How Much Urine Do We Make in a Day?
The Kidneys Filter 180 Litres and Return 99% to the Body
We pass about 1.5 litres of urine a day. Yet the kidneys filter more than 100 times that much fluid every day. They first push out almost everything, then pick back only what the body needs. This roundabout method has a good reason behind it.
You wake up and go to the toilet, go again at midday, and again in the evening. A few trips a day is perfectly ordinary.
Measured in bottles, the daily total is a little less than one large bottle. It is easy to assume that is all the body makes.
In fact, that 1.5 litres is just the dregs of the water the kidneys handle in a day.
The kidneys work in two stages
Blood is passed through a fine sieve, and nearly everything except blood cells and large proteins is filtered out. That comes to about 180 litres a day, close to a full bathtub at home.
As the filtered fluid travels along a long tube, most of the water, salt and sugar is returned to the body. The less than 1% that is never taken back becomes urine.
The kidneys do not choose what to throw away. They choose what to keep. Let us look at it step by step.
The kidneys wash your blood plasma about 60 times a day
About one fifth of the blood the heart pumps out is said to flow into the kidneys. You have one kidney on each side, each about the size of a fist. Each is said to hold around a million tiny filters.
At the entrance of each filter, capillaries are curled up like a ball of yarn. Blood pressure pushes fluid out through gaps in the wall. The gaps are fine enough to block blood cells and large proteins. Water, salt, sugar and small waste products slip straight through.
The amount filtered is about half a cup per minute. Over a day, that adds up to about 180 litres. All the plasma in your body (the fluid part of blood, once the cells are removed) is only about 3 litres. So the same fluid is filtered again and again, around 60 times a day.
Look at Figure 1. Blood enters from the left, and the round filter in the middle strains out a huge amount of fluid. Most of it is returned to the body, as shown by the upward arrows. What collects in the bucket at lower right is the urine.
The filtering force is the "balance" of several pressures
What pushes fluid out inside the filter is blood pressure from the heart. But not all of that push is put to use.
There is also pressure pushing back from the capsule where the filtered fluid collects. In addition, the proteins left in the blood try to pull water back into the vessel. In Figure 2, the long bar on the left is the pushing force, and the two bars on the right are the forces that cancel it. What is left after subtraction is said to be about one sixth of the original blood pressure.
If the remaining force is small, how can so much be filtered? The filter wall is very thin, full of gaps, and its total area is huge. Even a weak force can move a lot of fluid through a wide passage. Conversely, if blood pressure drops sharply, the filtering force disappears quickly. That is why the kidneys are said to have a regulating system that keeps the filtered amount steady even when blood pressure changes somewhat.
Why let everything out and then take it back?
If 99% is going to be returned anyway, it seems simpler to release only the things that are not needed. But then the kidneys would have to recognise everything that should be thrown away.
Countless substances enter the body: food components, drugs, and wastes made inside the body. Some are met for the first time. Building a system that identifies and discards each one separately would be a huge task.
So the kidneys do the opposite. They let all the small things out first, then take back only what the body is known to need (water, salt, sugar, amino acids and so on). Any substance with no recovery system is automatically sent out. The more unfamiliar a substance, the more surely it is discarded.
The price is that taking things back needs a lot of energy. The kidneys weigh only about 0.5% of body weight, yet they are said to use nearly a tenth of the oxygen the body consumes. Much of that is thought to go into pumping salt back toward the body side.
Normally, blood sugar is filtered out but almost all of it is taken back. When blood sugar gets too high, the recovery system cannot keep up. The sugar that could not be returned then appears in the urine. Doctors long ago noticed this, and it became the origin of the disease's name.
On days when you sweat a lot, urine is scarce and dark. The body is saving water by raising the return fraction slightly above 99%. As you can see with the slider in Figure 1, a small change in the return fraction changes the amount of urine a lot. For why we need the toilet more often when it is cold, see Why Do We Need the Toilet More Often When It Gets Cold?
Summary
The kidneys filter about 180 litres of fluid a day and return over 99% of it to the body. The 1.5 litres of urine is the part that could not be returned. Because the method is to choose only what to keep, even unfamiliar substances are not missed and get discarded. Adjusting the amount also needs only a shift in the last 1%.
The kidneys do not hunt for what to throw away.
They choose what to keep and let everything else flow out.
How blood travels around the body is covered in How Did We Find Out That Blood Circulates Around the Body?, and the link between blood pressure and body height in Why Is a Giraffe's Blood Pressure Twice as High as a Person's?
- Measure one visit with a measuring cup (rinse it lightly to keep it clean), then multiply by the number of visits a day to estimate your daily amount.
- Divide that amount by 180 litres to work out what percentage of the filtered fluid left the body.
- Compare the amount and colour on a day you sweated a lot with a cool day when you drank plenty of water.
For most people it should come to about 0.5 to 1%. If you have several days of very low output, or are concerned about changes in colour or amount, please consult a medical professional.
For those who want more: terms, formulas and links to textbooksEach part is labelled by level, from junior high science to university specialist courses
- Junior highCovered in junior high school science
- High schoolCovered in high school biology
- High school+Advanced high school material, or textbook sidebar content
- UniversityUniversity specialist courses (physiology, nephrology) not taught in high school
- ResearchTopics researchers are still investigating, not yet taught as settled fact even at university
Junior highTerms: this phenomenon has names
- Glomerulus: the filter inside the kidney, where capillaries are curled up like a ball of yarn. Plasma is filtered out here.
- Primary urine: the fluid just after it is filtered at the glomerulus. About 180 litres are made per day.
- Reabsorption: taking water, salt, sugar and so on back into the body from primary urine in the tubules.
- Nephron: the unit made of a glomerulus and the tubule attached to it. Each kidney is said to have about a million.
Junior highHigh schoolChecking with a formula: daily urine from the filtered amount and the return fraction
The amount of urine is set by two things: how much was filtered, and how much of it was returned.
| In symbols | V ＝ G × t × （1 − r） |
| In words | Daily urine ＝ filtered per minute × minutes in a day × (1 − return fraction) |
| Where it comes from | The water balance (conservation of mass). The filtered amount must split into "returned to the body" and "leaving as urine", with nowhere else to go. |
| V | Daily urine volume (L) |
| G | Glomerular filtration rate: the amount filtered per minute (mL/min) |
| t | Time (min). One day is 1440 min |
| r | Fraction of the filtered amount returned to the body (0 to 1) |
| Filtered per minute (typical adult value) | about 125 mL |
| Minutes in a day | 1440 min |
| Fraction returned (typical value) | about 99.2 ％ |
| Total plasma in the body | about 3 L |
| Sodium per litre of plasma | about 140 mmol |
| Mass of 1 mole of table salt | about 58.5 g |
| Filtered per day (mL) | 125 × 1440 ＝ 180000 |
| Convert to litres | 180000 ÷ 1000 ＝ 180 |
| Fraction not returned (1 − 0.992) | 1 − 0.992 ＝ 0.008 |
| Daily urine (L) | 180 × 0.008 ＝ 1.44 |
| Times plasma is filtered per day | 180 ÷ 3 ＝ 60 |
| Sodium filtered per day (mmol) | 180 × 140 ＝ 25200 |
| As table salt (25.2 mol, g) | 25.2 × 58.5 ＝ 1474.2 |
From the filtered 180 litres, we get about 1.4 litres, close to a typical day's urine. If the return fraction is 98%, then "1 − 0.98 ＝ 0.02" and urine becomes "180 × 0.02 ＝ 3.6" litres. It is the same calculation as the slider in Figure 1. The salt filtered also comes to about 1.5 kilograms a day as table salt. That is more than 100 times the salt we eat, sent out and taken back every day.
High schoolHigh school+Where, and what, is taken back?
High schoolIn high school biology you learn that primary urine, filtered from the glomerulus into Bowman's capsule, is reabsorbed as it passes through the renal tubule and collecting duct. Sugar and amino acids are normally all returned, and most of the water and salt too. Urea is only partly returned, so it becomes tens of times more concentrated in urine than in plasma. Comparing this "concentration ratio" substance by substance shows which ones the kidneys choose to keep.
High school+How much water is returned is regulated by vasopressin (antidiuretic hormone) from the pituitary gland in the brain, acting on the collecting duct. When the body is short of water, more of this hormone is released, more water channels (aquaporins) appear in the duct wall, and more water is taken back. Alcohol makes you need the toilet more because it is said to suppress the release of this hormone.
UniversityNet filtration pressure, clearance and countercurrent flow
The balance in Figure 2 corresponds to the Starling equation (Starling forces), which describes fluid flow across a capillary wall. For the glomerulus, the "net filtration pressure" is the hydrostatic pressure in the glomerular capillary minus the hydrostatic pressure in Bowman's capsule and the colloid osmotic pressure of plasma. Multiply this by a coefficient combining wall permeability and area (the ultrafiltration coefficient) and you get the glomerular filtration rate. In clinical practice, the filtration rate is measured by the renal clearance of inulin, which is filtered but neither reabsorbed nor secreted. In routine tests, a value estimated from blood creatinine (the estimated glomerular filtration rate) is used. Urine can be made more concentrated than plasma because of the "countercurrent multiplier system" created by the loop of Henle, which makes the salt concentration higher deeper in the kidney.
📖 For derivations and further reading: Japanese Wikipedia, "Glomerular filtration rate" / Japanese Wikipedia, "Nephron"
ResearchWhat is still not clear
- Why does the number of nephrons differ several-fold between people? Large individual differences have been reported, from a few hundred thousand to over 2 million per kidney. One idea is that conditions before birth play a part, and it is being studied.
- How do the controls that keep the filtered amount steady fit together? The blood vessels themselves are said to respond to pressure, and the tubules are said to sense salt and signal the filter. How the two share the work, and how they break down in disease, is still under study.
- How much of the decline in filtration with age is unavoidable? It is said to fall gradually with age, but there is debate about where disease begins and natural change ends.
In other words, this article too is "an explanation of what is known so far". Numbers such as 180 litres and 99% are typical values that vary with build, age and how much water you drink that day.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Junior high | Science, year 2: "Structure and function of animal bodies (excretion)" | The kidneys filter waste from the blood |
| High school | Biology: "Maintaining the internal environment (kidneys)" | Primary urine, reabsorption, concentration ratio, calculating daily urine |
| High school+ | Biology: "Hormonal regulation" | Vasopressin and the control of water return |
| University | Physiology, nephrology | Net filtration pressure, renal clearance, countercurrent multiplier |
| Research | Nephrology, developmental biology | Individual differences in nephron number, regulating filtration, ageing changes |
| ― | Links to daily life | Urine colour on sweaty days, kidney function values in health checks |
- Hall, Guyton and Hall Textbook of Medical Physiology (Japanese translation of the 13th edition), Elsevier Japan ― chapters on glomerular filtration and net filtration pressure
- Ken'ichi Honma (ed.), Standard Physiology (標準生理学), Igaku-Shoin ― kidney chapter
- Japanese Wikipedia, "Glomerular filtration rate"
- Japanese Wikipedia, "Nephron"
- Japanese Society of Nephrology (日本腎臓学会), ed., Evidence-Based Clinical Practice Guideline for CKD, Tokyo Igakusha
* This article is a general-audience science explainer. The numbers given are rough guides to help you understand how things work. For judgments about your own body or illness, please consult a doctor or other professional.