(d) the control of the water potential of the blood

The idea

Blood water potential is controlled by negative feedback centred on osmoreceptors in the hypothalamus. If water potential falls (blood too concentrated), osmoreceptors shrink and stimulate the posterior pituitary to release more ADH. ADH increases the permeability of the collecting duct to water by inserting aquaporins, so more water is reabsorbed by osmosis and a smaller volume of more concentrated urine is produced; water potential rises back towards normal. If water potential rises (blood too dilute), less ADH is released, the collecting duct is less permeable, less water is reabsorbed and a larger volume of dilute urine is produced. The loop of Henle and the countercurrent multiplier create the medullary solute gradient that allows this osmotic reabsorption.

What earns the marks

• Osmoreceptors in the hypothalamus detect changes in blood water potential.
• The posterior pituitary releases ADH in response to stimulation by the hypothalamus.
• ADH increases collecting duct permeability to water by inserting aquaporins.
• More water is reabsorbed by osmosis when ADH is high, producing small volumes of concentrated urine.
• Low ADH gives reduced water reabsorption and large volumes of dilute urine.
• The loop of Henle establishes a solute gradient in the medulla enabling osmotic water reabsorption.

Common mistakes and how to put them right

• Stating that ADH is released by the anterior pituitary.

  Put it right: Is that it is released by the posterior pituitary.

• Claiming ADH actively pumps water.

  Put it right: Is that ADH increases permeability and water moves by osmosis.

• Confusing the direction of the response.

  Put it right: Is that low water potential triggers more ADH and more reabsorption.

Exam tips

• Link each change in ADH to a change in urine volume and concentration.
• Name the site of osmoreceptors and the site of ADH release precisely.
• Use the term osmosis and refer to aquaporins when explaining water movement.