How do the kidneys regulate water balance and remove waste?
Explain how the kidney and ADH regulate water balance through filtration, reabsorption and negative feedback
The kidney regulates water balance by filtering blood and reabsorbing water and solutes in the nephron; ADH adjusts water reabsorption by negative feedback to control urine concentration.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
You need to explain the kidney's role in water balance, outline filtration and reabsorption in the nephron, and explain how ADH adjusts water reabsorption as a negative feedback loop.
What osmoregulation is
Osmoregulation is the control of the water and solute (salt) balance of the blood and body fluids. If the blood becomes too concentrated or too dilute, water moves into or out of cells by osmosis, damaging them. The kidneys maintain this balance while also removing nitrogenous waste (urea).
The nephron and how the kidney works
Each kidney contains about a million tiny tubes called nephrons, where blood is processed in two key steps:
- Filtration. Blood enters a knot of capillaries (the glomerulus) under high pressure. Small molecules - water, glucose, salts and urea - are forced out into the nephron, forming filtrate. Large molecules such as proteins and blood cells stay in the blood.
- Selective reabsorption. As filtrate passes along the nephron, useful substances are reabsorbed back into the blood: all the glucose, much of the salt, and a regulated amount of water. Urea and excess water and salts remain to form urine.
The amount of water reabsorbed is adjustable, which is how the body fine-tunes water balance.
ADH and the feedback loop
The hormone ADH (antidiuretic hormone), released by the pituitary gland under the control of the hypothalamus, controls how much water is reabsorbed from the nephron.
The hypothalamus detects the water concentration of the blood (its solute concentration):
- Blood too concentrated (dehydrated): the hypothalamus triggers more ADH release. The nephron walls become more permeable to water, so more water is reabsorbed into the blood. Urine becomes small in volume and concentrated. Blood water returns toward the set point.
- Blood too dilute (over-hydrated): less ADH is released. Less water is reabsorbed, so a large volume of dilute urine is produced, removing the excess water.
Exam-style practice questions
Practice questions written in the style of SACE Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2019 SACE Stage 22 marksA graph shows urine osmolality against plasma ADH concentration. Describe the change in urine osmolality as the concentration of ADH in the plasma increases.Show worked answer →
For 2 marks, describe the trend and its shape.
As plasma ADH concentration increases, urine osmolality increases (the urine becomes more concentrated).
The increase is steep at first and then levels off at high ADH concentrations, approaching a maximum. This is because more ADH causes more water to be reabsorbed from the kidney tubules, leaving the urine more concentrated until reabsorption nears its limit.
2019 SACE Stage 23 marksDescribe the effect of ADH on the kidneys and urine osmolality.Show worked answer →
Three points cover the marks.
ADH increases the permeability of the walls of the distal tubule and collecting duct of the nephron to water.
More water is therefore reabsorbed from the filtrate back into the blood.
As a result, a smaller volume of more concentrated urine is produced, so urine osmolality rises. (Low ADH has the opposite effect: less water reabsorbed, so a larger volume of dilute urine.)
2019 SACE Stage 23 marksCentral diabetes insipidus (CDI) can result from an inability to produce ADH, and some cases are caused by an injury to the head. Explain why an injury to the head might lead to CDI.Show worked answer →
Link the anatomy of ADH production to the injury.
ADH is produced by the hypothalamus and released from the posterior pituitary gland, both of which are located in the brain.
A head injury can damage the hypothalamus or pituitary gland, so ADH can no longer be produced or released.
Without ADH, the kidney cannot reabsorb enough water, so large volumes of dilute urine are produced, which is the central diabetes insipidus described.