The formation of urine, which in colloquial speech is called 'pee,' is a vital process that primarily occurs in the kidneys. Through complex filtration of blood, the kidneys remove everything unnecessary from the body, converting excess and breakdown products into a liquid for subsequent excretion.
Urine consists of approximately 95% water, and also contains uric acid, urea, and sodium chloride—these are products that the body excretes as a result of cellular activity. On average, an adult produces about 1.5 liters of urine daily, although this volume can vary significantly. Factors such as the amount of water consumed, ambient temperature, and individual metabolic characteristics influence daily output. Approximately 300 ml of fluid is excreted with each trip to the toilet.
When we drink a liquid, such as a glass of water, it passes through the esophagus and stomach, after which it is absorbed by the walls of the small intestine. This fluid is then transported via the bloodstream to the kidneys. There, some of the fluid is absorbed by the body, while the rest is directed into the complex urinary system, initiating the urine formation process.
The kidneys (there are two) are shaped like large peas, measuring about 12 cm in length, 7 cm in width, and 5 cm in thickness. Each of these vital organs contains approximately one million nephrons—functional units responsible for filtering blood and actually producing urine.
Inside the nephrons, blood enters under high pressure and undergoes primary filtration in a network of capillaries called the glomerulus. A significant portion of the liquid component of the blood, plasma, exits through the fine vessels, forming what is called glomerular filtrate, which can be described as 'pre-urine.' About 125 ml of plasma is filtered during this process every minute.
While the cleaned blood returns to the body, the 'pre-urine' accumulates in Bowman's capsule—a reservoir surrounding the glomerulus. From this point, the fluid begins its journey through the complex tubular system.
'Pre-urine' contains not only metabolic waste but also substances beneficial to the body, such as glucose, amino acids, and mineral salts. As the fluid moves through the nephron capillaries, these important components are reabsorbed, and new wastes present in the blood are secreted into the filtrate.
Next, the fluid enters the collecting duct—the final segment of the nephron. Reabsorption of some water may occur here, returning to the bloodstream. The amount of absorbed water directly depends on the body's hydration level: the more water you drink, the less reabsorption occurs, and the more diluted the final urine will be.
The yellow color of urine is due to the presence of urinary pigments, such as urochrome and urobilin, which are produced by the liver.
The finished urine is then transported to the bladder via the ureters—two tubes 20 to 30 cm long and straw-like in diameter. The bladder not only stores urine but is also responsible for expelling it from the body. It can hold up to half a liter of fluid, but the sensation of needing to urinate usually arises when the volume reaches approximately 300 ml, which is enough to activate the surrounding nerves.
When the brain receives a signal from the bladder nerves that the organ is full and needs emptying, a command is sent to the internal sphincter—a muscular ring located at the exit of the bladder. It opens, the bladder contracts, and urine can begin to flow.
For urine to actually be released, a second valve—the external sphincter, located slightly lower—must also open. Unlike the internal sphincter, the external sphincter is consciously controlled, allowing the process of urination to be postponed until a suitable moment. Ultimately, urine is expelled through the urethra—a channel that passes through the genitals. The male urethra is longer, about 20 cm, whereas the female urethra is only 4 cm long.