Discontinuous gas exchange

Discontinuous gas-exchange cycles (DGC), also called discontinuous ventilation or discontinuous ventilatory cycles, follow one of several patterns of arthropod gas exchange that have been documented primarily in insects; they occur when the insect is at rest. During DGC, oxygen (O2) uptake and carbon dioxide (CO2) release from the whole insect follow a cyclical pattern characterized by periods of little to no release of CO2 to the external environment. Discontinuous gas exchange is traditionally defined in three phases, whose names reflect the behaviour of the spiracles: the closed phase, the flutter phase, and the open phase. Discontinuous gas-exchange cycles (DGC), also called discontinuous ventilation or discontinuous ventilatory cycles, follow one of several patterns of arthropod gas exchange that have been documented primarily in insects; they occur when the insect is at rest. During DGC, oxygen (O2) uptake and carbon dioxide (CO2) release from the whole insect follow a cyclical pattern characterized by periods of little to no release of CO2 to the external environment. Discontinuous gas exchange is traditionally defined in three phases, whose names reflect the behaviour of the spiracles: the closed phase, the flutter phase, and the open phase. Until recently, insect respiration was believed to occur entirely by simple diffusion. It was believed that air entered the tracheae through the spiracles, and diffused through the tracheal system to the tracheoles, whereupon O2 was delivered to the cells. However, even at rest, insects show a wide variety of gas exchange patterns, ranging from largely diffusive continuous ventilation, to cyclic respiration, of which discontinuous gas exchange cycles are the most striking. Discontinuous gas exchange cycles have been described in over 50 insect species, most of which are large beetles (order Coleoptera) or butterflies or moths (order Lepidoptera). As the cycles have evolved more than once within the insects, discontinuous gas exchange cycles are likely adaptive, but the mechanisms and significance of their evolution are currently under debate. Discontinuous gas exchange cycles are characterized by a repeating pattern of three phases. These phases are named according to the behaviour of the spiracles and are most commonly identified by their CO2 output, primarily observed using open flow respirometry. During the closed phase of discontinuous gas exchange cycles, the spiracle muscles contract, causing the spiracles to shut tight. At the initiation of the closed phase, the partial pressure of both O2 and CO2 is close to that of the external environment, but closure of the spiracles drastically reduces the capacity for the exchange of gases with the external environment. Independent of cycles of insect ventilation which may be discontinuous, cellular respiration on a whole animal level continues at a constant rate. As O2 is consumed, its partial pressure decreases within the tracheal system. In contrast, as CO2 is produced by the cells, it is buffered in the haemolymph rather than being exported to the tracheal system. This mismatch between O2 consumption and CO2 production within the tracheal system leads to a negative pressure inside the system relative to the external environment. Once the partial pressure of O2 in the tracheal system drops below a lower limit, activity in the nervous system causes the initiation of the flutter phase. During the flutter phase of discontinuous gas exchange cycles, spiracles open slightly and close in rapid succession. As a result of the negative pressure within the tracheal system, created during the closed phase, a small amount of air from the environment enters the respiratory system each time the spiracles are opened. However, the negative internal pressure also prevents the liberation of CO2 from the haemolymph and its exportation through the tracheal system. As a result, during the flutter phase, additional O2 from the environment is acquired to satisfy cellular O2 demand, while little to no CO2 is released. The flutter phase may continue even after tracheal pressure is equal to that of the environment, and the acquisition of O2 may be assisted in some insects by active ventilatory movements such as contraction of the abdomen. The flutter phase continues until CO2 production surpasses the buffering capacity of the haemolymph and begins to build up within the tracheal system. CO2 within the tracheal system has both a direct (acting on the muscle tissue) and indirect (through the nervous system) impact on the spiracle muscles and they are opened widely, initiating the open phase. A rapid release of CO2 to the environment characterizes the open phase of discontinuous gas exchange cycles. During the open phase, spiracular muscles relax and the spiracles open completely. The open phase may initiate a single, rapid release of CO2, or several spikes declining in amplitude with time as a result of the repeated opening and closing of the spiracles. During the open phase, a complete exchange of gases with the environment occurs entirely by diffusion in some species, but may be assisted by active ventilatory movements in others. The great variation in insect respiratory cycles can largely be explained by differences in spiracle function, body size and metabolic rate. Gas exchange may occur through a single open spiracle, or the coordination of several spiracles. Spiracle function is controlled almost entirely by the nervous system. In most insects that demonstrate discontinuous gas exchange, spiracle movements and active ventilation are closely coordinated by the nervous system to generate unidirectional air flow within the tracheal system. This coordination leads to the highly regulated bursting pattern of CO2 release. Building CO2 levels during the flutter phase may either directly affect spiracular opening, affect the nervous system while being pumped through the haemolymph, or both. However, the effects of CO2 on both spiracles and the nervous system do not appear to be related to changes in pH. Variability in discontinuous gas exchange cycles is also dependent upon external stimuli such as temperature and the partial pressure of O2 and CO2 in the external environment. Environmental stimuli may affect one or more aspects of discontinuous cycling, such as cycle frequency and the quantity of CO2 released at each burst. Temperature can have massive effects on the metabolic rate of ectothermic animals, and changes in metabolic rate can create large differences in discontinuous gas exchange cycles. At a species-specific low temperature discontinuous gas exchange cycles are known to cease entirely, as muscle function is lost and spiracles relax and open. The temperature at which muscular function is lost is known as the chill coma temperature.