Q.Explain the role of ADH (vasopressin) in regulating the volume and concentration of urine, and describe what happens when its secretion is deficient.

[!TLDR]

ADH increases DCT/collecting-duct water permeability, concentrating urine; its deficiency causes diabetes insipidus (large dilute urine, excess thirst). …

ADH (antidiuretic hormone, vasopressin), produced by the hypothalamus and released from the posterior pituitary, is secreted in response to a rise in blood osmolarity or a fall in blood volume. It acts on the distal convoluted tubule and, especially, the collecting duct, increasing their permeability to water; this allows more water to be reabsorbed from the tubular fluid as it passes through the concentrated osmotic gradient of the medulla, producing a smaller volume of more concentrated urine and helping restore normal blood osmolarity. When ADH secretion is deficient, or when the kidney's collecting ducts fail to respond to circulating ADH, the collecting duct remains largely impermeable to water regardless of the body's hydration state. The result is diabetes insipidus: a very large volume of dilute, watery uri …

Describe ADH's normal action first (water permeability -> reabsorption -> concentrated urine), then state what specifically fails when it is deficient (imperm …

Describing ADH as acting on the PCT -- ADH's water-permeability effect is specific to the DCT and collecting duct, the sites of facultative, hormone-re …