The urinary system removes waste from the blood, controls the amount of water and certain minerals in the body, and helps maintain a stable internal environment. It also contributes to blood pressure regulation, red blood cell production, and bone health.
Its main organs are the kidneys, ureters, bladder, and urethra. Together, they make urine, move it out of the body, store it, and eventually release it.
The urinary system is best known for producing urine, but urine is only part of the story. The kidneys are sophisticated filters and regulators that continuously adjust the composition of the blood.
The main job of the urinary system
The urinary system helps keep the body’s internal chemistry within a workable range. Every cell depends on a relatively stable environment, including appropriate amounts of water, electrolytes, acids, and dissolved substances.
The kidneys perform most of this regulation. They filter the blood and then selectively return useful substances to the bloodstream while leaving excess water and unwanted substances to become urine.
The urinary system therefore has several closely related functions:
- Removing waste: The kidneys eliminate metabolic wastes and other substances the body needs to get rid of.
- Balancing water: They adjust how much water is retained or lost in urine.
- Regulating electrolytes: The kidneys control levels of substances such as sodium, potassium, calcium, and phosphate.
- Maintaining acid-base balance: They help keep blood acidity within a narrow range by regulating hydrogen ions and bicarbonate.
- Supporting blood pressure control: The kidneys influence blood pressure partly by controlling sodium and water balance and by releasing hormones involved in blood-pressure regulation.
- Supporting red blood cell production: The kidneys produce erythropoietin, a hormone that signals the bone marrow to make red blood cells when the body needs more oxygen-carrying capacity.
- Supporting bone health: The kidneys convert vitamin D into its active form, which helps the body absorb calcium and maintain healthy bones.
These functions happen continuously, whether or not a person is consciously aware of them.
How the kidneys make urine
The two kidneys sit toward the back of the abdomen, below the lower ribs. Each contains roughly a million microscopic filtering units called nephrons.
A nephron is the basic functional unit of the kidney. It consists of a filtering structure called the glomerulus and a series of tiny tubules that modify the filtered fluid.
Blood enters the glomerulus through small blood vessels. Blood pressure pushes water and small dissolved substances across the filtering barrier, producing a fluid called filtrate. Blood cells and most proteins are normally too large to pass through this barrier, so they remain in the bloodstream.
Filtration, however, is only the first step. If the kidneys simply filtered everything and discarded it, the body would rapidly lose substances it needs.
Instead, the nephron carefully processes the filtrate.
Reabsorption returns useful substances to the blood
As filtrate moves through the kidney tubules, the body reclaims substances such as water, glucose, sodium, and other electrolytes. These substances move from the tubular fluid back into the surrounding blood vessels.
The amount reclaimed is regulated according to the body’s needs. For example, when the body is short of water, the kidneys can conserve more water and produce a smaller volume of concentrated urine. When there is excess water, they can excrete more dilute urine.
Secretion adds additional substances to the urine
The tubules also move certain substances from the blood into the tubular fluid. This process, called secretion, helps the kidneys eliminate particular wastes, excess ions, and some drugs and their metabolites.
The final urine is therefore not simply blood that has been filtered. It is the result of filtration, reabsorption, and secretion working together.
What is normally in urine?
Urine is mostly water, but it also contains dissolved waste products and electrolytes.
One important waste product is urea, which is produced when the body breaks down proteins and amino acids. The kidneys also eliminate creatinine, a waste product generated by normal muscle metabolism, along with other substances.
Urine composition changes with hydration, diet, hormone levels, kidney function, and other conditions. The kidneys continually adjust urine composition so that substances the body needs are conserved while excesses and wastes are removed.
Normally, urine contains very little protein and no significant amount of blood or glucose. Finding these substances in urine can sometimes indicate an underlying problem, although interpretation depends on the circumstances and the specific test.
What the other urinary organs do
The kidneys make urine, but the remaining organs of the urinary system handle its transport, storage, and elimination.
Ureters move urine to the bladder
The ureters are two narrow muscular tubes, one from each kidney. They carry urine from the kidneys to the bladder.
Urine does not simply fall through the ureters because of gravity. Rhythmic contractions of the ureter walls, called peristalsis, help propel urine toward the bladder.
The bladder stores urine
The bladder is a muscular, expandable organ that temporarily stores urine.
As the bladder fills, its wall stretches. Nerves communicate information about this stretching to the brain, contributing to the sensation that it is time to urinate. The bladder can store urine because its muscular wall accommodates increasing volume without an equivalent rise in pressure during normal filling.
The urethra carries urine out of the body
During urination, the bladder’s muscular wall contracts while sphincter muscles relax, allowing urine to pass through the urethra and leave the body.
The urethra is shorter in females than in males. In males, it also carries semen during ejaculation, although urine and semen do not normally pass through it at the same time.
How the urinary system helps control water balance
Water balance is one of the kidneys’ most important regulatory tasks.
The body loses water through several routes, including urine, sweat, breathing, and bowel movements. The kidneys adjust urinary water loss according to how much water is available to the body.
A key hormone in this process is antidiuretic hormone (ADH), also called vasopressin. When the body needs to conserve water, ADH increases the kidneys’ ability to reabsorb water. More water returns to the bloodstream, and urine becomes more concentrated.
When the body has sufficient water, less ADH is released. The kidneys then allow more water to remain in the tubular fluid, producing a larger volume of more dilute urine.
This system helps prevent both excessive water loss and excessive accumulation of water.
How the kidneys regulate electrolytes and blood acidity
The kidneys do more than control water. They precisely regulate electrically charged minerals called electrolytes, including sodium and potassium.
Sodium is particularly important because it influences the amount of water in the bloodstream and therefore affects blood volume. The kidneys can retain sodium when necessary or excrete more of it when there is an excess.
Potassium requires especially careful regulation because abnormal blood potassium levels can interfere with the electrical activity of muscles and nerves, including the heart.
The kidneys also help maintain the body’s acid-base balance. Cells continually produce acids as part of normal metabolism. The kidneys remove hydrogen ions and conserve or produce bicarbonate, an important buffer. Along with the lungs, this helps keep blood pH within a narrow range.
The kidneys have important hormonal functions
The kidneys are not simply filters. They also act as endocrine organs, meaning they produce or activate hormones.
Erythropoietin is made primarily by the kidneys. It stimulates the bone marrow to produce red blood cells. When kidney function is severely impaired, inadequate erythropoietin production can contribute to anemia.
The kidneys also participate in regulating blood pressure through the renin-angiotensin-aldosterone system. Specialized kidney cells release renin when certain conditions, such as reduced blood flow or reduced sodium delivery, signal that blood-pressure and fluid regulation need adjustment. This system ultimately affects blood-vessel constriction and the retention of sodium and water.
In addition, the kidneys convert vitamin D into calcitriol, its biologically active form. Calcitriol helps regulate calcium and phosphate balance and supports normal bone formation and maintenance.
What happens when the kidneys are not working properly?
Because the kidneys perform so many regulatory functions, significant kidney dysfunction can affect many parts of the body.
Waste products can accumulate in the blood. Fluid and electrolytes can become unbalanced. Acid can build up, and abnormalities in blood pressure, red blood cell production, and mineral metabolism can develop.
Kidney disease can occur suddenly or develop gradually. Acute kidney injury is a relatively rapid decline in kidney function, while chronic kidney disease involves persistent abnormalities or reduced kidney function over time.
Early kidney disease may cause few or no noticeable symptoms. That is one reason kidney function is often assessed with laboratory tests rather than symptoms alone. Blood tests can measure substances such as creatinine, while urine tests can detect abnormalities such as excess protein.
The bladder and urinary tract can also develop problems independently of the kidneys. Urinary tract infections, kidney stones, urinary obstruction, and disorders affecting bladder control are examples of conditions involving different parts of the urinary system.
The urinary system works with other organs
The urinary system does not maintain the body’s internal balance by itself.
The lungs, for example, help regulate blood acidity by controlling how much carbon dioxide is exhaled. The kidneys provide slower, longer-term control by adjusting hydrogen ions and bicarbonate.
The heart and blood vessels influence kidney blood flow and blood pressure, while the kidneys in turn influence blood volume and vascular regulation.
The endocrine system also communicates extensively with the kidneys through hormones such as ADH and aldosterone. This coordination allows the body to respond to changes in hydration, blood pressure, electrolyte levels, and other physiological demands.
In this sense, urine production is one visible result of a much larger process: the urinary system is continually adjusting the composition of the body’s internal fluids to keep conditions suitable for cells and organs to function.
