1. Kidneys Make About 100 Times More Fluid Than the Urine You Pass
Kidneys are bean-shaped organs located on the back side of your abdomen, one on each side. They are about 10 cm (4 inches) long, 5 cm (2 inches) wide, and weigh between 120 and 150 grams (about 4 to 5 ounces). The US National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) describes them as fist-sized beans [1][3]. Inside these small organs, blood is filtered at a rate of about 150 liters per day. This filtered liquid is called "primary urine" or glomerular filtrate. Meanwhile, the actual urine that leaves your body is only about 1.5 liters per day [1]. This means there is a difference of roughly 100 times (150 divided by 1.5).
Documents vary slightly on the exact amount of primary urine. Japanese hospital materials and general explanations cite about 150 liters [1][2], while the MSD Manual medical reference states about 180 liters [4]. We can think of this range as 150 to 180 liters. Note that NIDDK mentions "about 150 quarts" (where 1 quart is about 0.95 liters) of blood passing through the kidneys, which refers to a different measurement than the primary urine volume [3]. Each kidney contains about 1 million tiny units called nephrons that make urine [1]. These 1 million units work together to filter that large amount of liquid. But why return most of it?
2. Kidney Tubules Control Water and Salt by Adjusting What Returns to the Blood
The primary urine filtered by the glomerulus passes through narrow tubes called renal tubules. Here, water and necessary components are returned to the blood. This process is called reabsorption, and about 99% of the primary urine is recovered here [1]. The kidneys have four main jobs: regulating water and electrolytes (like salt), balancing acid and alkaline levels in the body, removing waste products, and producing hormones [1].
On days you drink a lot of water versus days you sweat heavily, your body’s water intake changes. By adjusting how much is returned to the blood, the kidneys are thought to maintain the correct levels of water and salt inside the body. The detailed mechanisms of this regulation are not covered here. From a testing perspective, if creatinine (a waste substance) increases in the blood, it is considered one clue that kidney function has declined [1]. This number can help indicate whether the kidneys are doing their job of removing waste. The last job—producing hormones—is less well known. Kidneys even send commands to make blood.
3. Kidneys Make Substances That Help Control Red Blood Cells, Blood Pressure, and Bones
According to a team at Tohoku University, interstitial fibroblasts in the kidney produce erythropoietin, a hormone that increases red blood cells. Juxtaglomerular cells produce renin, a substance that raises blood pressure [5]. Erythropoietin acts on bone marrow to encourage red blood cell production, while renin helps regulate blood pressure [4]. Additionally, the kidneys convert vitamin D (from skin or food) into its active form in the body [4]. NIDDK also explains that kidneys make hormones to regulate blood pressure, help make red blood cells, and keep bones strong [3].
Tohoku University’s announcement notes that when kidney disease occurs, the function of these two hormones can become disordered, potentially leading to anemia or abnormal blood pressure [5]. So, kidneys do more than just filter blood; they make hormones for red blood cells and blood pressure, and help maintain bones by activating vitamin D. Now, let’s look at animals in places where water is hard to find.
4. How Kangaroo Rat Kidneys Save Water When It Cannot Drink
The kangaroo rat (*Dipodomys merriami*) can concentrate its urine to over 6000 mosmol/kgH2O, which is reported to be nearly twice the concentration of laboratory rats [6]. "mosmol" measures the concentration of particles dissolved in urine. This means the kangaroo rat’s urine concentration is almost double that of a rat’s. In this study, captive kangaroo rats maintained their body weight without being given water, eating only bird feed and spinach [6]. While we cannot say they never drink in the wild, the numbers show they are excellent at saving water.
A separate table cited by Kyoto University lecture materials lists the maximum urine concentration for humans as 1.4 Osm/L [7]. Since this is a different source and unit system, do not compare it directly to the kangaroo rat’s number; view them as separate values. The same table lists beavers at about 520, pigs at about 1,100, cats at about 3,100, and sand mice at about 6,300 (all in mosmol/L) [7]. Because this table includes only a limited number of species, use it to understand that urine concentration varies greatly between animals. It is safe to read each source’s values separately with their specific species names and units.
5. Why Drinking Seawater Makes Humans Lose Water but Lets Whales Gain It
If a vertebrate animal drinks seawater, the salt level in its body fluids rises. To remove this salt from the body, water must also be excreted with urine. In the table cited by Kyoto University lecture materials, if a human drinks 1 liter of seawater, they produce 1350 milliliters of urine, resulting in a net loss of 350 milliliters of water. A whale produces only 650 milliliters of urine, resulting in a net gain of 350 milliliters of water [7].
The difference comes from the kidney’s ability to concentrate urine. Whales can make more concentrated urine than humans, so under these conditions (drinking 1 liter of seawater), they produce less urine volume [7]. However, this table is an experimental calculation for "if 1 liter of seawater is drunk." A thesis abstract from Hokkaido University states that most cetaceans maintain body fluid osmotic pressure and salt levels at about one-third of seawater, similar to land mammals. They are thought to rarely drink seawater, relying instead on metabolic water produced from food [8]. So, while the calculation shows whale kidneys can reduce urine volume, they likely do not drink much seawater in daily life.
6. Why Chronic Kidney Disease Is Hard to Notice When Kidney Function Weakens
Chronic Kidney Disease (CKD) is a term for diseases where kidney function gradually declines. Based on the Japanese Society of Nephrology’s "CKD Clinical Practice Guideline 2024," it is estimated that about 20 million people in Japan have CKD, or 1 in 5 adults [9]. It is said that there are few symptoms in the early stages, so many cases progress without being noticed [9].
Estimates change depending on when they were measured and what criteria were used. The same society’s 2009 guideline estimated 13.3 million people, or 1 in 8 adults [10]. Because estimates vary by version, it is important to check which version a number comes from rather than memorizing just one figure. As we have seen, kidneys work quietly but handle a massive amount of work. Early detection through tests can be a clue for protecting this function.
7. How to Record Your Daily Water Intake at Home
You can record your daily water intake using items found at home: a cup with a known volume, paper, and a pencil. Multiply the number of cups you drink by the cup’s capacity to calculate your total daily intake. Also write down how many times you go to the toilet. Compare days when you sweat a lot versus days when you do not, noting changes in drink amount, toilet visits, and urine color. You do not need to measure the exact volume of urine.
Here is another mental calculation: 150 liters is equal to 100 bottles of 1.5-liter water. Imagine placing one 1.5-liter bottle from your home on a table, then imagining 100 of them lined up. This helps you visualize the amount your kidneys filter daily. The urine you actually release is equivalent to just 1 or 2 of those bottles. If you want to learn more about kidneys and these numbers, NIDDK’s English explanations are rich with diagrams and can be compared with Japanese figures [3].