Two Students, One Water Bottle
Two students of the same age and sex take part in a hydration study on a warm day. From 12:00, Student P drinks 1.5 dm³ of water over 20 minutes; Student Q drinks nothing extra and eats a large packet of salted crisps at lunch. Urine is collected hourly from 12:00 to 16:00 and measured.
Student P: 260 cm³, 300 cm³, 180 cm³, 90 cm³. Urine is pale straw coloured.
Student Q: 45 cm³, 40 cm³, 50 cm³, 55 cm³. Urine is dark yellow with a stronger smell.
Both students have normal blood glucose and no medical conditions. Blood samples show that P's blood is slightly more dilute than normal at 13:00, while Q's is slightly more concentrated.
1. Explain, using ADH and negative feedback, why Student P produces a large volume of dilute urine after drinking.
Read the worked answer
Drinking 1.5 dm³ of water raises the water content of the blood, so its solute concentration (and osmotic pressure) falls. This is detected by osmoreceptors in the hypothalamus, which reduce the amount of ADH released from the posterior pituitary.
Less ADH means the collecting ducts and distal tubules of the kidney become less permeable to water, so less water is reabsorbed into the blood by osmosis. More water therefore stays in the urine, giving a large volume of dilute, pale urine.
The loop is negative feedback: the water lost in urine gradually raises the blood concentration back towards normal, osmoreceptors fire more, ADH rises again and urine volume falls. P's figures show exactly this - the peak at 13:00 (300 cm³) is followed by 180 and then 90 cm³ as the blood returns to normal.
2. Explain Student Q's small volume of concentrated urine, and state what would happen to Q's urine after drinking 1 dm³ of water.
Read the worked answer
The salted crisps add a lot of solute, so Q's blood becomes more concentrated. Osmoreceptors stimulate the pituitary to release more ADH, making the collecting ducts more permeable to water. More water is reabsorbed, so a small volume of concentrated, dark urine is produced - 40–55 cm³ per hour, compared with P's 90–300 cm³.
After drinking 1 dm³ of water, the same negative feedback would operate: blood dilution lowers ADH, the collecting ducts become less permeable, and within about 30–60 minutes Q's urine volume would rise and the colour would lighten. Because Q started from a more concentrated state, some of the extra water would first correct the deficit, so the large dilute urine would appear slightly later and be less extreme than P's.
3. Both students spend the afternoon playing sport in the sun. Using the data, judge which student is at greater risk of dehydration and justify your answer.
Read the worked answer
Student Q is at greater risk, even though Q drinks less.
Q's hourly urine volume is already low (40–55 cm³) and the urine is dark and concentrated, which shows the body is conserving water and that Q is already short of water before exercise begins. Q's blood was already more concentrated, so there is less reserve before the concentration rises to a level that impairs performance and temperature control.
P has taken in 1.5 dm³, has dilute blood and is losing the excess in urine; by 16:00 the urine volume has fallen to 90 cm³, showing that the excess has been dealt with and ADH is rising again. P's starting position is therefore better.
Judgement should be qualified: if P's large urine output continued while sweating heavily, P could also become dehydrated. Urine colour and volume are useful indicators but not proof - thirst and body mass change before and after exercise would be better evidence.