How the Kidney Makes Urine, Explained

One organ, a million tiny filters

Each kidney contains about a million microscopic filtering units called nephrons, and every nephron carries out the same two-stage job on a small stream of blood: first it filters, then it reabsorbs. Understanding urine formation really means understanding what happens inside a single nephron, since the whole kidney is just a million of these units working side by side.

It helps to think of the process the wrong way round from how it might sound. The kidney does not carefully pick out only the waste products to remove.

Instead, it filters out almost everything small, waste and useful substances together, and then works to reclaim the useful substances afterward. Whatever is not reclaimed becomes urine.

Stage one: ultrafiltration at the glomerulus

1. Blood enters a tight ball of capillaries called the glomerulus through a fairly wide blood vessel.
2. Blood leaves the glomerulus through a narrower blood vessel, so blood can get in more easily than it can get out.
3. This mismatch builds up a high pressure inside the glomerulus.
4. The high pressure forces water and small dissolved substances, glucose, amino acids, mineral salts and urea, out through the capillary walls into a cup-shaped structure called the Bowman's capsule.
5. Blood cells and large plasma proteins are too big to be forced out, so they stay in the blood and continue on their way.

Stage two: selective reabsorption along the tubule

The fluid that was pushed out of the glomerulus, called the filtrate, now flows along a long, coiled tube called the renal tubule. This is where the kidney gets selective: it reabsorbs, back into the surrounding blood, whatever the body still needs.

Normally all the glucose is reabsorbed, since it is far too valuable to lose, along with a large amount of water and useful mineral ions. Urea, a waste product from the breakdown of excess amino acids, is not reabsorbed in any meaningful amount, so it stays in the tubule.

By the time the fluid reaches the end of the tubule and the collecting duct, it has lost almost all of its useful glucose and much of its water, and what remains, urea, excess water, and excess salts, is now urine. This urine drains into the renal pelvis, then down the ureter to the bladder for storage.

Why filtering everything first is actually efficient

It might seem wasteful to filter out useful substances only to reabsorb most of them afterward, but this two-step approach is actually what makes the kidney so effective at removing waste. Because filtration at the glomerulus is based purely on the size of a molecule, not on whether it is useful, the kidney does not need a separate, complicated system to specifically identify and remove urea.

Instead, urea is swept out automatically along with everything else, and the tubule only needs to work at reclaiming the substances the body wants back. This is also why the amount of urine and its concentration can be finely adjusted, by controlling exactly how much water is reabsorbed, without needing to change how filtration happens at the glomerulus.

Common mix-ups to avoid

How does the kidney control the water content of the blood?

The kidney does more than remove waste, it is the main organ that keeps the water content of the blood steady, a job called osmoregulation. Special cells in the hypothalamus of the brain constantly monitor the water potential of the blood.

When the blood is too concentrated, such as after sweating or drinking little, the pituitary gland releases more antidiuretic hormone (ADH). ADH makes the walls of the collecting duct more permeable to water, so more water is reabsorbed back into the blood and the urine produced is smaller in volume and more concentrated.

When the blood is too dilute, such as after drinking a large amount of water, less ADH is released. The collecting duct becomes less permeable, less water is reabsorbed, and the kidney produces a larger volume of dilute urine.

In this way the amount of water reabsorbed is adjusted from moment to moment, without changing how filtration happens at the glomerulus.

Where does the urea in urine come from?

Urea is not made in the kidney; it arrives already dissolved in the blood. It comes from the liver, where excess amino acids that the body cannot store are broken down in a process called deamination.

The amino group removed during deamination forms ammonia, which is very toxic, so the liver quickly converts it into the far less harmful urea. This urea is released into the blood and carried to the kidneys, which filter it out at the glomerulus and leave it behind in the urine.

Seeing the kidney and liver as partners here makes the whole excretory picture clearer: the liver produces the urea, and the kidney removes it. This is why a question on urine formation sometimes reaches back to what the liver does with excess protein.

What can the contents of urine reveal?

Because a healthy kidney reabsorbs all the glucose and lets no plasma protein through in the first place, neither should appear in normal urine. Testing urine for these substances is therefore useful.

Glucose in the urine can indicate that the blood glucose level was too high for all of it to be reabsorbed, which is associated with diabetes. Protein in the urine can indicate that the filtering membranes in the glomerulus are damaged and are letting large molecules through that should have stayed in the blood.

This is why urine composition appears in the syllabus alongside the mechanism of urine formation, what is present, and what is absent, both carry meaning about how well the two stages of filtering and reabsorption are working.

Key facts to remember about urine formation

• Each kidney contains about a million nephrons, each carrying out filtration and then reabsorption.
• Ultrafiltration at the glomerulus is driven by high blood pressure and separates molecules by size, not by usefulness.
• Selective reabsorption along the tubule takes back glucose, useful ions, and much of the water.
• Urea is made in the liver by deamination of excess amino acids, not in the kidney.
• How much water is reabsorbed is controlled by ADH, which adjusts the concentration of the urine.
• Normal urine contains urea, excess water, and excess salts, but no glucose or plasma protein.

Frequently asked questions