Composition of alveolar liquid in the foetal lamb

1. Experiments were performed on foetal lambs at gestations between 125 days and term. The foetus was exteriorized at Caesarean section with the umbilical cord and placental attachment maintained intact. Samples of liquid from the alveolar parts of the lung were withdrawn through a tracheal cannula and samples of lung lymph, plasma and amniotic liquid were also obtained. Measurements were made of total osmolality, concentrations of electrolytes and urea, pH and P CO 2 . Titrations were carried out with N /10 HCl and N /10 NaOH. The water content of the liquids was estimated and concentrations expressed per kg H 2 O. 2. In alveolar liquid [H + ], [K + ] and [Cl − ] were higher and [Ca 2+ ], [phosphates] and [HCO 3 − ] were lower than in plasma or lymph. In amniotic liquid osmolality [Na + ], [Cl − ] and [Ca 2+ ] were lower and [phosphates] higher than in plasma or lymph. Alveolar liquid/plasma ratios of [HCO 3 − ], [Ca 2+ ], [Cl − ] and [K + ] differed from ultra filtrate/plasma ratios of these ions. 3. Titration curves demonstrated a very small amount of buffering in alveolar liquid at its in vivo pH of 6·27 mostly due to HCO 3 − at an average concentration of 2·8 m M /kg H 2 O. 4. It is concluded that foetal alveolar liquid is not an ultrafiltrate of plasma nor a mixture of amniotic liquid and plasma ultrafiltrate, but a special material elaborated by the foetal lung.