Mailing List Archive Mailing List Archive
Gentlemen:
Recently I received a query (with about 20 pages of email from
this network address) about inherent unstauration, oxygen window,
thermodynamic descompression, and other comments form Hans Roverud.
The enclosure describes elements of the inherent unsaturation (biological)
and oxygen window (diving related to inherent unsaturation). I hope to answer
some more of the questions on thermodynamic decompression, bubbles,
and the relationship to phase separation in a later mailing -- but
let me point out here that the thermodynamic approach of taking up
the inherent unsaturation does not prevent bubble formation nor does
it eliminate later phase separation. Bubbles are an enigma all unto
themselves -- and must be coupled to dissolved phase transfer carefully.
The thermodynamic approach does not really take bubbles directly into
consideration. Similarly, the oxygen window and inherent unsaturation
can be related to diffusion gradients across bubble interfaces, as
well as their use in the original Hill's concept of phase equilibration
as a staging mechanism
Also, caution to those of you diving with the thermodynamic protocol.
Dropout at 30 $fsw$ (the tail of the Haldanian curve) can be hazardous
especially for shallow saturations -- recall that the Tektite experiments
in shallow saturation diving clearly indicate the need for decompression
for depths as shallow as 20 $fsw$.
So here, to add to the confusion, consider the following.
(ENCLOSURE)
GAS TRANSFER NETWORKS
The pulmonary and circulatory systems are connected gas transfer networks, as
Figure 6.1
suggests. Lung blood absorbs oxygen from inspired air in the alveoli (lung air
sacs), and releases carbon dioxide into the alveoli. The surface area for
exchange is enormous, on the order of a few hundred square meters. Nearly
constant values of alveolar partial pressures of oxygen and carbon dioxide
are maintained by the respiratory centers, with ventilated alveolar volume
near 4 $l$ in adults. The partial pressure of inspired oxygen is usually
higher than the partial pressure of tissue and blood oxygen, and the partial
pressure of inspired carbon dioxide less, balancing metabolic requirements
of the body.
Gas moves in direction of decreased concentration in any otherwise homogeneous
medium with uniform solubility. If there exist regions of varying solubility,
this is not necessarily true. For instance, in the body there are two
tissue types, one predominantly aqueous (watery) and the other (lipid),
varying in solubility by a factor of five for nitrogen. That is, nitrogen
is five times more soluble in lipid tissue than aqueous tissue. If aqueous
and lipid tissue are in nitrogen equilibrium, then a gaseous phses exists
in equilibrium with both. Both solutions are said to have a nitrogen tension
equal to the partial pressure of the nitrogen in the gaseous phase, with
the concentration of the dissolved gas in each species equal to the product
of the solubility times the tension according to Henry's law. If two nitrogen
solutions, one lipid and the other aqueous, are placed in contact, nitrogen
will diffuse towards the solution with decreased nitrogen tension. The
driving force for the transfer of any gas is the pressure gradient, whatever the
phases involved, liquid-to-liquid, gas-to-liquid, or gas-to-gas. Tensions and
partial
pressures have the same dimensions. The volume of gas that diffuses under any
gradient is a function of the interface area, solubility of the media, and
distance traversed. The rate at which a gas diffuses is inversely proportional
to the square root of its atomic weight. Following equalization, dissolved
volumes of gases depend upon their individual solubilities in the media.
Lipid and aqueous tissues in the body exhibit inert gas solubilities
differing by factors of roughly five, in addition to different uptake
and elimination rates. Near $standard$ temperature and pressure (32 $F sup o $,
and 1 $atm$), roughly 65% of dissolved nitrogen gas will reside in aqueous
tissues, and the remaining 35% in lipid tissues at equilibration, with the
total weight of dissolved nitrogen about .0035 $lb$ for a 150 $lb$ human.
The circulatory system, consisting of the heart, arteries, veins, and
lymphatics, convects blood throughout the body. Arterial blood leaves the
left heart via the aorta (2.5 $cm$), with successive branching of arteries
until it reaches arterioles (30 $microns$), and then systemic capillaries (8
$microns$)
in peripheral tissues. These capillaries join to form venules (20 $microns$),
which in turn connect with the vena cava (3 $cm$), which enters the right
heart. During return, venous blood velocities increase from 0.5 $cm/sec$ to
nearly 20 $cm/sec$. Blood leaves the right heart through the pulmonary
arteries on its way to the lungs. Following oxygenation in the lungs, blood
returns to the left heart through the pulmonary veins, beginning renewed
arterial circulation.
Blood has distinct components to accomplish many functions. Plasma is the
liquid part, carrying nutrients, dissolved gases (excepting oxygen), and
some chemicals, and makes up some 55% of blood by weight. Red blood cells
(erythrocytes) carry the other 45% by weight, and through the protein,
hemoglobin, transport oxygen to the tissues. Enzymes in red blood cells also
participate in a chemical reaction transforming carbon dioxide to a bicarbonate
in blood plasma. The average adult carries about 5 $l$ of blood, 30-35%
in the arterial circulation (pulmonary veins, left heart, and systemic
circulation),
and 60-65% in the venous flow (veins and right heart). About 9.5 $ml$ of
nitrogen are transported in each liter of blood. Arterial and venous tensions
of metabolic gases, such as oxygen and carbon dioxide differ, while blood and
tissue tensions of water vapor and nitrogen are the same. Oxygen tissue
tensions
are below both arterial and venous tensions, while carbon dioxide tissue
tensions exceed both. Arterial tensions equilibrate with alveolar (inspired
air) partial pressures in less than a minute. Such an arrangement of tensions
in the tissues and circulatory system provides the necessary pressure head
between alveolar capillaries of the lungs and systemic capillaries pervading
extracellular space.
Tissues and venous blood are typically unstaurated with respect to
inspired air and arterial tensions, somewhere in the vicinity of 8-13%
of ambient pressure. That is, summing up partial pressures of inspired gases
in air, total venous and tissue tensions fall short in that percentage range.
Carbon dioxide produced by metabolic processes is 25 times more
soluble than oxygen consumed, and hence exerts a lower partial pressure
by Henry's law. That tissue debt is called the biological $inherent$
$unsaturation$,
or $oxygen$ $window$, in diving applications
OXYGEN WINDOW
Inert gas transfer and coupled bubble growth are subtly influenced by
metabolic oxygen consumption. Consumption of oxygen and production of
carbon dioxide drops the tissue oxygen tension below its level in the lungs
(alveoli), while carbon dioxide tension rises only slightly because carbon
dioxide is 25 times more soluble than oxygen. Figure 6.2 compares the
partial pressures of oxygen, nitrogen, water vapor, and carbon dioxide
in dry air, alveolar air, arterial blood, venous blood, and tissue (cells).
Arterial and venous blood, and tissue, are clearly unsaturated with respect
to dry air at 1 $atm$. Water vapor content is constant, and carbon dioxide
variations are slight, though sufficient to establish an outgradient between
tissue and blood. Oxygen tensions in tissue and blood are considerably below
lung oxygen partial pressure, establishing the necessary ingradient for
oxygenation and metabolism. Experiments also suggest that the degree
of unsaturation increases linearily with pressure for constant composition
breathing mixture, and decreases linearily with mole fraction of inert gas
in the inspired mix. A rough measure of the inherent unsaturation, $ DELTA sub
u $,
is given as a function of ambient pressure, $P$, and mole fraction, $f sub { N
sub 2 } $,
of nitrogen in the air mixture, in $fsw$
DELTA sub u ~=~ ( ~ 1 ~-~ f sub {N sub 2 } ~ ) ~ P ~-~ 2.04 ~ f sub { N sub 2 }
~-~ 5.47 ~~.
Since the tissues are unsaturated with respect to ambient pressure at
equilibrium,
one might exploit this $window$ in bringing divers to the surface. By
scheduling the ascent strategically, so that nitrogen (or any other inert
breathing gas) supersaturation just takes up this unsaturation, the
total tissue tension can be kept equal to ambient pressure. This approach
to staging is called the zero supersaturation ascent.
Bruce Wienke brw@la*.go*
Catch you all later.
Navigate by Author:
[Previous]
[Next]
[Author Search Index]
Navigate by Subject:
[Previous]
[Next]
[Subject Search Index]
[Send Reply] [Send Message with New Topic]
[Search Selection] [Mailing List Home] [Home]