The Respiratory System

Overview

The respiratory system is responsible for the exchange of gases between the body and the environment. It consists of both upper and lower respiratory structures, each with specialized functions for air conduction, filtration, and gas exchange.

Functional Anatomy of the Respiratory System

Upper Respiratory System

The upper respiratory system includes the nose, nasal cavity, paranasal sinuses, and pharynx. These structures warm, humidify, and filter incoming air, providing protection and preparing air for entry into the lower respiratory tract.

• Nose: Provides an airway for respiration, moistens and warms air, filters and cleans incoming air, serves as a resonance chamber for speech, and houses olfactory receptors.

• Nasal Cavity: Divided by the nasal septum; lined with skin (anteriorly) and mucous membranes (posteriorly). Contains olfactory mucosa (for smell) and respiratory mucosa (with goblet cells for mucus production).

• Nasal Mucus: Contains lysozyme (defensive enzyme) and defensins (natural antibiotics).

• Nasal Conchae: Protrusions that increase mucosal surface area, enhancing air filtration, heating, and moistening.

• Paranasal Sinuses: Located in frontal, maxillary, sphenoid, and ethmoid bones; lighten the skull, warm and moisten air, and produce mucus.

• Pharynx: Connects nasal cavity and mouth to larynx and esophagus. Divided into:

  • Nasopharynx: Air passageway; contains pharyngeal tonsil for trapping pathogens.

  • Oropharynx: Passageway for air and food; houses palatine and lingual tonsils.

  • Laryngopharynx: Passageway for air and food; continuous with esophagus.

Table: The Upper Respiratory System

Lower Respiratory System

The lower respiratory system includes the larynx, trachea, bronchi, and lungs. It is divided into conducting and respiratory zones, facilitating air movement and gas exchange.

• Larynx: Attaches to hyoid bone and trachea; provides open airway, routes food and air, and produces sound via vocal cords. Composed of hyaline cartilages (thyroid, cricoid, arytenoid, corniculate, cuneiform) and elastic cartilage (epiglottis).

• Epiglottis: Closes off larynx during swallowing to prevent aspiration.

• Trachea: Descends from larynx; lined with ciliated pseudostratified epithelium to propel mucus upward. Supported by C-shaped cartilaginous rings; contains trachealis muscle for diameter regulation.

• Bronchi and Subdivisions: Conducting zone includes primary, secondary, and tertiary bronchi, branching into bronchioles. Respiratory zone begins at terminal bronchioles, leading to alveolar ducts and alveoli.

• Respiratory Membrane: Formed by type I alveolar cells and capillary endothelium; site of gas exchange. Type II alveolar cells secrete surfactant to reduce surface tension.

• Alveoli: Surrounded by elastic fibers, contain pores for air flow, and house macrophages for defense.

Table: The Lower Respiratory System

Lungs and Pleurae

Each lung occupies its own pleural cavity and is divided into lobes and bronchopulmonary segments. The pleurae are double-layered serous membranes that facilitate friction-free movement during breathing.

• Lungs: Right lung is larger than left; each lobe contains bronchopulmonary segments served by their own vessels and bronchi.

• Stroma: Elastic connective tissue supporting air spaces.

• Pleurae: Parietal pleura lines thoracic wall; visceral pleura covers lung surface. Pleural fluid lubricates and prevents friction.

• Pleural Cavities: Divide thoracic cavity into chambers, limiting organ interference and infection spread.

Respiratory Physiology

Pressure and Volume Changes in Breathing

Breathing is driven by changes in volume and pressure within the thoracic cavity, causing air to move in and out of the lungs.

• Respiratory Pressures: Measured relative to atmospheric pressure. Negative pressure indicates lower than atmospheric.

• Intrapulmonary Pressure: Pressure in alveoli; fluctuates during breathing, equalizes with atmospheric pressure.

• Intrapleural Pressure: Pressure in pleural cavity; always about 4 mm Hg less than intrapulmonary pressure due to opposing forces (lung recoil vs. chest wall expansion).

• Transpulmonary Pressure: Difference between intrapulmonary and intrapleural pressure; determines lung volume.

Formula:

Pulmonary Ventilation

Pulmonary ventilation is the mechanical process of moving air into and out of the lungs, governed by Boyle's law.

• Boyle's Law: At constant temperature, gas pressure varies inversely with volume.

• Quiet Inspiration: Diaphragm and intercostals contract, increasing thoracic volume and decreasing intrapulmonary pressure, causing air inflow.

• Forced Inspiration: Accessory muscles further increase thoracic volume.

• Quiet Expiration: Passive process relying on elastic recoil.

• Forced Expiration: Active process using abdominal muscles.

• Nonrespiratory Movements: Include coughing, sneezing, crying, laughing, hiccups, and yawning.

Physical Factors Influencing Pulmonary Ventilation

• Airway Resistance: Friction in airways; generally insignificant due to large diameter and numerous small airways.

• Alveolar Surface Tension: Water in alveoli draws walls together; surfactant reduces surface tension, preventing collapse.

Protective Mechanisms of the Respiratory System

• Mucus and Cilia: Trap and remove particles and pathogens.

• Tonsils: Trap and destroy airborne pathogens.

• Alveolar Macrophages: Remove debris and pathogens from alveoli.

• Surfactant: Prevents alveolar collapse by reducing surface tension.

Summary Table: Respiratory Passageway (Descending Order)

Additional info: The notes expand on brief points with academic context, including definitions, examples, and formulas. Tables are recreated for clarity and completeness.