Anatomic and physiologic dead space

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Anatomic and physiologic dead space


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Anatomic and physiologic dead space

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USMLE® Step 1 style questions USMLE

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A 65-year-old man comes to the clinic for evaluation of chronic exercise intolerance. He has to stop and catch his breath after walking two blocks or climbing 2 flights of stairs. The patient has a 55-pack-year smoking history. He undergoes pulmonary function testing and the results are shown below. Which of the following best represents the total volume of gas in this patient’s airways that does not participate in gas exchange?  

Calculated tidal volume (VT)  
500 mL
Upper airway volume  
80 mL
Conducting airway volume  
100 mL  
Inspired CO2 pressure (PICO2)  
1 mmHg  
Arterial CO2 pressure (PACO2)  
50 mmHg  
Expired CO2 pressure (PT)  
20 mmHg  

External References

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Alveolar dead space p. 688

Anatomic dead space p. 688

Dead space (lung) p. 688

Physiologic dead space p. 688, 733


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 and chest muscles contract to pull open the chest and that sucks in air like a vacuum cleaner, and then during an exhale - the muscles relax, allowing the lungs to spring back to their normal size pushing that air out.

But as it turns out, not all the air that we breathe in, ends up participating in gas exchange.

As we inhale, air enters the nasal cavity or the oral cavity and travels down the trachea and then splits into the two mainstem bronchi and enters the two lungs.

Within the lungs, the bronchi divide into progressively smaller and smaller bronchioles until air gets down to tiny thin-walled air-sacs called alveoli which are surrounded by tiny capillaries. This is the site of gas exchange.

So, the part of the respiratory tree prior to these alveoli, starting from the nose, or the mouth, right up to the tiny terminal tiny bronchioles without these alveoli, merely acts to conduct or transport air to the alveoli. This part is known as the conducting zone and it does not take part in gas exchange.

The volume of air contained in this conducting zone is known as anatomic dead space.

‘Dead’ sounds kind of ominous but it basically reflects the fact that this air is as good as dead to the body, because you can’t extract oxygen from it.


  1. "Medical Physiology" Elsevier (2016)
  2. "Physiology" Elsevier (2017)
  3. "Human Anatomy & Physiology" Pearson (2018)
  4. "Principles of Anatomy and Physiology" Wiley (2014)
  5. "Relationship between the humidity and temperature of inspired gas and the function of the airway mucosa" Critical Care Medicine (1996)
  6. "An Algebraic Solution to Dead Space Determination According to Fowler's Graphical Method" Computers and Biomedical Research (1999)

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