Gas exchange variables were measured in unanaesthetized domestic geese fitted with rubber facemasks and indwelling air-sac and arterial catheters. The results were analysed on the basis of functional models.

  1. Ventilation was characterized by low frequency (8·4 min−1) and high tidal volume (29·3 ml kg−1).

  2. Average values (± S.E.) of arterial blood variables were as follows: kPa, pH = 7·52 ± 0·01. Body temperature was 41·4 ± 0·2°C.

  3. The gas exchange ratio (calculated with reference to inspired gas) of caudal air sacs (average 1·09) was higher, and that of cranial air sacs (0·73) lower, than that of mixed-expired (0·82) or end-expired gas (0·78). This pattern can be explained by a higher effective ventilation/perfusion ratio in the neopulmo than in the paleopulmo.

  4. During inspiration, the neopulmo was estimated to contribute about 7% to the overall inspiratory O2 uptake, and about 18% to the CO2 output. Total inspiratory gas exchange was twice that during expiration.

  5. Arterial , was close to, but lower than, the partial pressure in cranial air sacs and in end-expired gas. This can be explained on the basis of a crosscurrent gas exchange system with unequal distribution of ventilation to perfusion between functional compartments.

After the experimental confirmation of unidirectional gas flow through the avian lung, a number of theoretical and experimental studies have been devoted to further analysis of gas exchange in avian lungs (reviewed by Scheid, 1979, 1982). Only a few experimental studies have been conducted on unanaesthetized birds, although it is accepted that anaesthesia affects respiration. Measurements of ventilation and of respiratory gases in expired air, in air sacs and in blood have been performed in unanaesthetized, lightly restrained pigeons (Bouverot et al. 1976), ducks (Bouverot et al. 1979; Jones & Holeton, 1972), hens (Piiper et al. 1970) and geese (Cohn & Shannon, 1968). Notable are the studies on ducks (Kiley et al. 1979) and hens (Brackenbury et al. 1981, 1982) running on the treadmill. None of these experiments was designed, however, for a complete analysis of gas exchange on the basis of the currently accepted pattern of respiratory gas flow and the cross-current model for parabronchial gas exchange.

In this study, we investigated avian gas exchange by simultaneous analysis of CO2 and O2 in the various air sacs, in expired gas and in arterial blood. This allowed partitioning of CO2 output and O2 uptake between paleopulmo and neopulmo and between inspiration and expiration. Particular attention was paid to maintaining the awake birds in resting conditions with as little disturbance as possible.

Measurements were performed on 11 Embden breed domestic geese (Anser anser) weighing 3·5–7·6kg (average 5·0kg). Initially, measurements were made during one or more days on intact, awake, resting geese. The birds were then anaesthetized, and indwelling air-sac and vascular catheters were installed. The animals were allowed to recover for at least 2 days before measurements, including determination of air-sac gas concentration and arterial blood values on birds in the awake, resting state, were resumed. The birds were deprived of food for approximately 12h before measurements, but water was always available up to the time of measurement.

Indwelling air-sac catheters were inserted under halothane anaesthesia (1–2% in O2). Perspex tubes, 4–10 cm long, with an inside diameter of 8 mm, were tightly sealed in the clavicular, left cranial thoracic, left caudal thoracic and left abdominal air sacs. The air-sac membranes were tied around the tubes, and the skin was sutured to form an airtight seal. A Perspex cap was screwed to the tube to seal the sac. Intramuscular injections of antibiotic (Ampicillin) were given to help prevent infection.

Prior to measurement of air-sac gas concentrations, the cap on each air-sac cannula was removed and a plastic rod, in which was embedded a PE 50 catheter, was inserted into the air sac and sealed in place by a cap and two rubber washers. The polyethylene catheter could be connected to the input of a mass spectrometer (Scheid, 1983) to provide continuous measurements of air-sac gas composition. Necropsy of the birds following the experiments verified the correct placement of the catheters.

A polyethylene catheter was placed in the left brachial artery for sampling of arterial blood.

Measurements

On the days of the experiments, the birds were blindfolded and lightly restrained in a metal stand in such a way as to allow normal breathing movements.

Two polyethylene catheters (PE 50) were taped to the bill (Fig. 1). One was inserted into the left nostril for measurement of expired and profiles; the other was placed in the dorsal midline, proximal to the nostrils, for measurement of inspired gas composition. A foam rubber facemask was placed over the bill and secured with rubber bands around the back of the head. The mask was sealed airtight to the face with machine grease. The polyethylene catheters were connected to the input capillary of the mass spectrometer for measurement of fractional concentrations of CO2 and O2 in inspired and end-expired gas.

Fig. 1.

Set-up for measurement of gas exchange in the awake goose. For details, see text. V·, flow, measured by pneumotachograph; Ft, FE′, fractions (of O2 and CO2) in gas flowing to the face mask and obtained from the nostril. Air sacs with indwelling Perspex tubes: Clav, clavicular; CrTh, cranial thoracic; CdTh, caudal thoracic; Abd, abdominal.

Fig. 1.

Set-up for measurement of gas exchange in the awake goose. For details, see text. V·, flow, measured by pneumotachograph; Ft, FE′, fractions (of O2 and CO2) in gas flowing to the face mask and obtained from the nostril. Air sacs with indwelling Perspex tubes: Clav, clavicular; CrTh, cranial thoracic; CdTh, caudal thoracic; Abd, abdominal.

The mask contained a mid-dorsal port through which room air was passed at a constant flow (5–61 min−1) that was maintained slightly above peak inspiratory flow to prevent reinhalation of expired gas. The air passed over the bill and exited at the tip of the mask through a hose connected to a Fleisch pneumotachometer (size 0) and a Godart-Statham differential manometer.

The constant-flow signal was subtracted from the pneumotachogram by an analog computer (Electronic Associates, Inc., model TR-20) and deviations from this flow, caused by inspiration and expiration, were integrated to yield the tidal volume (VT). The output signals from the mass spectrometer and the analog Somputer were recorded on a multichannel pen recorder (Gould-Brush, model 81).

Gas flowing out of the mask was collected into a 201, gas-tight bag, and the volume was measured in a calibrated spirometer. To check that no air leaked from the mask, the inflow line was detached from the mask, the bird was temporarily supplied with air from a second source, and gas flow from the detached inflow line was directly measured with the spirometer.

Protocol

Determinations of ventilation and gas exchange were made on nine geese before installation of the air-sac and vascular catheters. These measurements were repeated on successive days in five of the birds. Up to three sets of measurements were accomplished on a given day. Following cannulation, measurements including air-sac gas and blood gas determinations were made on seven birds, of which five had been studied before air-sac cannulation. These measurements were repeated on five successive days in one bird, and on two or more days in five birds. The measurement sessions lasted from 2 to 4 h and they were discontinued if the goose was not perfectly quiet.

Calculations

Partial pressures of O2 and CO2 and pH in arterial blood samples were measured at body temperature with a conventional electrode system (BMS III, Radiometer, Copenhagen, Denmark). This electrode system and the mass spectrometer were calibrated with gases provided by precision gas-mixing pumps (Type M301/a-F, W östhoff, Bochum, FRG). Blood gas correction factors for were determined for each experiment by equilibrating blood samples with gases of known partial pressure. Body temperature was monitored using a thermistor inserted 10cm into the colon. Fractions of respiratory gases were converted to partial pressures at the animal’s body temperature and the actual barometric pressure, which averaged 99·7 kPa (S.D. =0·4kPa).

O2 uptake and CO2 output were calculated using the inspired and bag gas concentration difference, the volume of gas collected, and the time of collection. Total (inspired) ventilation was measured from the sum of the inspired tidal volumes (Vi) during the collection period. Mixed expiratory concentrations (FĒ) were calculated from the relationship:
formula
where VB is the total gas volume collected in the bag, and Fi and FB are the fractional concentrations of inspired and bag gas, respectively.
The Bohr dead space (VD) for CO2 was calculated from the relationship:
formula
where VT is the mean tidal volume, and FE′ is the fractional concentration in end-expired gas, read from the mass spectrometer output when sampling through the catheter in the nostril, and averaged over this collecting period.
The gas exchange ratio, R, for mixed-expired, end-expired and air-sac gases (site x) with reference to inspired gas was determined according to the relationship:
formula
where was calculated as 1. It is important to note that R denotes the ratio CO2 output/O2 uptake only when applied to mixed-expired gas and, with reasonable approximation, when applied to end-expired gas. R for air sacs is a formal index from which information on the CO2 output/ O2 uptake ratio for the paleopulmo and neopulmo during inspiration and expiration can be derived (see Discussion).

Mean values were calculated for all measurements in a given bird before and after air-sac cannulation. Overall mean values were calculated by averaging the means from individual birds, giving each mean the same weight irrespective of the number of measurements involved. This calculation of an overall mean meant that birds with a large number of measurements did not influence the results more than those with fewer measurements. A t-test was used to determine if mean values before and after cannulation were significantly different. A one-way analysis of variance was applied to determine if differences among arterial, cranial air-sac and end-expired and values were significant (P ⩽5 0·05).

After implanting indwelling air-sac and arterial cannulae, respiratory frequency (f) and ventilation were significantly decreased, and body temperature was significantly increased (Table 1). No other measured variables were significantly changed by the cannulation procedure. Most values were less variable after the cannulations.

Table 1.

Gas exchange variables in nine geese before and seven geese after implantation of indwelling air sac and vascular cannulae; five geese were investigated both before and after cannulation

Gas exchange variables in nine geese before and seven geese after implantation of indwelling air sac and vascular cannulae; five geese were investigated both before and after cannulation
Gas exchange variables in nine geese before and seven geese after implantation of indwelling air sac and vascular cannulae; five geese were investigated both before and after cannulation

The CO2 and O2 partial pressures and the exchange ratios in expired gas and in air sacs are shown in Table 2. There were only minor (but consistent) differences between the air sacs of the cranial group (i.e. clavicular and cranial thoracic air sacs) and between those of the caudal group (i.e. caudal thoracic and abdominal air sacs), but there were large differences between the cranial and the caudal air-sac groups. The cranial air sacs had higher , lower and lower R values than the caudal air sacs. R values for end-expired and for mixed expired gases were intermediate between those for caudal and cranial air sacs, but closer to the latter. was significantly lower in arterial blood than in end-expired (by 0·7 kPa on average) and cranial air-sac gas (by 0·9 kPa on average) (Table 3). Significant differences between cranial air-sac and end-expired were not observed. in arterial blood was not significantly different from that in cranial air-sac or end-expired gas, but end-expired was significantly higher than that in the cranial air sac.

Table 2.

Partial pressure of CO2 and O2 and exchange ratio (R), calculated according to equation 3, for expired gas and air sacs in seven geese

Partial pressure of CO2 and O2 and exchange ratio (R), calculated according to equation 3, for expired gas and air sacs in seven geese
Partial pressure of CO2 and O2 and exchange ratio (R), calculated according to equation 3, for expired gas and air sacs in seven geese
Table 3.

Partial pressures (in kPa) of CO2 and O2 in arterial blood, cranial air sacs ( = clavicular and cranial thoracic) and end-expired gas, and arterial blood pH in seven geese

Partial pressures (in kPa) of CO2 and O2 in arterial blood, cranial air sacs ( = clavicular and cranial thoracic) and end-expired gas, and arterial blood pH in seven geese
Partial pressures (in kPa) of CO2 and O2 in arterial blood, cranial air sacs ( = clavicular and cranial thoracic) and end-expired gas, and arterial blood pH in seven geese

Physiological state

Measurements of respiration in unanaesthetized animals are physiologically more meaningful than values measured under anaesthesia. However, there are problems because of confinement of the animal, leading to unsteady condition and departures from undisturbed resting values. Although there was no evidence that the geese habituated to the experimental situation, the values measured later, i.e. in cannulated animals, may more closely represent resting values of undisturbed geese. This is supported by the smaller variation of respiratory values after cannulation than before.

The arterial and pH were in the range of values measured in awake, unanaesthetized birds (Table 4). The specific O2 uptake was somewhat lower than values measured in other geese and in ducks.

Table 4.

Oxygen uptake and arterial blood gases and pH measured in unanaesthe-tized, unrestrained birds

Oxygen uptake and arterial blood gases and pH measured in unanaesthe-tized, unrestrained birds
Oxygen uptake and arterial blood gases and pH measured in unanaesthe-tized, unrestrained birds

Analysis of gas exchange based on inspired, expired and air-sac gas

In the following analysis we will use the simplified functional avian lung-air sac model of Fig. 2, and will make the following, in part problematic, assumptions.

Fig. 2.

Model for calculations of gas exchange in the neopulmo and paleopulmo during inspiration (A) and expiration (B). The air flow is shown by the arrows. The shaded areas show the paleopulmo (pp) and neopulmo (np). CrS, cranial air sacs, CdS, caudal air sacs; Ī, mixed or effective inspired; Eep, end-parabronchial during expiration; E′, end-expired (in the trachea); M, gas exchange rate during inspiration (insp) or expiration (exp).

Fig. 2.

Model for calculations of gas exchange in the neopulmo and paleopulmo during inspiration (A) and expiration (B). The air flow is shown by the arrows. The shaded areas show the paleopulmo (pp) and neopulmo (np). CrS, cranial air sacs, CdS, caudal air sacs; Ī, mixed or effective inspired; Eep, end-parabronchial during expiration; E′, end-expired (in the trachea); M, gas exchange rate during inspiration (insp) or expiration (exp).

  1. The model contains a single cranial and a single caudal air sac, whose and are taken as averages of the values for clavicular and cranial thoracic air sacs, and for caudal thoracic and abdominal air sacs, respectively (Table 5).

    Table 5.

    Partial pressure values (in kPa) measured and derived in the analysis of gas exchange (see Discussion)

    Partial pressure values (in kPa) measured and derived in the analysis of gas exchange (see Discussion)
    Partial pressure values (in kPa) measured and derived in the analysis of gas exchange (see Discussion)

  2. Moreover, it is assumed that there are no differences in air-sac gases between the two sides of the body.

  3. Steady-state gas exchange is assumed, in spite of the very low breathing frequency. In particular, respiratory variations of air-sac gas composition are neglected, as are gas concentration gradients within the air sacs, although they have been shown to exist in ducks (Torre-Bueno et al. 1980).

  4. Gas exchange is assumed to occur in the parabronchi only, not in the air sacs. This appears justified since, according to Magnussen et al. (1976), only about 2% of CO2 exchange and less than 5% of O2 exchange occurred in the air sacs of the muscovy duck.

  5. Perfect unidirectional flow by aerodynamic valving is assumed, meaning that (a) during inspiration there is no flow through the ventrobronchi into the cranial air sacs, as confirmed by Powell et al. (1981) for the duck and by Banzett et al. (1987) for the goose, and (b) during expiration there is no shunting of gas from the caudal air sacs through the main bronchus (a shunt of about 12% was estimated in muscovy ducks by Powell et al. 1981).

  6. An equal partitioning of ventilation to cranial and caudal air sacs is assumed, based on experimental results obtained in spontaneously breathing muscovy ducks (Scheid et al. 1974).

Inspiratory phase

Because end-expired gas (E′) remains in the dead space (VD) from the preceding expiration, the mean partial pressures in the gas before entering the parabronchi, i.e. the effective inspired partial pressures, PI, are:
formula
The Pī values shown in Table 5 are obtained using the values from Tables 1 and 2.
When N2 equilibrium (no net transfer) is assumed, the gas transfer rate (of CO2 and O2) in a gas-exchanging compartment (paleopulmo or neopulmo) during inspiration, Ṁinsp, is:
formula
where the subscripts in and out refer to the inflow and outflow ends of the respective gas-exchanging compartment; βg is the gas phase capacitance coefficient (= 8·55 ml l−1 kPa−1 at the mean body temperature of 41·4°C; Piiper et al. 1971), and fN2 is the ‘N2 correction’ factor:
formula
According to assumption 5, to both neopulmo and paleopulmo is half of the total ventilation, V. The transfer rate in the neopulmo is thus:
formula
and that in the paleopulmo:
formula
where CdS and CrS refer to the caudal and cranial air-sac groups. It should be noted that equation 7 does not require that all gas inspired into caudal air sacs should pass through the neopulmo (see Fig. 2).

With the values from Table 5, equations 7 and 8 yield the following values for fractional neopulmonic transfer during inspiration, Ṁnp/(Ṁnp+Ṁpp): CO2, 18%; O2, 7%. The combined values of Ṁinsp for the neopulmo and paleopulmo suggest that 59% of the total CO2 output and 65% of the total O2 uptake occurred during inspiration.

According to equation 5, the exchange ratio is:
formula
With the values from Table 5, one obtains the following inspiratory R values: for the neopulmo, 1·9; for the paleopulmo, 0·66; for the whole lung, 0·75. The high R value for the neopulmo indicates it has a high ratio. From inspection of the lines calculated for domestic ducks by Hastings & Powell (1986), one can estimate from these R values that the ratio should be about 2–2·5 times higher in the neopulmo than in the paleopulmo, and this is in agreement with our findings.

Expiratory phase

During expiration, approximately equal gas flows are assumed to exit from caudal air sacs via the lungs (both paleopulmo and neopulmo) and from the cranial air sacs (assumptions 4 and 5, above). Therefore, the end-parabronchial partial pressures during expiration (PEep) may be calculated as the average value between PCrs and PE′ (Table 5). The R value calculated for the paleopulmo during expiration, using the PCds and PEep values and equation 9, is 0·73. This value is very similar to the value calculated above for the paleopulmo during inspiration, 0·75.

Unfortunately, the role of neopulmonic and paleopulmonic gas exchange cannot be separated for the expiratory phase, because this would require measurement at the entrance to the paleopulmonic parabronchi. But it is possible to estimate the total gas exchange occurring during expiration (ṀexP), using a relationship that corresponds to equation 5 and considering that the flow from the caudal air sacs through the neopulmo and then through the paleopulmo is again half of the total ventilation:
formula
The results indicate that 29% of the total CO2 output and 33% of the total O2 uptake occurred during expiration. Although inspiratory and expiratory CO2 exchange add up to only 88% of the total, the disagreement is not too large in view of the assumptions made and of the experimental errors. For O2, the rates add up to 98%.

The data further suggest that about two-thirds of gas exchange occurred during inspiration and one-third during expiration. This is somewhat surprising since inspiratory time was only half expiratory time. However, the gas entering neopulmonic and paleopuhnonic parabronchi during the inspiratory phase is closer in composition to the inhaled air than it is during the expiratory phase.

Arterial blood vs end-expired and cranial air-sac gas

In their study on the anaesthetized goose, Cohn & Shannon (1968) found end-tidal and clavicular air-sac to be very similar to arterial and concluded that the parabronchial air flow was unidirectional. The flow pattern proposed by these authors has indeed later been corroborated by more direct techniques (see Scheid, 1979). We consistently measured values of arterial that were significantly below end-tidal and clavicular , however. A lower value for arterial than end-expired has also been reported in earlier studies on avian gas exchange (Piiper et al. 1970; Scheid & Piiper, 1972; Powell et al. 1978). This arterial-to-end-expired difference, ‘anomalous’ from a mammahan point of view, is predictable as a result of the functional cross-current arrangement of air and blood flow in avian lungs (Scheid, 1979).

Based on the average difference of –0·9 kPa (Table 3) between arterial blood and cranial air-sac gas, end-parabronchial appears to be close to the mixed venous -A higher hi end-expired gas than in mixed venous blood has been reported in the chicken in hypercapnia (Davies & Dutton, 1975; Meyer et al. 1976), and can be explained by the action of the Haldane effect in a crosscurrent avian lung model (Meyer et al. 1976).

In contrast to , there was no significant difference between the of arterial blood and end-expired or cranial air-sac gas. In a cross-current system, a negative end-expired-to-arterial difference is possible. However, the unequal distribution of ventilation to perfusion (/ Q+̇ inhomogeneity), which has been shown by the multiple inert gas elimination technique to be present in bird lungs (Powell & Wagner, 1982), can easily increase this to zero or to a positive value. Indeed, in most studies on birds breathing air, a positive value of A has been found (Piiper et al. 1970; Jones & Holeton, 1972). It is important to note that the effect of a given inhomogeneity is much stronger on than on A, except in the case of very high corresponding to functional dead space ventilation (Powell & Wagner, 1982).

In addition to the influence of inhomogeneities, diffusion limitation offered by the gas exchange tissue and by pulmonary capillary blood is also expected to produce a positive value of , and a less negative or even positive value of .

In conclusion, the partial pressures of CO2 and O2 measured in the air sacs, expired gas and arterial blood can be explained on the basis of the currently accepted model of gas flow pattern in avian lungs. Neopulmonic gas exchange is Estimated to constitute a small fraction of overall gas exchange, particularly for O2.

We thank Drs R. E. Burger, J. A. Estavillo, J. Geiser and R. K. Gratz for their help in part of the experiments. This study was supported by a Senior Scientist Award from the Alexander von Humboldt-Stiftung to MRF and, in part, by a grant-in-aid from the American Heart Association, Kansas Affiliate, Inc. Contribution no. 87-125-3 from the Kansas Agricultural Experiment Station, Kansas State University, Manhattan, KS66506, USA.

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