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48.1 Respiratory gases diffuse from areas of higher partial pressure to areas of lower partial pressure.

48.1 Rising temperature decreases oxygen solubility in water and increases the metabolism of aquatic ectotherms.

48.2 In insects, efficient gas exchange is facilitated by an internal network of air passages.

Original Paper: Hetz, S. K. and T. J. Bradley. 2005. Insects breathe discontinuously to avoid oxygen toxicity. Nature 433: 516−519.

Insects’ air-delivery system consists of tracheae that branch throughout the body, facilitating the exchange of O2 and CO2 in and out of tissues. The finest of these tubes are less than a micrometer in diameter. Tracheal openings on the body’s surface are guarded by valve-like spiracles that can be opened, closed, or “fluttered.”

To study tracheal function, microscopic plastic tubes were surgically inserted into the tracheae of the pupal stages of Attacus atlas moths. The microtubes were attached to sensors that recorded the rate of CO2 released from the tracheae and the intratracheal concentrations of O2. Pupae were placed in chambers with normal atmospheric levels of gases, and the spiracle behavior, rate of CO2 release, and O2 concentrations in the tracheae were recorded (Figure A). Kilopascals (kPa) was the measure of gas pressure.

To test how the spiracles performed in differing atmospheric conditions and what effects this might have on the rate of CO2 release and O2 concentrations, researchers exposed pupae to differing levels of atmospheric O2 (labeled red line in graphs). They then measured levels of CO2 release and O2 concentrations during the fluttering stage (Figure B).

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Questions

Question 1

If 1 kPa = 7.5 mm Hg, calculate the change in PO2 in units of mm Hg in a typical open–close–fluttering cycle. What is the approximate mm Hg PO2 in the atmosphere and inside the tracheal tubes in each of the three experiments shown in Figure B?

In Figure A, in the 15 minutes that the spiracles were open, the intratracheal O2 level increased from 19 to 142 mm Hg. In the 22–23 minutes in which the spiracles were closed, the intratracheal level of O2 dropped from 142 to approximately 37.5 mm Hg. In the approximately 70 minutes that the pupae entered the fluttering phase, the intratracheal O2 level fluctuated between 37.5 and 18.75 mm Hg. In Figure B, the first chamber’s atmospheric O2 concentration was 48 mm Hg, but the fluttering phase kept the intratracheal O2 levels at a fairly consistent 36.75 mm Hg. In the second chamber, the atmospheric O2 levels were 159 mm Hg. The fluttering phase allowed some fluctuation in the intratracheal O2 levels, from a high of roughly 38.25 mm Hg to a low of roughly 36 mm Hg. In the third chamber, atmospheric levels of O2 were almost double normal atmospheric O2 levels, at 301.5 mm Hg. But the fluttering phase kept the intratracheal O2 level at an almost constant 30 mm Hg.

Question 2

Based on your understanding of the purpose of insect tracheae, explain why the PO2 rises to such a high level inside the tracheae when the spiracle is open, and then decreases over time. What effect does the fluttering of spiracles have on PO2?

When the tracheal tubes open, the atmospheric O2 diffuses into the tubes, bringing the intratracheal O2 level close to normal atmospheric O2 levels (159 mm Hg). However, over the approximately 23 minutes that the spiracles are closed, the O2 levels are slowly depleted as more O2 is dissolved into the hemocoel and distributed to all of the cells to be used in aerobic respiration. We could predict that this O2 level would continue to decline, approaching zero, if it were not for the fluttering phase. The fluttering phase appears to allow CO2 to escape into the atmosphere while also allowing small amounts of O2 to enter the tracheal tubes and maintain O2 levels at a fairly constant level.

Question 3

One function of the fluttering of spiracles might be to prevent the PO2 from reaching a toxic level. Based on the data in Figure B, is this a viable hypothesis? Explain your answer.

A delicate balance must be maintained between intratracheal O2 levels and atmospheric O2 levels. When atmospheric levels of O2 are normal (middle graph in Figure B), the fluttering phase allows CO2 release and replacement of O2 to seek a fairly constant level of O2 within the trachea. In a low-oxygen environment (first graph in Figure B), the fluttering phase is increased, compared with a normal atmospheric O2 level. Beginning at a lower level of O2 at the time of spiracle closure would make it necessary to increase the amount of fluttering to keep O2 levels at an appropriate intratracheal concentration for adequate aerobic respiration. However, when atmospheric O2 levels are abnormally high, the fluttering phase is greatly diminished (third graph in Figure B). Opening the spiracles as often as what is seen under normal atmospheric O2 levels could expose cells to higher than normal O2 levels. These levels could prove detrimental to insect cells. Therefore decreasing the amount of fluttering would be one way to prevent too much O2 from entering the tracheal tubes and reaching a toxic cellular level.

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