The kidney’s design looks wasteful at first. It squeezes a huge volume of fluid out of the blood, including everything useful, and then spends energy taking most of it back. But that is exactly what makes it so precise: filter indiscriminately, then reclaim exactly what you need and let the rest go.
📚 What you need to know
The renal artery supplies the kidney; the renal vein drains it; the ureter carries urine to the bladder.
The kidney has three regions: cortex, medulla and renal pelvis. The functional unit is the nephron.
Ultrafiltration in the glomerulus and Bowman’s capsule forces small molecules out of the blood at high pressure. Large proteins and blood cells stay behind.
The barrier has three layers: fenestrated capillary endothelium, basement membrane, and podocytes.
Selective reabsorption in the proximal convoluted tubule takes back glucose, amino acids, salts and most water.
The loop of Henle creates a solute gradient in the medulla, allowing production of urine more concentrated than the blood.
ADH from the posterior pituitary controls the permeability of the distal convoluted tubule and collecting duct by inserting aquaporins.
The plumbing, briefly
Blood arrives through the renal artery carrying urea and salts, and leaves through the renal vein with those removed. The filtrate that the kidney produces becomes urine, which travels down a ureter to the bladder, where it is stored temporarily before leaving through the urethra.
Structure
Function
Renal artery
Carries oxygenated blood containing urea and salts to the kidney
Renal vein
Carries deoxygenated blood, with urea and excess salts removed, away from the kidney
Kidney
Filters the blood and regulates its water content
Ureter
Carries urine from the kidney to the bladder
Bladder
Stores urine temporarily
Urethra
Releases urine outside the body
Inside one kidney
The kidney is wrapped in a tough outer layer, the fibrous capsule. Beneath it are three regions: the cortex on the outside, the medulla beneath it, and the renal pelvis, the funnel where all the nephrons drain before the ureter.
Each kidney contains thousands of tiny tubules called nephrons. These are the functional unit — the structure that actually makes urine. Different parts of a nephron sit in different regions, and that geography matters enormously.
Only the loop of Henle and the collecting duct dip into the medulla. Everything else — capsule, glomerulus and both convoluted tubules — stays up in the cortex. Examiners ask about this directly.
Stage 1: ultrafiltration
Blood reaches each glomerulus through an afferent arteriole, which splits into a ball of capillaries. Those capillaries rejoin to form the efferent arteriole, which is narrower than the afferent one. Squeezing the same blood into a narrower exit raises the pressure inside the glomerulus far above that of a normal capillary bed.
That high pressure forces small dissolved molecules out of the capillaries and into the Bowman’s capsule. The fluid produced is the glomerular filtrate. Large molecules such as proteins stay in the blood.
The basement membrane is the layer doing the real sorting. It is a mesh of collagen and glycoproteins that acts as a sieve, letting small molecules through and holding back large proteins.
The three layers in words
Capillary endothelium. There are gaps between the endothelial cells called fenestrations. Fluid passes through them, but blood cells are too large.
Basement membrane. A network of collagen protein and glycoproteins, arranged in a mesh that acts as a sieve. This is what stops large proteins from leaving the plasma.
Bowman’s capsule epithelium. These cells, called podocytes, have foot-like projections that wrap around the capillary. Small molecules pass through the gaps between the projections.
Substance
In blood plasma
In glomerular filtrate
What this tells you
Urea
Present
Present at the same concentration
Small enough to be filtered freely
Glucose
Present
Present at the same concentration
Filtered freely, so must be reabsorbed later
Sodium ions
Present
Present, almost unchanged
Small ions pass easily
Proteins
High concentration
Almost none
Blocked by the basement membrane
If a patient’s urine contains protein, the filtration barrier itself has been damaged — usually the basement membrane. If it contains glucose, filtration was fine but reabsorption could not keep up, which points at diabetes. Two different substances, two completely different diagnoses.
Stage 2: selective reabsorption
Much of what was filtered out is useful, so it is taken back into the blood as the filtrate travels along the nephron. It is called selective reabsorption because not everything is reclaimed — urea and other unwanted solutes are left behind.
Most of it happens in the proximal convoluted tubule, though the loop of Henle and collecting duct also reabsorb water and salts. The substances taken back are water, salts, glucose and amino acids.
🧩 How the proximal convoluted tubule does it
Sodium ions are pumped out of the epithelial cells into the surrounding tissue by active transport, using ATP. This lowers the sodium concentration inside the cell.
Sodium then moves in from the filtrate down its concentration gradient, through co-transporter proteins in the luminal membrane — and each of these drags glucose or an amino acid in with it.
Chloride ions follow the positively charged sodium by diffusion, down the electrical gradient.
Water follows by osmosis, because all these solutes moving into the surrounding tissue have lowered its water potential.
Urea moves out by diffusion too, but only a little — most stays in the filtrate.
Everything that has left the tubule then moves into the nearby capillaries down its concentration gradient.
Notice that glucose is not pumped directly. It rides along with sodium, which is why this is called cotransport — and why it is still classed as an active process even though the glucose itself moves passively. The ATP was spent one step earlier.
Adaptation of the epithelial cell
How it helps reabsorption
Many microvilli on the luminal membrane
Greatly increase the surface area available for reabsorption
Many co-transporter proteins in the luminal membrane
Each type carries one specific solute, such as glucose or a particular amino acid
Many mitochondria
Supply ATP for the sodium-potassium pumps in the basal membrane
Cells tightly packed together
No fluid can pass between cells, so everything reabsorbed must pass through them
Stage 3: the loop of Henle
The loop of Henle exists to solve one problem: how do you produce urine that is more concentrated than your own blood? Its trick is not to concentrate the urine directly. It concentrates the tissue around the tubule, and then lets osmosis do the work.
The two limbs are doing opposite things on purpose. The ascending limb spends ATP salting the medulla; the descending limb then loses water to that salty medulla for free. This arrangement is called a counter-current multiplier.
The loop in sentences
Sodium and chloride ions are pumped out of the ascending limb into the surrounding medulla, lowering the water potential of the medulla.
The ascending limb is impermeable to water, so water cannot follow those ions out. The filtrate inside it therefore becomes more dilute as it rises back towards the cortex.
The descending limb is permeable to water but has few ion transport proteins. Water moves out of it by osmosis into the salty medulla, so the filtrate inside becomes more concentrated as it descends.
The low water potential the loop creates in the medulla also allows water to be reabsorbed from the collecting duct as it passes down through the medulla.
Water and ions that leave the loop enter the vasa recta, the capillary that runs alongside it, which also supplies oxygen to the respiring cells of the loop.
🧠
Which limb does what
Ascending pumps out Actively and blocks water. Descending just lets water Drain out. If you remember which one is impermeable to water, the rest follows.
Stage 4: ADH and the collecting duct
Everything so far happens all the time. This last stage is the adjustable one — the part that actually performs osmoregulation.
Osmoreceptors in the hypothalamus monitor the composition of the blood flowing past, and also receive signals from receptors elsewhere in the body. The hypothalamus then controls how much antidiuretic hormone (ADH) is released from the posterior pituitary gland.
ADH’s target cells are in the distal convoluted tubule and collecting duct. It changes how permeable their walls are to water by controlling the number of aquaporins — channel proteins for water — in their cell surface membranes. The aquaporins are stored in the membranes of vesicles; ADH causes those vesicles to fuse with the cell surface membrane, adding the channels.
When ADH levels fall, the cell surface membrane is pinched inwards to reform the vesicles, taking the aquaporins back out of the membrane. The change is reversible in both directions.
When blood water content is low
This happens after reduced water intake, sweating or diarrhoea. It can also be described as a high blood solute concentration, and taken far enough it causes dehydration.
The drop is detected by osmoreceptors in the hypothalamus.
The hypothalamus causes the posterior pituitary to secrete ADH into the blood.
ADH increases the permeability of the distal convoluted tubule and collecting duct by adding aquaporins.
More water is reabsorbed into the blood, all the way down the collecting duct, because the loop of Henle has made the medulla increasingly concentrated.
Blood water content rises, and a small volume of concentrated urine is produced.
When blood water content is high
This happens after drinking a lot, or after losing salts in sweat. Taken far enough it causes overhydration.
The rise is detected by the hypothalamus, which no longer stimulates the pituitary to release ADH, so ADH levels in the blood drop.
The walls of the distal convoluted tubule and collecting duct become less permeable to water as aquaporins are removed.
Less water is reabsorbed, so it stays in the duct and passes into the renal pelvis with the rest of the filtrate.
Blood water content falls, and a large volume of dilute urine is produced.
The word that trips people up. ADH stands for anti-diuretic hormone. A diuretic makes you produce more urine, so an anti-diuretic makes you produce less. If you can remember that, you can never get the direction of this loop wrong.
Worked examples
WE 1
Explaining ultrafiltration
Explain how the structure of the glomerulus and Bowman’s capsule allows ultrafiltration to occur. (4 marks)
Step 1: the pressure
The afferent arteriole is wider than the efferent arteriole, so blood in the glomerular capillaries is at unusually high pressure.
Step 2: the first two layersFenestrations between the capillary endothelial cells let fluid through but not blood cells, and the basement membrane acts as a mesh-like sieve that holds back large proteins.
Step 3: the third layer
The podocytes of the capsule wall have foot-like projections, and small molecules pass through the gaps between them.
Step 4: the result
Water, glucose, amino acids, urea and mineral ions are forced into the capsule to form the glomerular filtrate.
Pressure pushes; the three layers decide what gets throughname all three layers in order. Each is usually worth a mark on its own.
WE 2
Comparing plasma, filtrate and urine
The table shows measurements from one patient. Blood plasma: glucose 5.0, protein 74, urea 5.0. Glomerular filtrate: glucose 5.0, protein 0.0, urea 5.0. Urine: glucose 0.0, protein 0.0, urea 300. All values are relative units. Explain the three values for glucose, and the value for urea in the urine. (4 marks)
Step 1: glucose in the filtrate
Glucose is the same in plasma and filtrate (5.0), because it is a small molecule that is filtered freely at the glomerulus.
Step 2: glucose in the urine
Glucose falls to zero because it is completely reabsorbed in the proximal convoluted tubule by co-transport with sodium ions.
Step 3: urea in the urine
Urea rises from 5.0 to 300 — a 60-fold increase — not because more urea was added, but because water has been reabsorbed from the filtrate, concentrating what remains.
Step 4: the check
Protein is zero in both filtrate and urine, confirming the basement membrane is working normally.
Glucose is taken back; urea is left behind while the water goesthe urea trap catches many students. Concentration can rise with no extra solute if the solvent is removed.
WE 3
Predicting the ADH response
A runner completes a long race in hot weather without drinking. Describe and explain the changes to their ADH level and urine. (4 marks)
Step 1: the change in the blood
Water is lost in sweat, so blood water content falls and the solute concentration of the blood rises.
Step 2: detection
This is detected by osmoreceptors in the hypothalamus, which stimulates the posterior pituitary to release more ADH.
Step 3: the effect on the nephron
ADH causes vesicles containing aquaporins to fuse with the membranes of cells in the distal convoluted tubule and collecting duct, increasing their permeability to water.
Step 4: the outcome
More water is reabsorbed into the blood by osmosis, so a smaller volume of more concentrated urine is produced, and blood water content returns to normal.
Less water in, more ADH out, less urine made“describe and explain” means give the change and the mechanism. Doing only one halves your marks.
💡 Exam tips
Learn which parts sit in the cortex (capsule, glomerulus, both convoluted tubules) and which in the medulla (loop of Henle, collecting duct).
For ultrafiltration, mention afferent wider than efferent — it is the reason the pressure is high.
Say co-transport with sodium ions for glucose reabsorption, not “active transport of glucose”.
For the loop of Henle, always state that the ascending limb is impermeable to water. Most of the marks hang off that fact.
ADH changes permeability. It does not pump water and it does not act on the loop of Henle.
When comparing filtrate and urine data, look for what has been removed as well as what has been concentrated.
⚠ Common mistakes
Saying the glomerulus filters “waste”. It filters by size. Glucose and amino acids are filtered out too.
Saying protein is reabsorbed. Protein never enters the filtrate in the first place.
Saying the loop of Henle concentrates the urine. It concentrates the medulla. The collecting duct then concentrates the urine.
Getting the limbs the wrong way round. Water leaves the descending limb; salts leave the ascending limb.
Saying ADH is made in the pituitary. It is produced under hypothalamic control and released from the posterior pituitary.
Writing that ADH makes more urine. Anti-diuretic. It makes less.
Up next: Changing Blood Supply to Organs. The last page in this topic returns to vasodilation and vasoconstriction, but this time as a way of deciding where your blood goes rather than how much heat you lose.
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