The Distal Tubule and Collecting Duct Respond to Systemic Signals to Alter Salt Excretion as Required
In the proximal tubule, filtered solutes and water are generally reabsorbed regardless of the animals physiological state. In contrast, the distal tubule and collecting duct control the ultimate rate of excretion of electrolytes and water to maintain homeostasis despite variations in dietary intake and extrarenal losses of salts and water.
I he specific homeostatic responses of these segments are controlled in large part by several hormones, including aldosterone, angiotensin II, antidiuretic hormone, endothelin, atrial natriuretic peptide, parathyroid hormone, lα,25-(OH)2-vitamin D3, and calcitonin. In birds the relative importance of the reptilian-type and the mammalian-type nephrons in regulation of salt balance has not been established. Furthermore, in many species of birds, particularly marine and desert species, sodium balance is regulated largely by secretion of NaCl by the nasal (supraorbital) gland, rather than by regulation of renal excretion. Finally, ureteral urine delivered to the cloaca moves in a retrograde manner into the digestive tract, where additional salt reabsorption occurs.
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