Seasonal changes in the cardiorespiratory responses to hypercarbia and temperature in the bullfrog, Rana catesbeiana

We assessed the seasonal variations in the effects of hypercarbia (3 or 5% inspired CO2) on cardiorespiratory responses in the bullfrog Rana catesbeiana at different temperatures (10, 20 and 30 degrees C). We measured breathing frequency, blood gases, acid-base status, hematocrit, heart rate, blood pressure and oxygen consumption. At 20 and 30 degrees C, the rate of oxygen consumption had a tendency to be lowest during winter and highest during summer. Hypercarbia-induced changes in breathing frequency were proportional to body temperature during summer and spring, but not during winter (20 and 30 degrees C). Moreover, during winter, the effects of CO2 on breathing frequency at 30 degrees C were smaller than during summer and spring. These facts indicate a decreased ventilatory sensitivity during winter. PaO2 and pHa showed no significant change during the year, but PaCO2 was almost twice as high during winter than in summer and spring, indicating increased plasma bicarbonate levels. The hematocrit values showed no significant changes induced by temperature, hypercarbia or season, indicating that the oxygen carrying capacity of blood is kept constant throughout the year. Decreased body temperature was accompanied by a reduction in heart rate during all four seasons, and a reduction in blood pressure during summer and spring. Blood pressure was higher during winter than during any other seasons whereas no seasonal change was observed in heart rate. This may indicate that peripheral resistance and/or stroke volume may be elevated during this season. Taken together, these results suggest that the decreased ventilatory sensitivity to hypercarbia during winter occurs while cardiovascular parameters are kept constant.     

The relationship between rate of oxygen consumption, heart rate and thermal conductance of the dik-dik antelope (Rhynchotragus kirkii) at various ambient temperatures

1. The extent of cardiovascular adjustments to heat and cold were investigated between ambient temperatures of 5 and 45 degrees C by measuring conductance and the rates of oxygen consumption and heart beats. 2. Minimum heart rate was observed at 25 degrees C (114 +/- 9 beats/min). In the heat at 45 degrees C heart rate was observed to increase only slightly (127 +/- 12 beats/min) but in the cold -5 degrees C heart rate nearly doubled that at 25 degrees C. 3. Thermal conductance was on average 0.031 mlO2 (g. hr. degrees C)-1 below 25 degrees C but increased by more than 20 times at 40 degrees C. 4. A positive correlation between heart rate and rate of oxygen consumption was demonstrated below 25 degrees C and the relation may be of practical use.     

Effects of temperature and dissolved oxygen on heart rate,ventilation rate and oxygen consumption of spangled perch,Leiopotherapon unicolor (Günther 1859), (Percoidei,Teraponidae)

Summary Heart, ventilation and oxygen consumption rates ofLeiopotherapon unicolor were studied at temperatures ranging from 5 to 35°C, and during progressive hypoxia from 100% to 5% oxygen saturation. Biopotentials recorded from the water surrounding the fish corresponded to ventilation movements, and are thought to originate from the ventilatory musculature. Cardio-respiratory responses to temperature and dissolved oxygen follow the typical teleost pattern, with bradycardia, increased ventilation rate and reduced oxygen consumption occurring during hypoxia. However, ventilation rate did not increase at 15°C and below. Ventilation rate showed a slower response to increasing temperature (normoxic Q₁₀=1.39) than heart rate and oxygen consumption (normoxic Q₁₀=2.85 and 2.38).L. unicolor is unable to survive prolonged hypoxia by utilising anaerobic metabolism, but has a large gill surface area which presumably facilitates oxygen uptake in hypoxic environments. Periodic ventilation during normoxia in restingL. unicolor may improve ventilation efficiency by increasing the oxygen diffusion gradient across the gills.Abbreviations EBG electrobranchiogram - ECG electrocardiogram     

Exposure to hypoxia primes the respiratory and metabolic responses of the epaulette shark to progressive hypoxia

The majority of vertebrates are not tolerant to hypoxia but epaulette sharks (Hemiscyllium ocellatum) living on shallow reef platforms appear to tolerate hypoxic periods during tidal fluctuations. The effects of progressive hypoxia on the metabolic and ventilatory responses of these elasmobranchs were examined in a closed respirometer. In order to determine whether repeated exposure to hypoxia primes these sharks to alter their metabolism, one group of sharks was exposed to repeated sub-lethal hypoxia, at 5% of air saturation, prior to respirometry. In response to falling oxygen concentration [O(2)], the epaulette shark increased its ventilatory rate and maintained its O(2) consumption rate (VO(2)) down to 2.2 mg O(2) l(-1) at 25 degrees C. This is the lowest critical [O(2)] ([O(2)](crit)) ever measured for any elasmobranch. After reaching the [O(2)](crit), the shark remained in the respirometer for a further 4-5 h of progressive hypoxia. Only after the [O(2)] fell to 1.0 mg l(-1) was there a decrease in the ventilatory rate followed by a rise in blood lactate levels, indicating that the epaulette shark responds to severe hypoxia by entering a phase of metabolic and ventilatory depression. Interestingly, hypoxia tolerance was dynamic because hypoxic pre-conditioning lowered the VO(2) of the epaulette shark by 29%, which resulted in a significantly reduced [O(2)](crit) (1.7 mg O(2) l(-1)), revealing that hypoxic pre-conditioning elicits an enhanced physiological response to hypoxia.     

Cardiovascular correlates of exercise in snapping turtles, Chelydra serpentina

1. Heart rate increased with a rise in body temperature (10-30 degrees C) and with induced physical exercise in snapping turtles. 2. Maximum heart rate increment occurred at 30 degrees C. 3. Standard oxygen pulse did not change with a rise in temperature. 4. Oxygen pulse during exercise and oxygen pulse increment were maximal at 10 degrees C and minimal at 20 degrees C. 5. The increase in heart rate with exercise accounted for only 9-22% of the increase in oxygen transport during activity; the remainder was provided by a rise in cardiac stroke volume and/or A-V difference.     

Diving response and arterial oxygen saturation during apnea and exercise in breath-hold divers.

This study addressed the effects of apnea in air and apnea with face immersion in cold water (10 degrees C) on the diving response and arterial oxygen saturation during dynamic exercise. Eight trained breath-hold divers performed steady-state exercise on a cycle ergometer at 100 W. During exercise, each subject performed 30-s apneas in air and 30-s apneas with face immersion. The heart rate and arterial oxygen saturation decreased and blood pressure increased during the apneas. Compared with apneas in air, apneas with face immersion augmented the heart rate reduction from 21 to 33% (P < 0.001) and the blood pressure increase from 34 to 42% (P < 0.05). The reduction in arterial oxygen saturation from eupneic control was 6.8% during apneas in air and 5.2% during apneas with face immersion (P < 0.05). The results indicate that augmentation of the diving response slows down the depletion of the lung oxygen store, possibly associated with a larger reduction in peripheral venous oxygen stores and increased anaerobiosis. This mechanism delays the fall in alveolar and arterial PO(2) and, thereby, the development of hypoxia in vital organs. Accordingly, we conclude that the human diving response has an oxygen-conserving effect during exercise.     

Ventilatory response to hyperoxia in the chick embryo

In the avian embryo at term we measured the ventilatory response to hyperoxia, which lowers the chemoreceptor activity, to test the hypothesis that the peripheral chemoreceptors are tonically functional. Measurements of pulmonary ventilation (VE) were conducted in chicken embryos during the external pipping phase, at 38 degrees C, during air and hyperoxia, and during hypercapnia in air or in hyperoxia. Hyperoxia (95% O2) maintained for 30 min lowered VE by 15-20%, largely because of a reduction in breathing frequency (f). The oxygen consumption and carbon dioxide production of the embryo were not altered. The hyperoxic drop of VE was more marked in those embryos, which had higher values of normoxic VE. Hypercapnia, whether 2 or 5% CO2, increased VE, almost exclusively because of the increase in tidal volume (VT). The increase in VT was less pronounced when hypercapnia was associated with hyperoxia, and f slightly decreased. Hence, in hyperoxia, the VE response to CO2 was less than in air. The results are in support of the hypothesis that in the avian embryo, after the onset of breathing, the peripheral chemoreceptors exert a tonic facilitatory input on . This differs from neonatal mammals, where the chemoreceptors have minimal or no activity at birth, presumably because the increased arterial oxygenation with the onset of air breathing is a much more sudden phenomenon in mammals than it is in birds.     

Cold-induced changes in breathing pattern as a strategy to reduce respiratory heat loss

Thermoregulatory benefits of cold-induced changes in breathing pattern and mechanism(s) by which cold induces hypoventilation were investigated using male Holstein calves (1-3 mo old). Effects of ambient temperatures (Ta) between 4 and 18 degrees C on ventilatory parameters and respiratory heat loss (RHL) were determined in four calves. As Ta decreased, respiratory frequency decreased 29%, tidal volume increased 35%, total ventilation and RHL did not change, and the percentage of metabolic rate attributed to RHL decreased 26%. Total ventilation was stimulated by increasing inspired CO2 in six calves (Ta 4-6 degrees C), and a positive relationship existed between respiratory frequency and expired air temperature. Therefore, cold-exposed calves conserve respiratory heat by decreasing expired air temperature and dead space ventilation. Compared with thermoneutral exposure (16-18 degrees C), hypoventilation was induced by airway cold exposure (4-6 degrees C) alone and by exposing the body but not the airways to cold. Blocking nasal thermoreceptors with topical lidocaine during airway cold exposure prevented the ventilatory response but did not lower hypothalamic temperature. Hypothalamic cooling (Ta 16-18 degrees C) did not produce a ventilatory response. Thus, airway temperature but not hypothalamic temperature appears to control ventilation in cold-exposed calves.     

The effect of corrective surgery on energy expenditure during ambulation in children with cerebral palsy

Mechanical efficiency, heart rate, blood lactate, and some other variables were studied in six children with cerebral palsy who walked on a treadmill before and after corrective surgery. During each test, conducted at each child's naturally selected speed, two situations were studied: steady state level walking for 9 min, and then walking at an increasing inclination up to 20% for another 10 min. During the test the subjects were allowed to hold on to a handrail to eliminate the risk of falling off the treadmill. The corrective surgery resulted in a 5% reduction in oxygen consumption per kg body mass during level walking. The subjects' levels of physical fitness, as estimated from oxygen pulse, however, were unchanged. These results are indicative of a biomechanical improvement due to the corrective surgery. While walking at a 20% inclination the subjects off loaded themselves to different degrees on the handrail which influenced the results. Their feeling of exhaustion at this load was probably due to local factors, since heart rate was well below maximal values, and blood lactate, respiratory exchange ratio and ventilatory equivalent also indicated that they were below their anaerobic thresholds (50-60% of maximal oxygen uptake).     

Effect of thermoperiod on diapause intensity in Pyrrhocoris apterus (Heteroptera Pyrrhocoridae)

The intensity of adult diapause in Pyrrhocoris apterus was measured in two series of experiments as the duration of pre-oviposition period at a constant temperature of 25 degrees C after transfer from short (12L:12D) to long day conditions (18L:6D). Higher diapause intensity was induced with a thermoperiod than at constant temperatures. After the induction throughout larval instars 3-5 and during 4 weeks of adult life at short days and a thermoperiod of 25/15 degrees C the pre-oviposition period was 30+/-4 and 26+/-3 days. After induction at constant 25 degrees C the pre-oviposition period was 22+/-3 and 23+/-4 days, while after induction at constant 20 degrees C it was 17+/-4 and 19+/-4 days. Induction at a lower constant temperature of 20 degrees C was thus followed by a less intense diapause than the induction at a higher constant temperature of 25 degrees C. These counterintuitive results are discussed. The oxygen consumption rate measured at experimental temperatures prior to transfer from short to long days was higher at thermoperiodic conditions than at constant temperatures and it was similar at constant 20 and 25 degrees C. Thus, the oxygen consumption rate measured prior to the transfer was highest (indication of the least intense diapause) in the insects that showed later, after the transfer to long days, the longest pre-oviposition period (indication of the most intense diapause). Within the first two days after transfer to constant 25 degrees C, oxygen consumption rate measured at 25 degrees C decreased in the thermoperiodic insects, while it transiently increased in insects from constant 20 degrees C. Two days and later after the transfer, oxygen consumption rate was similar in all groups. Cold hardiness was not correlated with diapause intensity. The low lethal temperature in diapausing insects was correlated with the night temperature during diapause induction.     

栉孔扇贝耗氧率和排氨率的研究

1999年 4～ 6月 ,采用室内实验生态学方法对栉孔扇贝的耗氧率和排氨率进行了研究 .结果表明 ,在适宜的温度范围内 ,栉孔扇贝的耗氧率和排氨率均与温度成正比 ,而与体重呈负相关关系 .在实验室温度 (8～ 2 8℃ )条件下 ,栉孔扇贝的耗氧率为 0 .48～ 9.0 9mg·g-1·h-1,排氨率为 0 .0 5～ 1 0 1mg·g-1·h-1.其中耗氧率在 2 3℃时达到最高值 ,2 8℃时开始下降 ,而排氨率则呈持续升高趋势 .栉孔扇贝的日常代谢明显高于标准代谢 ,耗氧率和排氨率平均值分别提高约 35 .8%和 75 .9% .     

Ventilatory accommodation of oxygen demand and respiratory water loss in a large bird,the emu (Dromaius novaehollandiae), and a re-examination of ventilatory allometry for birds

Ventilation was studied in the emu, a large flightless bird of mass 40kg, within the range of ambient temperatures from-5 to 45°C. Data for the emu and 21 other species were used to calculate allometric relationships for resting ventilatory parameters in birds (breath frequency=13.5 mass^-0.314; tidal volume=20.7 mass^1.0). At low ambient temperatures the ventilatory system must accommodate the increased metabolic demand for oxygen. In the emu this was achieved by a combination of increased tidal volume and increased oxygen extraction. Data from emus sitting and standing at-5°C, when metabolism is 1.5x and 2.6x basal metabolic rate, respectively, indicate that at least in the emu an increase in oxygen extraction can be stimulated by low temperature independent of oxygen demand. At higher ambient temperatures ventilation was increased to facilitate respiratory water loss. The emu achieved this by increased respiratory frequency. At moderate heat loads (30–35°C) tidal volume fell. This is usually interpreted as a mechanism whereby respiratory water loss can be increased without increasing parabronchial ventilation. At 45°C tidal volume increased; however, past studies have shown that CO₂ washout is minimal under these conditions. The mechanism whereby this is possible is discussed.Abbreviations BMR basal metabolic rate - BTPS body temperature, ambient pressure, saturated - EO ₂ oxygen extraction - EWL evaporative water loss - f _R ventilation frequency - RH relative humidity - RHL respiratory heat loss - SEM standard error of the mean - SNK student-Newman-Keuls multiple range test - STPD standard temperature and pressure, dry - T _a ambient temperatures(s) - T _b body temperature(s) - T _ex expired air temperature(s) - T _rh chamber excurrent air temperature - V _J ventilation - VO₂ oxygen consumption - V _T tidal volume - V/Q air ventilation to blood perfusion ratio     

In situ and laboratory assessment of heart rate in a Mediterranean limpet using a noninvasive technique

Heart rate of the Mediterranean limpet Patella caerulea L. was investigated on the natural shore and in the laboratory by using a technique based on infrared phototransducers. Field recording occurred in the Gulf of Trieste (northern Adriatic) during March and June 1997. A consistent dependence of heart rate on temperature was observed in limpets both when submerged and when exposed to air in the two periods, but thermal acclimation was evident. During spontaneous activity at high tide, heart rate increased 1.5-1.7 times the values observed during resting in water at corresponding temperatures. The dependence of heart rate on temperature (10 degrees, 16 degrees, and 22 degrees C) and size (wet weight <1.25 and >1.30 g) in submerged limpets from different populations (northern Adriatic and Tyrrhenian) was tested in the laboratory by adopting a factorial design. The results showed a marked effect of temperature, body weight, and their interaction, independent from the site of origin. Smaller limpets showed a linear increase of heart rate in the whole range of temperature tests, while in the larger ones the increase between 10 degrees and 16 degrees C was greater than between 16 degrees and 22 degrees C. Heart rate decreased with increasing body size at control (16 degrees C) and high (22 degrees C) temperature, while at lower temperature (10 degrees C) no effect of body size was evident. When removed from their home scar, limpets increased heart rate to about 1.5 times the reference value. Finally, correlation of oxygen consumption with heart rate of submerged limpets maintained at a different temperature (10 degrees -22 degrees C) was statistically significant.     

The effect of extrinsic and intrinsic factors on oxygen consumption by the southern rock lobster, Jasus edwardsii

The oxygen consumption rate of the southern rock lobster, Jasus edwardsii, was evaluated in response to body weight, temperature, activity, handling, diurnal rhythm, feeding and oxygen saturation level. There was a positive relationship between standard oxygen consumption (M(O(2))) and both body weight and water temperature. The relationship between total oxygen consumption and wet whole body weight was described by the equation: LogM(O(2))=0.595log W-0.396 (r(2)=0.83). The relationship between weight-specific oxygen consumption and temperature was described by the equation: LogM(O(2))=0.047T-2.25 (r(2)=0.94). Activity had a significant influence on the oxygen consumption rate, causing a three-fold increase above the standard rate at the temperature of acclimation (13 degrees C). However, at temperatures approaching the upper and lower extremes, lobsters had a decreased ability to increase their oxygen consumption rates during activity. Lobsters took 4.5-5 h to return to standard oxygen consumption rates after a period of emersion and handling. A strong diurnal rhythm to oxygen consumption was recorded. J. edwardsii displayed a classic postprandial increase in oxygen consumption. A peak (1.72 times standard M(O(2))) occurred 10-13 h after feeding with an increase above standard M(O(2)) being maintained for 42 h. In its rested state J. edwardsii was an oxygen regulator down to a critical oxygen tension of 58 Torr, whilst activity resulted in the critical oxygen tension increasing to 93 Torr.     

Effect of ambient temperature on metabolism and heart rate of resting albino rats]

Heart rate and oxygen consumption were measured simultaneously in albino rats. These measurements were carried out in the resting animal at different temperatures between 18 degrees and 33 degrees C. The metabolism evolution with the environmental temperature allowed to place the thermal neutrality at 29 degrees C. The resting heart rate varies as metabolism. It shows the lowest values round the thermoneutrality and increases quickly as the environment is cooling. This result shows the effect of the thermal environment upon the resting heart rate level. On the other hand, the non linear relationship between metabolism and heart rate points out that the heart rate increase is not the only factor that allows an increased oxygen consumption during the body temperature regulation.     

Tolerance, behaviour and oxygen consumption in the sand goby, Pomatoschistus minutus (Pallas), exposed to hypoxia

Behavioural and metabolic reactions of Pomatoschistus minutus (Pallas) exposed to various degrees of hypoxia were studied. At 15°C and 20‰ mortality was 50% at 15.2% oxygen saturation. Avoidance and oxygen saturation showed a linear inverse relationship. At levels lower than 60% saturation increased activity occurred; avoidance was significant at 30% saturation. Active, routine and standard MO₂ correlated linearly with weight at 6 and 15°C (salinity = 19‰). During hypoxia at 15°C routine MO₂ rose significantly at 60–50% and 40–30% saturation expressed either as MO₂ during longer periods at night or MO₂ at shorter intervals during the day. Standard MO₂ was unaffected by hypoxia at 15°C. Haemoglobin concentration was significantly increased when P. minutus was acclimatized to 35% saturation.     

Basal oxygen consumption,ventilatory frequency,and heart rate during the protracted spawning run of the Southern Hemisphere lamprey Geotria australis

Summary Basal oxygen consumption, ventilatory frequency, and heart rate were recorded at four different times during the unusually protracted 15–16-month spawning run of the Southern Hemisphere lamprey Geotria australis. At 15°C, the mean basal oxygen consumption of G. australis caught immediately after they had left the sea and embarked on the spawning run (45 l · g^-1 · h^-1) was less than in young adults about to commence their marine feeding phase (64 l · g^-1 · h^-1), but greater than in large ammocoetes (26.5 l · g^-1 · h^-1). Basal oxygen consumption fell progressively during the spawning-run of to 33 l · g^-1 · h^-1 after 5 months and 25 l · g^-1 · h^-1 after 10 months, before rising to 35 l · g^-1 · h^-1 after 15 months when the animals were approaching sexual maturity. The downwards trend in basal oxygen consumption contrasts with that recorded during the spawning run of Lampetra fluviatilis. Furthermore, these values for spawning-run of G. australis are far lower than those measured at any time during the upstream migration of L. fluviatilis or during the parasitic phase of landlocked Petromyzon marinus. A low and declining metabolic rate during much of the spawning run of G. australis would facilitate the conservation of energy reserves during this very long non-feeding period. Trends shown by ventilatory frequency and heart rate essentially parallel those of basal oxygen consumption. The Q₁₀s for basal oxygen consumption, ventilatory frequency and heart rate over the temperature range 5–25°C were 1.6, 1.6, and 1.7, respectively. The trends shown by basal oxygen consumption during metamorphosis and the upstream migration did not parallel those exhibited by circulating thyroid hormones.     

Reduced stress- and cold-induced increase in energy expenditure in interleukin-6-deficient mice

Interleukin-6 (IL-6) deficient (-/-) mice develop mature onset obesity. Pharmacological studies have shown that IL-6 has direct lipolytic effects and when administered centrally increases sympathetic outflow. However, the metabolic functions of endogenous IL-6 are not fully elucidated. We aimed to investigate the effect of IL-6 deficiency with respect to cold exposure and cage-switch stress, that is, situations that normally increase sympathetic outflow. Energy metabolism, core temperature, heart rate, and activity were investigated in young preobese IL-6-/- mice by indirect calorimetry together with telemetry. Baseline measurements and the effect of cage-switch stress were investigated at thermoneutrality (30 degrees C) and at room temperature (20 degrees C). The effect of cold exposure was investigated at 4 degrees C. At 30 degrees C, the basal core temperature was 0.6 +/- 0.24 degrees C lower in IL-6-/- compared with wild-type mice, whereas the oxygen consumption did not differ significantly. The respiratory exchange ratio at 20 degrees C was significantly higher and the calculated fat utilization rate was lower in IL-6-/- mice. In response to cage-switch stress, the increase in oxygen consumption at both 30 and 20 degrees C was lower in IL-6-/- than in wild-type mice. The increase in heart rate was lower in IL-6-/- mice at 30 degrees C. At 4 degrees C, both the oxygen consumption and core temperature were lower in IL-6-/- compared with wild-type mice, suggesting a lower cold-induced thermogenesis in IL-6-/- mice. The present results indicate that endogenous IL-6 is of importance for stress- and cold-induced energy expenditure in mice.     

The influence of ambient temperature on metabolism and body temperature of newborn and growing reindeer calves (Rangifer tarandus tarandus L.)

Thermoregulatory capacities of 51 reindeer calves (Rangifer tarandus tarandus L.) aged 1-35 days were studied at -26.5 to +35.0 degrees C ambient temperatures at Kaamanen reindeer research station, Finland (69 degrees 10' N) during calving periods in May 1981 and May-July 1982. The newborn calves aged 1-4 days maintained a high body temperature (Tre) (mean +40.2 degrees C) even at the lowest experimental temperature of -22.5 degrees C by increasing their metabolic rate five-fold above the level at +11.0 degrees C. Heat production of the new-born calves was largely based on the metabolism of brown adipose tissue, stimulated by cold-induced discharge of the sympathetic nervous transmitter, noradrenaline (NA). Sensitivity of the calves to exogenous NA disappeared during the first 3-4 weeks of life. Thermal conductance of the calves was low at low ambient temperatures, but rose strongly as Ta increased above +10 degrees C. The extensive peripheral cooling, especially in the feet, was demonstrated in the calves aged 1-10 days. The lowest foot temperature (+10.5 degrees C) was measured in a 4-day-old calf at -14.5 degrees C. Slight shivering thermogenesis was recorded in the calves aged 1-4 days and occasionally in the older calves at low values of Ta. Shivering appears to be a reserve mechanism against severe cold. At about +20 degrees C and above the calves increased their Tre (approximately 1 degree C), oxygen consumption and heart rate. In the newborn calves oxygen consumption rose four- to five-fold and in 1-month-old calves about two-fold. Fast growing calves (maximum 400 g/day) appear to be more stressed by heat than by cold exposure.     

Comparison of buffer and red blood cell perfusion of guinea pig heart oxygenation

Myocardial mean myoglobin oxygen saturation was determined spectroscopically from isolated guinea pig hearts perfused with red blood cells during increasing hypoxia. These experiments were undertaken to compare intracellular myoglobin oxygen saturation in isolated hearts perfused with a modest concentration of red blood cells (5% hematocrit) with intracellular myoglobin saturation previously reported from traditional buffer-perfused hearts. Studies were performed at 37 degrees C with hearts paced at 240 beats/min and a constant perfusion pressure of 80 cmH2O. It was found that during perfusion with a hematocrit of 5%, baseline mean myoglobin saturation was 93% compared with 72% during buffer perfusion. Mean myoglobin saturation, ventricular function, and oxygen consumption remained fairly constant for arterial perfusate oxygen tensions above 100 mmHg and then decreased precipitously below 100 mmHg. In contrast, mean myoglobin saturation, ventricular function, and oxygen consumption began to decrease even at high oxygen tension with buffer perfusion. The present results demonstrate that perfusion with 5% red blood cells in the perfusate increases the baseline mean myoglobin saturation and better preserves cardiac function at low oxygen tension relative to buffer perfusion. These results suggest that caution should be used in extrapolating intracellular oxygen dynamics from buffer-perfused to blood-perfused hearts.