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Overview of Pulmonary Physiology

Pulmonary physiology involves the study of the respiratory system, focusing on the mechanics of breathing, gas exchange, and the regulation of respiration. The lungs are responsible for supplying oxygen to the blood and removing carbon dioxide, a waste product of metabolism.

Key Concepts

• Anatomy of the Respiratory System:
  • Upper Respiratory Tract: Includes the nose, nasal cavity, pharynx, and larynx.
  • Lower Respiratory Tract: Includes the trachea, bronchi, bronchioles, and alveoli.
  • Lungs: Consist of lobes (three in the right lung, two in the left lung) and are encased in the pleural membranes.
• Mechanics of Breathing:
  • Inspiration (Inhalation): Diaphragm and intercostal muscles contract, increasing thoracic volume and decreasing pressure, drawing air into the lungs.
  • Expiration (Exhalation): Diaphragm and intercostal muscles relax, decreasing thoracic volume and increasing pressure, pushing air out of the lungs.
  • Involves the concepts of compliance (lung stretchability) and elasticity (tendency to return to original shape).
• Gas Exchange:
  • Occurs in the alveoli, where oxygen diffuses into the blood, and carbon dioxide diffuses out of the blood.
  • Dependent on partial pressures of gases (PO2 and PCO2) and the principles of diffusion.
  • Oxygen transport: Primarily bound to haemoglobin in red blood cells.
  • Carbon dioxide transport: Dissolved in plasma, bound to haemoglobin, or as bicarbonate ions.
• Regulation of Respiration:
  • Controlled by the respiratory centres in the brainstem (medulla oblongata and pons).
  • Chaemoreceptors: Monitor levels of CO2, O2, and pH in the blood and cerebrospinal fluid.
  • Mechanoreceptors: Respond to changes in lung volume and pressure.

Detailed Processes

• Inspiration and Expiration:
  • Inspiration:
    • Diaphragm contracts, moving downward, and external intercostal muscles lift the ribs upward and outward.
    • Thoracic cavity volume increases, leading to a decrease in intrapulmonary pressure, allowing air to flow into the lungs.
  • Expiration:
    • Diaphragm relaxes, moving upward, and internal intercostal muscles and abdominal muscles may contract to expel air.
    • Thoracic cavity volume decreases, leading to an increase in intrapulmonary pressure, pushing air out of the lungs.
• Gas Exchange and Transport:
  • Alveolar Gas Exchange:
    • Oxygen diffuses from alveoli into pulmonary capillaries, driven by the difference in partial pressures (high PO2 in alveoli, low PO2 in blood).
    • Carbon dioxide diffuses from pulmonary capillaries into alveoli (high PCO2 in blood, low PCO2 in alveoli).
  • Oxygen Transport:
    • Oxygen binds to haemoglobin, forming oxyhaemoglobin, which is transported to tissues.
    • Oxygen dissociation from haemoglobin is influenced by factors such as pH (Bohr effect), temperature, and CO2 levels.
  • Carbon Dioxide Transport:
    • CO2 is transported in three forms: dissolved in plasma, bound to haemoglobin (carbaminohaemoglobin), and as bicarbonate ions (HCO3-).
    • Bicarbonate formation: CO2 reacts with water to form carbonic acid, which dissociates into bicarbonate and hydrogen ions (catalyzed by carbonic anhydrase).
• Regulation of Respiration:
  • Central Control:
    • Medulla oblongata: Contains the dorsal respiratory group (DRG) and ventral respiratory group (VRG), which control rhythmic breathing.
    • Pons: Contains the pneumotaxic centre and apneustic centre, which modulate the respiratory rhythm.
  • Chaemoreceptors:
    • Central chaemoreceptors: Located in the medulla, sensitive to changes in CO2 and pH in cerebrospinal fluid.
    • Peripheral chaemoreceptors: Located in the carotid bodies and aortic bodies, sensitive to changes in O2, CO2, and pH in arterial blood.
  • Mechanoreceptors:
    • Respond to stretch and pressure changes in the lungs and airways, providing feedback to the respiratory centres.

Clinical Relevance

• Chronic Obstructive Pulmonary Disease (COPD):
  • Includes conditions such as chronic bronchitis and emphysema, characterized by airflow obstruction and difficulty in breathing.
  • Caused primarily by smoking and exposure to pollutants.
• Asthma:
  • Chronic inflammatory disorder of the airways, leading to episodes of wheezing, shortness of breath, and coughing.
  • Triggered by allergens, exercise, cold air, and respiratory infections.
• Pulmonary Fibrosis:
  • Condition characterized by scarring of lung tissue, leading to stiff and thickened lungs, and reduced gas exchange.
  • Causes include environmental pollutants, radiation, and certain medications.
• Pneumonia:
  • Infection that inflames the air sacs in one or both lungs, which may fill with fluid or pus.
  • Caused by bacteria, viruses, or fungi.
• Acute Respiratory Distress Syndrome (ARDS):
  • Severe condition characterized by rapid onset of widespread inflammation in the lungs, leading to respiratory failure.
  • Often caused by trauma, sepsis, or severe pneumonia.

Summary

Pulmonary physiology encompasses the mechanics of breathing, gas exchange, and the regulation of respiration. Understanding these processes is essential for diagnosing and treating respiratory disorders, ensuring effective oxygen delivery and carbon dioxide removal to maintain homeostasis.