Acute Hypercapnia and Eggshell Formation

Abstract

THE domestic fowl forms an eggshell of about 5 g in 20 h and during this process the oviduct derives about 3 g of carbonate ions from the blood stream. This results in a fall in both pH and bicarbonate content of the blood and a state of metabolic acidosis¹. The bird partially compensates for this condition by hyperventilating, thereby lowering the carbon dioxide tension of its blood and reducing the fall in pH². Obviously, this does not restore the bicarbonate content of the blood, and a number of attempts have recently been made to do this on the theory that the availability of this ion might be a limiting factor during shell secretion. The most common technique which has been used for this purpose is to allow the bird to breathe various concentrations of carbon dioxide during the time when it is forming the eggshell. Unfortunately, the results obtained from birds made hypercapnic in this way are contradictory, as some workers find an increase³ whereas others obtain a decrease in the thickness of the shells⁴. This has led to some discussion as to whether it is the pH or the bicarbonate content of the blood which affects the process of shell formation most.