Adv Physiol Educ 30: 30 –32, 2006;
doi:10.1152/advan.00046.2005.
How We Teach
Resorption of gas trapped in body cavities: comparison of alveolar and
pleural space with inner ear and paranasal sinuses
Zdravko Ivezić, Sven Kurbel, Sanda Škrinjarić-Cincar, and Radivoje Radić
Osijek Medical Faculty, University of Osijek, Osijek, Croatia
Submitted 29 July 2005; accepted in final form 5 October 2005
paranasal sinusitis; otitis media; closed pneumothorax; atelectasis; air
trapping
pain, because the surrounding pressure reduces cavity volume
and thus makes intracavitary pressure to be 760 mmHg. This
compression increases partial pressures in the cavity above
pressures in the venous blood. Reabsorption of gases continues
until no gas is left in the collapsible cavity. In closed bone
cavities, subatmospheric pressure develops due to partial resorption of oxygen by venous blood. Blood is forced to enter
mucosa by the subatmospheric pressure that sucks it inside.
Mucosal interstitial hydrostatic pressure is also more negative
than normal, and a painful mucosal edema develops. An added
volume of interstitial water in mucosal tissue together with
secretion of free fluid from the edematous mucosa into the
cavity reduces the negativity of the intracavitary pressure.
Further absorption of trapped gas will stop when the balance of
partial pressures with mucosal blood is once reached.
We can conclude that low total gas pressure in venous blood,
mainly due to tissue clearance of oxygen, is important in
resorption of entrapped air in body cavities.
TEXT FOR STUDENTS
our attempt to devise a short text aimed at
improving students’ understanding of gas resorption in body
cavities. The following text is given to medical students as
reading material for a discussion that is usually scheduled for
the next week. The students can use their textbooks (1, 2) or
other references (3). In most cases, this is the Croatian edition
of Medical Physiology by Guyton and Hall (2), used for
seminars.
During preparation of this text in English, to omit possible
differences among American (pressures in mmHg) and Croatian editions (pressures in kPa), all cited figures are taken from
the last international edition of textbook by Ganong (1).
Students are expected to understand the mechanisms behind
paranasal sinusitis, otitis media, closed pneumothorax, and
atelectasis of collapsed lung tissue, all used as examples. In
case of any doubt, they are encouraged to ask questions during
seminars.
On the basis of the interpretation that during pneumothorax
resorption, gases diffuse down pressure gradients into the
blood, students are encouraged to calculate tables of pressure
gradients (shaded columns of Tables 1-6) for the abovementioned pathological conditions.
After finishing the tables, students should understand that
due to tissue metabolism, venous blood is poor in oxygen and
rich in CO2, making the total pressure of all gases reduced (in
tables, 706 mmHg). In collapsible cavities (pleural cavity, lung
tissue, etc.), there is no subatmospheric pressure and almost no
THIS PAPER DESCRIBES
Address for reprint requests and other correspondence: S. Kurbel, Osijek
Medical Faculty, J Huttlera 4, 31000 Osijek, Croatia (e-mail: [email protected]).
30
Resorption of Gas Trapped in Body Cavities: How Does the
Body Get Rid of It?
Please use your textbooks and other sources. Pneumatized
body cavities can cause serious health problems if their connection with external air is somehow closed. A few conditions
may be familiar.
Paranasal sinusitis. In cases of paranasal sinusitis, the swollen mucosa can close the sinus opening, and soon pain develops, which can be relieved by forced allowance of some air to
enter the closed cavity. In most cases, a large portion of the
cavity is filled with fluid.
Otitis media. Middle ear infections, particularly in small
children, often close the Eustachian tube. Resorption of entrapped air reduces the pressure of gases in the middle ear to
subatmospheric levels and causes pain. Air pressure from
outside pushes the tympanic membrane inward.
Closed pneumothorax. In cases of pneumothorax, when
some air is allowed to enter the pleural space, the lung on the
affected side collapses, but when the hole is closed, the
entrapped air will be fully reabsorbed in following days. In the
first half of the twentieth century, a common practice in Europe
was to repeatedly inject air in the pleural space of patients with
pulmonary tuberculosis over weeks and months. The condition
of deliberately sustained lung collapse due to iatrogenic pneumothorax helped scarring of the diseased lung tissue.
Atelectasis. In cases of bronchial cancer, the primary tumor
can seal the involved bronchus. The entrapped air is fully
reabsorbed, and the condition of the collapsed lung tissue is
called atelectasis (see the computerized tomography scan of the
collapsed lung in Fig. 1).
1043-4046/06 $8.00 Copyright © 2006 The American Physiological Society
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Ivezić, Zdravko, Sven Kurbel, Sanda Škrinjarić-Cincar, and
Radivoje Radić. Resorption of gas trapped in body cavities: comparison
of alveolar and pleural space with inner ear and paranasal sinuses. Adv
Physiol Educ 30: 30 –32, 2006; doi:10.1152/advan.00046.2005.—This
paper describes our attempt to devise a short text aimed at improving
students’ understanding of gas resorption in body cavities. Students
are expected to understand the mechanisms behind paranasal sinusitis,
otitis media, closed pneumothorax, and atelectasis of collapsed lung
tissue, all used as examples. On the basis of the interpretation that
during pneumothorax resorption, gas diffuses down pressure gradients
into the blood, students are encouraged to calculate tables of pressure
gradients for the above-mentioned pathological conditions. After
answering a few questions, students need to analyze and eventually
accept the following conclusion: in cases of air trapping in collapsible
body cavities, all gases will be fully reabsorbed without pain. Air
trapping in bone cavities leads only to partial reabsorption of gases
and results in subatmospheric intracavity pressure. Partial vacuum
causes painful mucosal edema and free fluid secretion.
How We Teach
PLEASE SUPPLY
31
Table 2. Partial gas pressures in alveolar air and venous blood
Gradient Direction
Alveolar Gas
Venous Blood
Difference
To blood
To alveoli
40
46
47
573
706
40
46
47
573
706
0
0
0
0
0
No
No
No
No
No
No
No
No
No
No
PO2
PCO2
PH2O
PN2
Total
Values are partial pressures (in mmHg). No gradient exists.
The usual interpretation of the closed pneumothorax disappearance is as follows: “since gas is at atmospheric pressure, its
total pressure, PO2 and [partical pressure of nitrogen] PN2 are
greater than the corresponding values in venous blood (comTable 1. Partial gas pressures in alveolar air and venous blood
Table 3. Partial pressures of gases in alveolar
gas and venous blood
Alveolar Gas
Gradient Direction
PO2
PCO2
PH2O
PN2
Total
Alveolar Gas
Venous Blood
Difference
To blood
100
40
47
573
760
40
46
47
573
706
60
⫺6
0
0
54
Yes
No
No
Yes
To alveoli
Yes
No
No
Values are partial pressures (in mmHg). Values are for an unventilated lung
portion that will soon become atelechtatic.
PO2
PCO2
PH2O
PN2
Total
Gradient Direction
Pa
760/706 ⫻ Pa
Venous
Blood
40
46
47
573
706
43.06
49.52
50.60
616.83
760.00
40
46
47
573
706
Difference
To blood
3.06
3.52
3.60
43.83
54
Yes
Yes
Yes
Yes
Yes
To alveoli
Values are partial pressures (in mmHg). Pa, atmospheric pressure. The
correction due to external compression of alveolar gas is shown.
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Fig. 1. Computerized tomography scan of the collapsed lung portion (gray
areas) surrounded with normally ventilated lung tissue (dark areas). The
collapsed portion is compressed with the surrounding pressure, and inside
pressure equals atmospheric pressure.
pare values for air and venous blood). Gas diffuses down these
gradients into the blood, and after 1–2 weeks all of the gas
disappears.” (2)
We will try to test this interpretation on air trapped in an
unventilated lung portion that will soon become atelectatic (as
seen in Fig.1). Partial gas pressures in alveolar air and venous
blood are compared in Table 1.
Try to complete Table 1. Values for alveolar gas and venous
gas are taken from Ref. 2. Which gas(es) is trying to leave
alveoli and which is trying to leave blood? How do you explain
the subatmospheric total of only 706 mmHg for all gases in
venous blood? Is it important for the transport of gases? YES
or NO.
Why should we expect that all air is going to be reabsorbed,
according to the above-cited explanation? Discuss this with
your colleagues.
Because partial pressures of gases in the venous blood are
stable, soon the air in the closed lobule equals pressures in the
venous blood around the unventilated lobule.
Try to complete Table 2. Values are again taken from Ref. 2.
No gradient exists. Why should we still expect that all air is
going to be reabsorbed? Discuss this with your colleagues.
As shown in Fig. 1, the atelectatic lobule is surrounded by
normal lung tissue filled with alveolar air that comes through
normal airways. In them, pressure is near the outside pressure
of 760 mmHg. The sealed lobule is, because of that, compressed with surrounding ventilated lung tissue, and it collapses. Its inside pressure now equals pressures in the surrounding tissue (760 instead of 706 mmHg). We can correct
partial pressures by multiplying them with the ratio 760/706.
Why?
Try to complete Table 3. The uncorrected values are again
taken from Ref. 2.
Which gas(es) is trying to leave alveoli and which is trying
to leave blood?
How We Teach
32
PLEASE SUPPLY
Table 4. Partial gas pressures for the closed pneumothorax
Table 6. Gas dynamics due to paranasal sinusitis
Gradient Direction
Initial Content of
Pneumothorax Gas
Venous
Blood
Difference
To blood
158
0.3
5.7
596
760
40
46
47
573
706
118
⫺45.7
⫺41.3
23
54
Yes
PO2
PCO2
P H 2O
PN 2
Total
Gradient Direction
To pleural
cavity
Yes
Yes
Yes
Yes
Initial Content of
Intracavitary Gas
Venous
Blood
Difference
To blood
158
0.3
5.7
596
760
40
46
47
573
706
118
⫺45.7
⫺41.3
23
54
Yes
PO2
PCO2
PH2O
PN2
Total
To cavity
Yes
Yes
Yes
Yes
Values are partial pressures (in mmHg).
Values are partial pressures (in mmHg). Pneumothorax gas, air trapped in
the pleural space.
Table 5. Partial gas pressures for the closed pneumothorax
Pneumothorax
PO2
PCO2
PH2O
PN2
Total
Gradient Direction
Pp
ⱕ760/706 ⫻ Pp
Venous
Blood
40
46
47
573
706
ⱕ43.06
ⱕ49.52
ⱕ50.60
ⱕ616.83
ⱕ760.00
40
46
47
573
706
Difference
To blood
ⱕ3.06
ⱕ3.53
ⱕ3.60
ⱕ43.83
ⱕ54
Yes
Yes
Yes
Yes
Yes
To alveoli
Values are partial pressures (in mmHg). Pp, pleural pressure. The correction
due to external compression of the pneumothorax is shown.
REFERENCES
1. Ganong WF. Review of Medical Physiology (21th ed.). Stamford, CT:
Appleton & Lange, 2005.
2. Guyton AC and Hall JE. Medical Physiology. Philadelphia, PA: Saunders,
2000, p. 660.
3. Richardson DR, Randall DC, and Dexter DF. Cardiopulmonary System.
Madison, CT: Fence Creek, 1998.
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Do you find the subatmospheric total of only 706 mmHg for
all gases in venous blood important for the transport of gases?
YES or NO and WHY?
Now complete Tables 4 and 5 for the closed pneumothorax
caused by instillation of outside air in the pleural cavity. As
mentioned before, it was used 50 years ago as a physical
measure that helped in the treatment of lung tuberculosis.
Air trapped in the pleural space (pneumothorax gas in our
tables) is compressed with atmospheric pressure coming from
the collapsed lung because it is still connected with outside air
through the unobstructed bronchial tree. The actual pressure
transferred from the collapsed lung is reduced by the force of
lung tissue elastic recoils. So, in the closed pneumothorax, total
pressure in the pleural cavity is ⬍760 but more than 706
mmHg.
The pneumothorax can be filled with external air as well
with alveolar gas. Which one is supposed to be reabsorbed
faster? Look for the inspired air values in Ref. 2. They equal
outside air pressures.
Now complete Table 6, which describes gas dynamic during
paranasal sinusitis, or otitis media. In both cases, inflamation
seals the entrance of air to the bone cavity. Values are again
taken from Ref. 2. Intracavitary gas in the open cavity equals
inspired air.
Bone cavities are incompressible, so, theoretically, after a
balance of pressures with venous blood is achieved, the total
pressure of gases in them can be 54 mmHg less than the outside
air pressure. This never happens because the subatmospheric
pressure alters Starling forces in the mucosa that cover cavity
surfaces. Blood is forced to enter the mucosa by the subatmospheric pressure that sucks it inside. Mucosal interstitial hydrostatic pressure is also more negative than normal, and
painful mucosal edema develops. The added volume of interstitial water in mucosal tissue together with secretion of free
fluid from the edematous mucosa into the cavity reduces the
negativity of the intracavitary pressure. Further absorption of
trapped gas will stop when the balance of partial pressures with
mucosal blood is once reached.
Does this make sense to you? YES or NO.
If YES, can you accept the following conclusion:
In cases of air trapping in collapsible body cavities, all gases
will be fully reabsorbed without pain. Air trapping in bone
cavities leads only to partial reabsorption of gases and results
in subatmospheric intracavitary pressure. Partial vacuum
causes painful mucosal edema and free fluid secretion.