Question

Mammals can eliminate hypotonic or hypertonic urine according to body needs. Explain.

Answer 1

Mammals have hormones and countercurrent mechanism that helps in the elimination of hypotonic or hypertonic urine according to body needs.

Elimination of hypertonic and hypotonic urine is under control of antidiuretic hormone (ADH) and aldosterone. Osmoreceptors in the body are activated by changes in blood volume, body fluid volume and ionic concentration. An excessive loss of fluid from the body can activate these receptors which stimulate the hypothalamus to release antidiuretic hormone (ADH) or vasopressin from the neurohypophysis. ADH facilitates water reabsorption from latter parts of the tubule, thereby preventing diuresis and producing hypertonic urine. An increase in body fluid volume can switch off the osmoreceptors and suppress the ADH release to complete the feedback. Switching off of ADH release facilitates production of hypotonic urine. While aldosterone acts mainly at the renal tubules and stimulates the reabsorption of Na+ and water and excretion of K+ and phosphate ions. This, in turn, helps in the production of hypertonic urine.