Difference Between Anatomical and Physiological Dead Space

The key difference between anatomical and physiological dead space is that the anatomical dead space refers to the volume of air that fills the conducting zone of respiration made up by the nose, trachea, and bronchi without penetrating the gas exchange regions of the lung. Meanwhile, the physiological dead space refers to the anatomical dead space together with the portion of the air that reaches gas exchange regions of the lung, but does not participate in gas exchange (alveolar dead space).

Lung dead space is the volume of ventilated air that does not undergo gas exchange. Thus, dead space is a portion of each tidal volume which does not participate in gas exchange. There are two ways to describe lung dead space. They are anatomical dead space and physiological dead space. Anatomic dead space describes the volume of air that does not penetrate gas exchange regions of the lung while physiological dead space describes the anatomical dead space plus the volume of air that penetrates gas exchange regions but does not undergo gas exchange.

In a healthy individual, both values are roughly equal. But under a disease condition, physiological dead space can be considerably larger than the anatomical dead space. Therefore, compared to anatomical dead space, physiological dead space is clinically significant.

CONTENTS

1. Overview and Key Difference
2. What is Anatomical Dead Space
3. What is Physiological Dead Space
4. Similarities Between Anatomical and Physiological Dead Space
5. Side by Side Comparison – Anatomical vs Physiological Dead Space in Tabular Form
6. Summary

What is Anatomical Dead Space?

Anatomical dead space is the volume of air contained within the conductive airways of the respiratory system. These parts are nose, trachea, and bronchi. This volume of air does not penetrate gas exchanging regions such as respiratory bronchioles, alveolar duct, alveolar sac, and alveoli. Hence, anatomical dead space does not participate in gas exchange.

From the normal tidal volume (500 mL), anatomical dead space occupies 30%. Therefore, the normal value ranges between 130 – 180 mL depending on the size and posture. The average value is 150 mL.

What is Physiological Dead Space?

Physiological dead space refers to the volume of air that fills conducting airways plus the volume of air that penetrates gas exchanging regions but does not involve in gas exchange. In simple words, physiological dead space is the combination of anatomical dead space and alveolar dead space. Therefore, physiological dead space is the sum of all parts of the tidal volume that do not participate in gas exchange.

Generally, in a healthy individual, alveolar dead space is negligible or zero. Thus, physiological dead space and anatomical dead space are equal. But under disease states, alveolar dead space has a value. Hence, physiological dead space becomes larger than the anatomical dead space. Compared to the anatomical dead space, physiological dead space is clinically important since it indicates the lung status.

What are the Similarities Between Anatomical and Physiological Dead Space?

• Anatomical and physiological dead space are two different ways to define lung dead space.
• Both represent the air that does not participate in gas exchange.
• In healthy individuals, the anatomical and physiological dead spaces are roughly equivalent.
• Combination of anatomical dead space and alveolar dead space give the physiological dead space.

What is the Difference Between Anatomical and Physiological Dead Space?

Anatomical dead space is the air-filled in conducting airways and does not participate in gas exchange. Meanwhile, physiological dead space is the sum of all parts of the tidal volume that does not participate in gas exchange. So, this is the key difference between anatomical and physiological dead space. The average value of anatomical dead space is 150 mL, while the normal value of physiological dead space is also150 mL. But, physiological dead space becomes larger under disease conditions.

Anatomical dead space does not include the air that penetrates into gas exchange regions. In contrast, physiological dead space includes the air that penetrates into gas exchange regions.  Therefore, this is another difference between anatomical and physiological dead space.

Summary – Anatomical vs Physiological Dead Space

Lung dead space is the portion of the tidal volume that does not participate in gas exchange. Anatomical dead space and physiological dead space are two ways of defining the lung dead space. Anatomical dead space is the volume of air that is in the conducting zone of the lung. Physiological dead space is the combination of anatomical dead space plus alveolar dead space. Alveolar dead space is the volume of air that fills the gas exchanging regions of the lung but does not participate in gas exchange. In a healthy individual, alveolar dead space is zero. Therefore, it indicates a disease condition. So, this summarizes the difference between anatomical and physiological dead space.

Reference:

1. Intagliata, Sal. “Physiology, Lung Dead Space.” StatPearls [Internet]., U.S. National Library of Medicine, 15 May 2019, Available here.
2. Robertson, H. Thomas. “Dead Space: the Physiology of Wasted Ventilation.” European Respiratory Society, European Respiratory Society, 1 June 2015, Available here.

Image Courtesy:

1. “Blausen 0865 TracheaAnatomy” By Blausen.com staff (2014). “Medical gallery of Blausen Medical 2014”. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. – Own work (CC BY 3.0) via Commons Wikimedia
2. “Lungvolumes Updated” By Kapwatt at English Wikipedia (CC BY-SA 3.0) via Commons Wikimedia