External Gas Transport Chapters 20 & 21

Respiration The process of acquiring oxygen and releasing carbon dioxide

Why do animals require oxygen? Aerobic respiration –C 6 H 12 O 6 + 6O 2  6CO 2 + 6H 2 O –Enough Energy for 30 ATP Anaerobic respiration –C 6 H 12 O 6  2C 3 H 6 O 3 –Enough Energy for 2 ATP All known metazoans require oxygen

Why do animals need to release carbon dioxide? CO 2 tends to react with water CO 2 + H 2 O  H 2 CO 3  H + + HCO 3 -  2H + + CO 3 -  [H + ],  pH Low pH’s can disrupt physiological function (e.g., enzyme activity)

Air Composition (Dry Air): –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. P O2 = 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 = 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) –P 1 and P 2 = particle concentration at points 1 and 2 –X = distance between points 1 and 2 J = K × P 1 – P 2 X

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:

Pressure Amount of gas dissolved is proportional to the pressure (tension) of the gas Henry’s Law: –V g =  (P g /760)*V H2O V g = volume of gas dissolved (ml) P g = pressure of the gas (mmHg) V H2O = 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

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 O 2 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 O 2 ) Use of air can lead to water loss through evaporation