High blood pressure in the glomerulus pushes water and small molecules through the filtration membrane into Bowman’s capsule, while blood cells and large proteins stay in the blood.
Every day your blood passes through your kidneys again and again, dropping off wastes and getting its water and salt balanced. The work is done by about a million tiny tubes per kidney called nephrons. The trick is simple to state: first squeeze a lot of watery fluid out of the blood, then take back almost everything worth keeping. What is left over — mostly water with dissolved wastes — becomes urine.
Blood enters a tangle of leaky capillaries, the glomerulus, under high pressure. That pressure forces a protein-free fluid, the filtrate, into Bowman’s capsule — this is filtration. It is huge and unselective: humans filter about $125\ \text{mL/min}$, roughly $180\ \text{L/day}$, far more than the body’s $\sim 3\ \text{L}$ of plasma. Clearly we cannot lose that much, so the long tubule downstream does reabsorption: it returns over $99\%$ of the water plus all the glucose and most ions to the blood in the surrounding capillaries. A third process, secretion, actively dumps extra wastes such as $\text{H}^+$ and $\text{K}^+$ from the blood into the tubule. Filter a lot, take back what you need, add a few extras — that is the whole strategy.
How can urine be made more concentrated than blood? The hairpin loop of Henle sets it up. Its ascending limb pumps salt into the surrounding medulla but keeps water in, building an osmotic gradient that climbs from about $300$ to $1200\ \text{mOsm/L}$ deep in the kidney. The collecting duct then runs back down through that salty zone. The hormone ADH decides how leaky the duct’s walls are to water: high ADH opens water channels, so water is pulled out and you make a little very concentrated urine (up to $\sim 1200\ \text{mOsm/L}$); low ADH keeps the walls sealed, so you make a lot of dilute urine. Roughly, the body must excrete a fixed solute load $\approx 600\ \text{mOsm/day}$, so urine volume $\approx 600 / \text{(urine osmolarity)}$.
Press Play and watch the particles: blue water and green glucose leaving the tubule for the blood during reabsorption, orange wastes entering during secretion. Drag ADH to the Dehydrated preset and watch the urine-osmolarity readout climb toward $1200\ \text{mOsm/L}$ while the urine volume shrinks; switch to Over-hydrated and see the opposite. Then drag Blood glucose past about $180\ \text{mg/dL}$ (the High glucose preset) and watch glucose appear in the urine — the proximal tubule’s transporters have been overwhelmed.
| Segment | What happens |
|---|---|
| 1. Glomerulus & Bowman’s capsule | Filtration. A wide afferent and narrow efferent arteriole keep glomerular pressure high, forcing a cell-free, protein-free filtrate into the capsule. Water, ions, glucose, amino acids and urea pass; cells and large proteins stay. GFR $\approx 125\ \text{mL/min}$; filtrate $\approx 300\ \text{mOsm/L}$. |
| 2. Proximal convoluted tubule | Reabsorption (bulk). About $65\%$ of filtered water and $\text{Na}^+$, and essentially $100\%$ of glucose and amino acids, are reabsorbed into the peritubular capillaries. Reabsorption is isosmotic, so osmolarity stays $\approx 300$ while volume falls sharply. |
| 3. Loop of Henle — descending limb | Reabsorption (water). The descending limb is permeable to water but not salt. As filtrate descends into the salty medulla, water leaves by osmosis and the filtrate concentrates, reaching up to $\approx 1200\ \text{mOsm/L}$ at the hairpin tip. |
| 4. Loop of Henle — ascending limb | Reabsorption (salt). The thick ascending limb is impermeable to water but actively pumps out $\text{Na}^+$, $\text{K}^+$ and $\text{Cl}^-$. The filtrate becomes dilute ($\approx 100\ \text{mOsm/L}$), and the exported salt maintains the medullary gradient — the countercurrent multiplier. |
| 5. Distal convoluted tubule | Secretion & fine-tuning. Hormone-controlled reabsorption of $\text{Na}^+$ (aldosterone) and secretion of $\text{H}^+$, $\text{K}^+$ and some drugs from the blood into the tubule adjust the urine’s exact composition and the blood’s pH. |
| 6. Collecting duct | ADH-controlled water reabsorption. The duct runs back through the salty medulla. ADH inserts aquaporins: high ADH → much water reabsorbed → small volume of concentrated urine (up to $\approx 1200\ \text{mOsm/L}$); low ADH → little reabsorbed → large volume of dilute urine ($\approx 50$–$100\ \text{mOsm/L}$). |
The loop of Henle is the kidney’s clever trick for concentrating urine. The ascending limb constantly pumps salt out into the medulla without letting water follow, so the deeper medulla becomes increasingly salty. Because the descending limb sits in that same salty tissue, water keeps leaving it — which makes the fluid reaching the bottom even saltier, which the ascending limb then pumps out even harder. This self-reinforcing “multiplier” builds and holds a steep gradient from $\approx 300\ \text{mOsm/L}$ in the cortex to $\approx 1200$ deep in the medulla. The collecting duct passes through this gradient, so whenever ADH makes its walls permeable, water is drawn out and the urine can be concentrated far above the blood.
It helps to keep the two transport directions straight. Reabsorption moves substances out of the tubule and into the blood — reclaiming water, glucose, amino acids and most ions. Secretion moves substances out of the blood and into the tubule — getting rid of $\text{H}^+$, excess $\text{K}^+$, ammonia, and many drugs. Filtration is the bulk first pass; reabsorption recovers the good stuff; secretion is a targeted top-up for things the body wants gone. The final urine is what is filtered, minus what is reabsorbed, plus what is secreted.
A nephron is the microscopic functional unit of the kidney, and each human kidney contains about a million of them. Every nephron has two parts: a renal corpuscle that filters the blood and a long tubule that processes the filtrate. The renal corpuscle is the glomerulus, a tuft of capillaries, cupped inside Bowman’s capsule. From the capsule the fluid flows into the proximal convoluted tubule, then down and up the hairpin loop of Henle, into the distal convoluted tubule, and finally into a collecting duct shared by several nephrons. Blood vessels run alongside: the afferent arteriole feeds the glomerulus, the efferent arteriole drains it, and the peritubular capillaries wrap the tubule to collect what is reabsorbed.
Key takeaway: a nephron is one filtering unit — a glomerulus inside Bowman’s capsule plus a tubule (proximal tubule, loop of Henle, distal tubule, collecting duct) — with blood vessels running alongside.Filtration happens at the glomerulus inside Bowman’s capsule. Blood arrives through a wide afferent arteriole and leaves through a narrower efferent arteriole, and that mismatch keeps the pressure inside the glomerulus high. The high pressure forces fluid through a three-layer filtration membrane — capillary wall, basement membrane, and the slit pores between podocyte foot processes — which sorts by size and charge, not by usefulness. Water, ions, glucose, amino acids and small wastes like urea pass through into the capsule; blood cells and large plasma proteins such as albumin are too big and stay in the blood. The fluid that crosses is essentially plasma without its cells and proteins, near 300 milliosmoles per litre. Humans filter about 125 millilitres per minute, roughly 180 litres per day.
Key takeaway: high pressure pushes a cell-free, protein-free filtrate into Bowman’s capsule, sorting molecules by size and charge rather than by whether the body needs them.Reabsorption returns useful substances from the filtrate in the tubule back into the blood of the peritubular capillaries; it is the reason we do not lose the 180 litres we filter daily. Most happens in the proximal convoluted tubule, which reclaims about 65 percent of the filtered water and sodium and essentially all of the glucose and amino acids; here it is isosmotic, so water follows solute and osmolarity stays near 300 while volume shrinks. The loop of Henle adds more: the descending limb loses water by osmosis in the salty medulla, while the thick ascending limb pumps out sodium, potassium and chloride. The distal tubule and collecting duct then do fine, hormone-controlled reabsorption. In total the kidney reclaims more than 99 percent of the filtered water.
Key takeaway: reabsorption moves water, glucose and ions from the tubule back into the blood, mostly in the proximal tubule, recovering over 99 percent of what was filtered.Secretion moves selected substances from the blood in the peritubular capillaries into the filtrate in the tubule — the opposite direction to reabsorption. It happens mainly in the proximal and distal tubules and the collecting duct, and it handles hydrogen ions (for pH control), potassium ions (regulated by aldosterone), ammonia, and many drugs and toxins such as penicillin. It differs from filtration in two ways: filtration is a bulk, unselective, pressure-driven push at the glomerulus that sweeps almost all small molecules out at once, while secretion is selective and often uses active transport to add specific substances that were not filtered efficiently or must be removed in larger amounts. Filtration is the coarse first pass; secretion is the targeted top-up.
Key takeaway: secretion actively adds specific wastes and ions from the blood into the tubule to fine-tune the urine, whereas filtration is the bulk, pressure-driven removal of small molecules at the glomerulus.The kidney can make urine more dilute or more concentrated than blood using the loop of Henle and the hormone ADH. The loop works as a countercurrent multiplier: the ascending limb pumps salt out into the medulla without letting water follow, building a steep gradient that rises from about 300 milliosmoles per litre in the outer kidney to about 1200 in the deep medulla. The collecting duct runs down through this salty region, and ADH (vasopressin) — released by the pituitary when the body is short of water — controls how many aquaporin water channels sit in the duct wall. High ADH makes the wall permeable, so water is drawn out into the salty medulla and a small volume of concentrated urine, up to 1200, results. Low ADH keeps the duct waterproof, so a large volume of dilute urine, as low as 50 to 100, is produced.
Key takeaway: the loop of Henle builds a salty medullary gradient and ADH sets how much water the collecting duct reabsorbs — high ADH gives little concentrated urine, low ADH gives lots of dilute urine.In a healthy person there is no glucose in the urine, even though glucose is freely filtered, because the proximal tubule reabsorbs every bit of it using sodium-glucose cotransporters. Those transporters have a limited capacity, the transport maximum. As long as the filtered glucose stays below that limit, all of it is reclaimed. The renal threshold — the blood-glucose level at which the transporters start to be overwhelmed — is around 180 milligrams per decilitre, roughly 10 millimoles per litre. In untreated diabetes mellitus blood glucose climbs well above this, so more glucose is filtered than can be reabsorbed and the excess spills into the urine, called glucosuria. That unreabsorbed glucose also holds water in the tubule by osmosis, which is why people with untreated diabetes produce a lot of urine and feel very thirsty.
Key takeaway: glucose appears in urine only when blood glucose climbs above the renal threshold of about 180 milligrams per decilitre and overwhelms the proximal-tubule transporters, as in untreated diabetes.The glomerular filtration rate, GFR, is the total volume of filtrate that all the glomeruli of both kidneys produce per minute. In a healthy adult it is about 125 millilitres per minute, which adds up to roughly 180 litres per day. That is striking because it is far more than the body’s 3 litres or so of blood plasma, meaning the kidneys filter the entire plasma volume many times over each day. Almost all of it is reabsorbed, so only about 1 to 1.5 litres leaves as urine. GFR is one of the most important measures of kidney health: if it falls, wastes build up in the blood, which is why doctors estimate GFR from a blood creatinine test.
Key takeaway: GFR is about 125 millilitres per minute, or 180 litres per day; the kidneys filter the whole plasma volume many times daily, and over 99 percent is reabsorbed so only 1 to 1.5 litres becomes urine.