Your body makes urine continuously as your kidneys filter your blood, remove substances the body does not need, and carefully adjust the amount of water and dissolved chemicals that remain in your bloodstream. Urine is the final product of that process.
Although urine is often described simply as “filtered blood,” that is not quite accurate. The kidneys do much more than strain waste out of the bloodstream. They filter a large amount of fluid, then selectively take useful substances and much of the water back into the blood. They also add certain substances to the fluid that will become urine. This allows the kidneys to control the body’s water, electrolyte, acid-base, and waste balance at the same time.
Where urine is made
Urine is made in the kidneys, two organs located toward the back of the abdomen, on either side of the spine. Each kidney contains about a million microscopic structures called nephrons, which are the functional units that make urine.
A nephron has several parts, but the process can be understood as three major steps:
- Filtration: Fluid and small dissolved substances move from the blood into the nephron.
- Reabsorption: The body takes useful substances and needed water back into the blood.
- Secretion: Some additional substances move from the blood into the nephron for elimination.
The fluid that remains after these processes becomes urine.
Step 1: Blood is filtered in the kidney
Blood enters each kidney through a renal artery and eventually reaches the tiny blood vessels of individual nephrons. At the beginning of each nephron is a knot of capillaries called the glomerulus.
The glomerulus sits inside a cup-shaped structure called Bowman’s capsule. Blood pressure within the glomerular capillaries pushes water and many small dissolved substances through a filtration barrier and into Bowman’s capsule.
This process is called glomerular filtration.
The filtration barrier allows substances such as water, glucose, amino acids, sodium, and urea to pass through. Larger components, including blood cells and most proteins, normally remain in the bloodstream.
The resulting fluid is called filtrate. It resembles blood plasma in many ways, but it normally contains little protein and no significant number of blood cells.
Importantly, filtration does not mean that the kidneys immediately discard everything that enters the nephron. Much of what is filtered is valuable and will be recovered.
Step 2: The kidney takes useful substances back
The filtrate flows from Bowman’s capsule into a long, winding tube called the renal tubule. As the fluid travels through this tubule, the kidney selectively moves substances from the tubule back into nearby blood vessels.
This process is called tubular reabsorption.
The proximal tubule, the first major section of the renal tubule, reabsorbs much of the filtered water and sodium. Under normal conditions, it also reabsorbs essentially all of the filtered glucose and amino acids.
Other parts of the nephron fine-tune how much water and electrolytes are recovered. This is important because the body does not simply need to remove waste; it needs to maintain the right concentrations of substances in the blood.
For example, if the body needs to conserve water, the kidneys can increase water reabsorption. If there is excess water, they can allow more water to remain in the tubular fluid and ultimately leave the body as dilute urine.
The loop of Henle helps control urine concentration
After the proximal tubule, the fluid enters the loop of Henle, a hairpin-shaped section of the nephron that extends into the kidney’s inner region.
The loop has descending and ascending portions with different properties. The descending portion allows substantial movement of water, whereas the ascending portion moves salts out of the tubular fluid but is relatively impermeable to water.
This arrangement helps establish a concentration gradient within the kidney’s inner tissue. That gradient is essential for the kidney’s ability to produce concentrated urine when the body needs to conserve water.
The loop of Henle therefore plays a major role in regulating how much water ultimately leaves the body.
Step 3: Some substances are added to the forming urine
Reabsorption is not the only way the kidney modifies the filtrate. The kidney also actively moves certain substances from the blood into the renal tubule.
This process is called tubular secretion.
Substances secreted into the tubule include hydrogen ions, potassium under certain conditions, and various drugs and metabolic waste products. Secretion helps the kidneys regulate blood chemistry and eliminate substances that were not removed sufficiently through filtration.
The kidneys also use secretion to help control the body’s acid-base balance. By regulating the handling of hydrogen ions and bicarbonate, they help keep blood pH within a narrow range.
How the kidney decides how much water to save
The amount of water in urine is not fixed. The kidneys continually adjust water reabsorption according to the body’s needs.
A major regulator is antidiuretic hormone (ADH), also called vasopressin. When the body needs to conserve water, ADH levels rise. ADH makes certain kidney tubules and collecting ducts more permeable to water, allowing more water to move from the forming urine back into the bloodstream.
When less ADH is present, less water is reabsorbed in these areas, so more water remains in the urine.
This is why urine can range from relatively concentrated to quite dilute. The change is not simply a matter of the kidneys “filtering more” or “filtering less”; it reflects active regulation of water reabsorption.
The collecting ducts give urine its final composition
After passing through the nephron’s tubule, the fluid enters a collecting duct. Collecting ducts run through the kidney and receive fluid from many nephrons.
Here, the final adjustments to water and electrolyte balance take place. Hormones and other signals influence how much water and certain ions are reabsorbed.
By the time the fluid leaves the collecting ducts, it is considered urine.
Urine then flows from the kidneys into the ureters, two narrow muscular tubes that carry it to the urinary bladder. The bladder temporarily stores urine. When urination occurs, muscles in the bladder contract and urine passes through the urethra and out of the body.
What is actually in urine?
Normal urine consists primarily of water, along with dissolved substances that the body needs to eliminate or regulate.
One important waste product is urea, which is produced when the body breaks down proteins and amino acids. The kidneys remove urea from the blood and excrete it in urine.
Urine also normally contains varying amounts of electrolytes and other dissolved substances. Its exact composition changes according to hydration, diet, hormones, metabolism, medications, and kidney function.
The yellow color of normal urine largely comes from pigments produced during the body’s breakdown of hemoglobin. Urine can become more concentrated and therefore darker when less water is available to the body.
Why glucose normally does not appear in urine
Glucose illustrates how selective the kidney’s process is.
Glucose is small enough to pass through the glomerular filter, so it enters the initial filtrate. Under normal circumstances, however, the proximal tubule reabsorbs essentially all of that filtered glucose.
As a result, little or no glucose normally reaches the final urine.
If the amount of glucose filtered becomes greater than the tubule’s capacity to reabsorb it, glucose begins to appear in urine. This can occur when blood glucose levels are sufficiently elevated, as in uncontrolled diabetes.
This example shows why urine cannot be understood simply as “whatever the kidneys filter out.” The kidneys first filter substances and then selectively recover many of them.
How the kidneys keep the blood balanced while making urine
Urine production is closely tied to homeostasis, the body’s ongoing effort to keep its internal environment within appropriate ranges.
The kidneys help regulate:
- Water balance, by changing how much water is excreted or conserved.
- Electrolytes, including sodium, potassium, and other ions.
- Blood acidity, by controlling hydrogen ions and bicarbonate.
- Waste products, including urea and other substances produced by metabolism.
- Blood volume and pressure, partly through their control of sodium and water and through hormone systems involving the kidneys.
- Certain hormones and hormone-related processes, including mechanisms involved in red blood cell production and bone-mineral regulation.
Because these functions are interconnected, urine production is not merely a waste-disposal system. It is one of the principal ways the body maintains a stable internal chemical environment.
From blood to urine: the whole process
The process can be followed as a continuous path:
Blood → glomerular filtration → renal tubule → reabsorption and secretion → collecting duct → urine → ureter → bladder → urethra
At the glomerulus, blood pressure drives filtration. Along the nephron, the kidney recovers substances the body needs, adds selected substances for removal, and adjusts water and electrolyte levels. The remaining fluid becomes urine and is transported out of the kidneys for storage and eventual elimination.
The remarkable part of urine formation is not simply that the kidneys remove waste. It is that they do so while constantly deciding what the body should keep.