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Kidney
Avian kidneys functions in almost the same way as the more extensively studied mammalian kidney, but with a few important adaptations; while much of the anatomy remains unchanged in design, some important modifications have occurred during their evolution. A bird has paired kidneys which are connected to the lower gastrointestinal tract through the ureters. Depending on the bird species, the cortex makes up around 71-80% of the kidney's mass, while the medulla is much smaller at about 5-15% of the mass. Blood vessels and other tubes make up the remaining mass. Unique to birds is the presence of two different types of nephrons (the functional unit of the kidney) both reptilian-like nephrons located in the cortex and mammalian-like nephrons located in the medulla. Reptilian nephrons are more abundant but lack the distinctive loops of henle seen in mammals. The urine collected by the kidney is emptied into the cloaca through the ureters and then to the colon by reverse peristalsis.

Osmotic Homeostasis

The role of osmoregulation, that is maintaining a precise balance of solute and water concentrations within the body, is performed by a melody of bodily functions working together. In flamingos, the kidney, the lower GI tract, and the salt glands work together to maintain the homeostasis between ions and fluids. In mammals, the kidneys and urinary bladder are the primary organs used to control osmoregulation; birds, however, lack a urinary bladder and must compensate for this. They do so through the of mechanisms of these three systems.