Glomerular filtration in birds--its control.

Morphologically there are marked differences between the avian and mammalian kidneys. It is apparent that these differences affect the means by which these organs carry out their homeostatic function of maintaining the internal environment constant. One of the morphological differences is a larger range in size of nephrons within the avian kidney. It is clear that there is a significant degree of intermittent filtration within this very heterogeneous population of nephrons. This leads to larger variation in the total-kidney glomerular filtration rate in birds than in mammals. The glomerular intermittence in birds appears to be controlled by the antidiuretic hormone arginine vasotocin acting at the level of the afferent arteriole. The process of ultrafiltration at the glomerulus in birds is most likely governed by the same parameters that have been quantified for the glomerulus of mammals, because ultrastructural studies indicate that the filtration barrier is similar in both groups. However, for birds it is not known whether certain hormones and other biochemical messengers can effect the filtration process at the level of the glomerulus as demonstrated for mammals. Moreover, it is not known whether feedback regulation occurs at the level of the individual nephron. The morphological attributes of the individual nephrons within the kidney are consistent with the theories of feedback regulation, but the physiological occurrence of this has not been tested.