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Physiology Of The Respiratory System

Unlock the secrets of the human respiratory system and discover the fascinating inner workings that keep us breathing and alive.

Physiology of the Respiratory System


The respiratory system plays a vital role in the exchange of oxygen and carbon dioxide in the body. Understanding the physiology of the respiratory system is essential for medical professionals, especially for those preparing for the United States Medical Licensing Examination (USMLE). This guide aims to provide a comprehensive overview of the physiology of the respiratory system, focusing on key concepts and mechanisms.

1. Anatomy of the Respiratory System

The respiratory system consists of various organs, including the nasal cavity, pharynx, larynx, trachea, bronchi, and lungs. Understanding the anatomical structures and their functions is crucial to comprehend the physiology of respiration.

2. Pulmonary Ventilation

Pulmonary ventilation refers to the process of breathing, which involves inspiration (inhalation) and expiration (exhalation). The diaphragm and intercostal muscles play a crucial role in this process. Inspiration is an active process, while expiration is generally passive. The regulation of ventilation is controlled by the respiratory centers in the brain, mainly the medulla oblongata.

3. Gas Exchange

Gas exchange occurs in the lungs, specifically in the alveoli. Oxygen from inhaled air diffuses into the bloodstream, while carbon dioxide is eliminated from the blood into the alveoli to be exhaled. The process of diffusion is facilitated by differences in partial pressures between alveolar air and blood.

4. Oxygen Transport

The majority of oxygen in the blood is carried by hemoglobin molecules within red blood cells. Hemoglobin binds with oxygen in the lungs and releases it in tissues with lower oxygen concentrations. The oxygen-hemoglobin dissociation curve illustrates the relationship between oxygen saturation and partial pressure of oxygen.

5. Carbon Dioxide Transport

Carbon dioxide is transported in the blood in three forms: dissolved in plasma, as bicarbonate ions, and bound to hemoglobin. The majority of carbon dioxide is transported as bicarbonate ions, which are formed through the carbonic anhydrase reaction in red blood cells.

6. Control of Respiration

Respiration is regulated by various factors, including chemical, neural, and hormonal mechanisms. The medulla oblongata and pons in the brainstem play a significant role in controlling the rate and depth of breathing. Chemical factors such as carbon dioxide levels and pH in the blood influence the respiratory centers.

7. Pulmonary Circulation

The lungs have a unique circulatory system known as the pulmonary circulation. This system ensures that blood is properly oxygenated and carbon dioxide is eliminated. The pulmonary arteries carry deoxygenated blood from the heart to the lungs, while the pulmonary veins return oxygenated blood to the heart.

8. Lung Compliance and Elasticity

Lung compliance refers to the distensibility or stretchiness of the lungs, while elasticity refers to the ability of the lungs to recoil after stretching. These properties are crucial for efficient ventilation and gas exchange. Various factors, such as surfactant production and lung diseases, can affect lung compliance and elasticity.

9. Respiratory Disorders

Understanding the physiology of the respiratory system is essential for diagnosing and managing respiratory disorders. Common respiratory disorders include asthma, chronic obstructive pulmonary disease (COPD), pneumonia, and pulmonary embolism. Familiarity with the pathophysiology of these conditions is crucial for the USMLE.


A strong understanding of the physiology of the respiratory system is essential for medical professionals. This guide has provided an overview of key concepts related to the anatomy, ventilation, gas exchange, oxygen and carbon dioxide transport, regulation, pulmonary circulation, lung compliance, and common respiratory disorders. By studying and comprehending these concepts, medical professionals can effectively diagnose and manage respiratory conditions.

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