Respiratory system anatomy and physiology

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Respiratory system anatomy and physiology


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Respiratory system anatomy and physiology

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USMLE® Step 1 style questions USMLE

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A segment of lung tissue is examined. On the gross examination, the hilum of the lung is identified. Which of the following is true regarding the structures contained in the hilum of the lungs? 

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The main job of the lungs is gas exchange, pulling oxygen into the body and getting rid of carbon dioxide. Normally, during an inhale - the diaphragm contracts to pull downward and chest muscles contract to pull open the chest, which helps suck in air like a vacuum , and then during an exhale - the muscles relax, allowing the lungs to spring back to their normal size pushing that air out.

When you breathe in, air flows through the nostrils and enters the nasal cavity which is lined by cells that release mucus. That mucus is salty, sticky, and contains lysozymes, which are enzymes that help kill bacteria. Nose hairs at the entrance of the nasal cavity get coated with that mucus and are able to trap large particles of dust and pollen as well as bacteria, forming tiny clumps of boogers.

The nasal cavity is connected to four paranasal sinuses which are air-filled spaces inside the bones that surround the nose. There’s the frontal, ethmoid, sphenoid, and maxillary sinus. The paranasal sinuses help the inspired air to circulate for a bit so it has time to get warm and moist. The paranasal sinuses also act like tiny echo-chambers that help amplify the sound of your voice, which is why you sound so different when they’re clogged with mucus during a cold!

So the relatively clean, warm, and moist air goes from the nasal cavity into the pharynx or throat, the region connecting the two is called the nasopharynx, and the part connecting the pharynx to the oral cavity is called - you guessed it - the oropharynx. The soft palate, the softer portion of the roof of your mouth behind the hard part that you can feel with your tongue, and the pendulum-like uvula hanging at its end move together to form a flap or valve that closes the nasopharynx off when you eat to prevent food from going up into the nasopharynx. Finally, there’s the laryngopharynx, the part of the pharynx that’s continuous with the larynx or the voice box.


The respiratory system is the system of the body responsible for breathing, which is the process of taking in oxygen and expelling carbon dioxide. Structures of the respiratory system include the nose, paranasal sinuses, pharynx, larynx, trachea, bronchi, bronchioles, alveoli, pleura, and lungs. The lungs are the main organs of the respiratory system, are located in the thoracic cavity, and are protected by the rib cage. They are two: the right lung and the left lung, and are separated by the mediastinum.

The structure of the lungs is subdivided into a conducting zone and a respiratory zone. The respiratory and conducting zones have different functions and are, therefore, lined with different structures to perform these functions. Oxygen in the air is inhaled and makes its way through the pharynx, larynx, trachea, large upper airways, conducting bronchioles, respiratory bronchioles, the alveoli, and finally the capillaries to be sent to the body's tissue. Then Carbon dioxide makes the reverse journey to eventually be exhaled into the world.


  1. "Medical Physiology" Elsevier (2016)
  2. "Physiology" Elsevier (2017)
  3. "Human Anatomy & Physiology" Pearson (2018)
  4. "Principles of Anatomy and Physiology" Wiley (2014)
  5. "Lung Structure and the Intrinsic Challenges of Gas Exchange" Comprehensive Physiology (2016)
  6. "LUNG FUNCTION STUDIES. II. THE RESPIRATORY DEAD SPACE" American Journal of Physiology-Legacy Content (1948)

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