The avian glomerulus is smaller, less vascular, and has fewer capillary loops than the mammalian, so it can be expected to filter less per glomerulus. The concentration of substances in the urine may be the same, higher, or lower than that in the blood plasma. A lower concentration usually indicates the substance is being reabsorbed by the kidney tubules. Glucose, which is completely filterable, must be reabsorbed. The threshold for glucose to spill over into the urine varies with species.
Studies suggest that acetylcholine stimulates the valves to open.
The avian urinary organs consist of paired symmetrical reddish brown kidneys and ureters which transport urine to the cloaca. They are located in the bony depressions of the pelvis on either side of the spine caudal to the lungs. The kidneys have no cortex or medulla and each is made up of 3 lobes, the cranial, middle, and caudal. In each lobule is a central vein around which radiate the nephrons. Wastes, salts, glucose and water filter out of the blood into renal tubules.