 levels.

• If CO2 expression increases to 800 ml/min, then alveolar ventilation must quadruple to maintain normal CO2 pressures.

Key Relationships Between Gases

• The partial pressure of carbon dioxide in the alveoli increases directly with its expression rate while decreasing inversely with increased ventilation.

• These principles mirror those for oxygen but focus on absorption for O2 and expression for CO2 during respiration processes.

Summary of Gas Exchange Pressures

• Venous blood shows a transition from a CO2 pressure of 45 mmHg down to arterial levels at approximately 40 mmHg due to effective gas exchange.

• A graphical representation illustrates these changes clearly: venous O2 at 40 mmHg becomes arterial O2 at 104 mmHg while venous CO2 drops from 45 mmHg to arterial levels at around 40 mmHg.

Air Composition During Exhalation

• Expired air is a mixture that includes dead space air and true alveolar air; initial exhalation primarily consists of dead space air before transitioning into more concentrated alveolar air.

Respiratory Membrane Structure

Gas Exchange Mechanisms in the Respiratory System

Structure of the Alveolar-Capillary Membrane

• The alveolar-capillary membrane consists of six layers:

• Alveolar epithelium

• Epithelial basement membrane

• Interstitial space

• Capillary basement membrane

• Endothelial layer of capillaries

Diffusion Capacity of Gases

• Oxygen diffusion capacity is typically 21 ml/min, increasing to 65 ml/min during exercise.

• Carbon dioxide diffusion capacity ranges from 400 to 500 mL/min, potentially reaching up to 1200 mL/min during exercise, making it approximately 20 times more diffusible than oxygen.

Factors Influencing Gas Diffusion Rate

Thickness of the Membrane

• The rate of gas diffusion is inversely proportional to the thickness of the membrane; increased thickness reduces diffusion speed.

• In cases like pulmonary edema, fluid accumulation increases membrane thickness, slowing down oxygen diffusion from a normal time of 0.25 seconds to 0.75 seconds.

Surface Area for Gas Exchange

• The surface area available for gas exchange (hematosis area in alveoli) significantly impacts diffusion rates.

• Conditions such as emphysema reduce surface area due to destruction of alveolar walls, leading to decreased gas exchange efficiency.

Diffusion Coefficient and Pressure Differences

• The diffusion coefficient indicates how readily gases diffuse; carbon dioxide has a higher coefficient than oxygen.

• The net direction of gas diffusion depends on partial pressure differences between gases in the alveoli and blood:

• Higher oxygen pressure in alveoli drives oxygen into blood.

• Higher carbon dioxide pressure in blood drives carbon dioxide into alveoli.

Conclusion and Future Topics