Question 3d
Suggest why the results during the second day were so different from those on the first day. [2]
Answer:
The results on the second day (line B) were different because the dilute salt solution has a similar water potential to blood plasma, which affects how much water is reabsorbed by the kidneys. Here’s a breakdown of the reason:
- Role of ADH: The presence of salt in the solution on the second day may cause the body to release less antidiuretic hormone (ADH), as the water potential does not increase as it did with plain water. Less ADH means more water is retained, reducing urine output.
- Water Absorption and Osmoregulation: On the first day, drinking plain water increases the water potential of blood, leading the body to excrete more water through urine to maintain osmotic balance. This results in a larger volume of dilute urine shortly after drinking.
- Salt Solution Impact: On the second day, because the dilute salt solution has a similar water potential to blood, it does not disrupt the osmotic balance in the same way. This means the kidneys do not need to excrete as much water, leading to lower urine output.