I. Introduction to Oxygen and Carbon Dioxide –cells need oxygen to live and produce carbon dioxide as a waste material
II. Diffusion– unicellular organisms can exchange gases by simple diffusion; multicellular organisms use diffusion plus convection.
A. Diffusion is the movement of molecules from a location of higher concentration to another of lower concentration solely as a result of random motion. It only works for short distances.
B. Fick diffusion equation allows calculation of the rate of diffusion.
Factors include diffusion coefficient, cross sectional area, and concentration gradient. The diffusion coefficient indicates how likely it is that a substance will move by diffusion. For example a gas will have a bigger diffusion coefficient than a viscous material, because it moves more easily by diffusion.
C. Flux is the flow (amount of material moving in an amount of time) of materials by diffusion and is affected by concentration gradient, temperature, mass of molecules and surface area.
D. Convection is the movement of fluid in bulk; such as blood circulation driven by the heart creating a pressure gradient. This is needed for multicellular animals.
III. Air breathing animals (mammals)
A. Two convection mechanisms –air flow or alveolar ventilation; blood flow or circulation
B. Two diffusion mechanisms –alveolar gas exchange; tissue gas exchange
IV. Water breathing animals (fish)
A. Gill perfusion (water pump) replaces alveolar ventilation
V. Oxygen and Carbon Dioxide in air
A. 21% Oxygen, 0% Carbon Dioxide, 79% Nitrogen gas
B. Atmospheric or Barometric Pressure– gases exert a force due to their mass and acceleration resulting in a pressure. At sea level atmospheric pressure is 760 torr = mm Hg
C. Barometric pressure is equal to the sum of all of the partial pressures of each gas in the atmosphere
1. PO2= .21 x 760 = 159 mm Hg
2. PCO2= .00 x 760 = 0 mm Hg
3. PN2= .79 x 760 = 600 mm Hg
D. Barometric pressure in Denver is about 630 mm Hg.
1. PO2 in Denver is 132 mm Hg
2. Gravity pulls more gases towards the ground and fewer at high altitudes, thus a lower concentration of gases at higher altitudes and a lower atmospheric pressure
VI. Oxygen and Carbon Dioxide in Solution
A. Gases in solution do not exert atmospheric pressure, so if a PO2 is 100 torr in solution, this really means that if this concentration of oxygen were in equilibrium with the gas, the gas would have a partial pressure of 100 torr
B. Concentration of gas in a solution depends on the solubility of the gas in solution and the partial pressure of the gas. This is measured by Henry’s Law: [G] = Pgas x Sgas which describes how gases dissolve in liquids. Pgas is the pressure gradient of the gas. Sgas is the solubility coefficient. Oxygen has a lower solubility coefficient than carbon dioxide, so it is much less soluble.
C. Note that solubility, and therefore concentration, of oxygen decreases as temperature increases. Additionally, as salinity increases, oxygen solubility decreases. Therefore, it is toughest for saltwater fish to get oxygen when living in very warm water.