trouble understanding thoracic pressures

[zacarias, ASN, RN]

"Breathing consists of two phases, inspiration and expiration. During inspiration, the diaphragm and the intercostal muscles contract. The diaphragm moves downwards increasing the volume of the thoracic (chest) cavity, and the intercostal muscles pull the ribs up expanding the rib cage and further increasing this volume. This increase of volume lowers the air pressure in the alveoli to below atmospheric pressure. Because air always flows from a region of high pressure to a region of lower pressure, it rushes in through the respiratory tract and into the alveoli. This is called negative pressure breathing, changing the pressure inside the lungs relative to the pressure of the outside atmosphere. In contrast to inspiration, during expiration the diaphragm and intercostal muscles relax. This returns the thoracic cavity to it's original volume, increasing the air pressure in the lungs, and forcing the air out."

Does this mean that when you have inhaled and still have air in your chest that the atmospheric pressure and thoracic pressure are equal?

I'm just having trouble wrapping my brain around this concept. Any ideas?

[akulahawkRN, ADN, RN, EMT-P]

As long as the airway is open and patent, any time you stop generating airflow in or out, the pressures will be equal. Air "wants" to flow from high pressure to low pressure because it always wants to reach equilibrium. When you inhale, your muscles increase the intrathoracic volume, so the air pressure in there drops, in effect, you now have a little bit of a vacuum. The outside air rushes in. The expansion of the thorax also puts the tissues under tension. When you exhale, you relax your muscles and the tissues act like a spring or elastic band and they want to return to their relaxed state. As they "spring back" to their resting state, this increases intrathoracic pressure and the air wants to flow out to a lower pressure area, in this case, outside the body. Once all the tissues are relaxed and no longer under tension and airflow has ceased, the pressures in the entire airway circuit have once again reached an equilibrium with the atmosphere.

[nurse2033, MSN, RN]

akulahawk, yeah! Atmospheric pressure is equal at the end of expiration. Inhalation builds positive pressure that is released passively as the chest expansion naturally returns to baseline. Blow up a balloon and let it go. When it is limp again, this is equal pressure. So no, to answer your question, pressure is higher than atmosphere at the end of inspiration. You are primed balloon at this point. Release the breath and your chest relaxes, air rushes out, and until you begin to inspire again, you are in equilibrium with the atmosphere. Chest expansion, which takes energy, sucks air in. Natural recoil, which takes no energy, releases air.

[sharkdiver]

akulahawk, yeah! Atmospheric pressure is equal at the end of expiration. Inhalation builds positive pressure that is released passively as the chest expansion naturally returns to baseline. Blow up a balloon and let it go. When it is limp again, this is equal pressure. So no, to answer your question, pressure is higher than atmosphere at the end of inspiration. You are primed balloon at this point. Release the breath and your chest relaxes, air rushes out, and until you begin to inspire again, you are in equilibrium with the atmosphere. Chest expansion, which takes energy, sucks air in. Natural recoil, which takes no energy, releases air.

Actually, assuming that the airway is open, the pressures are equal at both the end of inspiration and end of expiration - simple law of physics...the airway is an orifice, and when there is no flow through an orifice the pressure on both sides has to be equal.

A balloon is not really a good example - a syringe would be a better one. When you draw out the plunger (inspiration) it creates a negative pressure and air flows into the syringe...when you stop drawing it out, the pressure equalizes and the airflow stops. When you push the plunger in (expiration) the internal pressure increases and air flows out of the syringe....when you stop pushing it in the pressure equalizes and the airflow stops.