Respiratory exchanges in the incubated egg of the domestic guinea fowl.

The daily O2 uptake (MO2) and CO2 production (MCO2) rates were measured in guinea fowl eggs with a wide range of mass specific shell water vapor conductance, spGH2O [0.108 to 0.365 mg.(g.Torr.day)-1], from day 10 to the end of incubation, day 28. The respiration rate showed a plateau, from day 22 to day 24, typical of a precocial bird: MO2 approximately 21.4 and MCO2 approximately 15.2 mmol.day-1. The plateau MO2 value was that predicted by allometric relation. At the plateau, respiratory exchanges appeared significantly limited by spGH2O. The limiting effect of spGH2O on respiratory metabolism began as soon as day 18 (64% of incubation time). The respiratory ratio was 0.75 at day 10, decreased to 0.65-0.66 on days 12-14 and stabilized at 0.70-0.71 on days 20-28.

Gordon memorial lecture. Physics and physiology of incubation.

1. The earliest mention of artificial incubation occurs in Aristotle's Historia Animalium written in the 4th century BC. A brief survey of the history of incubation is given from that time to the present. 2. Artificial incubation is also practised by birds belonging to the family of the Megapodes: the Brush Turkey and the Mallee Fowl build a mound and maintain the required temperature of the eggs laid in it. 3. The importance of functional eggshell porosity and incubator ventilation rate for maintaining optimal gas tensions in the embryonic medium (the gas space below the shell) is discussed. 4. Among the early scientific studies (reviewed by Landauer, Lundy, Freeman) particular attention is paid to Barott's (1937) systematic work on temperature, relative humidity and oxygen concentration. 5. The requirements of the embryo with regard to temperature, humidity and gaseous environment are defined. The importance of using gas tensions instead of gas concentrations is once again emphasised. 6. The problems of incubation at high altitude are explained and a successful method for hatching eggs at any terrestrial altitude is described. 7. Although hens can be selected for the functional porosity of their eggs, the procedure does not offer any worthwhile advantages. 8. If functional eggshell porosity and embryonic oxygen uptake are known, then optimal incubator ventilation rate can be predicted when a given optimum gas space oxygen tension is assumed.

Air space: the embryonic medium.

Normalization of gas exchange and growth of chicken embryos incubated under abnormal conditions with regard to eggshell conductance and barometric pressure is directed to restoring the normal water loss from the eggs and the normal pattern of air space gas tensions during development. The latter is achieved by maintaining an O2 tension of 149 torr in the gas mixture supplied to the incubator and by reducing the incubator ventilation rate to compensate for increased, standard or effective, eggshell conductances. Thus, for maintenance of normal air space gas tensions, a decrease of the diffusive resistance of an egg requires a similar increase of the convective resistance so that the total resistance against gas transport remains normal. The latter must be set at a value given by (149-PA)/M, where PA is the normal air space O2 tension, torr, and M is the normal O2 uptake, ml(STPD)/day. This value is independent of embryonic age, because PA is directly proportional to M during development. Thus the convective resistance required for maintenance of normal PA and M values is given by the difference between (149-PA)/M and the diffusive resistance. Such PA and M values can be maintained in eggs with the minimum diffusive conductance, M/(149-PA), thus requiring an infinite convective conductance, to infinite diffusive conductance, which requires a minimum convective conductance, M/(149-PA). This minimum is predicted for all shell-less eggs, viz., for the continuous replacement of diffusive by convective gas transport.

Altitude hypocapnia at 2,800 m does not affect development of the chicken embryo.

Eggs laid at sea level and incubated at high altitude are subject to hypoxia, hypocapnia, and excessive water loss, resulting in retarded development and poor hatchability. The effect of altitude hypocapnia alone was studied in two series of eggs incubated at a simulated altitude of 2,800 m, PB = 542 torr; the incubator was ventilated at a low flow rate with O2-enriched air; the relative humidity was 70-74%, PH2O 34.4-36.4 torr; ambient PO2 about 130 torr at the plateau stage. In the normocapnic series, CO2 produced by the embryos increased ambient PCO2 to 14 torr at 18-19 days; altitude hypoxia, hypocapnia, and excessive water loss were practically compensated for. In the hypocapnic series, ambient CO2 was almost completely absorbed by soda lime, so that only hypocapnia was not compensated for. In 17-19-day eggs with similar sea level mass specific shell conductances [sp GH2O = 0.26-0.25 mg [g.d.torr]-1], the measured PO2 in the gas space, hematocrit, hemoglobin concentration, lengths of beak and third toe, and masses of body and brain were essentially the same in both series. The masses of heart, liver, and left wing were slightly different on day 19. Altitude hypocapnia alone, without altitude hypoxia and excessive water loss, had almost no significant effect on the embryos' development and hatchability.

Variability of shell conductance and gas exchange of chicken eggs.

In 395 fertilized chicken eggs (strain Warren) obtained from a commercial hatchery the following coefficients of variation (SD/mean) were found: egg shell conductance for water vapor, GH2O, 22%; freshly laid egg weight, W, 8%; the specific conductance, gH2O (= GH2O/W), 22%. In 20 eggs selected for widely varying gH2O (range 57%-195% of the mean, 0.246 mg/(day X torr X g), the specific O2 uptake and the CO2 output, measured on days 16-19 of incubation, showed a maximum at medium gH2O values, decreasing at both lower and higher gH2O. The variations of gH2O in the selected eggs were shown to cause a variation of water loss up to hatching from 9 to 26% (average 15%), and a variation of the O2 tension from 84 to 123 torr, and of the CO2 tension from 18 to 54 torr, in the perichorioallantoic air space on days 16-19 of incubation. Mechanisms responsible for the observed changes in the metabolic rate and for the physiological adjustments to varied egg shell conductance are discussed.

Gas exchange and hatchability of chicken eggs incubated at simulated high altitude.

Chicken eggs laid at sea level were incubated at sea level (control conditions), at a simulated altitude of 5.5 km without any further measures (natural conditions), and at a simulated altitude of 5.7 km at optimal incubator gas composition (optimal conditions). Under optimal conditions the incubator relative humidity was 70% throughout incubation, the gas mixture supplied to the incubator contained 45% O2-55% N2, and the ventilation rate was reduced to 6% of control in order to maintain the normal air-space gas tensions and to compensate for the increased eggshell conductance at altitude. The embryos that developed under control conditions showed a normal CO2 production with 94% hatchability of fertile eggs. Under natural conditions at altitude all embryos died within a few days. Optimal conditions resulted in an almost normal gas exchange and in an improvement of hatchability from 0 to 81% of fertile eggs.

Blood gases and acid-base status in chicken embryos with naturally varying egg shell conductance.

In chicken eggs selected for widely varying values of specific water vapor conductance, gH2O (= water vapor conductance per freshly laid egg mass), PCO2, pH, PO2 and hematocrit were measured in arterialized blood sampled from an allantoic vein (after 16 days of incubation) or in blood termed 'venous', sampled from an allantoic artery (after 18 days of incubation). Both arterialized and 'venous' PCO2 were inversely related to gH2O. Since the variations of blood plasma pH with PCO2 were smaller than predicted for true plasma, partial compensation by appropriate non-respiratory changes of plasma bicarbonate concentration must have occurred. Only with extremely high and low gH2O a definite alkalosis and acidosis, respectively, were observed. Both arterialized and 'venous' PO2 tended to diminish with decreasing gH2O. The hematocrit value showed a tendency to increase with decreasing gH2O and with decreasing arterialized PO2.

Gas exchange, blood gases and acid-base status in the chick before, during and after hatching.

To study the transition from chorioallantoic to pulmonary gas exchange in birds, blood gases and acid--base variables were measured in chicks of domestic fowl before, during and after hatching. Measurements were made in samples of 'venous' blood (from allantoic arteries or the right ventricle, respectively) entering the gas exchanger (chorioallantois or lungs, respectively) and arterialized blood (from allantoic veins or the left ventricle, respectively) leaving the gas exchanger. Also, O2 uptake was measured and blood flow of the gas exchanger was determined according to the Fick principle. During the last days of incubation PO2 decreased PCO2 increased in both arterialized and 'venous' blood, but the changes of pH were small due to a concomitant increase in bicarbonate concentration, in accordance with the results of previous studies. After external pipping and hatching pronounced hypocapnia developed, but the respiratory alkalosis was partiallY compensated by a transitory non-respiratory reduction of bicarbonate. In spite of arterial hypoxia at the end of incubation and some loss of blood during hatching, blood O2 transport was not seriously impaired during pipping and hatching as revealed by 'venous' blood gases. The blood gases and pH of 17-day-old chicks were close to those of adult chickens.

Replacement of diffusive by convective gas transport in the developing hen's egg.

Comparison of diffusive and convective transport equations for O2, CO2 or H2O in the gas-phase shows that for the same gas flux (ml . day-1) and the same pressure difference (Torr) one can substitute an effective air ventilation, VA (ml . day-1), for a given diffusive conductance, G, (ml . day-1. Torr-1). The theoretical relation is: VA = (RT) . G. Thus air ventilation of 865 ml . day-1 (BTPS) is required for each unit of G. This relation was tested in the developing chick embryo (days 15 to 19) by continuously ventilating the air space beneath the shell while the egg was maintained at 37.8 degrees C in air or submerged. During this exposure normal development, oxygen uptake, and gas tensions were maintained, verifying the theoretical prediction.

Renal hemodynamics and proteinuria in running and swimming beagle dogs.

In beagle dogs swimming, in contrast to treadmill running, was found to cause an increase in urine flow and urinary protein excretion. Renal blood flow measured by electromagnetic flow probes decreased by 13.0 +/- 4.9% when the treadmill gradient was 15% and arterial pressure was elevated by 11.6 +/- 4.9%. Immersion resulted in an immediate decrease in renal blood flow of 8.8 +/- 5.1% and a 24.6 +/- 6.9% increase in arterial pressure. Acid-base status indicated a respiratory alkalosis in all running experiments, no net change in five swimming experiments in which hyperventilation occurred, but a metabolic acidosis in eight swimming experiments without hyperventilation. During running there was a threefold increase in oxygen consumption. We conclude that swimming possibly induces more sympathetic nervous activity than treadmill running in dogs, while an alkalosis is consistently present during running, but acid-base response is variable during swimming.

Carbon dioxide in the chick embryo towards end of development: effects of He and SF6 in breathing mixture.

Using an implanted CO2 electrode in the air cell of the chicken egg, its PCO2 could be followed continuously during the prenatal and paranatal period until hatching occurred. CO2 elimination rate was followed simultaneously. At various stages such eggs were subjects to 75 : 25% He/O2 and SF6/O2 atmosphere, resulting in a large decrease and increase, respectively, in air cell CO2 tension, indicating that during the prenatal stage all CO2 exchange was by gas phase diffusion transport across the pores of the shell. Measurements of the change in PCO2 as well as the CO2 output allowed one to calculate the effective diffusion coefficient for CO2 in the He and SF6 mixtures, which agreed well with the theoretical values calculated according to Wilke (1950). From the CO2 release or retention following exposure, respectively, to He or SF6 the CO2 capacitance values for blood and tissue could be calculated and agreed with values established in mammals. During the last period of development, the paranatal period, the change of PCO2 after replacement of N2 by He gradually declined, indicating that pulmonary ventilation was replacing diffusion through egg shell pores.

The independent effects of atmospheric pressure and oxygen partial pressure on gas exchange of the chicken embryo.

CO2 production and air cell PCO2 were continuously measured during late development in the chicken egg while acutely exposed from one to three hours to various O2 concentrations ranging from 11 to 39%. A small but significant increase in metabolism, ca. 8%, was found when O2 concentration was above normal values, while a reduction to 70% was observed when O2 concentrations were below normal, and fell to 50% when maintained for three hours. These values were also compared with metabolic rates reported by Lokhorst and Romijn (1965, 1967)) who incubated eggs continuously at reduced O2 concentrations as well as under reduced barometric pressure, and showed that at the same ambient PO2 the metabolism was significantly higher in the eggs at reduced barometric pressure. We attribute this difference to the increased diffusion coefficient of O2 which is inversely related to the barometric pressure. It illustrates that the ambient partial pressure of O2 and ambient atmospheric pressure exert an independent effect upon gas exchange of the avian embryo.