The collecting duct receives filtrate from the distal convoluted tubules of several nephrons. It runs from the cortex down through the medulla to the renal pelvis, passing through the gradient of water potential set up by the loops of Henle. Its job is to control how much water is reabsorbed, and so how concentrated the urine will be.
Filtrate entering the collecting duct is dilute. The tissue fluid of the medulla around it has a lower water potential, which gets lower (more concentrated) deeper in the medulla. So, if the duct wall is permeable, water leaves the filtrate by osmosis all the way down the duct and is carried away by the blood capillaries.The permeability of the collecting duct wall to water can be changed. The cells contain water channel proteins called aquaporins. When many aquaporins are inserted into the membrane facing the lumen, the wall is very permeable and a lot of water is reabsorbed. When few are present, the wall is almost impermeable and little water is reabsorbed. The number of aquaporins is controlled by the hormone ADH (explained in a later lesson).• High permeability: much water is reabsorbed, giving a small volume of concentrated urine.
• Low permeability: little water is reabsorbed, giving a large volume of dilute urine.Urea also diffuses out of the lower collecting duct into the medulla, which adds to the low water potential there.Because the urine in the collecting duct can come close to equilibrium with the deepest part of the medulla, the maximum concentration of urine depends on the concentration at the tips of the loops of Henle.
Filtrate entering the collecting duct is dilute. The tissue fluid of the medulla around it has a lower water potential, which gets lower (more concentrated) deeper in the medulla. So, if the duct wall is permeable, water leaves the filtrate by osmosis all the way down the duct and is carried away by the blood capillaries.The permeability of the collecting duct wall to water can be changed. The cells contain water channel proteins called aquaporins. When many aquaporins are inserted into the membrane facing the lumen, the wall is very permeable and a lot of water is reabsorbed. When few are present, the wall is almost impermeable and little water is reabsorbed. The number of aquaporins is controlled by the hormone ADH (explained in a later lesson).• High permeability: much water is reabsorbed, giving a small volume of concentrated urine.• Low permeability: little water is reabsorbed, giving a large volume of dilute urine.Urea also diffuses out of the lower collecting duct into the medulla, which adds to the low water potential there.Because the urine in the collecting duct can come close to equilibrium with the deepest part of the medulla, the maximum concentration of urine depends on the concentration at the tips of the loops of Henle.