Final answer:
The ability of ADH to cause the excretion of concentrated urine is contingent upon the presence of aquaporins, which are water channel proteins that ADH regulates to control the permeability of the kidney's collecting ducts to water.
Step-by-step explanation:
The ability of ADH to cause excretion of concentrated urine depends on the presence of aquaporins. ADH, or antidiuretic hormone, is crucial for regulating water balance in the body by affecting the kidney's absorption of water. When ADH levels increase, it stimulates the insertion of water channels, known as aquaporins, into the membranes of kidney cells. This process makes the renal collecting ducts more permeable to water, leading to more water being reabsorbed into the bloodstream and, consequently, a more concentrated urine output. Conversely, when ADH levels decrease, fewer aquaporins are present in the cell membranes, resulting in less water reabsorption and more dilute urine.