Fisiología respiratoria: ¿cómo se realiza el intercambio gaseoso de O2 y CO2 entre aire y sangre?
Gas Exchange at the Pulmonary Level
Introduction to Gas Exchange
- Mauricio Giuliodori introduces the topic of gas exchange, focusing on the pulmonary level and how carbon dioxide and oxygen are exchanged between mixed venous blood and alveolar air.
Mechanism of Gas Exchange
- Gas exchange occurs at both lung and tissue levels, involving mixed venous blood in pulmonary capillaries and alveolar air.
- The process of hematosis involves simple diffusion driven by concentration differences between air and blood, facilitated by pressure gradients for oxygen and carbon dioxide.
Partial Pressures of Gases
- A detailed comparison of partial pressures shows variations in atmospheric, inspired, alveolar, and expired air for oxygen (O2) and carbon dioxide (CO2).
- Atmospheric air: 158 mmHg O2; Inspired air: 140 mmHg O2; Alveolar air: 114 mmHg O2; Expired air: 123 mmHg O2.
- CO2 levels also vary significantly across these states.
Reasons for Differences in Air Composition
- Inspired air has less oxygen than atmospheric due to saturation with water vapor, which reduces the partial pressures of other gases.
- Alveolar air contains more CO2 and less O2 than inspired air because gas exchange (haematosis) has occurred in the alveoli.
Understanding Expired Air Composition
- Expired air has more oxygen and less carbon dioxide compared to alveolar air due to stale versus fresh air dynamics during breathing.
- Inhaled volume includes a mix of fresh (300 ml causing hematosis) and stale (150 ml from previous breaths), affecting gas concentrations upon exhalation.
Structure of the Respiratory Membrane
Layers Involved in Gas Exchange
- The respiratory membrane consists of six layers that gases must traverse:
- Alveolar fluid,
- Alveolar epithelium (type 1 & type 2 pneumocytes),
- Alveolar basement membrane,
- Interstitial space,
- Basement membrane of pulmonary capillary,
- Endothelium of pulmonary capillary.
Carbon Dioxide Diffusion Dynamics
Gas Exchange and Fick's Law of Diffusion
Arterial Blood Gas Pressures
- The arterial blood leaves the lungs with a carbon dioxide pressure of 40 mmHg, while oxygen has an alveolar pressure of 100 mmHg and a venous pressure of 40 mmHg, resulting in a 60 mmHg gradient for oxygen diffusion.
Fick's Law of Diffusion
- Fick's Law, established in 1855, states that the net diffusion of a gas through a membrane is proportional to the diffusion coefficient and the pressure gradient across the membrane. It also depends on the diffusion area and inversely on the thickness of the membrane.
- The law highlights three factors that favor net diffusion (diffusion coefficient, pressure gradient, surface area) against one factor that opposes it (thickness).
Modifying Factors for Enhanced Gas Diffusion
- To increase net diffusion during heightened metabolic activity, organisms enhance gas exchange by increasing both alveolar ventilation and cardiac output—maximizing air and blood flow to the lungs.
Impact of Respiratory Diseases on Diffusion
- Respiratory diseases can adversely affect gas exchange by altering diffusion areas; conditions like emphysema (alveoli rupture) reduce surface area while atelectasis leads to collapsed airways. Additionally, thrombosis can impair lung irrigation.