why is glomerular filtrate hypertonic and hypotonic in the descending and ascending limb of loop of henle ? What is the effect of ADH on collecting tubules

The glomerular filtrate is hypertonic in descending limb of the loop of Henle because the descending limb is impermeable to electrolytes but permeable to water. The ascending limb of the loop of Henle, on the other hand, is impermeable to water and hence, is hypotonic.

Antidiuretic hormone helps in the reabsorption of water back into the circulation by binding to receptors present in collecting ducts. If the hormone is absent, the collecting ducts becomes impermeable to water and all the water flows out of the body as urine. 

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