Theory on axial pressure gradient in kidney, inspired by Robert W. Berliner and Homer Smith

Berliner mentions in his 1958 article an important observation that supports my theory.

Reduction in glomerular filtration impairs ability to concentrate urine.

My theory is that the axial pressure gradient is normal pressure, not osmotic. High input area to output area ratio. The urine will equilibrate with parenchyma along axial pressure gradient, result is an osmotic pressure gradient as well but it is an effect, not a cause.

U-shaped vasa recta the logical way for blood to reach deep into medulla. Countercurrent exchange along it also prevents dilution of osmotic gradient, but, secondary. The "loop of Henle" mainly a scaffold for vascularization of medulla, like Homer Smith said, a vestige of embryology.

The "milking" effect of papilla central. Knepper's equilibrium for cation retention in hyaluronan during contraction as well. Water tends to follow axial pressure gradient in general, pelvic wall smooth muscle adds to it.

Vasoconstrictive effects of ADH and aldosterone, collecting ducts likely surrounded by smooth muscle.

I think that is why newborns cannot concentrate urine, they have very low blood pressure.

The kidney anatomically concentrates urine, like a reverse of the lung but with a narrow trachea.