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    • What is it:
      • Pulmonary parenchyma refers to:
        • The functional tissue of the lungs,
        • primarily responsible for gas exchange.
      • It includes the:
        • Alveoli,
        • Alveolar sacs,
        • Alveolar ducts,
        • Respiratory bronchioles, and
        • Associated capillaries.
    • Etymology:
      • Derived from the Greek word parenchyma, meaning “that which is poured in,” reflecting the tissue’s role in gas exchange.
    • Parts:
      • Structural unit:
        • Secondary pulmonary lobule: The smallest unit of lung structure visible on imaging.
          • Composed of 3 to 25 acini, depending on the location:
            • Smaller secondary lobules: Found near the pleura, containing fewer acini.
            • Larger secondary lobules: Found near the hilum, containing more acini.
      • Functional unit:
        • Acinus: The smallest functional unit of the lung where gas exchange occurs.
          • Includes:
            • Respiratory bronchioles,
            • Alveolar ducts,
            • Alveolar sacs, and
            • Alveoli.
      • Additional components:
        • Alveolar sacs: Terminal structures composed of clusters of alveoli.
        • Alveoli: Small sac-like structures where gas exchange occurs.
        • Alveolar ducts: Passages connecting respiratory bronchioles to alveolar sacs.
        • Respiratory bronchioles: Smallest airways involved in gas exchange.
          • Size:
            • Pulmonary parenchyma:
              • Makes up about 90-95% of the thoracic space.
              • Includes the vast network of alveoli, acini, secondary lobules, and associated capillary beds essential for gas exchange.
            • Transporting airways (conducting zone):
              • Represent only about 5-10% of the thoracic space.
              • Include the trachea, bronchi, and bronchioles up to the terminal bronchioles (which do not participate in gas exchange).

            The parenchyma’s dominance reflects its role as the functional unit of the lung, designed to maximize the surface area for gas exchange

            • The surface area of the lungs in a healthy adult is approximately 70 square meters, which is roughly equivalent to the size of one tennis court.
          • Shape: Sponge-like network of air-filled spaces.
        • Position:
          • Found throughout the lungs, distal to the terminal bronchioles.
    • Character:
      • Highly elastic and vascularized to facilitate efficient gas exchange.
    • Blood supply:
      • Arterial supply:
        • Pulmonary arteries supply deoxygenated blood for gas exchange.
        • Bronchial arteries provide oxygenated blood to lung tissues.
      • Venous drainage:
        • Pulmonary veins carry oxygenated blood back to the heart.
        • Bronchial veins drain into systemic circulation.
    • Lymphatic drainage:
      • Drains into the subpleural and interstitial lymphatic networks, which eventually connect to hilar lymph nodes.
    • Nerve supply:
      • Autonomic innervation via the vagus nerve (parasympathetic) and sympathetic nerves.
    • Embryology:
      • Derived from the endoderm (epithelial lining) and mesoderm (vascular and connective tissues).
    • Applied anatomy:
      • The parenchyma’s structure, and particularly the understanding of the secondary lobule is key to understanding diseases like:
        • Emphysema: Loss of alveolar walls.
        • Fibrosis: Thickened parenchymal tissue.
        • Pneumonia: Consolidation of alveoli.
      • Imaging application:
        • Parenchymal abnormalities are assessed on imaging modalities such as:
          • Chest X-ray: Detects opacities or hyperinflation.
          • Chest CT: Identifies fine details like ground-glass opacities, honeycombing, or nodules.
          • PET CT