About counter current mechanism

[keep@science]

As medulla is hyperosmotic with respect to medullary collecting tubules (I.e. main site for water absorption )water flows out of the collecting tubules and urine get concentrated ...But how it enters vasa
recta since vasa recta is also going through same hyperosmotic medulla ,how it absorbs The water ?

 

[TeethWhitener]

This isn't my area of expertise, so take this with a grain of salt, but...

It's not just osmotic pressure that forces water out of the glomerulus and into the Bowman's capsule. It's also hydrostatic pressure (basically, plain old blood pressure). So the total (osmotic + hydrostatic) pressure gradient in the renal corpuscle pushes water and small molecules out of the bloodstream and into the urinary ultrafiltrate.

However, once the filtered blood passes out of the glomerulus, the hydrostatic pressure in the bloodstream drops and the osmotic pressure between the peritubular capillaries and the collecting tubules dominates the net direction of water flow. Since the blood in the peritubular capillaries is hyperosmotic compared with the collecting tubules, water flows back into the bloodstream, concentrating the urine.