Light microscopic morphometry of the kidneys of fourteen avian species

Kidney volume and the volume proportions of the cortex, medulla, blood vessels larger than capillaries and ureter and ureteral ducts were investigated in 61 birds from 14 species representing passeriformes, psittaciformes, podicipediformes, anseriformes, galliformes, and columbiformes. The kidneys were fixed in situ by perfusion via their arterial supply. Kidney volume and body weight were strongly correlated (r = 0.991); the allometric function relating kidney volume (Vk) to body weight (W) was: Vk = 13.96W0.902. Histological sections of one kidney from each bird were analysed by point counting with a Zeiss integrating eyepiece to estimate the volume proportions and subsequently the absolute volumes of the main components of the kidney. The avian kidney was found to consist of a very large cortex (range 71-81%), a relatively small medulla (range 5-15%), blood vessels larger than capillaries (range 10-13%). For all the birds, the mean volume proportions of the kidney components were as follows: cortex, 77%; medulla, 10%; blood vessels larger than capillaries, 12%; ureter and ureteral ducts, 1%. The values for the absolute volume of each of the main components of the kidney were well correlated with body weight. The allometric equations and correlation coefficients (r) relating: (a) the volume of cortex (Vc) and the body weight (W) was Vc = 10.06W0.914, r = 0.991; (b) the volume of medulla (Vm) and body weight was Vm = 2.27W0.795, r = 0.974; (c) the volume of blood vessels larger than capillaries (Vb) and body weight was Vb = 1.76W0.894, r = 0.988. All the birds were divided into two groups in the following five sets: (a) passeriform and non-passeriform orders; (b) galliform and non-galliform orders; (c) small and large birds weighing less than and more than 500 g respectively; (d) aquatic and non-aquatic birds; (e) desert and non-desert birds. In the two groups of each set the differences between kidney volume per gram body weight and those between the respective percentage proportions of cortex, medulla and blood vessels larger than capillaries were assessed by Student's t-test. The differences between kidney volume per gram body weight were significant for all sets except desert and non-desert birds; those between the respective percentage proportions of cortex and medulla were significant for all sets; those between the percentage proportions of blood vessels larger than capillaries were significant only in desert and non-desert birds.(ABSTRACT TRUNCATED AT 400 WORDS)