BIOA02H3 Lecture Notes - Mount Everest, Partial Pressure, Cellular Respiration
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48.1 What Physical Factors Govern Respiratory Gas
The respiratory gases that animals must exchange are oxygen (O2) and carbon dioxide
(CO2). Cells need to obtain O2 from the environment to produce an adequate supply of
ATP by cellular respiration. CO2 is an end product of cellular respiration, and it must be
removed from the body to prevent toxic effects.
Diffusion is the only means by which respiratory gases are exchanged between
the internal body fluids of an animal and the outside medium (air or water).
There are no active transport mechanisms to move respiratory gases across
biological membranes. Because diffusion is a physical process, knowing the physical
factors that influence rates of diffusion helps us understand the diverse adaptations of
gas exchange systems.
Diffusion is driven by concentration differences
Because diffusion results from the random motion of molecules, the net movement of a
molecule is always down its concentration gradient. One way biologists express the
concentrations of different gases in a mixture is by the partial pressures of those
gases. First, we have to know what the total pressure is, and we measure that with an
instrument called a barometer. At sea level, the pressure exerted by the atmosphere will
support, and therefore be equal to, a column of mercury in the tube that is about 760
mm high. Therefore, barometric pressure (atmospheric pressure) at sea level is 760
millimeters of mercury (mm Hg). Because dry air is 20.9% O2, the partial pressure of
oxygen (PO2) at sea level is 20.9% of 760 mm Hg.
Describing the concentration of respiratory gases in a liquid such as water is a little
more complicated because another factor is involved—the solubility of the gas in the
liquid. Thus, the actual amount of a gas in a liquid depends on the partial pressure of
that gas in the gas phase in contact with the liquid as well as on the solubility of that
gas in that liquid. However, the diffusion of the gas between the gaseous phase and the
liquid still depends on the partial pressures of the gas in the two phases.
Fick’s law applies to all systems of gas exchange
Diffusion is a physical phenomenon that can be described quantitatively with a simple
equation called Fick’s law of diffusion. All environmental variables that limit
respiratory gas exchange and all adaptations that maximize respiratory gas exchange
are reflected in one or more components of this equation. Fick’s law is written as
Q is the rate at which a gas diffuses between two locations.
D is the diffusion coefficient, which is a characteristic of the diffusing substance, the
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medium, and the temperature. (For example, perfume has a higher D than motor oil
vapor, and all substances diffuse faster at higher temperatures, as well as diffusing
faster in air than in water.)
A is the cross-sectional area through which the gas is diffusing.
P1 and P2 are the partial pressures of the gas at the two locations.
L is the path length, or distance, between the two locations.
Therefore, (P1 − P2)/L is a partial pressure gradient.
Animals can maximize D for respiratory gases by using air rather than water as their
gas exchange medium whenever possible; doing so greatly increases Q. All other
adaptations for maximizing respiratory gas exchange must influence the surface area
(A) for gas exchange or the partial pressure gradient [(P1 − P2)/L] across that surface
Air is a better respiratory medium than water
Oxygen can be obtained more easily from air than from water for several reasons:
The O2 content of air is much higher than the O2 content of an equal volume of water.
The maximum O2 content of a bubbling stream in equilibrium with air is less than 10 ml
of O2 per liter of water. The O2 content of the air over the stream is about 200 ml of O2
per liter of air.
Oxygen diffuses about 8,000 times more rapidly in air than in water. That is why the
O2 content of a stagnant pond can be zero only a few millimeters below the surface.
When an animal breathes, it does work to move water or air over its specialized gas
exchange surfaces. More energy is required to move water than to move air because
water is 800 times more dense than air and about 50 times more viscous.
The slow diffusion of O2 molecules in water affects air-breathing animals as well as
water-breathing ones. Eukaryotic cells carry out cellular respiration in their
mitochondria, which are located in the cytoplasm—an aqueous medium. Cells are
bathed in extracellular fluid—also an aqueous medium. In addition, all respiratory
surfaces must be protected from desiccation by a thin film of fluid through which O2
must diffuse. O2 must diffuse in water to reach cells. The slow rate of O2
diffusion in water limits the efficiency of O2 distribution from gas exchange
surfaces to the sites of cellular respiration even in air-breathing animals.
Diffusion of O2 in water is so slow that even animal cells with low rates of metabolism
can be no more than a couple of millimeters away from a good source of environmental
O2. Therefore, there are severe size and shape limits on the many species of
invertebrates that lack internal systems for distributing O2. Most of these species are
very small, but some have grown larger by evolving a flat, thin body with a large
external surface area. Still others have very thin bodies that are built around a central
cavity through which water circulates. A critical factor enabling larger, more complex
animal bodies has been the evolution of specialized respiratory systems with large
surface areas for enhancing respiratory gas exchange.
High temperatures create respiratory problems for aquatic
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Animals that breathe water are in a double bind when environmental temperatures rise.
Most water breathers are ectotherms—their body temperatures are closely tied to the
temperature of the water around them. As the temperature of the water gets warmer,
the ectotherm’s body temperature and metabolic rate rise. Thus, water breathers need
more O2 as the water gets warmer. But warm water holds less dissolved gas than cold
water does. In addition, if the animal performs work to move water across its gas
exchange surfaces (as fish do), the energy the animal must expend to breathe increases
as water temperature rises. Therefore, as water temperature goes up, the water
breather must extract more and more O2 from an environment that is increasingly O2
deficient, and a lower percentage of that O2 is available to support activities other than
O2 availability decreases with altitude
Just as a rise in temperature reduces the supply of O2 available to water breathers, an
increase in altitude reduces the O2 supply for air breathers. For example, at an altitude
of 5,800 m, barometric pressure is only half what it is at sea level, so the PO2 at that
altitude is only about 80 mm Hg (760/2 x 0.21). Since the movement of O2 across
respiratory gas exchange surfaces and into the body depends on diffusion, its rate of
movement depends on the PO2 difference between the air and the body fluids. Therefore,
the drastically reduced PO2 in the air at high altitudes constrains O2 uptake. Because of
these constraints, mountain climbers who venture to the heights of Mount Everest
usually breathe O2 from pressurized bottles.
CO2 is lost by diffusion
Respiratory gas exchange is a two-way process: CO2 diffuses out of the body as O2
diffuses in. The direction and rate of diffusion of the respiratory gases across the
respiratory exchange surfaces depend on the partial pressure gradients of the gases. The
partial pressure gradients of O2 and CO2 across these gas exchange surfaces are quite
different. The amount of CO2 in the atmosphere is extremely low (0.03%), so for air-
breathing animals there is always a large concentration gradient for diffusion of CO2
from the body to the environment. Whereas the partial pressure gradient for O2
decreases with altitude, the partial pressure gradient for CO2 does not. The
partial pressure of CO2 in the atmosphere is close to zero both at sea level and on top of
In general, getting rid of CO2 is not a problem for water-breathing animals
because CO2 is much more soluble in water than is O2. Even in stagnant water,
where the PCO2 is higher than in fresh water, the lack of O2 becomes a problem for the
animal long before CO2 exchange difficulties arise.
Respiratory gases are exchanged by diffusion only. Air is a better respiratory
medium than water because there is more O2 in a given volume of air than in
the same volume of water, O2 diffuses faster in air than in water, and it
requires less work to move air over respiratory exchange surfaces than water.
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