ADH increases the permeability of the collecting duct to water, and this means water flows from the Collecting duct into the blood to decrease the volume of the fluid in the collecting duct.
ADH is a hormone that increases permeability to water in the collecting ducts and therefore increases water reabsorption from the urine. This decreases the volume of water in the urine.
ADH travels in the bloodstream to its target organ, the kidneys. It causes the kidney tubules to become more permeable . This means more water can leave the kidney tubule to be reabsorbed back into the blood during selective reabsorption.
If it were to be one giant cell, it would be hard to transport materials across the membrane. Being multicellular allows transporting materials faster and more efficient.
