The nephron is the microscopic filtering unit of the kidney. Each human kidney contains about one million nephrons. Blood is forced through a knot of capillaries under high pressure, small molecules are filtered out, and then the body selectively reclaims what it needs before producing urine from what remains.
What is the nephron and where is it found?
A nephron is the functional unit of the kidney — the structure that actually filters blood and produces urine. Each kidney contains approximately one million nephrons, tightly packed into the outer cortex and inner medulla of the kidney.
The nephron consists of several connected regions, each with a distinct function:
- Glomerulus — a tangled knot of capillaries inside the Bowman's capsule
- Bowman's capsule — a cup-shaped structure that surrounds the glomerulus and collects filtered fluid
- Proximal convoluted tubule — the first coiled section of the tubule, where most reabsorption occurs
- Loop of Henle — a hairpin loop descending into the medulla and back up; creates a concentration gradient in the tissue
- Distal convoluted tubule — a second coiled section where fine adjustment of ions and pH occurs
- Collecting duct — carries urine to the renal pelvis; water reabsorption here is controlled by ADH
What happens during ultrafiltration?
Ultrafiltration is the first stage of urine formation. It occurs in the glomerulus and Bowman's capsule:
- Blood enters the glomerulus via an afferent arteriole (wide) and leaves via an efferent arteriole (narrower). The difference in diameter creates high hydrostatic pressure inside the glomerulus.
- This high pressure forces small molecules out of the blood through tiny pores in the capillary walls and the basement membrane.
- The fluid that collects in the Bowman's capsule is called the filtrate.
What passes into the filtrate:
- Water
- Glucose
- Urea
- Mineral ions (e.g. sodium, potassium, chloride)
What stays in the blood:
- Red and white blood cells (too large)
- Plasma proteins such as albumin (too large)
The filtrate is essentially blood plasma minus the large proteins. At this stage it contains many substances the body still needs — which is why the next stage (reabsorption) is critical.
What is selective reabsorption?
Selective reabsorption is the process by which the body reclaims useful substances from the filtrate before it becomes urine. It occurs mainly in the proximal convoluted tubule:
| Substance | Reabsorbed? | Mechanism |
|---|---|---|
| Glucose | Yes — 100% | Active transport (carrier proteins + ATP) |
| Water | Yes — most of it | Osmosis |
| Mineral ions | Most reabsorbed | Active transport and diffusion |
| Urea | Partially | Passive diffusion only |
| Creatinine | No | Remains in filtrate to be excreted |
Glucose is completely reabsorbed by active transport into the surrounding capillaries. Because glucose must move from the filtrate (low concentration after some reabsorption has occurred) back into the blood (where glucose concentration may already be moderate), this requires ATP and carrier proteins. This is why proximal tubule cells contain very many mitochondria.
How does the Loop of Henle concentrate urine?
The Loop of Henle creates a high concentration of sodium ions in the tissue fluid surrounding the collecting duct. This is important for concentrating the urine:
- The descending limb is permeable to water but not to ions — water leaves by osmosis into the salty tissue.
- The ascending limb is impermeable to water but actively pumps sodium and chloride ions out — making the surrounding tissue even saltier.
- The combined effect is a salt concentration gradient in the medulla.
When filtrate flows down the collecting duct past this salty medulla, water is drawn out by osmosis, concentrating the urine. The steeper the salt gradient (created by a longer Loop of Henle), the more concentrated the urine can become. Desert-adapted animals (such as kangaroo rats) have extremely long Loops of Henle, allowing them to produce very concentrated urine and conserve water.
How does ADH control urine concentration?
ADH (antidiuretic hormone) is released by the pituitary gland when blood water potential is too low (i.e. the blood is too concentrated).
- ADH travels in the blood to the kidneys.
- It increases the permeability of the collecting duct to water.
- More water is reabsorbed by osmosis back into the blood.
- A smaller volume of more concentrated urine is produced.
When blood water potential is high (e.g. after drinking a lot of water), less ADH is released, less water is reabsorbed, and a larger volume of dilute urine is produced. This is a negative feedback loop that keeps blood water potential within a narrow range.
What does the final urine contain?
By the time filtrate has passed through the entire nephron, its composition has changed drastically:
| Substance | In blood plasma | In filtrate (Bowman's) | In urine |
|---|---|---|---|
| Water | ~92% | ~99% | ~96% |
| Glucose | ~0.1% | ~0.1% | 0% |
| Urea | ~0.03% | ~0.03% | ~2% |
| Mineral ions | ~0.72% | ~0.72% | ~1.5% |
Urea is produced in the liver from the breakdown of excess amino acids. It is toxic in high concentrations and must be excreted — the kidneys filter it out and concentrate it in the urine.
Frequently asked questions
What is the difference between filtration and reabsorption in the kidney?
Filtration (ultrafiltration) is the non-selective pressure-driven process that pushes small molecules from the blood into the Bowman's capsule — everything small enough passes through, including glucose, water, urea, and ions. Reabsorption is the selective process by which useful substances (glucose, most water, many ions) are reclaimed from the filtrate and returned to the blood. Only what is not needed is left to become urine.
Why is glucose not normally found in urine?
All glucose in the filtrate is reabsorbed by active transport in the proximal convoluted tubule. The carrier proteins can handle normal blood glucose levels entirely. Glucose only appears in urine when blood glucose is very high (as in poorly controlled diabetes mellitus) — the filtrate contains more glucose than the carrier proteins can reabsorb, so the excess passes through into the urine.
What does ADH stand for and what does it do?
ADH stands for antidiuretic hormone. It is released by the pituitary gland when blood water potential falls (blood becomes too concentrated, usually due to sweating or insufficient fluid intake). ADH increases the permeability of the collecting duct to water, so more water is reabsorbed by osmosis from the filtrate back into the blood. The result is a smaller volume of more concentrated urine, and blood water potential rises back towards its set point.
Why do people on dialysis need the machine several times a week?
A dialysis machine mimics the kidney's filtration function, but it cannot replicate the kidney's fine hormonal control. Blood is passed over a partially permeable membrane next to a dialysis fluid matched to the correct concentration of useful substances (glucose, ions). Waste products such as urea diffuse out by osmosis and diffusion. Because dialysis removes wastes but does not reabsorb selectively the way a nephron does, patients must attend regularly — typically three times a week — to prevent dangerous build-up of urea and excess fluid.
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