## Dead Space and Its Effect on Alveolar Ventilation

Some of the air a person breathes never reaches the gas exchange areas but simply fills respiratory passages where gas exchange does not occur, such as the nose, pharynx, and trachea. This air is called dead space air because it is not useful for gas exchange.

On expiration, the air in the dead space is expired first, before any of the air from the alveoli reaches the atmosphere. Therefore, the dead space is very disadvantageous for removing the expiratory gases from the lungs.

Measurement of the Dead Space Volume. A simple method for measuring dead space volume is demonstrated by the graph in Figure 37-7. In making this measurement, the subject suddenly takes a deep breath of oxygen.This

Air expired (ml)

Figure 37-7

Record of the changes in nitrogen concentration in the expired air after a single previous inspiration of pure oxygen. This record can be used to calculate dead space, as discussed in the text.

Rate of Alveolar Ventilation fills the entire dead space with pure oxygen. Some oxygen also mixes with the alveolar air but does not completely replace this air. Then the person expires through a rapidly recording nitrogen meter, which makes the record shown in the figure. The first portion of the expired air comes from the dead space regions of the respiratory passageways, where the air has been completely replaced by oxygen. Therefore, in the early part of the record, only oxygen appears, and the nitrogen concentration is zero. Then, when alveolar air begins to reach the nitrogen meter, the nitrogen concentration rises rapidly, because alveolar air containing large amounts of nitrogen begins to mix with the dead space air. After still more air has been expired, all the dead space air has been washed from the passages, and only alveolar air remains. Therefore, the recorded nitrogen concentration reaches a plateau level equal to its concentration in the alveoli, as shown to the right in the figure. With a little thought, the student can see that the gray area represents the air that has no nitrogen in it; this area is a measure of the volume of dead space air. For exact quantification, the following equation is used:

Gray area x VE Pink area + Gray area where VD is dead space air and VE is the total volume of expired air.

Let us assume, for instance, that the gray area on the graph is 30 square centimeters, the pink area is 70 square centimeters, and the total volume expired is 500 milliliters. The dead space would be

+2 -1

## Essentials of Human Physiology

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### Responses

• Eleleta Tewelde
4 years ago
• eyob
How does dead space affect alveolar ventilation?
4 years ago
• demet
Why is dead space very disadvantageous for removing expiratory gases from lungs?
3 years ago
• Jeffery
What is the effects of increased anatomical dead space on alveolar ventilation?
3 years ago
• Tarja Pakarinen
How does anatomical dead space volume affect alveolar ventilation?
3 years ago
• Frank
How to calculate alveolar dead space and anatomic dead space?
3 years ago
• linda
Why is dead space a concern in medical?
2 years ago
• diana achen
What increasinging anatomical dead space does to ventilation efficiency?
5 months ago
• jouko
Why does pulmonary respiratory rate change with dead space?
4 months ago
• Susanne
What is the disadvantageous effect of the human pharynx?
4 months ago
• Ralph
Where does deadspace ventilation occur?
1 month ago
• ubalda
Is alveolar ventilation equal to dead space ventilation?
10 days ago