Urine: Structure, Function & Diseases

Urine is a sterile blood serum concentrate. Through urine, the organism gets rid of various metabolic waste products. A healthy adult produces between 1 and 1.5 liters of urine daily.

What is urine?

Urine is an excretory product, which is permanently formed in the paired kidneys in a complicated process of resorption and filtration. Urine is formed by the filtering function of the kidneys. Blood continuously flows through the renal artery into the kidneys, where it is cleansed of toxins, waste products and metabolic waste products. From up to 150 liters of the so-called primary urine per day, about 1.5 liters of final urine are formed by diuresis and filtration; this is then excreted via the bladder over the course of 24 hours. From the kidneys, urine is passed through the ureters directly into the bladder. The bladder has the function of a urine collection basin and is equipped with a particularly protective mucous membrane. Only when it reaches a certain level of filling do the receptors on the inner wall of the bladder signal that urination is necessary. This physiological process of emptying the bladder via the urethra and the external bladder sphincter is also known as micturition. In addition to the metabolic waste products that the body excretes through urine, urine mainly contains water as a body fluid that is no longer needed.

Anatomy and structure

Human urine is a complex mixture of a wide variety of organic and inorganic substances. In its composition, urine always reflects the current physical condition. This is because every pathological process in the organism also has a direct effect on the urine composition. The main component of urine is water. In aqueous solution, it contains primarily the metabolic end products urea, uric acid and creatinine. These are also referred to as urinary substances. Urea is the end product of protein metabolism, uric acid the end product of cell nuclear metabolism, and creatinine the end product of muscle metabolism. In addition, urine contains vitamins, organic acids, hormones, proteins and dyes, so-called urochromes, which give the final urine its characteristic amber appearance. The preparation of urine takes place according to the anatomical structure of the kidney in 3 steps. The anatomical functional unit of the kidney is called the nephron. Each nephron consists of the glomerulus, the filtering unit, and the loop of Henle, the tubule system. Each human kidney has about 1 million such nephrons. During glomerular filtration, the first stage of urine preparation, blood is squeezed into the glomeruli and cleansed of coarse-molecular protein bodies. During tubular reabsorption, the second stage of urine preparation, metabolic waste products, electrolytes, or breakdown products of drugs are actively secreted from the flowing blood into the primary urine. In the third and final stage of urine preparation, tubular secretion, about two-thirds of the primary urine obtained is reabsorbed, that is, recovered.

Function and tasks

The main function of urine is to excrete metabolic waste products dissolved in it by the kidney via the bladder. During the 3 steps of urine preparation, a sophisticated system always takes care to maintain the so-called homeostasis. This refers to the constant maintenance of various vital parameters, without which metabolism could not function. This applies in particular to the pH value of the blood, which is always around 7.4. By adjusting the individual steps in urine preparation, it is possible to keep this and other vital parameters constant at all times. The higher-level processes required for this are controlled by certain areas of the brain. The adjustment takes place in particular via the excreted quantity of urine. Depending on the overall situation of the organism, the excreted urine can be rather acidic or alkaline. A reduction in the amount drunk also reduces the volume of excreted final urine, which is then also more concentrated and therefore has a deep yellow to brownish color. If large quantities of urine are excreted, it may also be water-clear. Correspondingly few solid components and urinary substances are then found in it.

Diseases and complaints

Human final urine is also an important diagnostic tool in daily practice.Not only urologists, but also general practitioners and other medical specialists analyze an abundance of urine samples every day. The composition of the urine and the distribution of the individual constituents quickly provide information, especially about diseases of the urogenital tract. For example, in the case of inflammation of the bladder or urinary tract, urine typically contains leukocytes or nitrite, as a reliable indicator of pathogenic nitrite-forming bacteria. Blood in the urine may also indicate inflammatory processes or even a malignant renal tumor, hypernephroma. In the traditional urine examination of antiquity, conclusions could already be drawn about pathological conditions in the body from the appearance of the urine. Today, urine examination has fallen into oblivion and has been completely replaced by so-called multi-strip diagnostics. Such a urine test strip contains up to 12 different individual parameters that can be tested after a brief dip in a urine sample. These include test fields for erythrocytes, nitrite, protein, leukocytes or urobilinogen. General metabolic diseases, for example diabetes mellitus, can also be diagnosed by the final urine. As soon as the so-called renal threshold of glucose, 180 milligrams per milliliter of blood, is exceeded, the blood glucose passes into the urine and is then detectable in the urine, a sure diagnostic sign of diabetes. Typical complaints for diseases of the genitourinary system include burning during urination, reduced or increased urine excretion, urinary urgency, and even nausea and vomiting in the case of kidney involvement. If the kidney is no longer able to fulfill its filtering function due to disease, urinary substances accumulate in the blood, which is also known as uremia. Only dialysis can then save a patient’s life.