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External Gas Transport
Chapters 20 & 21
Respiration
The process of acquiring oxygen and
releasing carbon dioxide
Why do animals require oxygen?
• Aerobic respiration
– C6H12O6 + 6O2  6CO2 + 6H2O
– Enough Energy for 30 ATP
• Anaerobic respiration
– C6H12O6  2C3H6O3
– Enough Energy for 2 ATP
• All known metazoans require oxygen
Why do animals need to release
carbon dioxide?
• CO2 tends to react with water
CO2 + H2O  H2CO3  H+ + HCO3-  2H+ + CO3-
•  [H+],  pH
• Low pH’s can disrupt physiological
function (e.g., enzyme activity)
Air
• Composition (Dry Air):
Gas
Oxygen
Carbon dioxide
Nitrogen
Argon
%
20.95
0.03
78.09
0.93
– May vary in some environments (burrows, soil)
– Water vapor can also alter this composition
Principles of Gas Diffusion in Air
• Gases Exert Pressures
– Atmospheric Pressure (760 mmHg (1 atm) at sea level)
• The relationship between the concentration of a gas and
pressure is described in the Ideal Gas Law
PV = nRT
–
–
–
–
–
P = pressure
V = volume
n = moles of gas
R = universal gas constant (8.314 J/mol*°K)
T = temperature (°K)
Principles of Gas Diffusion
• Dalton’s Law of Partial Pressures
– the total pressure exerted by a gas mixture is the sum of
individual pressures exerted by each gas
– e.g. PO2 = 20.95% * 760 mmHg = 159 mmHg
• Diffusion (for Gases)
– movement of a gas from an area of high partial
pressure to one of low partial pressure
– Movement of a gas can occur against a concentration
gradient as long it flows along the partial pressure
gradient
Factors Influencing Gas Diffusion
in Air
J=K×
P1 – P2
X
– J = net rate of diffusion between point 1 and point 2
– K = Krogh’s diffusion coefficient
(depends on medium permeability, temperature, and
the specific gas being transported)
– P1 and P2 = particle concentration at points 1 and 2
– X = distance between points 1 and 2
Diffusion rate increases with…
•
•
•
•
Bigger differences in concentration
Shorter diffusion differences
Greater medium permeability
Higher temperatures
Water
• Gases are soluble in water
• Amount of a gas in water depends on:
–
–
–
–
solubility of the gas
the pressure of the gas in the gas phase
temperature
presence of other solutes
Solubility
• Measured as the solubility coefficient ()
– volume of gas dissolved in 1 L when the
pressure of the gas = 1 atm
• Different gases have different solubilities:
Gas
α (ml/L)
Oxygen
34.1
Nitrogen
16.9
Carbon Dioxide
1019
Pressure
• Amount of gas dissolved is proportional
to the pressure (tension) of the gas
• Henry’s Law:
– Vg = (Pg/760)*VH2O
•
•
•
•
Vg = volume of gas dissolved (ml)
Pg = pressure of the gas (mmHg)
VH2O = volume of water (L)
 = solubility coefficient
Temperature and Other Solutes
• Temperature
–  temperature,  solubility of gases
– Opposite of solid solutes
• Other Solutes
–  [solid solutes],  solubility of gases
– [other gases] does not affect solubility
Temperature and Other Solutes
Temperature (C)
0
10
15
20
30
Fresh Water
Sea Water
(ml O2 /L water)
(ml O2 /L water)
10.29
7.97
8.02
6.35
7.22
5.79
6.57
5.31
5.57
4.46
Partial Pressures in Water
• Amt of gas in water corresponds to a
specific gas pressure in the gas phase
• Gas Tension
– pressure of a gas in an atmosphere with which
the solution is in equilibrium
Air vs. Water
• Air contains more O2 than water (30x)
• Air is less dense than water (1/800th)
– easier to move over respiratory surfaces
– less energy expenditure
• Respiratory gases diffuse much more quickly in
air than in water (~10,000x greater for O2)
• Use of air can lead to water loss through
evaporation