Homeostasis and the kidney

Sample questions

Original practice questions written in the style of the exam.

1. Question 15 marks

   After a long football match on a hot day, a player's blood water potential falls.

   Explain how the body responds to return blood water potential to normal.

   Mark scheme

   • Osmoreceptors in the hypothalamus detect the fall in water potential (1)
   • The posterior pituitary releases more ADH into the blood (1)
   • ADH binds to receptors on cells of the collecting duct (and distal convoluted tubule) (1)
   • Vesicles containing aquaporins fuse with the cell membrane, so the walls become more permeable to water (1)
   • More water is reabsorbed by osmosis into the blood, producing a small volume of concentrated urine (1)
2. Question 24 marks

   Explain how the loop of Henle produces a low water potential in the medulla of the kidney.

   Mark scheme

   • Sodium and chloride ions are actively transported out of the ascending limb into the medulla (1)
   • The ascending limb is impermeable to water, so water cannot follow the ions (1)
   • This lowers the water potential of the medulla tissue fluid, so water leaves the descending limb by osmosis (1)
   • The countercurrent multiplier makes the water potential lowest at the tip of the loop / deep in the medulla (1)