Lecture 1: Gases in Air and Water

Most animals satisfy their energy requirements by

Oxidation of food materials in the process forming CO2 and H2O

Define respiration. Why is it needed?

The process of oxygen uptake and release of carbon dioxide, nessecary for providing cells with oxygen for aerobic respiration

Why is oxygen important?

Used in metabolism to extract energy from food, 30x more oxygen in the air than in water, very different processes for extracting oxygen in land vs air animals

How does oxygen get to the cells?

Bulk movement takes it to the site of diffusion (lungs), where hemoglobin in red blood cells carries it to its destination

Brownian motion, particularly in the body

Slow, random, undirected. Oxygen travels too slow for diffusion to be reliable for oxygen transfer.

What is diffusion and how does it work?

Brownian motion moves particles from high to low concentrations due to particles bouncing off eachother at a faster rate than in the lower concentration side

What is the most important process in the movement of oxygen from the external medium to the cells?

Diffusion

Bulk movement (convection)

aids diffusion, but concentration gradients remain the fundamental driving force for moving respiratory gases

Proportions of dry atmospheric air

20.95% oxygen, 0.04% carbon dioxide, 79.01% nitrogen

Characteristics of the atmosphere

Constant gas composition (except for water vapor) due to nonstop movement. Excludes microenvironments like burrows and open spaces between soil particles.

Why is the proportion of oxygen in dry atmosphere air globally the same?

The atmosphere is in constant motion and therefore thoroughly mixed

Water Vapor Pressure

As temperature of air increases it holds more water

What is the water vapor pressure for the body’s natural temperature?

A natural temperature of 37C can hold 143.9 mg water per liter of air

Dalton’s Law of Partial Pressure

Each gas in a mixture creates pressure as if the other gases were not present. The total pressure is the sum of the pressures created by the gases in the mixture

Partial Pressure Equation for Dry Air

PTotal = PO2 + PCO2 + PN2

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Dry Partial Pressure Example

• Po2 = Pt (0.2095) = 159.22 mmttg

• PCO2 = Pt (0.0004) = 0.304

• PNO2 = Pt ( 0.7901) = 600.48

Why do we adjust for moist air in partial pressure? What is the equation for moist air?

Because atmospheric air is rarely dry. PT = PO2 + PCO2 + PN2 + PH20

Partial Pressure Equation for moist air

(PT - PH20 ) x % concentration of gas

Find the partial pressure of oxygen in a room at 20*C at sea level with 40% humidity.

• PO2 = (Pt - PH2O) 0.2095

• Cont…

Find the partial pressure of a person’s lung.

• Pt = 760

• Ph2o = 46.9

• 760 - 46.9 x 0.209 = 149 mmttg

What does water vapor do in lungs?

Reduces partial pressure of oxygen

Compare the solubility of oxygen, nitrogen, and carbon dioxide

N2 is only 1/2 as soluble as O2, but CO2 is roughly 30x as
soluble as oxygen

Solubility Gas Constant

the volume of gas dissolved in 1 liter of water when the pressure of the gas is 1 atm (1 atm = 760 mm Hg)

Henry’s Law

Vg = A (Pg /760) 1LH20
Vg = volume of gas (ml) in solution; Pg is partial pressure (mm Hg);
VH20 = volume of water (l); A = solubility constant (ml l-1)

Always calculating for 1 L H2O

What does the amount of gas dissolved in water depend on?

The waters partial pressure and solubility coefficient

Describe gas solubility

Decreases with increased temperature or increased solutes (dissolved particles), gases do not affect the solubility of each other like solutes and gases do

If you pour a cold soda and a warm soda, which will fizz more? Why?

The warm soda, because warmer air holds less dissolved gas, leaving the carbon dioxide to find equilibrium, causing the fizz

Tension of gases in solution

The partial pressure of that particular gas in an atmosphere in equilibrium with the solution

Describe the solubility of CO2 in water

30x higher in water than O2, BUT because CO2 is not abundant in the atmosphere, total quantity dissolved in water is very small

Solubility of CO2 in water reaction

CO₂ + H₂O <> H₂CO₃ <> H⁺ <> HCO_3- <> 2H⁺ + CO₃²^-

Why is carbon dioxide highly dissolvable in the ocean?

Because it does not only exist as molecular carbon dioxide, allowing the amount of carbon dioxide in the water to remain large as it is converted into other things

Rate of Diffusion

Inversely proportional to the square root of moleuclar weight

Does carbon dioxide diffuse faster than oxygen? Why or why not?

No, its high solubility in water makes it appear to diffuse faster

What size do you have to be to develop respiratory structures? (What is the limit of diffusion-based respiration?)

Approximately 1mm

Characteristics of larger animals relying on diffusion

Have large surface areas and short diffusion distances. Flattened, threadlike, or have complex surfaces

Air vs. Water as a Respiratory Medium

Air has more 30x oxygen than water and less mass. Water is 50x more viscous than air making it 50x more costly to move/pump, diffusion is 100000x faster in air than water, BUT evaporation is high in air

Why don’t land animals do unidirectional breathing?

This would require another exit hole, which would make us lose water and increase the risk of dehydration

You are at an elevation of 1500 m (air pressure = 630 mm Hg), the relative humidity is 80%, the air temperature is 20ºC, and water vapor pressure over a free surface is 18 mm Hg at 20ºC, and the solubility constant at 20ºC = 31 ml O2 / 1L H20).
1) What is the partial pressure of oxygen in air?
2) There is a pool of water at the site above. What is the oxygen content of the water in the pool?

1)

PO2 = (PT - PH2O)%O2

PT = PO2 + PCO2+PN2+PH2O

PO2 = (630-(0.8×18))0.2095

PO2= 129 mm Hg

2)

Vg=31(PO2/760) x 1L H2O

Vg = 31(129 mm Hg / 760 )× 1

Vg = 5.3 mL O2 / L H2O