Respiratory gases are exchanged only by diffusion. All factors influencing respiratory gas exchanges are components of Fick’s law of diffusion. Air is a better respiratory medium than water because a given volume of air has more O2 than the same volume of water. O2 diffuses faster in air than in water, and less work is required to move air over respiratory exchange surfaces. CO2 exchange with the external environment occurs readily because of a large concentration gradient from the body to the environment and because CO2 is highly soluble in water.
learning outcomes
You should be able to:
Analyze changes in partial pressure of a gas at different altitudes and at different depths under water.
Apply Fick’s law to explain animal adaptations that influence respiratory gas exchange.
Describe three factors that require a fish to increase its gill ventilation in response to an increase in its environmental temperature.
Explain how altitude influences respiratory gas exchange in terms of partial pressures of gases.
What is the partial pressure of oxygen in inhaled air at sea level; at 2,000 meters above sea level, where atmospheric pressure is 80 percent that of sea level; and while breathing air at the same pressure as the environment at 10 meters (= an additional 1 atmosphere of pressure) underwater?
At sea level, assuming a normal atmospheric pressure of 760 mm Hg, the PO2 will be 20.9 percent of that, or 159 mm Hg. Therefore the PO2 at 2,000 meters will be 127 mm Hg. When breathing air at two times the atmospheric pressure, the PO2 will be 318 mm Hg.
Flatworms (see Figure 48.1A) are invertebrates that do not have a respiratory system. What two components of Fick’s law enable them to satisfy their need for oxygen?
Flatworms have a flat, thin body form, so all cells of the body are close enough to the surrounding environment that O2 and CO2 can diffuse directly between the cells and the environment. The two components of Fick’s law reflected in these anatomical features are high surface area for diffusion, and minimal path length.
The O2 content of water decreases with an increase in water temperature, but the gill ventilation rate of a fish increases much more than is necessary to match that decrease in O2 availability. Why?
As water temperature increases, the O2 content of water decreases (O2 solubility decreases with increase in temperature), but since the fish’s body temperature is at equilibrium with the water temperature, its metabolism increases (Q10, effect). Therefore the fish has to ventilate its gills more to satisfy its O2 needs, and the work of gill ventilation increases O2 demand even more.
In terms of Fick’s law, what is the difference between the ability to exchange O2 and CO2 at sea level and at high altitude?
In going from sea level to high altitude, the partial pressure of O2 in the inhaled air decreases, so the partial pressure gradient causing O2 to diffuse into the body goes down. Since the partial pressure of CO2 is extremely low at sea level, it does not decrease significantly at high altitude. Therefore the partial pressure gradient for CO2 diffusing out of the body does not change.
Now that we have discussed the physical factors that influence diffusion rates of respiratory gases between animals and their environments, let’s look at some of the adaptations that have evolved for maximizing respiratory gas exchange.