Dynamics of glomerular ultrafiltration in the neonate kidney.

In the neonate kidney the glomerular filtration rate is generally depressed both in absolute terms and when calculated per gram kidney weight. Micropuncture studies have revealed that this phenomenon is due, not to changes in the driving pressures or the hydraulic conductivity of the glomerular capillary membrane, but to a retarded glomerular blood flow. A retarded flow will thus mean a steep rise in the protein concentration in parallel to the filtration of the protein-free filtrate. The corresponding steep rise of the colloid osmotic pressure will then induce cessation of the filtration in the distal parts of the glomerular capillary. In dehydrated states this phenomenon will be even more aggravated. An extracellular volume expansion with saline induces relaxation of the two arterioles with subsequent rise in the glomerular blood flow. The rise in the colloid osmotic pressure along the capillary is then less and the filtration can proceed along the whole of the glomerulus. Accordingly the glomerular filtration rate will be increased, reaching figures typical of the mature kidney.