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S5 Biology: Homeostasis (Paper 3)
S5 Biology: Homeostasis (Paper 3)
The third paper concentrates on water balance and ADH: the water budget, urine volume and concentration, solute conservation and the common ADH confusion.
Direct answer Intake must equal output; volume times concentration gives total solute, and ADH changes water but not solute; more ADH means more water reabsorbed and smaller, more concentrated urine.
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 |
| Suggested time | 25 |
| Difficulty mix | Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1 |
| Chapter | Chapter: S5 Biology: Homeostasis |
| Level | Four basic, three advanced, two challenge and one error-repair item |
02 Questions (10 items)
- FoundationThrough which routes does the body lose water?
- FoundationWhere is ADH made and where is it released?
- FoundationWhat does a rise in ADH do to urine?
- FoundationWhat are the first two responses to dehydration?
- IntermediateDaily intake is 2 500 cm³ with urine 1 500, sweat 500, exhaled 400 and faeces 100. Is the balance maintained?
- IntermediateAfter drinking 1 000 cm³ of water, urine rises from 1 500 to 1 800 cm³. What is the increase?
- IntermediateWhat does a daily urine volume below 500 cm³ usually indicate?
- ChallengeUrine is 400 cm³ at 1 200 mOsm with high ADH and 1 600 cm³ at 300 mOsm with low ADH. Is the solute output the same?
- ChallengeAfter losing 1.5 L of sweat, urine falls from 1 500 to 600 cm³. What percentage fall is that?
- Error redoA student says more ADH means more urine. What is wrong?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | Urine, sweat, exhaled air and faeces | The kidneys adjust urine volume and are the main adjustable outlet. |
| 2 | Made in the hypothalamus and released from the posterior pituitary | The hypothalamus senses plasma osmotic pressure and signals the posterior pituitary to release ADH. |
| 3 | More water is reabsorbed in the collecting duct, so urine volume falls and concentration rises | Water is retained, so urine becomes smaller in volume and more concentrated while plasma osmotic pressure falls. |
| 4 | Thirst and increased ADH secretion | Thirst drives drinking while ADH limits urinary water loss, so both conserve water. |
| 5 | Yes, the four losses total 2 500 cm³ | Fifteen hundred plus five hundred plus four hundred plus one hundred equals 2 500, so intake matches output. |
| 6 | An increase of 300 cm³ | Eighteen hundred minus 1 500 gives 300, and the rest stays in the body or leaves by other routes. |
| 7 | Dehydration or excess ADH, with the body conserving water | Small concentrated urine shows the kidney is reclaiming water, typical of dehydration or excess ADH. |
| 8 | Yes, both give 480 000 osmotic units, since 400 times 1 200 equals 1 600 times 300 | ADH changes water volume but not total solute, so the concentrated and dilute products are equal. |
| 9 | A fall of 900 cm³, which is 60 per cent | Fifteen hundred minus six hundred gives 900, and 900 divided by 1 500 is 0.6, or sixty per cent. |
| 10 | More ADH increases water reabsorption, so urine volume falls | ADH stands for antidiuretic hormone, and antidiuretic literally means reducing urine production. |
04 What to do after marking
- After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
- Repeat the same item two days later; move to the next paper only when both attempts are correct.
- Parents only need the answer and note columns; there is no need to explain every item.
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