The role of the kidney in electrolyte and nitrogen excretion in a large flightless bird, the emu, during different osmotic regimes, including dehydration and nesting

Summary

• 1. In birds the role of the kidney in excretion may be augmented by the cloaca-rectum, where the final modification of excreta may occur. The emu has a limited ability to concentrate urine but the capacity of its cloaca-rectum to absorb water and NaCl is considerable. Clarification of the characteristics of renal regulation of salt and water excretion is therefore required.

• 2. The functioning of the kidney was studied during dehydration, nesting, NaCl loading, and NaCl depletion. Glomerular filtration rate (GFR), urine flow rate (UFR), and the patterns of excretion of the major electrolytes were examined. GFR was decreased in response to dehydration, salt loading, and nesting, while UFR was lowered only during dehydration and nesting. A diuresis occurred in the NaCl-depleted birds.

• 3. Although the limited renal concentrating ability was confirmed, the kidney had excellent water and electrolyte reclaiming abilities. In control emus, 94.8% of water, 99.8% of Na, and 98.7% of Cl were reabsorbed. Even during NaCl loading, 99.5% of Na and 97.9% of Cl were reabsorbed by the kidney.

• 4. The excretion of nitrogenous compounds was similar to that described in other birds, with uric acid/urates being the major excretory product. Also examined was the binding of cations to precipitated uric acid/urates. In the ureteral urine cation trapping was only of minor significance, with K being the cation most involved, but marked modification of urine can occur in the cloacarectum because the ion binding in excreta can be much increased.