August 27, 2026 Together with the skin and the respiratory and digestive systems, the urinary system is responsible for the body excretory function , that is, the elimination of compounds that may be harmful or are present in excess in the body , with the aim of maintaining internal balance, known as homeostasis. The urinary system carries out its excretory function through a set of organs responsible for the production, storage, transport, and elimination of urine. Through urine, excess water and mineral salts, as well as waste products and residues from cellular metabolism and medications, are removed from the body. Blood passes through the kidneys about 60 times per day, which corresponds to the filtration of approximately 180 litres of fluid per day. An adult produces, on average, almost 1.5 litres of urine per day. The organs of the urinary system are: Kidneys (2); Ureters (2); Urinary bladder (1); Urethra (1). Kidneys In adults, the kidneys are paired organs (there are normally two), measuring approximately 10-12 cm in length and 5-7 cm in width, with a shape similar to that of a bean. They are located in the posterior part of the abdominal cavity, on either side of the vertebral column, at the level of the last thoracic vertebrae and the first lumbar vertebrae. The right kidney lies immediately below the liver and is positioned slightly lower than the left kidney. The kidneys play an essential role in maintaining the body's internal balance. Through processes of filtration, reabsorption, and secretion, they produce urine and regulate the levels of water, minerals, and electrolytes in the body . Each kidney consists of several anatomical regions, and more than one million functional units called nephrons . The outermost region of the kidney is called the renal cortex, while the innermost region is known as the renal medulla. Filtration of the blood entering the kidneys occurs in the glomeruli , structures within the nephrons located in the renal cortex. The fluid produced by this filtration process is known as the glomerular filtrate. In the deeper regions of the kidneys, the glomerular filtrate is concentrated through processes of reabsorption and secretion. Water and substances present in the filtrate that are needed by the body are reabsorbed and returned to the bloodstream. The remaining fluid continues its passage through the kidney, where mineral salts and other compounds, such as metabolic waste products and drug metabolites, may be added through secretion. The final urine consists mainly of water (about 95%) and also contains urea, creatinine, uric acid, mineral salts, and other substances that the body needs to eliminate. This urine is collected in a funnel-shaped structure called the renal pelvis, which then channels it into the ureters. In addition to producing urine, the kidneys have several other important functions . For example, they help regulate blood volume and blood pressure, maintain the acid-base balance (pH) of the blood, produce a hormone called erythropoietin, which stimulates the production of red blood cells, and participate in the activation of vitamin D by converting it into calcitriol, that regulates calcium and phosphorus levels in the blood. Ureters The ureters are tube-like muscular structures . The right ureter connects the right kidney to the bladder, and the left ureter connects the left kidney to the bladder. In adults, the ureters are approximately 25-30 cm long. The function of the ureters is to transport urine from the kidneys to the bladder . This occurs mainly through rhythmic contractions of their muscular walls, known as peristaltic movements. Urinary bladder The urinary bladder is a hollow muscular organ , similar to a sac, whose function is to store urine until it is eliminated. It has the ability to expand as urine accumulates. A healthy adult can typically store 300-500 mL of urine . Under normal conditions, bladder emptying is a voluntary process. As the bladder fills, signals are sent to the nervous system, creating the sensation of needing to urinate. The bladder can retain urine (urinary continence) until an appropriate opportunity arises for its elimination . Urethra The urethra is the canal that extends from the bladder and carries urine out of the body . It differs significantly between males and females: In females, the urethra is approximately 4 cm long and opens into the vulva; In males, it is much longer, measuring approximately 20 cm. It passes through the prostate gland and the penis, ending at the urinary meatus. In addition to transporting urine, the male urethra also carries semen during ejaculation, although urination and ejaculation do not occur simultaneously. The elimination of urine from the body is called urination . This process results from the contraction of the bladder and the coordinated relaxation of the urinary sphincters. Sources Kenhub. Rins, ureteres e glândulas suprarrenais . Kenhub. Sistema excretor . The content published in the Health Guide is intended for the general public and is provided for informational and educational purposes only. It does not replace individual medical assessment, diagnosis, or personalized clinical advice from a qualified healthcare professional. The content is produced or reviewed by physicians and other healthcare professionals at Hospital da Luz and is based on the knowledge available at the time of publication or the most recent update, as indicated at the beginning of each item. English translation generated with AI and reviewed by the editorial team.
