Chapter 8: Respiratory System
Function. Gas exchange (respiration): uptake of O₂ from air, removal of CO₂ to air.
Organs. Nasal and oral cavities, pharynx, larynx, trachea, bronchi, diaphragm, lungs.
Airways.
- Upper airway: external/internal nose plus nasal & oral parts of the pharynx up to the laryngeal inlet.
- Lower airway (respiratory tract): larynx → trachea → right & left main bronchi → lungs.
Key point. Lungs have no skeletal muscle. Their volume changes are driven by respiratory muscles—primarily the diaphragm and thoracic (intercostal) muscles—which move the chest wall and therefore the lungs.
Chest wall mechanics. At the costovertebral/costotransverse joints (ribs with thoracic vertebrae) and the sternum:
- Inspiration: thoracic dimensions increase.
- Ribs and sternum move upward (sternum lifts away from the spine: “pump-handle” motion → ↑ anteroposterior diameter).
- Ribs move outward (“bucket-handle” motion → ↑ transverse diameter).
- Diaphragm contracts and descends, increasing vertical diameter.
- Expiration: the reverse occurs.
- Ribs and sternum move downward/inward; diaphragm relaxes and ascends; thoracic volume decreases.
Observable result: during breathing, the chest and abdomen move (chest expansion and abdominal excursion reflect the action of the intercostals and diaphragm, respectively).
Respiratory Muscles
All respiratory muscles are skeletal muscles, built with the same types of fibers as limb muscles. Their fiber composition is crucial for their endurance and contractile properties. For example, the diaphragm—the most important respiratory muscle—contains a very high proportion of fatigue-resistant fibers, allowing it to contract continuously throughout life.
Primary Respiratory Muscles
- Diaphragm – the principal muscle of inspiration; contracts and descends to enlarge thoracic volume.
- Intercostal muscles – located between the ribs:
- External intercostals: elevate the ribs, assisting inspiration.
- Internal intercostals: depress the ribs, assisting forced expiration.
- Abdominal wall muscles – contract during forced expiration, pushing the diaphragm upward.
Accessory Respiratory Muscles
These muscles are recruited during heavy breathing, exercise, or respiratory distress:
- Sternocleidomastoid – elevates the sternum.
- Scalene muscles (neck) – elevate the upper ribs.
- Additional muscles of the back and shoulder girdle may also assist by stabilizing or elevating the thoracic cage.
👉 In quiet breathing, only the diaphragm and external intercostals are active. Accessory and abdominal muscles are engaged during forced inspiration and expiration.
Pulmonary Ventilation and Gas Exchange
The movement of air in and out of the lungs is driven by differences in atmospheric and intrathoracic pressure, which are created by the respiratory muscles of the thorax.
Role of the Respiratory Muscles
- Diaphragm:
- Contracts and moves downward, reducing intrathoracic pressure.
- At the same time, increases intra-abdominal pressure as the dome descends.
- This action is the primary driver of normal inspiration.
- Intercostal Muscles:
- Move the thoracic wall and assist with both inspiration and expiration.
- External intercostals and the chondral portion of the internal intercostals act as inspiratory muscles (lifting the ribs upward and outward).
- The interosseous portion of the internal intercostals functions as an expiratory muscle (pulling the ribs downward and inward).
- Scalenes: Now recognized as true inspiratory muscles, elevating the first two ribs.
- Sternocleidomastoids: Considered the most significant accessory inspiratory muscles, especially active during deep or labored breathing.
Gas Exchange
- Takes place in the alveoli of the lungs, where:
- Oxygen (O₂) passes from inhaled air into the bloodstream.
- Carbon dioxide (CO₂) moves from the blood into the alveoli to be exhaled.
Nasal Breathing
Inhalation through the nose provides three important physiological benefits:
- Humidification – increases the moisture of inspired air.
- Temperature regulation – warms or cools the air to body temperature.
- Filtration – traps dust, allergens, and pathogens before they reach the lungs.
Pulmonary Performance
Lung function is determined by:
- Tidal volume – the amount of air moved per breath.
- Respiratory frequency – the number of breaths per minute.
- The product of these two gives the minute ventilation, a key measure of respiratory efficiency.
Respiratory Volumes and Key Structures
Respiratory Volumes
- Tidal Volume (TV): The amount of air that enters and leaves the respiratory system during a normal resting breath. In adults, this volume averages around 500 ml.
- Residual Volume (RV): The volume of air that remains in the lungs after a maximal exhalation. This air cannot be expelled and is essential for keeping the lungs inflated.
Larynx
The larynx serves two primary functions:
- Airway passage – directing air between the pharynx and the trachea.
- Voice production – the vocal cords (folds of mucous membrane) vibrate with exhaled air to produce sound.
- Location: Connects superiorly to the pharynx and inferiorly continues as the trachea.
- Structure: Built from cartilage (which forms the framework), muscles (that move the cartilages), blood vessels, and nerves.
- Mucous Membrane: Lines the internal cavity and protects underlying tissues.
Trachea
The trachea is a fibrous and cartilaginous tube, serving as the continuation of the larynx.
- Length: Varies by sex, age, and body size (average: 12 cm in men, 11 cm in women).
- Division: Splits into the right and left bronchi at the level of the 4th thoracic vertebra.
Bronchi
The bronchi are two large branches that extend from the trachea into the lungs.
- Orientation: Pass obliquely downward and outward to enter the hilum of each lung.
- Right bronchus: Shorter, wider, and more vertical than the left, making it more prone to foreign body aspiration.
- Layers:
- Outer fibrous-cartilaginous layer.
- Middle muscular layer.
- Inner mucosal lining.
Diaphragm
The diaphragm is a dome-shaped musculotendinous sheet that separates the thoracic cavity from the abdominal cavity.
- Function: Primary muscle of respiration, driving diaphragmatic breathing.
- Openings: Contains foramina and fissures through which anatomical structures pass between the thorax and the abdomen (e.g., esophagus, aorta, vena cava).
- Action: On contraction, it flattens and descends, enlarging the thoracic cavity and reducing intrathoracic pressure to draw air into the lungs.
Lungs and Pleura
Lungs
The lungs are two cone-shaped organs located in the pleural cavities, one on each side of the thorax.
- Right lung: Larger, heavier, and divided into three lobes – upper, middle, and lower.
- Left lung: Smaller (to accommodate the heart) and divided into two lobes – upper and lower.
Morphological features:
- Base: Rests on the diaphragm.
- Apex: Extends slightly above the first rib into the root of the neck.
- Surfaces:
- External (costal) surface lies against the ribs.
- Internal (mediastinal) surface contains the hilum, where structures enter and leave.
- Borders: Three borders define the shape – anterior, posterior, and inferior.
Hilum of the lung: Each lung has a hilum on its mediastinal surface where:
- The bronchus enters,
- The pulmonary artery and bronchial arteries bring blood in,
- The pulmonary veins and bronchial veins exit,
- Lymphatic vessels and nerves pass through.
Inside, the lungs consist of:
- The bronchial tree (progressive branching of bronchi into bronchioles),
- The pulmonary tissue,
- A network of blood vessels and nerves.
Anatomy of the Pleura
The pleura is a thin double-layered membrane that surrounds each lung.
- Visceral pleura: The inner layer, closely adherent to the lungs, covering their surfaces.
- Parietal pleura: The outer layer, lining the thoracic wall, diaphragm, and mediastinum.
Between the two lies the pleural cavity, containing a very small amount of pleural fluid (normally <1 ml).
- This fluid acts as a lubricant, reducing friction during breathing movements.
- It also helps create surface tension, keeping the lungs adhered to the thoracic wall during inspiration and expiration.