AQA A-level Biology: Osmoregulation and the Kidney (7402) – Revision Notes

These notes condense AQA A-level Biology (7402) section 3.6.4.3, Control of blood water potential, from the AQA AS and A-level Biology specification for first teaching 2015. The section is A-level only. For full explanations start with the study guide, then use the practice questions. Course pages: AQA A-level Biology hub and the printable checklist. If you are unsure where to begin, take a free diagnostic.

Key definitions

Term	Meaning
Osmoregulation	Control of the water potential of the blood
Water potential	The tendency of water to move; pure water is the highest, solutes lower it
Osmoreceptor	A cell in the hypothalamus that detects blood water potential
ADH	Antidiuretic hormone, made in the hypothalamus, released from the posterior pituitary
Ultrafiltration	Filtration under pressure through the glomerulus into the Bowman’s capsule
Glomerular filtrate	Fluid in the capsule: water, glucose, amino acids, urea, ions; no cells or large proteins
Aquaporin	A water channel protein in a cell membrane
Countercurrent multiplier	The loop of Henle arrangement that builds a sodium ion gradient in the medulla

The ADH pathway in order

Dehydration, low blood water potential:

1. Osmoreceptors in the hypothalamus shrink.
2. Neurosecretory cells send impulses down their axons to the posterior pituitary.
3. More ADH is released into the blood.
4. ADH binds to receptors on DCT and collecting duct cells.
5. More aquaporins are inserted into the membranes, which become more permeable.
6. More water is reabsorbed by osmosis, giving a small volume of concentrated urine.
7. Blood water potential rises, osmoreceptors stop being stimulated, less ADH is released. This is negative feedback.

Too much water, high blood water potential: every step reverses. Less ADH, fewer aquaporins, a large volume of dilute urine.

Nephron map

Part	Where	Key facts
Afferent arteriole	Cortex	Wider; brings blood to glomerulus
Glomerulus	Cortex	Capillary knot; high hydrostatic pressure
Efferent arteriole	Cortex	Narrower; causes the pressure build-up
Bowman’s capsule	Cortex	Wall has podocytes
PCT	Cortex	Microvilli, many mitochondria; glucose and water reabsorbed
Loop of Henle	Medulla	Ascending limb impermeable to water; builds sodium ion gradient
DCT	Cortex	Water reabsorbed under ADH control
Collecting duct	Cortex to medulla	Water reabsorbed under ADH control; carries urine to the pelvis

Ultrafiltration: method in steps

1. Efferent arteriole narrower than afferent, so hydrostatic pressure in the glomerulus is high.
2. Fluid is forced out through capillary endothelium gaps, the basement membrane and between podocytes.
3. The basement membrane is the main barrier. Blood cells and plasma proteins stay in the blood.
4. Plasma proteins lower the water potential of blood, which opposes outward movement.
5. Net outward pressure = capillary hydrostatic pressure − capsule pressure − pressure due to plasma proteins (when these values are given as data).

PCT reabsorption: method in steps

1. Sodium-potassium pumps in the PCT cell surface next to the blood move sodium ions out of the cell by active transport.
2. Sodium ion concentration inside the cell is low.
3. Sodium ions enter from the lumen by co-transport with glucose.
4. Glucose leaves the cell into tissue fluid by facilitated diffusion, then enters the blood.
5. Water potential of the filtrate rises compared with the blood, so water follows by osmosis.

Adaptations: microvilli (surface area), many mitochondria (ATP), folded membrane next to the capillary (surface area), carrier proteins. Glucose cannot be reabsorbed once all the carriers are in use.

Loop of Henle: method in steps

1. Ascending limb: sodium ions (and chloride ions) actively transported out into the medulla. The wall is impermeable to water.
2. Medulla tissue fluid has a low water potential.
3. Descending limb: permeable to water, which leaves by osmosis. Filtrate becomes concentrated.
4. Opposite flow in the two limbs multiplies the effect, so the gradient is steepest at the base of the loop.
5. Collecting duct: water leaves by osmosis all the way down, if its walls are permeable.

A longer loop gives a greater gradient over a greater depth, and a more concentrated urine.

Must-know distinctions

Compare	Point of difference
Ultrafiltration and selective reabsorption	Filtration is passive, driven by pressure, and non-selective by size. Reabsorption uses carriers and ATP
Ascending and descending limb	Ascending: ions out, impermeable to water. Descending: water out
Dehydrated and hydrated	Dehydrated: more ADH, small volume, concentrated urine. Hydrated: less ADH, large volume, dilute urine
Filtrate and urine	Filtrate has glucose; urine of a healthy person does not
Cortex and medulla	Cortex: capsules and convoluted tubules. Medulla: loops and collecting ducts

Small worked reminders

Percentage reabsorbed: with 152 dm³ of filtrate formed in a day and 1.4 dm³ of urine passed, reabsorbed = 150.6 dm³; 150.6 ÷ 152 × 100 = 99.1%.

Filtered load = plasma concentration × filtration rate. For plasma glucose 4.5 g dm⁻³ and filtration of 0.120 dm³ min⁻¹: 4.5 × 0.120 = 0.540 g min⁻¹ = 540 mg min⁻¹.

Reading the filtrate and the urine

Substance	In glomerular filtrate	In urine of a healthy person	Reason
Water	Yes	Yes, less	Reabsorbed in PCT, descending limb, DCT and collecting duct
Glucose	Yes	No	All reabsorbed in the PCT by co-transport
Amino acids	Yes	No	Reabsorbed in the PCT
Urea	Yes	Yes, more concentrated	Not reabsorbed as much as the water
Plasma proteins	No	No	Too large to pass the basement membrane
Red blood cells	No	No	Too large to be filtered

Protein or blood cells in urine point to damage to the filter. Glucose in urine suggests the carriers were overloaded.

Data questions: a routine

1. Read the axis labels and units before describing anything.
2. Quote values from the table or graph, with units, for both ends of the change.
3. For a percentage change, divide the difference (new minus old) by the old value, multiply by 100, and keep the sign.
4. Link the figure to a mechanism: ADH, aquaporins, osmosis, the sodium ion gradient.
5. For comparisons between species, say what the data do and do not show: a trend does not prove a cause.

Example: loop length 12 mm gives urine 4.0 times plasma concentration, and loop length 28 mm gives 8.5 times. The longer loop goes with the more concentrated urine, which fits a steeper gradient in the medulla. Three species cannot show proportionality.

Quick self-test

1. Name the part of the brain that contains osmoreceptors.
2. Where is ADH stored and released?
3. State which two parts of the nephron have walls whose water permeability is controlled by ADH.
4. Explain why the efferent arteriole is narrower than the afferent arteriole.
5. Name the three layers that fluid crosses to enter the capsule.
6. Why is there no protein in glomerular filtrate?
7. Describe how sodium ions help glucose to enter PCT cells.
8. The ascending limb of the loop of Henle is impermeable to water. Explain why this is important.
9. Plasma glucose is 4.5 g dm⁻³ and filtrate forms at 0.120 dm³ min⁻¹. Calculate the glucose filtered per minute, in mg.
10. A person’s urine flow rises from 0.8 to 2.0 cm³ min⁻¹ after drinking. Calculate the percentage increase.
11. State the effect of a longer loop of Henle on urine concentration.
12. Why does glucose appear in urine when the plasma glucose is very high?

Answers

1. Hypothalamus.
2. Stored and released in the posterior pituitary gland (made in the hypothalamus).
3. The distal convoluted tubule and the collecting duct.
4. It causes a build-up of hydrostatic pressure in the glomerulus, which drives ultrafiltration.
5. Capillary endothelium, basement membrane, podocytes (the capsule wall cells).
6. The proteins are too big to cross the basement membrane.
7. Sodium ions enter down a concentration gradient through co-transport proteins and carry glucose in with them. The gradient is kept by active transport of sodium ions out of the cell.
8. Ions leave but water cannot follow, so the medulla tissue fluid gets a low water potential, which is the gradient that draws water out of the descending limb and collecting duct.
9. 4.5 × 0.120 = 0.540 g min⁻¹ = 540 mg min⁻¹.
10. (2.0 − 0.8) ÷ 0.8 × 100 = +150%.
11. A steeper gradient over a greater depth, so more water is reabsorbed and the urine is more concentrated.
12. The filtered glucose exceeds what the carriers can reabsorb, so some stays in the filtrate.

Where marks are usually lost

• Saying ADH “is released by the hypothalamus” when it is released from the posterior pituitary. Examiners tend to look for both structures named correctly.
• Writing that the kidney “filters out” what the body needs. State that useful substances are filtered and then reabsorbed.
• Describing water as being “pumped” or “actively transported” out of the filtrate.
• Leaving out the word “osmosis”, or not stating the water potential gradient between filtrate and medulla tissue fluid.
• Saying only that the loop of Henle “concentrates urine”. Give the steps: ions out, limb impermeable to water, low water potential in the medulla.
• Mixing up high and low water potential. Dehydration means lower blood water potential, not higher.
• Omitting units or the sign of a percentage change, such as a decrease written as a positive value.
• Naming a hormone effect without the target: ADH acts on the DCT and collecting duct, not the PCT.

Official syllabus

AQA, AS and A-level Biology specification (7401 and 7402), for first teaching 2015, section 3.6.4.3 Control of blood water potential (A-level only). Published by AQA.