Distal convoluted tubule

The glomerular filtrate flows into the distal convoluted tubule where controlled amount of water is reasorbed into the blood by osmosis .This process is enhanced in 2 ways:

(i) Due to the active intake of sodium salt into the blood at the loop of henle which increases the osmotic potential of the blood.

(ii) A hormone known as antidiuretic hormone (ADH)/vasopressin. This hormone is secreted by the pituitary gland.

ADH increases the permeability of the tubule and blood capillaries to water. When there is excess water in the body

eg  

• As a result of excessive intake of fluids, osmotic potential of the blood falls causing the pituitary gland to reduce its secretion of ADH into the blood. Water reabsorption in the tubule is thereby reduced as the tubules becomes less permeable to water and because very little water is removed from the filtrate it results in the production of large amounts of dilute urine.
• If the body loses a lot of water through sweating, the blood pressure is raised hence the pituitary gland release more ADH which results in increased water reabsorption as the tubule becomes more permeable to water. Because a lot of water if removed from the filtrate it results in the production of little amounts of concentrated urine.

NB Adaptations of distal  convoluted tubule are similar to those of proximal convoluted tubule.

Read the text given above and explain how little and concentrated urine is produced in mammals.