Question

Can you please describe the mechanism by which the kidney makes urine which is (a)hypotonic, and (b) hypertonic, with respect to plasma.

Answer 1

a) excess water excretion is leading the formation of hypotonic urine. countercurrent mechanism and hormones are helping the elimination of hypotonic urine formation. the increase of body fluids volume is switching  off the osmoreceptors and thereby Suppressing the ADH release form the hypothalamus. due to the switching off of ADH is facilitating the production of hypotonic urine.

b) excess excretion of solutes results in the formation of hypertonic urine. countercurrent mechanism and hormones are helping the elimination of hypertonic urine formation. fluids are excessively losing from the body and they activating the receptors which are stimulating the hypothalamus to produce ADH or Vasopressin. from the neurohypophysis these enzymes are produced. ADH is the hormone facilitating water reabsorption from the tubules. filtering of  too much body fluid from kidney , this condition is called diuresis is prevented by the ADH hormone. thereby as a result hypertonic urine is produced.

when aldosterone acts in the tubules, there will be reabsorption of NA + and water and excretion of K+ and phosphorous ions. this also helping for the production of hypertonic urine.