Effect of Water Intake on Urine Volume

Aim

To investigate, as a supervised class activity, how the volume of water a person drinks affects the volume of urine they produce over a fixed period of time. This activity should only be carried out with student volunteers under teacher supervision, using moderate, reasonable volumes of water.

Variables

• Manipulated variable: the volume of water consumed by student volunteers at the start of the activity (e.g. 0 mL, 500 mL, 1000 mL groups).
• Responding variable: the total volume of urine produced by each volunteer over a fixed period (e.g. 2-3 hours), with urine colour noted as a secondary observation.
• Controlled variables: the length of the observation period, no other fluids or food consumed during that period, similar activity level among volunteers, and room temperature, all carried out under teacher supervision.

Materials and apparatus

• Drinking water
• Measuring cylinder
• Stopwatch or clock
• Collection containers
• Record sheet

Procedure

1. Under teacher supervision, divide student volunteers into groups, with each group drinking a different, fixed and reasonable volume of water at the start of the activity (e.g. 0 mL, 500 mL, 1000 mL).
2. Record the exact time each group drinks its assigned volume of water.
3. Ask all volunteers not to drink or eat anything else during the observation period.
4. At the end of the fixed period, ask each volunteer to collect their urine and measure its total volume using a measuring cylinder.
5. Record the volume produced by each volunteer and note the colour of the urine as an indicator of how dilute or concentrated it is.
6. Calculate the mean urine volume for each group and compare the results across the groups.

Expected results

Volunteers who drink a larger volume of water generally produce a larger volume of paler, more dilute urine over the observation period, while volunteers who drink little or no water produce a smaller volume of darker, more concentrated urine. Typical class results are shown below.

Conclusion

The volume of urine produced increases with the volume of water consumed. When a large volume of water is drunk, the blood plasma becomes more dilute, so the pituitary gland secretes less antidiuretic hormone (ADH).

With less ADH acting on the kidney nephrons, less water is reabsorbed from the filtrate, so a larger volume of more dilute urine is produced. When little water is drunk, the blood plasma becomes more concentrated, more ADH is secreted, more water is reabsorbed by the nephrons, and a smaller volume of more concentrated urine is produced.

This meets the aim of showing how water intake affects urine formation.

Safety

Common mistakes

Paper 3-style questions

Question 1 (hypothesis). A class investigates how water intake affects urine volume. State a hypothesis linking the volume of water drunk to the volume of urine produced.

Model answer. Hypothesis: the greater the volume of water drunk, the greater the volume of urine produced. It is supported if the group that drinks the most water produces the largest mean volume of paler, more dilute urine over the fixed period.

Question 2 (variables and fair test). State the manipulated and responding variables and one variable that must be controlled in this class activity.

Model answer. The manipulated variable is the volume of water drunk at the start; the responding variable is the volume of urine produced over the fixed period. A controlled variable is the length of the observation period, kept the same for every group so the comparison is fair.

Question 3 (tabulation and graph). Suggest how the class results should be recorded and what a graph would show.

Model answer. Results are recorded in a table with volume of water drunk / mL against mean volume of urine produced / mL. A graph of mean urine volume (y-axis) against water intake (x-axis) rises, showing that a larger water intake is associated with a larger volume of urine.

Question 4 (inference). The group that drank no water produced a small volume of dark urine. What inference can be made in terms of ADH?

Model answer. The inference is that, with little water drunk, the blood plasma became more concentrated, so more ADH was secreted and the nephrons reabsorbed more water. This left a small volume of concentrated, dark urine, showing how the body conserves water.

Source:SRC-DSKP-EN

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