How do the kidneys keep the water and salt content of the blood constant whether you drink a litre of water or sweat it out?
Explain how body fluid composition is regulated, including the role of the kidney and nephron, antidiuretic hormone and aldosterone in osmoregulation by negative feedback
A focused answer to the WACE Year 12 Human Biology Unit 3 dot point on osmoregulation. The nephron and the three stages of urine formation, the role of antidiuretic hormone in water balance, and aldosterone in salt balance, all as negative feedback.
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What this dot point is asking
WACE wants you to explain how the body keeps the concentration of its fluids constant, the structure of the nephron, and the hormones that fine-tune water and salt. The internal environment must stay at the right concentration so that cells neither swell nor shrink by osmosis. The organ that does this is the kidney.
Filtering the blood: the nephron
Each kidney contains about a million nephrons, the functional units that make urine. Urine formation has three stages:
- Filtration: blood is forced under pressure from the glomerulus (a knot of capillaries) into the Bowman's capsule. Small molecules (water, glucose, salts, urea) pass through; blood cells and large proteins stay in the blood. The fluid is now called filtrate.
- Reabsorption: as the filtrate passes along the tubule, useful substances are reabsorbed back into the blood. All glucose, most water and the needed salts are returned, mostly in the proximal tubule and the loop of Henle.
- Secretion and concentration: some additional waste is secreted into the tubule, and water is adjusted in the collecting duct. What remains, mainly urea, excess salts and water, leaves as urine.
Water balance: antidiuretic hormone
Water balance is controlled by antidiuretic hormone (ADH). Osmoreceptors in the hypothalamus monitor the concentration (osmotic pressure) of the blood.
If the blood becomes too concentrated (for example after sweating or not drinking), the hypothalamus detects this and stimulates the posterior pituitary to release more ADH. ADH makes the collecting ducts of the nephron more permeable to water, so more water is reabsorbed into the blood and a small volume of concentrated urine is produced. The blood becomes less concentrated, back toward the set point.
If the blood becomes too dilute (for example after drinking a lot of water), less ADH is released, the collecting ducts reabsorb less water, and a large volume of dilute urine is produced, raising the concentration back toward the set point.
Salt balance: aldosterone
Aldosterone, a hormone from the adrenal glands, controls the reabsorption of sodium ions in the kidney tubules. When blood sodium is low (or blood pressure falls), aldosterone is released and makes the kidney reabsorb more sodium, and water follows the sodium, which also helps restore blood volume and pressure. When sodium is high, less aldosterone is released and more sodium is excreted.
How this maps to the exam
Expect questions that ask you to label the nephron and describe filtration and reabsorption, write the ADH negative-feedback loop for a concentrated or dilute blood scenario, or explain how urine volume and concentration change after drinking water versus exercising. Aldosterone may appear as the salt-balance partner to ADH.
Exam-style practice questions
Practice questions written in the style of SCSA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WACE 20227 marksA person drinks a large volume of water. Explain how antidiuretic hormone (ADH) acts by negative feedback to return the water concentration of the blood toward its set point, naming the receptor, the gland and the target structure.Show worked answer →
A 7 mark response needs the full ADH loop applied to a fall in blood solute concentration.
- Stimulus
- After drinking, the water content of the blood rises, so the solute concentration of the blood falls below the set point (the blood becomes more dilute).
- Receptor
- Osmoreceptors in the hypothalamus detect the lower solute concentration.
- Gland and hormone
- The hypothalamus reduces signalling so the posterior pituitary releases less ADH.
- Target structure and effect
- ADH acts on the walls of the distal convoluted tubule and collecting duct of the nephron, making them permeable to water. With less ADH, these walls become less permeable, so less water is reabsorbed and more remains in the filtrate.
- Response and feedback
- A large volume of dilute urine is produced, removing the excess water, so the solute concentration of the blood rises back toward the set point. The response opposes the change, so it is negative feedback.
Markers reward osmoreceptors, reduced ADH, the collecting duct as the target, and dilute urine restoring the set point.
WACE 20244 marksExplain the role of aldosterone in regulating the sodium concentration of the blood, and identify the gland that secretes it.Show worked answer →
A 4 mark answer needs the gland, the trigger, the action and the result.
- Gland
- Aldosterone is secreted by the adrenal glands (adrenal cortex).
- Trigger
- A fall in blood sodium concentration or blood pressure stimulates its release.
- Action
- Aldosterone acts on the nephron, increasing the reabsorption of sodium ions from the filtrate back into the blood, with water following by osmosis.
- Result
- Blood sodium concentration and blood volume rise back toward the set point, a negative feedback response.
Markers reward the adrenal gland, increased sodium reabsorption, and the return toward the set point.