9.02 Structure and Function of the Lungs
1. Location and Protection of the Lungs
Thoracic Cavity
- Location: The lungs reside within the thoracic (chest) cavity.
- Protection:
- Pleural Membranes: Each lung is enveloped by pleural membranes, creating an airtight pleural space.
- Pleural Fluid: A thin layer of pleural fluid exists within the pleural space, facilitating friction-free movement of the lungs during breathing.
Ventilation
- Mechanism:
- Diaphragm and Ribs: Movement of the diaphragm and rib cage ventilates the lungs by expanding and contracting the thoracic cavity.
- Note: Detailed mechanics of diaphragm and rib movements are not required for this overview.
2. Gas Exchange Surface
Definition
- Gas Exchange Surface: A specialized area where gases (oxygen and carbon dioxide) are exchanged between the external environment and the organism’s body.
Alveoli: Primary Gas Exchange Surface in Humans
- Structure:
- Alveoli: Tiny air sacs located at the end of the respiratory tree.
Adaptations of Alveoli for Efficient Gas Exchange
- Single-layered Squamous Epithelium:
- Minimizes the diffusion distance for gases, enhancing the efficiency of gas exchange.
- Elastic Fibers:
- Enable alveoli to expand during inhalation and recoil during exhalation.
- Capillary Network:
- Each alveolus is surrounded by a dense network of capillaries, ensuring close proximity between air and blood for optimal gas exchange.
3. Structure and Pathway of Air Through the Lungs
Airway Structure from Trachea to Alveoli
| Airway Component | Number Approx. | Diameter | Cartilage | Goblet Cells | Smooth Muscle | Cilia | Site of Gas Exchange |
|---|---|---|---|---|---|---|---|
| Trachea | 1 | 1.8 cm | Yes | Yes | Yes | Yes | No |
| Bronchus | 2 | 1.2 cm | Yes | Yes | Yes | Yes | No |
| Terminal Bronchiole | ~48,000 | 1.0 mm | No | No | Yes | Yes | No |
| Respiratory Bronchiole | ~300,000 | 0.5 mm | No | No | No | Few | No |
| Alveolar Duct | ~9 million | 400 μm | No | No | No | No | Yes |
| Alveoli | ~3 billion | 250 μm | No | No | No | No | Yes |
4. Summary of Air Passage and Gas Exchange Process
Air Passage
Inhalation Pathway:
- Air Entry: Air enters through the trachea.
- Bronchi: The trachea divides into the left and right bronchi.
- Bronchioles: Bronchi further branch into smaller bronchioles.
- Alveolar Ducts and Alveoli: Bronchioles terminate in alveolar ducts and alveoli, where gas exchange occurs.
Gas Exchange
Location: Occurs in the alveoli.
Mechanism:
- Oxygen Diffusion: Oxygen moves from the alveolar air into the blood within pulmonary capillaries.
- Carbon Dioxide Diffusion: Carbon dioxide moves from the blood into the alveolar air to be exhaled.
Surface Area:
- Large Surface Area: The alveoli collectively provide a surface area of approximately 70–75 m², maximizing the capacity for gas exchange.
5. Key Terms
- Gas Exchange Surface:
- Definition: Part of an organism that facilitates the exchange of gases between the external environment and the body.
- Alveolus (plural: Alveoli):
- Definition: Tiny air sacs in the lungs characterized by a single layer of squamous epithelium, elastic fibers, and a surrounding capillary network.
- Function: Primary site for gas exchange in the respiratory system.
Additional Notes
- Structural Adaptations of Alveoli: The specialized structure of alveoli ensures efficient gas exchange, which is crucial for maintaining oxygen levels and removing carbon dioxide from the blood.