2.3 – Gaseous Exchange Mechanisms

Learning Outcomes

2.4. Compare counter- and crosscurrent exchange using specific respiratory system examples.

Most gas exchange in the respiratory system structures in animals takes place in counter-current arrangement. In short, this means that the medium that delivers the oxygen (air or water) and the structures that the oxygen is delivered to are moving in directions opposite to one another. An example of this is a fish gill system (Figure 2.12). Another way that gas exchange can happen is through cross-current exchange in which the air carrying the oxygen is moving through the respiratory structures (parabronchi), which are positined perpendicular to the movement of blood around the parabronchi. (Figure 2.12). Please note that in the figure below, the oxygen-rich and oxygen-poor blood flow direction with respect to the secondary lamellae should be reversed (Figure 2.12a). Additonally, the flow of blood within the barabronchus (Figure 2.12b, direction flow should be from oxygen-poor to oxygen-rich blood). The open textbook developers and author are currently working on updating this figure.

This figure describes differences between counter- and cross-current flow and differences between unidirectional and tidal ventilation.
Figure 2.12. Comparison of gas exchange in gills, parabronchi, and alveoli. This figure was created and designed by Christine Shan, a graduate student in Biomedical Communications Masters program at the University of Toronto Mississauga (developed as a project in MSC2001Y Visual Representation of Medical Knowledge course in the BMC MSc program).
Think Question 2.9.

Which one do you think is an example of counter-current and which one is an example of crosscurrent exchange?

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Introductory Animal Physiology by Sanja Hinic-Frlog is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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