Pressure in the alveoli during inspiration/expiration

[ieatshrimp24]

So I understand that the pleural space is always negative during both inspiration and expiration. However, I am reading TPR Biology and it says that during inspiration the pressure in the alveoli becomes negative. At the end of expiration, the alveoli pressure is zero, and just before expiration the alveoli pressure is zero. Does the alveoli pressure ever become positive?

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[Czarcasm]

So I understand that the pleural space is always negative during both inspiration and expiration. However, I am reading TPR Biology and it says that during inspiration the pressure in the alveoli becomes negative. At the end of expiration, the alveoli pressure is zero, and just before expiration the alveoli pressure is zero. Does the alveoli pressure ever become positive?

We often refer to it as "negative pressure" as a consideration of the pressure differences of two components. However, you should realize that there is actually an associated pressure in each compartment:

STEP 1: Intrapulmonary Pressure = Atmospheric Pressure
During (Quiet) Inhalation:
Intrapleural Pressure: 756 mmHg
Intrapulmonary Pressure: 760 mmHg

STEP 2: Intrapulmonary Pressure becomes less than Atmospheric Pressure; Air flows in.
During (Quiet) Inhalation:
Intrapleural Pressure: 754 mmHg
Intrapulmonary Pressure: 759 mmHg

STEP 3: Intrapulmonary Pressure = Atmospheric Pressure
During (Quiet) Expiration:
Intrapleural Pressure: 754 mmHg
Intrapulmonary Pressure: 760 mmHg

STEP 4: Intrapulmonary Pressure becomes greater than Atmospheric Pressure; Air flows out.
During (Quiet) Expiration:
Intrapleural Pressure: 756 mmHg
Intrapulmonary Pressure: 761 mmHg

Note: Atmospheric Pressure is 760 mmHg (something you should know); Don't memorize the other values but focus on the understanding.