The Respiratory System: Structure, Function, and Physiology

Lower Respiratory Tract

Larynx

The larynx is a cartilaginous structure in the lower respiratory tract that regulates the amount of air entering and leaving the lungs. It contains the epiglottis and glottis, which includes the vestibular folds, vocal cords, and the space between them. The epithelial lining transitions from stratified squamous epithelium to pseudostratified ciliated columnar epithelium with goblet cells.

• Epiglottis: Prevents food from entering the airway during swallowing.

• Glottis: Involved in sound production and airway protection.

• Vocal cords: Vibrate to produce sound; pitch depends on diameter, length, and tension.

• Transition of epithelium: Protects against abrasion and facilitates mucus production.

Example: The larynx is commonly referred to as the "voice box" due to its role in phonation.

Trachea

The trachea is a tube supported by 16-20 C-shaped pieces of hyaline cartilage connected by dense connective tissue (DCT). It provides protection, prevents collapse or overexpansion, and connects the larynx to the bronchi.

• Trachealis muscle and elastic connective tissue: Form a fibroelastic membrane for flexibility.

• Cartilage rings: Keep the airway open during breathing.

Example: The trachea is often called the "windpipe."

Bronchial Tree

Structure and Branching

The bronchial tree begins with the right and left primary bronchi, which split at the carina and enter the lungs at the hilum. Each primary bronchus divides into secondary bronchi (one per lung lobe), which further split into tertiary bronchi and eventually into bronchioles.

• Carina: Contains nerve tissue that triggers coughing if a foreign body is present.

• Bronchioles: Lack cartilage, composed mainly of smooth muscle.

• Branching:

  • Intralobular bronchioles: Small branches of tertiary bronchi.

  • Terminal bronchioles: Branches off intralobular bronchioles.

  • Respiratory bronchioles: Lead to alveolar ducts and terminate in alveolar sacs.

Example: The bronchial tree resembles an upside-down tree, with branches becoming progressively smaller.

Defense System of Conducting Zone

Mucosa and Mucus

The conducting zone is lined with respiratory epithelium containing goblet cells that produce mucus. Mucus traps particles and pathogens, while cilia move the mucus toward the pharynx for removal.

• Mucus: Sticky secretion that traps bacteria, fungi, and viruses.

• Cilia: Beat in one direction to move mucus out of the lungs or down from the nasal cavity.

• Smoking: Can paralyze cilia, leading to "smoker's cough."

• Stomach acid: Neutralizes pathogens if mucus is swallowed.

Example: The mucociliary escalator is a key defense mechanism in the respiratory tract.

Cystic Fibrosis

Cystic fibrosis is the most common lethal inherited disease among Caucasians of Northern European descent. It is characterized by the production of overly thick mucus, which the respiratory system cannot transport efficiently.

• Incidence: 1 in 2,500 births among Caucasians; less common in other ethnic groups.

• Symptoms: Thick mucus restricts airflow and traps bacteria, leading to infections.

• Prognosis: Median predicted lifespan is mid-30s.

Example: Patients with cystic fibrosis often experience chronic respiratory infections.

More Defense Systems

Conditioning the Air

The conducting zone also conditions incoming air. The lamina propria in the nasal cavity contains expandable veins that warm the air, causing water in mucus to evaporate and humidify the air before it reaches the lungs.

• Air temperature: Air is nearly body temperature and saturated with water before reaching the lungs.

• Reverse process: On exhalation, nasal mucosa reabsorbs heat and water, preventing loss.

• Inefficiency: Breathing through the mouth is less efficient for conditioning air.

Example: Nasal breathing is preferred for optimal air conditioning.

Respiratory Zone

Alveoli and Gas Exchange

The respiratory zone is the site of gas exchange and includes alveolar ducts and alveoli. Each bronchiole leads to an alveolar duct, which opens into clusters of alveoli.

• Alveoli: Air sacs for gas exchange; elastic walls stretch during air intake to increase surface area.

• Alveolar pores: Connect neighboring alveoli to equalize air pressure.

• Cell types:

  • Type I cells: Permeable to gases, site for diffusion.

  • Type II cells: Secrete surfactant, reducing surface tension and helping alveoli stay open.

  • Macrophages: Remove debris and pathogens.

• Respiratory membrane: Very thin barrier formed by alveolar and capillary membranes, site of gas exchange.

Example: Surfactant deficiency in premature infants can lead to respiratory distress syndrome.

Respiratory Physiology

External Respiration

External respiration is the exchange of carbon dioxide (CO2) and oxygen (O2) between blood, lungs, and the environment. It involves pulmonary ventilation and gas diffusion.

• Breakdown in external respiration: Can lead to hypoxia (low O2 levels) or anoxia (no O2 flow to tissue).

• Anoxia: Causes most damage in stroke or heart attack.

Example: Oxygen therapy is used to treat hypoxia in clinical settings.

Air Flow and Pressure

Air flows from areas of high pressure to low pressure. Atmospheric pressure at sea level is 760 mm Hg (1 atmosphere). Air pressure is analogous to the concentration of gas molecules, and air molecules move to equalize pressure.

• Pressure-volume relationship: As volume increases, pressure decreases, and vice versa.

Equation:

$P_1 V_1 = P_2 V_2$

Example: During inhalation, the thoracic cavity expands, decreasing pressure and drawing air into the lungs.

Pleural Membranes and Pressure Control

The lungs are surrounded by the parietal pleura (lining the thoracic cavity) and visceral pleura (covering the lungs), with pleural fluid in between. Pressure is controlled by the size of the thoracic cavity.

• At rest: Pressure inside and outside the cavity are equal; no air movement occurs.

• Pleural fluid: Bonds within pleural fluid keep lungs attached to thoracic walls, preventing collapse and allowing expansion.

Example: Pneumothorax (collapsed lung) occurs when air enters the pleural cavity, disrupting pressure balance.

Table: Bronchial Tree Branching

Branch Level	Structure	Cartilage	Function
Primary Bronchi	Right & Left Bronchi	Present	Conduct air to lungs
Secondary Bronchi	One per lung lobe	Present	Conduct air to lung lobes
Tertiary Bronchi	Branches of secondary	Present	Conduct air to bronchopulmonary segments
Bronchioles	Small branches	Absent	Regulate airflow, no cartilage
Terminal Bronchioles	End branches	Absent	Conduct air to respiratory bronchioles
Respiratory Bronchioles	Lead to alveolar ducts	Absent	Site of some gas exchange
Alveolar Ducts/Sacs	Clusters of alveoli	Absent	Main site of gas exchange

Additional info: Academic context was added to clarify the structure and function of the respiratory system, including definitions, examples, and a summary table for bronchial tree branching.