Question

Osmotic movement of water by proximal convoluted tubule cells is referred to as ______ (three words), whereas _____ (three words) is when water is reclaimed from filtrate in the collecting duct.

Answer 1

Osmotic movement of water by proximal convoluted tubule cells referred to as Obligatory water reabsorption because, in the proximal tubule water is reabsorbed through transcellular and paracellular pathway by osmosis. This reabsorption process of water can't be changed according to the need of the body. This is why this mechanism is called obligatory water reabsorption.This water reabsorption is also associated with reabsorption of some solvents (solvents drag).

Whereas Facultative water reabsorption is when water is reclaimed from filtrate in the collecting duct. Because, in the collecting duct water is reabsorbed in response to Anti diuretic hormone (ADH). In the absence of ADH the collecting duct is impermeable to water,. ADH increases the water permeability in the collecting duct by increasing water channels. So this water reabsorption in the collecting duct can be changed according to the need of the body. When excess water is lossed as sweat during summar the ADH increases the water reabsorption so that body compensate the water loss, but in winter when excess water can't be lossed from the body then ADH wouldn't act on collecting duct, so that the water reabsorption is stopped and water is lossed with urine from the body to maintain water balance in the body. SO the water reabsorption in the collecting duct can be changed according to the need of the body, that's why this water reabsorption is called facultative water reabsorption.