Acid-base relationships in the blood of the toad, Bufo marinus. II. The effects of dehydration

Cutaneous CO2 excretion is reduced as the skin dries during dehydration but an increase in breath frequency acts to regulate the arterial blood PCO2 and thus pHa. Moreover, the toad does not urinate and water is reabsorbed from the bladder to replace that lost by evaporation at the skin and lung surfaces. The animal does, however, produce a very acid bladder urine to conserve circulating levels of plasma [HCO3-] and this together with an increased ventilation effectively maintains the blood acid-base status for up to 48 h of dehydration in air. Water loss and acid production are presumably also reduced by the animal's behaviour; animals remain still, in a crouched position or in a pile if left in groups. Dehydrated toads are less able than hydrated toads to regulate blood pH during hypercapnia: they hyperventilate and mobilize body bicarbonate stores in much the same fashion as hydrated animals but due to the restrictions on cutaneous CO2 excretion and renal output, there is comparatively little reduction in the PCO2 difference between arterial blood and inspired gas thereby resulting in a more severe respiratory acidosis. These factors further contribute to the persistent acidosis which continues even when the animals are returned to air.