One action of the Loop of Henle is to make the renal medulla hyperosmotic. Why is this important in renal function (there is no active transport mechanism for water in the body)? We need to regulate water reabsorption to maintain homeostasis. How could we do this and how is this example analogous to another example that you have looked at?

The collecting duct which is the final part of the tubule system passes through the renal medulla. Because the medulla is hyperosmotic water will move from the collecting duct into the renal medulla down the osmotic gradient. We need to control the amount of water movement that is occurring and we could do that by using channels in the membranes of the cells that line the collecting duct. If we need to increase water reabsorption to maintain homeostasis (i.e. produce concentrated urine) we can insert more channels into the membranes, and if we wanted to increase water loss (i.e. produced islet urine) we could remove those channels. The channels are known as aquaporins and the insertion of aquaporins into the membranes is controlled hormonally. This system is analogous to the relationship between insulin and GLUTs that enables blood glucose levels to be regulated.