Chapter 5 - Physiology: Energetics, metabolism, and gas exchange

OBJECTIVETo describe the kinetics of metabolic gas exchange at the onset and offset of low level, constant work exercise in patients with chronic heart failure.SETTINGTertiary referral centre for cardiology.PATIENTS10 patients...

We compare standard metabolic rate (VCO2) and gas exchange patterns in the Colorado potato beetle (Leptinotarsa decemlineata Say) in winter diapause (i.e. lasting only one overwintering period) with those of beetles in prolonged diapause (i.e. diapause lasting 2 or 3 years). The length of diapause is estimated by the behaviour of the beetles: burrowing into soil as a sign of the beginning of diapause and emergence from the soil as the ending of diapause. Measurement with a flow‐through carbon dioxide (CO2) infrared gas analyzer reveals that most beetles in winter or prolonged diapause display distinct discontinuous gas exchange cycles at 23 °C. Beetles with cyclic gas exchange and continuous breathing do not survive the winter. Beetles in prolonged diapause are characterized by a three‐ to five‐fold lower standard metabolic rates, longer discontinuous gas exchange periods and shorter CO2 releases (open phase) than those in winter diapause.

The Grand River is a 7th-order 300-km river draining the largest watershed (6800 km2) in southern Ontario, Canada. The watershed has experienced large landuse changes during a period >100 y, resulting in increasing agricultural and urban nutrient inputs to the river. As a result, the Grand River is highly degraded from its source to mouth. We studied longitudinal and temporal changes in aquatic community metabolism (photosynthesis and respiration) and O2 gas exchange over 3 seasons using O2 and δ18O-O2. Diel changes in O2 saturation were >50 percentage points along the river. In some parts, the diel O2 change was >10 mg/L. Strong daily variation in δ18O-O2, up to 22‰, was observed at all 23 sampling sites in the river. Despite consistently high nutrient levels and high productivity from headwaters to mouth, wastewater treatment plant (WWTP) effluents strongly affected O2 saturation in all seasons. Modifications to river flow or nutrient inputs that affect the O2 gas-exchange coefficient or O2 demand could exacerbate current nighttime hypoxia problems. Strong diel variability in O2 may not directly indicate changes in metabolic rates because changes in gas-exchange coefficients alone are enough to mask changes in metabolic rates. Photosynthetic rates immediately downstream of WWTPs did not increase because nutrients were already high because of agricultural nutrient loading upstream of WWTPs. As a result, WWTP nutrients were exported downstream rather than used immediately below the WWTPs and the zone of impact from WWTP nutrients extended farther downstream than would otherwise be expected. O2 is the measure used by ecosystem managers, but metabolism and gas exchange must be managed to achieve the desired O2 outcomes.

Gas exchange in avian embryos and hatchlings

Respiratory water loss during metabolic gas exchange is an unavoidable cost of living for terrestrial insects. It has been suggested to depend on several factors, such as the mode of gas exchange (convective vs. diffusive), species habitat (aridity), body size and measurement conditions (temperature). We measured this cost in terms of respiratory water loss relative to metabolic rate (respiratory water cost of gas exchange; RWL/V˙CO2) for adults of two insect species, the speckled cockroach (Nauphoeta cinerea) and the darkling beetle (Zophobas morio), which are similar in their mode of gas exchange (dominantly convective), habitat (mesic), body size and measurement conditions, by measuring gas exchange patterns using flow-through respirometry. The speckled cockroaches showed both continuous and discontinuous gas exchange patterns, which had significantly a different metabolic rate and respiratory water loss but the same respiratory water cost of gas exchange. The darkling beetles showed continuous gas exchange pattern only, and their metabolic rate, respiratory water loss and respiratory cost of gas exchange were equivalent to those cockroaches using continuous gas exchange. This outcome from our study highlights that the respiratory water cost of gas exchange is similar between species, regardless of gas exchange pattern used, when the confounding factors affecting this cost are controlled. However, the total evaporative water cost of gas exchange is much higher than the respiratory cost because cuticular water loss contributes considerably more to the overall evaporative water loss than respiratory water. We suggest that the total water cost of gas exchange is likely to be a more useful index of environmental adaptation (e.g., aridity) than just the respiratory water cost.

Several controversies currently dominate the fields of arthropod metabolic rate, gas exchange and water balance, including the extent to which modulation of gas exchange reduces water loss, the origins of discontinuous gas exchange, the relationship between metabolic rate and life-history strategies, and the causes of Palaeozoic gigantism. In all of these areas, repeated calls have been made for the investigation of groups that might most inform the debates, especially of taxa in key phylogenetic positions. Here we respond to this call by investigating metabolic rate, respiratory water loss and critical oxygen partial pressure (Pc) in the onychophoran Peripatopsis capensis, a member of a group basal to the arthropods, and by synthesizing the available data on the Onychophora. The rate of carbon dioxide release (VCO2) at 20 degrees C in P. capensis is 0.043 ml CO2 h(-1), in keeping with other onychophoran species; suggesting that low metabolic rates in some arthropod groups are derived. Continuous gas exchange suggests that more complex gas exchange patterns are also derived. Total water loss in P. capensis is 57 mg H2O h(-1) at 20 degrees C, similar to modern estimates for another onychophoran species. High relative respiratory water loss rates ( approximately 34%; estimated using a regression technique) suggest that the basal condition in arthropods may be a high respiratory water loss rate. Relatively high Pc values (5-10% O2) suggest that substantial safety margins in insects are also a derived condition. Curling behaviour in P. capensis appears to be a strategy to lower energetic costs when resting, and the concomitant depression of water loss is a proximate consequence of this behaviour.

RATIONALEPreterm birth affects millions of infants each year, and survival rates are on the rise. Frequently, premature birth results in alveolar simplification that may persist into adulthood. We have previously shown that pulmonary gas exchange efficiency is lower during exercise in some adults born preterm compared with control subjects. However, it is unknown whether the presence of inhaled nitric oxide (iNO), a potent broncho‐ and vasodilator, has an effect on pulmonary gas exchange and other physiological measures during exercise in preterm birth survivors. The objective of this study was to determine the effect of iNO on alveolar to arterial oxygen difference (AaO2), respiratory exchange ratio (RER), and maximum wattage during exercise in adult survivors of preterm birth.METHODSFourteen young adults born preterm (recruited from the Newborn Lung Project, birth weight <1,500 g, gestational age <36wk, age 20–23 years) and 16 term‐born, age‐matched control subjects performed incremental exercise on a cycle ergometer to volitional exhaustion while breathing normoxic air with and without iNO. Arterial and venous blood samples were taken every two minutes. Ventilation, mixed expired gases, power output, and oxygen consumption were continuously measured during rest and exercise using LabChart. Max wattage, RER, and AaO2 were calculated. Statistics were done with two‐way ANOVA to compare control and preterm with and without iNO with Tukey's for multiple comparisons.RESULTSWith respect to max power, controls attained a higher wattage than preterms in normoxia (p= 0.01). This difference was attenuated in the presence of iNO (174.3 ± 28.4 W controls without iNO v 141.4 ± 27.2 W preterms with iNO, p = 0.21). With respect to RER, there was no significant difference between controls and preterms in normoxia. In the presence of iNO, preterms had a significantly higher RER than controls without iNO throughout graded exercise (0.92 ± 0.10 v 0.89 ± 0.12, p = 0.0013). Preterms had a significantly higher AaO2 than controls during graded exercise in normoxia without iNO, which was attenuated with iNO (9.3 ±6.1 mmHg control v 11.6 ±10.8 mmHg preterm, p =0.21).CONCLUSIONSWe have previously shown that pulmonary gas exchange efficiency is lower in some adult survivors of preterm birth during exercise compared with control subjects. Here we show that iNO in preterm adults attenuates many of the metabolic and pulmonary gas exchange differences between preterm and control adults during graded exercise, including AaO2 and max power. RER in the preterm + NO group was significantly higher than the controls without NO. Overall, this suggests iNO can improve gas exchange efficiency in preterm born young adults.Support or Funding InformationNIH NHLBI, R01 HL086897 (Eldridge)Nitric oxide was supplied by Mallinckrodt Pharmaceuticals, formerly Ikaria, Inc.

Energy Metabolism of Thoracic Surgical Patients in the Early Postoperative Period: Effect of Posture

Ecosystem metabolism and the contribution of carbon dioxide from lakes to the atmosphere can be estimated from free-water gas measurements through the use of mass balance models, which rely on a gas transfer coefficient ( k ) to model gas exchange with the atmosphere. Theoretical and empirically based models of k range in complexity from wind-driven power functions to complex surface renewal models; however, model choice is rarely considered in most studies of lake metabolism. This study used high-frequency data from 15 lakes provided by the Global Lake Ecological Observatory Network (GLEON) to study how model choice of k influenced estimates of lake metabolism and gas exchange with the atmosphere. We tested 6 models of k on lakes chosen to span broad gradients in surface area and trophic states; a metabolism model was then fit to all 6 outputs of k data. We found that hourly values for k were substantially different between models and, at an annual scale, resulted in significantly different estimates of lake metabolism and gas exchange with the atmosphere.

Ecosystem metabolism and the contribution of carbon dioxide from lakes to the atmosphere can be estimated from free-water gas measurements through the use of mass balance models, which rely on a gas transfer coefficient (k) to model gas exchange with the atmosphere. Theoretical and empirically based models of k range in complexity from wind-driven power functions to complex surface renewal models; however, model choice is rarely considered in most studies of lake metabolism. This study used high-frequency data from 15 lakes provided by the Global Lake Ecological Observatory Network (GLEON) to study how model choice of k influenced estimates of lake metabolism and gas exchange with the atmosphere. We tested 6 models of k on lakes chosen to span broad gradients in surface area and trophic states; a metabolism model was then fit to all 6 outputs of k data. We found that hourly values for k were substantially different between models and, at an annual scale, resulted in significantly different estimates of lake metabolism and gas exchange with the atmosphere.

Body Size and Metabolic Rate in Salamanders

We have developed a gas exchange simulation system (GESS) to assess the quality control in measurements of metabolic gas exchange. The GESS simulates human breathing from rest to maximal exercise. It approximates breath-by-breath waveforms, ventilatory output, gas concentrations, temperature and humidity during inspiration and expiration. A programmable motion control driving two syringes allows the ventilation to be set at any tidal volume (VT), respiratory frequency (f), flow waveform and period of inspiration and expiration. The GESS was tested at various combinations of VT (0.5-2.51) and f(10-60 stroke x min(-1)) and at various fractional concentrations of expired oxygen (0.1294-0.1795); and carbon dioxide (0.0210-0.0690) for a pre-set flow waveform and for expired gases at the same temperature and humidity as room air. Expired gases were collected in a polyethylene bag for measurement of volume and gas concentrations. Accuracy was assessed by calculating the absolute and relative errors on parameters (error=measured-predicted). The overall error in the gas exchange values averaged less than 2% for oxygen uptake and carbon dioxide output, which is within the accuracy of the Douglas bag method.

Metabolism and gas exchange patterns in Rhodnius prolixus

Dissolved O2 concentration and delta18O-O2 diel curves can be combined to assess aquatic photosynthesis, respiration, and metabolic balance, and to disentangle some of the confounding factors associated with interpretation of traditional O2 concentration curves. A dynamic model is used to illustrate how six key environmental and biological parameters interact to affect diel O2 saturation and delta18O-O2 curves, thereby providing a fundamental framework for the use of delta18O-O2 in ecosystem productivity studies. delta18O-O2 provides information unavailable from concentration alone because delta18O-O2 and saturation curves are not symmetrical and can be used to constrain gas exchange and isotopic fractionation by eliminating many common assumptions. Changes in key parameters affect diel O2 saturation and delta18O-O2 curves as follows: (1) an increase in primary production and respiration rates increases the diel range of O2 saturation and delta18O-O2 and decreases the mean delta18O-O2 value; (2) a decrease in the primary production to respiration ratio (P:R) decreases the level of O2 saturation and increases the delta18O-O2 values; (3) an increase in the gas exchange rate decreases the diel range of O2 saturation and delta18O-O2 values and moves the mean O2 saturation and delta18O-O2 values toward atmospheric equilibrium; (4) a decrease in strength of the respiratory isotopic fractionation (alphaR closer to 1) has no effect on O2 saturation and decreases the delta18O-O2 values; (5) an increase in the delta18O of water has no effect on O2 saturation and increases the minimum (daytime) delta18O-O2 value; and (6) an increase in temperature reduces O2 solubility and thus increases the diel range of O2 saturation and delta18O-O2 values. Understanding the interplay between these key parameters makes it easier to decipher the controls on O2 and delta18O-O2, compare aquatic ecosystems, and make quantitative estimates of ecosystem metabolism. The photosynthesis to respiration to gas exchange ratio (P:R:G) is better than the P:R ratio at describing and assessing the vulnerability of aquatic ecosystems under various environmental stressors by providing better constrained estimates of ecosystem metabolism and gas exchange.

On the inaccuracy of breath-by-breath metabolic gas exchange systems