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Respiratory System

Discover the intricate workings of the human respiratory system, its vital role in our overall health and the surprising ways it adapts to various environments.

USMLE Guide: Respiratory System


The respiratory system is a vital organ system responsible for gas exchange in the human body. It consists of several organs and structures that work together to ensure efficient oxygen intake and carbon dioxide elimination. Understanding the respiratory system is crucial for medical professionals, and it is an important topic to study for the United States Medical Licensing Examination (USMLE). This guide provides a comprehensive overview of key concepts related to the respiratory system to help you prepare for the USMLE.

Anatomy of the Respiratory System

Nasal Cavity

  • Functions: Filters, warms, and humidifies incoming air.
  • Important Structures: Nasal conchae, nasal septum, olfactory epithelium.


  • Functions: Acts as a common pathway for air and food.
  • Important Structures: Nasopharynx, oropharynx, laryngopharynx.


  • Functions: Protects the lower respiratory tract, houses vocal cords.
  • Important Structures: Epiglottis, thyroid cartilage, cricoid cartilage.


  • Functions: Conducts air to and from the lungs.
  • Important Structures: C-shaped tracheal cartilages, trachealis muscle.

Bronchial Tree

  • Functions: Distributes air to the lungs.
  • Important Structures: Primary bronchi, secondary bronchi, tertiary bronchi, bronchioles.


  • Functions: Facilitate gas exchange.
  • Important Structures: Lobes (right lung: 3 lobes, left lung: 2 lobes), pleura, alveoli.

Physiology of the Respiratory System

Pulmonary Ventilation

  • Definition: The process of moving air in and out of the lungs.
  • Key Concepts:
    • Inspiration: Active process involving diaphragm contraction and external intercostal muscles.
    • Expiration: Passive process relying on elastic recoil of the lungs and relaxation of inspiratory muscles.
    • Control of Ventilation: Regulated by respiratory centers in the brainstem (medulla and pons).

Gas Exchange

  • Definition: The transfer of oxygen from the lungs to the bloodstream and carbon dioxide from the bloodstream to the lungs.
  • Key Concepts:
    • External Respiration: Exchange of gases between alveoli and pulmonary capillaries.
    • Internal Respiration: Exchange of gases between systemic capillaries and body tissues.
    • Factors Affecting Gas Exchange: Partial pressure gradients, diffusion distance, surface area, and ventilation-perfusion matching.

Transport of Respiratory Gases

  • Oxygen Transport:
    • Majority bound to hemoglobin (oxyhemoglobin) in red blood cells.
    • A small fraction dissolved in plasma.
  • Carbon Dioxide Transport:
    • Dissolved in plasma.
    • Bound to hemoglobin.
    • As bicarbonate ions (HCO3-) in the plasma.

Regulation of Respiration

  • Key Concepts:
    • Chemoreceptors: Central and peripheral chemoreceptors monitor blood pH, carbon dioxide, and oxygen levels to regulate respiration.
    • Respiratory Centers: medulla oblongata and pons control the rate and depth of breathing.
    • Factors Affecting Respiration: Oxygen levels, carbon dioxide levels, and pH.

Common Respiratory Disorders


  • Definition: Chronic inflammatory disorder characterized by airway hyperresponsiveness, bronchoconstriction, and reversible airflow limitation.
  • Clinical Features: Wheezing, dyspnea, chest tightness, cough.
  • Treatment: Bronchodilators (short-acting β2 agonists), inhaled corticosteroids, leukotriene modifiers.

Chronic Obstructive Pulmonary Disease (COPD)

  • Definition: Progressive lung disease causing airflow limitation due to chronic bronchitis and/or emphysema.
  • Clinical Features: Dyspnea, chronic cough, sputum production, decreased exercise tolerance.
  • Treatment: Smoking cessation, bronchodilators, inhaled corticosteroids, oxygen therapy.


  • Definition: Infection of the lung parenchyma typically caused by bacteria, viruses, or fungi.
  • Clinical Features: Fever, productive cough, chest pain, dyspnea.
  • Treatment: Antibiotics (bacterial pneumonia), antivirals (viral pneumonia), antifungals (fungal pneumonia).

Pulmonary Embolism

  • Definition: Blockage of a pulmonary artery by a blood clot or other substances.
  • Clinical Features: Sudden onset dyspnea, pleuritic chest pain, tachycardia, hemoptysis.
  • Treatment: Anticoagulation therapy (heparin, warfarin), thrombolytic therapy (severe cases).


Understanding the respiratory system is crucial for medical

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