When the time filtrate reaches the distal convoluted tubule of the ascending Limb DCT(distal convoluted tubule) most of the solute have been reabsorbed. The filtrate entering DCT does has a high concentration of Urea and other wastages through it is hypoosmotic to the interstitial fluid as NaCl has been transported out of the Tubular lumen.
This reason is important for transportation of sodium hydrogen and NH3 into the tubule from the interstitial fluid and sodium chlorine out of the tubule into the interstitial fluid.
As the salts are pumped out of the lumen water also moves out passively.
If the body requires additional water it is reabsorbed here.
The activity of the cells of DCT depends on the conditions of osmotic stress faced by the body and is under normal hormonal control.
Under conditions of diuresis there is if no water has to be conserved in the body then the fluid passes an unchanged from the DCT into the collecting duct.
When water needs to be conserved then the hypothalamus secrets the antidiuretic hormone ADH which changes the permeability of the DCT was to allow water to move out of the tube use into the interstitial fluid of the kidney in order to be retained by the body.
When a DH titres are high in the body more aquaporins are required in the epithelial cells of the tubule water diffuses out into the body and the urine fluid becomes is-osmotic to blood when it enters the collecting duct.
In amphibians and reptiles there is no loop of henle so the fluid from the proximal tubule passes into the distal tubule directly and the final urine may be hypoosmotic or at best ISO osmotic to blood.
Urine in these animals is made hypoosmotic by the option of source and under dry conditions the volume is also reduced by these option of water in the distal tubule under the influence of ADH.
In lower vertebrates ADH is referred to as vasotocin.
Collecting duct
The fluid that reaches the collecting duct may be hypoosmotic or isosmotic depending on the water status of the animal.
Final adjustments in urine composition are made in the collecting ducts who’s was become permeable only under the influence of ADH.
In the absence of Ada secretion the hyperosmotic urine that enters the collecting ducts is excreted unchanged and sew a relatively large volume of urine is excreted.
Under situations demanding water conservation ADH is secreted and the walls of the collecting ducts become permeable to water.
The urine fluid passing through the collecting ducts become progressively concentrated and a urine hyperosmotic to blood and isosmotic to interstitial fluid is excreted.
Tubular secretion
Apart from potassium hydrogen and H3 some organic acids and organic bases that are not bound to the plasma proteins are secreted from the plasma into the Tubular lumen or excretion.
Organic and ions are situated along the sodium gradient that is established by Sodium Potassium pump.
These secretary mechanism are not specific and as a result many new substance including drug molecules and toxins can be secreted coupled to the transporting molecules.
Most of the Tubular secretion happens in the DCT but some occurs in the early part of the collecting duct. Kidneys also maintained and acid base balance by secreting excess h + ions