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The major adaptations that increase efficiency of respiratory gas exchange are large surface areas for exchange and maximized partial pressure gradients across those surfaces. Fish and avian respiratory systems have continuous and unidirectional ventilation of their respiratory exchange surfaces. Mammals have tidal ventilation, resulting in residual volumes of O2 that dilute the incoming fresh air.
learning outcomes
You should be able to:
Explain the concept and adaptive significance of countercurrent exchange, using the fish gills as the example.
Describe how the avian respiratory system maximizes the partial pressure gradient for O2 uptake.
Determine the total lung capacity (including residual volume) of a human.
Given constant rates of ventilation and perfusion, how can the directionality of blood and water flow on opposite sides of fish gill membranes influence the maximum O2 exchange that can occur?
If ventilation (water) and perfusion (blood) of the gills are in the same direction (concurrent flow), then the concentration gradient of O2 across the gills will gradually equilibrate somewhere between the maximum O2 concentration of the water coming in and the O2 concentration of the blood coming in. In countercurrent flow, it is possible for the O2 concentration of the blood leaving the gills to be almost as great as the O2 concentration of the water entering the gills.
How do birds maintain a constant and unidirectional flow of air through their lungs?
An important feature of avian lungs is the parabronchi, which enable air to travel unidirectionally through the lungs. When a bird inhales, the incoming air goes to the posterior air sacs. That fresh air flows into the parabronchi during the next exhalation. The subsequent inhalation causes air in the lungs to flow into the anterior air sacs while fresh air flows into the posterior air sacs. The subsequent exhalation empties the anterior air sacs to the environment while the air in the posterior air sacs enters the lungs. Thus the through-
Find the functional residual volume of a person who yields the following data while breathing from a flowmeter that has a volume of 30 liters (L) and an initial gas mixture of 20 percent O2, 75 percent nitrogen (N), and 5 percent helium (He). Beginning with an inhalation, the person inhales and exhales 20 times. The final concentration of He in the flowmeter is 4.6 percent.
The amount of He in the system remains the same but becomes equally distributed in the spirometer and lungs. So the initial amount of He is 0.05 × 30 L = 1.5 L. The final amount of He is still 1.5 L, but it is distributed over 30 L + the person’s FRV.
Thus,
1.5 L = 0.046 × (30 L + FRV)
1.0 L = 1.38 L + 0.046FRV
0.12 L = 0.046FRV
2.61 L = FRV
Despite their limitations, mammalian lungs serve the respiratory needs of mammals well. Offsetting the inefficiencies of tidal breathing, mammalian lungs have an enormous surface area and a very short path length for diffusion. Next we will look at the human respiratory system as an example.