Distal Convoluted Tubule
A hypoosmotic fluid from the ascending loop of Henle enters the distal convoluted tubule. Under conditions of water diuresis, i.e., when there is no need for water conservation, the hypoosmotic tubular fluid passes into the collecting tubule unchanged in its osmolarity. When water is to be conserved by the body; the hypothalamo- hypophyseal system secretes antidiueretic hormone (ADH) (also known as vasopressin) which makes the walls of the distal convoluted tubule permeable to water.
So, when the ADH titer is high in the blood, water diffuses out of the distal tubule and the fluid becomes isosmotic to blood by the time it enters the collecting tubule. In amphibians and reptiles, where-there is no loop of Henle, the fluid from the proximal tubule passes into the distal tubule directly and the final concentration of the ureteral urine may be hypoosmotic or at best isosmotic. Urine in these animals is made hypoosmotic by reabsorption of salt. Under dry conditions, in these animals, the urine is made less hypoosmotic and smaller in volume due to reabsorption of water in the distal tubule under the influence of ADH. ADH is referred to as vasotocin (not vasopressin) in lower vertebrates.