After leaving the glomerulus, blood has lost some pressure as it traverses the efferent arteriole and from there through the paratubular capillaries. These capillaries follow the tubules and one limb even follows the Loop of Henle. Because the blood still contains proteins and intertubular fluid does not, the blood is always slightly hypertonic to intertubular fluid even as it follows the Loop into the lower medulla. This means that the paratubular capillaries will always attract fluid into themselves. It is by this means that the large amounts of fluid accumulated by the intertubular space is removed.
Capillary endothelium is always permeable to water and salts.
The variations that terrestrial vertebrates , or desert vertebrates exhibit are most apparent when examining their mode of excretion. Amphibians, reptiles, and almost all birds can not produce a hypertonic urine to conserve water. The best that they can do is produce a urine isotonic with their blood. Water needed to get rid of nitrogenous wastes is the biggest water waster for these animals. Urea is much less toxic so that it can be concentrated in a little water and one has the benefit of two nitrogen atoms per molecule instead of one.