Introduction

The respiratory system is essential for oxygen delivery and carbon dioxide removal, processes crucial for cellular function and energy production. It is divided into two main components:

1. Upper respiratory tract: Manages airflow and initial air filtration.
2. Lower respiratory tract: Facilitates gas exchange in the lungs.

Anatomy of the Respiratory System

1. Upper Respiratory Tract

The upper respiratory tract includes:

• Nose and nasal cavity: Filters, warms, and moistens inhaled air.
• Mouth: Secondary entry point for air.
• Pharynx (throat): A passage for air and food.
• Larynx (voice box): Protects the trachea and houses vocal cords.
• Trachea (windpipe): Directs air to the lungs.

2. Lower Respiratory Tract

The lower respiratory tract consists of:

• Bronchi and bronchioles: Air passageways that branch into smaller structures.
• Alveoli: Tiny air sacs where gas exchange occurs.
• Lungs: The primary organ of the respiratory system, divided into lobes (3 in the right lung, 2 in the left lung) and surrounded by the pleura.

3. Pathway of Air

Airflow sequence:

1. Enters through the nose or mouth.
2. Passes through the pharynx and larynx.
3. Travels down the trachea to the main bronchi.
4. Divides into smaller bronchi, bronchioles, and finally into alveoli for gas exchange.

Tracheobronchial Tree

• The tracheobronchial tree branches 23 times, from the trachea (generation 0) to the alveolar sacs (generation 23).
• Conducting airways (generations 0–16) form the dead space volume (~150 mL), which does not participate in gas exchange.
• Respiratory airways (generations 17–23) form the acinus, the functional unit of the lungs where gas exchange occurs.

Respiratory Physiology

Ventilation

• Tidal volume (VT): Normal air volume moved per breath (~4–8 mL/kg).
• Alveolar ventilation: The portion of ventilation participating in gas exchange, matching cardiac output (~5 L/min).

Lung Volumes

• Vital capacity (VC): Max air exhaled after a deep breath (~4–5 L).
• Residual volume (RV): Air remaining in lungs post-max exhalation, preventing alveolar collapse.

Lung Compliance

• Indicates lung elasticity and ability to expand:
  • Normal compliance: 0.2–0.3 L/cm H2O.
  • Reduced compliance at extremes of functional residual capacity (FRC).

Gas Exchange

• Occurs in the alveoli, where:
  • Oxygen diffuses into the blood.
  • Carbon dioxide diffuses out of the blood.

Specialised Structures

Trachea

• Length: ~11–13 cm.
• Composition: 16–22 cartilage rings and smooth muscle.
• Bifurcates at the carina (T4-T5 level) into the right and left main bronchi.

Bronchi

• Right main bronchus: Wider and more vertical, making it prone to foreign body aspiration.
• Segmental bronchi serve bronchopulmonary segments, the functional subdivisions of lung lobes.

Clinical Relevance

1. Pharmacological Action:
   • Bronchodilators: Target smooth muscle in bronchioles to relieve obstruction.
   • Anti-inflammatory agents: Reduce swelling and mucus production in airways.
2. Lung Function Testing:
   • Spirometry: Measures vital capacity and airflow.
   • Arterial blood gases (ABG): Assesses oxygenation and ventilation.
3. Diseases:
   • Asthma: Affects bronchioles, causing airflow obstruction.
   • Chronic Obstructive Pulmonary Disease (COPD): Results in reduced compliance and impaired gas exchange.

Summary

The respiratory system’s anatomy and physiology are foundational for understanding respiratory health and disease. This knowledge enables healthcare professionals to manage respiratory conditions effectively and optimise pharmacological treatments.

References

1. Cleveland Clinic. (n.d.). Respiratory System: Function and Anatomy. Retrieved from https://my.clevelandclinic.org/health/body/21205-respiratory-system

2. University of Rochester Medical Center. (n.d.). The Respiratory System. Retrieved from https://www.urmc.rochester.edu/encyclopedia/content?contentid=p01300&contenttypeid=85

3. West, J. B. (2015). The remarkable respiratory system: Adaptations for life in an oxygen-limited world. American Journal of Physiology-Lung Cellular and Molecular Physiology, 309(6), L499-L516. Retrieved from https://pmc.ncbi.nlm.nih.gov/articles/PMC4613399/

4. Cleveland Clinic. (n.d.). Lungs: Function and Anatomy. Retrieved from https://my.clevelandclinic.org/health/body/8960-lungs

5. Schmitt, A., & Picard, F. (2023). Lymphatic function in the lungs: Implications for health and disease. Journal of Pulmonary and Respiratory Medicine, 27(3), 156-168. Retrieved from https://pmc.ncbi.nlm.nih.gov/articles/PMC10043242/#:~:text=Initial%20lymphatics%20in%20the%20lung,returned%20to%20the%20blood%20circulation.