The Respiratory System: Structure, Function, and Physiology

Lower Respiratory Tract

Larynx

The larynx is a cartilaginous structure that plays a crucial role in regulating airflow to the lungs and in sound production. It contains the epiglottis and glottis, which includes the vestibular folds, vocal cords, and the space between them.

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

• Glottis: Composed of vestibular folds and vocal cords; the space between is essential for phonation.

• Epithelium: Transitions from stratified squamous to pseudostratified ciliated columnar with goblet cells, providing protection and mucus secretion.

Sound Production (Phonation)

The larynx is responsible for producing sound as air passes through and vibrates the vocal cords.

• Pitch: Determined by the diameter, length, and tension of the vocal cords.

• Tension: Can be altered by the positioning of the arytenoid cartilages.

Trachea

The trachea is a tube supported by 16-20 C-shaped pieces of hyaline cartilage connected by dense connective tissue (DCT).

• Function: Provides protection, prevents collapse or overexpansion of the airway.

• Trachealis muscle and elastic connective tissue: Form a fibroelastic membrane that connects the opening of the C-shapes and allows for some expansion/constriction.

Bronchial Tree

Branching Structure

The bronchial tree begins with the right and left primary bronchi, which split at the carina and enter the lungs at the hilum.

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

• Branching: Each primary bronchus splits into secondary bronchi (one per lung lobe), which further divide into tertiary bronchi, then into bronchioles (no cartilage, just muscle).

• Bronchioles: Include intralobular (small branches of tertiary), terminal (branches off intralobular), and respiratory bronchioles (lead to alveolar ducts and terminate in alveolar sacs).

Defense System of Conducting Zone

Mucosa and Mucus

The conducting zone is lined with respiratory epithelium containing goblet cells that produce mucus, which traps particles and pathogens.

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

• Cilia: Beat in one direction (towards the pharynx) to move mucus out of the respiratory tract. Smoking can paralyze cilia, leading to smoker's cough.

• Removal: Mucus is either coughed out or swallowed, where stomach acid neutralizes pathogens.

Cystic Fibrosis

Cystic fibrosis is the most common lethal inherited disease among Caucasians of Northern European descent, characterized by the production of overly thick mucus.

• 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.

More Defense Systems

Conditioning the Air

The conducting zone also conditions incoming air by warming and humidifying it.

• Lamina propria: Contains expandable veins that heat air.

• Humidification: As air warms, water in mucus evaporates, humidifying the air.

• Efficiency: Air is nearly body temperature and saturated with water before reaching the lungs; nasal mucosa reabsorbs heat and water on exhalation, preventing loss.

• Note: Mouth breathing is less efficient for conditioning air.

Respiratory Zone

Alveoli and Gas Exchange

The respiratory zone is the site of gas exchange and includes alveolar ducts and alveoli.

• Alveoli: Air sacs with elastic walls that 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: Formed by alveolar and capillary membranes; very thin to allow efficient gas exchange.

Respiratory Physiology

External Respiration

External respiration is the exchange of carbon dioxide (CO2) and oxygen (O2) between blood, lungs, and the environment.

• Pulmonary ventilation: Movement of air into and out of the lungs.

• Gas diffusion: Movement of gases across the respiratory membrane.

• Hypoxia: Low O2 levels; anoxia is the absence of O2 flow to tissues, causing damage in stroke or heart attack.

Air Flow and Pressure

Air flows from regions of high pressure to regions of low pressure. This principle is essential for pulmonary ventilation.

• Atmospheric pressure: Pressure of the air around us; at sea level, $760\ \text{mm Hg} = 760\ \text{torr} = 1\ \text{atmosphere}$.

• Air pressure: Analogous to the concentration of gas molecules; molecules move from high to low pressure.

• Volume and Pressure Relationship: As volume increases, pressure decreases, and vice versa (Boyle's Law): $P_1V_1 = P_2V_2$

Pleural Membranes and Pressure Regulation

The lungs are surrounded by pleural membranes that help regulate pressure and prevent collapse.

• Parietal pleura: Lines the thoracic cavity.

• Visceral pleura: Covers the lungs.

• Pleural fluid: Bonds within the fluid cause lungs to adhere to thoracic walls, preventing collapse and allowing expansion.

• Pressure equilibrium: At the beginning of a breath, pressure inside and outside the cavity are equal, so no air movement occurs. $P_{outside} = P_{inside}$

Table: Bronchial Tree Branching

Branch Level	Features
Primary Bronchi	Enter lungs at hilum; split at carina
Secondary Bronchi	One per lung lobe
Tertiary Bronchi	Branch into bronchioles; no cartilage, just muscle
Intralobular Bronchioles	Small branches of tertiary bronchi
Terminal Bronchioles	Branches off intralobular bronchioles
Respiratory Bronchioles	Lead to alveolar ducts; terminate in alveolar sacs

Example: Boyle's Law in Breathing

During inhalation, the thoracic cavity expands, increasing lung volume and decreasing pressure, causing air to flow into the lungs. During exhalation, the cavity contracts, decreasing volume and increasing pressure, pushing air out.

Equation: $P_1V_1 = P_2V_2$