Structure of the Nephron — AQA A-level Biology

SPEC 3.6.4.3 - RESPONSES — HOMEOSTASIS

Osmoregulation: ADH, Nephron & Loop of Henle

Controlling blood water potential · Nephron structure & function · Ultrafiltration · Selective reabsorption · Loop of Henle · ADH & aquaporins

What is Osmoregulation?

Definition

Osmoregulation is the control of the water potential of the blood and body fluids. It is essential for maintaining the correct concentration of solutes in cells and tissues to prevent shrinkage (crenation) or bursting of cells.

The kidneys are the primary organ of osmoregulation in mammals. Each kidney contains approximately one million functional units called nephrons.

If blood water potential falls

Blood is too concentrated (e.g. after sweating or dehydration) → osmoreceptors in the hypothalamus detect this → more ADH released → more water reabsorbed → small volume of concentrated urine

If blood water potential rises

Blood is too dilute (e.g. after drinking water) → less ADH released → less water reabsorbed → large volume of dilute urine

Exam Tip

Always describe osmoregulation in terms of water potential, not "water concentration." Osmosis moves water from a region of higher water potential to a region of lower water potential.

The Nephron — Structure

Definition

The nephron is the functional unit of the kidney. Each nephron consists of: Bowman's capsule (surrounding the glomerulus), proximal convoluted tubule (PCT), loop of Henle, distal convoluted tubule (DCT), and collecting duct.

Ultrafiltration

Ultrafiltration occurs at the Bowman's capsule, driven by the high hydrostatic pressure of blood in the glomerulus.

How it works

The afferent arteriole (carrying blood to the glomerulus) is wider than the efferent arteriole (leaving) — this creates high hydrostatic pressure in the glomerular capillaries

High pressure forces small molecules out of the blood through the basement membrane (filter) into the Bowman's capsule, forming the glomerular filtrate

Passes into filtrate: water, glucose, urea, amino acids, ions (small molecules)

Retained in blood: plasma proteins, blood cells (too large to pass through basement membrane)

Definition

Glomerular filtrate is the fluid forced from the blood into the Bowman's capsule by ultrafiltration; it has a similar composition to blood plasma but lacks large proteins and blood cells.

Examiner Alert

⚠️ The driving force for ultrafiltration is hydrostatic pressure (blood pressure), not osmosis. Do not describe ultrafiltration as osmosis — it is a pressure-driven filtration process.

Selective Reabsorption — PCT

The proximal convoluted tubule (PCT) reabsorbs useful substances from the filtrate back into the blood.

What is reabsorbed in the PCT

All glucose — by active transport and co-transport (sodium-glucose co-transport) across the tubule wall into the blood

All amino acids — by active transport

~80% of water — by osmosis, following the active transport of solutes

Inorganic ions (Na⁺, Cl⁻ etc.) — by active transport

Structural adaptations of PCT cells

Microvilli (brush border) — greatly increase surface area for absorption

Many mitochondria — provide ATP for active transport

Close association with blood capillaries — short diffusion distance

Synoptic Link

Glucose reabsorption in the PCT uses the same co-transport mechanism as in the ileum (Topic 3.04): Na⁺ is actively pumped out of the epithelial cell → Na⁺ diffuses back in, carrying glucose with it.

The Loop of Henle

The loop of Henle acts as a countercurrent multiplier, creating an increasingly concentrated sodium ion gradient in the medulla. This gradient is essential for water reabsorption in the collecting duct.

1

Descending limb — permeable to water but not to salts. As filtrate descends into the increasingly concentrated medulla, water leaves by osmosis → filtrate becomes more concentrated.

2

Bottom of loop — filtrate is at its most concentrated here.

3

Ascending limb — impermeable to water. Na⁺ and Cl⁻ are actively pumped out into the medulla → medullary solute concentration is maintained high → filtrate becomes more dilute as it ascends.

4

Result — a steep gradient of sodium ion concentration is maintained in the medulla (increasingly concentrated towards the bottom). This gradient drives water out of the collecting duct when ADH is present.

Why loop length matters

A longer loop of Henle → steeper sodium gradient → greater water reabsorption capacity. Desert animals (e.g. kangaroo rats) have very long loops and can produce highly concentrated urine.

DCT & Collecting Duct

The distal convoluted tubule (DCT) and collecting duct are the sites of ADH-controlled water reabsorption.

Without ADH (low ADH)

Collecting duct wall has low permeability to water

Little water reabsorbed → large volume of dilute urine produced

Occurs when blood water potential is high (well-hydrated)

With ADH (high ADH)

ADH binds receptors on collecting duct cells → aquaporin channel proteins inserted into membrane

Collecting duct becomes highly permeable to water

Water passes by osmosis into the hypertonic medulla → small volume of concentrated urine

Occurs when blood water potential is low (dehydrated)

Examiner Alert

⚠️ Never write "ADH makes the collecting duct more permeable" without specifying more permeable to water via the insertion of aquaporin channel proteins. Both details are required for full marks.

ADH Mechanism — Step by Step

1

Trigger: Blood water potential falls (e.g. sweating, insufficient fluid intake) → blood becomes more concentrated (solute concentration rises)

2

Detection:Osmoreceptors in the hypothalamus detect the fall in blood water potential

3

Hormone release: The hypothalamus signals the posterior pituitary gland to release ADH (antidiuretic hormone) into the bloodstream

4

Target organ: ADH travels in the blood to the collecting duct cells

5

Effect: ADH binds to receptors on collecting duct cells → triggers insertion of aquaporin channel proteins into the cell surface membrane → collecting duct becomes more permeable to water

6

Outcome: Water moves by osmosis from the filtrate into the medullary tissue fluid and back into the blood → small volume of concentrated urine produced

7

Negative feedback: Blood water potential rises → osmoreceptors detect this → ADH secretion reduced → fewer aquaporins inserted → water reabsorption decreases

ADH Regulation — Diagram

Nephron — Function Summary

RegionProcessKey Details

Glomerulus + Bowman's capsuleUltrafiltrationHigh hydrostatic pressure forces small molecules into filtrate; proteins and cells remain in blood

Proximal convoluted tubule (PCT)Selective reabsorptionAll glucose and amino acids (active transport/co-transport); ~80% water (osmosis); ions (active transport)

Loop of HenleCountercurrent multiplierDescending: water leaves by osmosis; Ascending: Na⁺/Cl⁻ actively pumped out → high solute concentration in medulla

Distal convoluted tubule (DCT)ADH-dependent water reabsorptionADH acts on DCT and collecting duct; aquaporin insertion increases water permeability → water reabsorbed by osmosis

Collecting ductADH-dependent water reabsorptionIf ADH present: aquaporins inserted → water reabsorbed by osmosis into hypertonic medulla → concentrated urine

DEEP Chain — ADH Mechanism

Use this structure for any 4–6 mark question asking you to explain how ADH controls urine concentration:

DDefine: ADH (antidiuretic hormone) is released from the posterior pituitary gland; it controls the water permeability of the collecting duct, regulating blood water potential.

EExplain mechanism: Fall in blood water potential → osmoreceptors in hypothalamus detect change → posterior pituitary releases ADH → ADH binds receptors on collecting duct cells → aquaporin channel proteins inserted into membrane → collecting duct becomes more permeable to water → water reabsorbed by osmosis from filtrate into blood → small volume of concentrated urine.

EEvidence/Example: The negative feedback loop: rising blood water potential → reduced ADH → fewer aquaporins → less water reabsorbed → more dilute urine. Alcohol inhibits ADH release → dilute, high-volume urine (diuresis).

PPractical/Quantitative: Patients with diabetes insipidus lack functional ADH → cannot concentrate urine → produce very large volumes of dilute urine regardless of hydration status. This confirms ADH's essential role.

Practice Question

A person exercises vigorously in hot weather and does not drink any water. Describe and explain the changes in ADH secretion and urine production that occur as a result. [5 marks]

D:

Exercise causes sweating → water is lost from the blood → blood water potential falls (blood becomes more concentrated).

E:

Osmoreceptors in the hypothalamus detect the fall in blood water potential → hypothalamus signals the posterior pituitary gland → more ADH is released into the bloodstream.

E:

ADH binds to receptor proteins on collecting duct cells → aquaporin channel proteins are inserted into the cell surface membrane of the collecting duct → the collecting duct becomes more permeable to water.

P:

Water moves by osmosis from the filtrate into the medullary tissue fluid (down a water potential gradient created by the hypertonic medulla) → more water is reabsorbed into the blood → a small volume of concentrated urine is produced. This continues until blood water potential is restored to normal (negative feedback).

Loop of Henle — Countercurrent Multiplier

The loop creates a gradient of sodium ion concentration in the medulla — highest at the base of the loop, lowest near the cortex. This gradient is what draws water out of the collecting duct.

Descending Limb

Permeable to water but not to salts

Filtrate becomes progressively more concentrated as water leaves by osmosis into the hypertonic medulla

Water moves out down its water potential gradient

Ascending Limb

Impermeable to water

Na⁺ and Cl⁻ actively transported out into the medulla interstitial fluid

Filtrate becomes more dilute; medulla becomes more concentrated

Maintains the high salt gradient in the medulla

Why it's called "countercurrent"

Filtrate flows DOWN the descending limb and UP the ascending limb — in opposite directions. This counter-flow allows the gradient to be progressively multiplied along the length of the loop.

Examiner Alerts & Synoptic Links

Common Errors

"ADH makes the collecting duct permeable" — must say permeable to water, via insertion of aquaporin channel proteins

"Ultrafiltration uses osmosis" — it uses hydrostatic pressure; osmosis is not involved at this stage

"Glucose is excreted in urine" — all glucose is reabsorbed in the PCT under normal conditions; glycosuria only occurs when blood glucose exceeds the renal threshold

"ADH is released from the hypothalamus" — ADH is made in the hypothalamus but released from the posterior pituitary gland

Ascending limb permeability — the ascending limb is impermeable to water; water leaves in the descending limb only

Synoptic Link

Link to cell membranes (Topic 2.05): Aquaporin insertion is an example of protein trafficking — vesicles containing aquaporins fuse with the cell surface membrane. This links to the fluid mosaic model and membrane protein function.

Link to diabetes (Topic 6.08): Hyperglycaemia in diabetes causes glycosuria — glucose in the urine lowers the water potential of the filtrate, reducing water reabsorption and causing polyuria (osmotic diuresis).

Summary

Key Points — Lesson 6.09

TopicCore Fact

OsmoregulationControl of blood water potential; kidneys are the primary organ

UltrafiltrationHigh hydrostatic pressure → small molecules forced into Bowman's capsule; proteins and cells retained

PCTAll glucose + amino acids reabsorbed (active transport/co-transport); ~80% water by osmosis

Loop of HenleDescending: water out (osmosis); Ascending: Na⁺/Cl⁻ actively pumped out → high medullary solute gradient

ADH sourceMade in hypothalamus; released from posterior pituitary

ADH mechanismBinds collecting duct receptors → aquaporins inserted → more permeable to water → concentrated urine

Negative feedbackBlood water potential rises → less ADH → fewer aquaporins → dilute urine

Loop lengthLonger loop → steeper gradient → more concentrated urine possible

Exam Tip

ADH questions nearly always require the word "aquaporin" and the phrase "more permeable to water" for full marks. Loop of Henle questions require stating which limb does what — descending (water out), ascending (salts out, impermeable to water). Always use water potential language, not "concentration."