Every day the human body takes up to 2.5 liters of water Along with food and drink, up to 150 ml of water enters the body as a result of metabolism. In order for the body to maintain water balance, the influx of water must be equal to its consumption. The kidneys play the main role in the process of removing water from the body. Due to daily diuresis (urination), up to 1500 ml of liquid comes out. Some of the water is excreted by the lungs (up to 500 ml), the skin (up to 400 ml), a small amount is excreted in the feces.

Every minute up to 1.2 liters of blood, while the mass of the kidneys is only 0.43% of the human body weight, which confirms the very high level of renal blood supply. If it is recalculated per 100 g of tissue, then the blood flow of the kidneys is 430, the cardiac system - 66, and the brain - 53 ml/min. It is important that blood flow in the kidneys is not affected by even a twofold increase in blood pressure (for example, from 90 to 190 mm Hg). The renal arteries are connected to the abdominal aorta, so they constantly maintain the required high level of blood pressure.

How is primary and secondary urine formed?

The genitourinary system performs the main function of excreting metabolic products from the body. The process of urine formation is a very complex mechanism, consisting of two stages. During the first stage, primary urine is formed by filtration in the nephron capsule. It then passes through the convoluted tubule and loop of Henle, where up to 99% of the water with the amino acids, sugars and some mineral salts it contains is absorbed back into the blood.

What is the difference between primary and secondary urine?

Produces primary urine glomerulus, consisting of a huge number of capillaries. The blood passing through them is filtered, and the secreted liquid is directed into the capsule Shumlyansky-Bowman. This will be the primary urine. It does not contain blood cells and molecules of complex proteins, since the walls of the capillaries do not allow them to pass through, but molecules of amino acids, sugars, fats, etc. pass freely through them. Primary urine also contains water, which, passing through the convoluted tubules of the nephron, is absorbed due to increased osmotic pressure on the walls of the tubules (so-called reabsorption).

Every day the body produces 150–180 l of primary urine. All the beneficial compounds present in it are not lost, as they re-enter the body through the process of diffusion and the transport function of the tubular walls. The substances that remain after the diffusion process will be secondary urine. It first enters the collecting ducts, then into the small and large renal calyces, then collects in the renal pelvis, from which urine is discharged into the bladder through the ureters, and, after it is filled, exits the body through the urethra.

Secondary urine is more concentrated; in addition to water, it contains urea, uric acid, sodium, chlorine, potassium salts, sulfates and ammonia. They are what give urine its characteristic odor. The human body produces up to 1.5 liters of secondary urine every day, which is then released during urination. It is in the peculiarities of urine formation that the answer to the question of how primary and secondary urine differs from each other is contained.

Since primary urine is a liquid that was formed at the beginning of the urinary process, it is identical to blood plasma and contains only useful trace elements. Secondary urine contains remnants of the primary fluid, which, as a result of reabsorption, were not absorbed by the body.

conclusions

Both primary and secondary urine are stages of a single process; they are closely interconnected and their formation occurs through the gradual flow of one into the other. If the primary fluid is produced by the renal glomerulus, then secondary urine is formed in the capillaries that intertwine the urinary tubules. While most of the primary urine is reabsorbed by the body, secondary urine leaves the body completely.

A vital process in the kidneys is the process of urine formation. It includes several components - filtration, absorption, excretion. If for some reason the mechanism of production and subsequent excretion of urine is disrupted, various serious illnesses appear.

The composition of urine includes water and special electrolytes, in addition, an important component is the end products of metabolism in cells. The products of the last stage of metabolism enter the bloodstream from the cells while it circulates throughout the body and are excreted by the kidneys as part of urine. The mechanism of urine production in the kidneys is implemented by the functional unit of the kidney - the nephron.

The nephron is a unit of the kidney that ensures the formation of urine and its further excretion, due to its versatility. Each organ has about 1 million such units.

The nephron, in turn, is divided into:

• glomerulus
• Bowman-Shumlyansky capsule
• tubular system

The glomerulus is a whole network of capillaries that are embedded in the Bowman-Shumlyansky capsule. The capsule is formed of double walls and resembles a cavity with continuation into tubules. The tubules of the renal unit form a kind of loop, parts of which perform the necessary functions for the formation of urine. The parts of the tubules, convoluted and straight, adjacent directly to the capsule are called proximal tubules. In addition to these basic structural units of the nephron, there are also:

• rising and falling thin sections
• distant straight canaliculus
• thick afferent segment
• loops of Henle
• distant convolute
• connecting tubule
• collecting duct

Formation of primary urine

The blood that enters the nephron glomeruli, under the influence of the processes of diffusion and osmosis, is filtered through a specific glomerular membrane and in this process wastes most of the fluid. Filtered blood products subsequently enter the Bowman-Shumlyansky capsule.

All kinds of waste products, glucose, salts, water and various other biochemical substances filtered from the blood and found in Bowman's capsule are called primary urine. Primary urine contains a large amount of glucose, creatinine, amino acids, water and other low-molecular compounds. Filtration in both renal tubules is considered excellent and is 130 ml per minute. If you make simple calculations, it turns out that the nephrons that make up the kidneys filter approximately 185 liters in 24 hours.

This is a huge amount, because there is not a single case of excretion of such a large amount of fluid. What else lies in the mechanism of urine formation?

Secondary urine and its formation

Reabsorption is the second component factor in the mechanism that determines the formation of urine. This process consists of the movement of various filtered substances back into the capillaries and vessels of the circulatory system. The reabsorption process begins in the tubules adjacent to Bowman's capsule and continues in the loops of Henle, as well as distant convoluted tubules and the collecting duct.

The mechanism of secondary urine formation is quite complex and painstaking, however, about 183 liters of liquid per day from the tubules returns back to the bloodstream.

All valuable nutrients do not disappear along with urine; they all undergo a reabsorption mechanism.

Glucose necessarily returns to the blood, provided there are no disturbances in the body systems. If the glucose content in the bloodstream exceeds 10 mmol/l, then glucose begins to be excreted along with the urine.

In addition, various ions are returned, including sodium ions. The amount that the kidney resorbs per day directly depends on how much salty food the patient ate the day before. The more sodium ions enter the body with food, the more is absorbed from the primary urine.

In a healthy state of the body, urine should not contain protein, red blood cells, ketone bodies, glucose, or bilirubin. If various substances are contained in the excreted urine, this may indicate a malfunction of the liver, gastrointestinal tract, pancreas and many others.

The process of excretion of urine from the body

The third important process is tubular secretion. This is the mechanism of urine formation. During this process, ions of hydrogen, potassium, ammonia, and also some drugs are released from the capillaries next to the distant and collecting tubules, into the recess of the tubules, namely into the primary urine, by the method of active transfer and penetration. As a result of the absorption and excretion of primary urine in the renal tubules, secondary urine is formed, which normally should be from 1.3 to 2.3 liters.

Excretion in the kidney tubules plays a very important role in stabilizing the acid-base balance of the human body.

Accumulated urine in the bladder leads to increased pressure in the bladder itself. It is innervated by the autonomic nervous system and, in turn, irritation of the parasympathetic pelvic nerves leads to contraction of the walls of the bladder and subsequent relaxation of the sphincter, which entails the expulsion of urine from the bladder.

Urine formation largely depends on the level of blood pressure, blood supply to the kidneys, as well as the size of the lumen of the arteries and veins of the kidneys. A drop in blood pressure, as well as a narrowing of the lumen of the capillaries in the kidneys, entails a significant reduction in urine output, and the expansion of capillaries and, accordingly, increased blood pressure increases.

Urine is a physiological waste product of the body. Thanks to biological fluid, toxic compounds and end products of metabolic breakdown are eliminated. Urine is formed as a result of blood filtration by the kidneys, where its primary accumulation occurs. It is excreted through the urinary system.

The mechanism of urine formation is assigned to the kidneys, which in 1 minute purify more than 1.2 liters of blood, transporting waste products, salts and other compounds. During the day, more than 1500 liters of blood passes through the paired organs, from which, during filtration, urine is formed equal to 1/1000 of the blood volume.

Education and secretion stage

The purification process begins with the passage of plasma through the nephrons enclosed in a capsule. This is a functional structural kidney, consisting of a body that is responsible for ultrafiltration and tubules that perform the function of reverse absorption (reabsorption).

• Ultrafiltration is the process of direct purification of colloidal particles from the blood by renal nephrons. Glomeruli produce about 160 liters of primary urine per day. The formation of primary urine occurs as a result of high hydrostatic pressure in the vessels of the nephron (about 60-70 mm Hg) and low pressure around it (about 30 mm Hg). The pressure drop in and around the capillary is about 30-40 mm. rt. Art. Due to the pressure difference, plasma carrying carbon and inorganic compounds (uric acid, salts, urea) passes through the vessels and is purified in the nephron. Other compounds with a mass of more than 8 thousand atomic units, for example, white and red blood cells, platelets, protein compounds, do not penetrate the capillaries and remain in the vascular bed. If high molecular weight proteins and compounds appear in the urine, this indicates a disruption in the process of blood filtration by glomeruli and is a consequence of inflammatory and other pathological processes in the kidneys.
• The process of formation of secondary urine (reabsorption). The process of formation of secondary biological fluid is called reabsorption or reverse absorption, which is of two types: active and passive. The scheme for the formation of secondary urine is as follows. The biological fluid formed during ultrafiltration from the renal nephron descends into curved and straight tubules, where it is reabsorbed. The tubules have a complex structure with a significant number of blood vessels, which allows important compounds that are necessary for the functioning of the body (glucose, amino acids, water, etc.) to penetrate back into the blood. During the process of reabsorption, about 95% of the urine formed in the first stage of filtration is absorbed. As a result, from 160 liters of biological fluid obtained through ultrafiltration, a significantly smaller amount of secondary urine is obtained - 1.6 liters, which is 1/100 of the primary one.
• Secretion is the last stage of urine formation. In parallel with the process of formation of secondary biological fluid in the renal tubules, a secretion process occurs, which is similar to the reabsorption mechanism, but has the opposite direction. Thanks to secretion, it is possible to remove harmful compounds that are not involved in the cleansing process. These can be medications or toxic substances, for example, ammonia, which, if stored in the body, lead to intoxication. The process of secretion allows you to obtain the final product of blood purification - urine.

Read also on the topic

What does an increase in ascorbic acid (ASA) in urine mean?

What is the difference between primary and secondary urine

The biological fluid obtained during the ultrafiltration process consists of 99% water, in which organic and inorganic compounds are dissolved. Primary urine is similar in composition to blood plasma - it contains proteins, albumin, globulin, glucose, and lactic acid. The difference is that hemoglobin, albumin and protein are present in higher concentrations in plasma.

Secondary urine is more concentrated than biological fluid obtained by ultrafiltration. 95% consists of water, the remaining 5% includes ammonium salts, urea, creatinine, sodium, magnesium, uric acid, chlorine sulfates.

In addition to the composition, primary and secondary urine differ in the method of emission. In the first case, urine enters the tubules of the glomeruli of the kidney, where the process of its transformation occurs. In the second case, the release of physiological fluid occurs into the external environment.

What affects urination

The mechanism of formation of primary and secondary urine depends on:

• Hemostasis is a special biological system that is entrusted with the function of maintaining the liquid state of the blood, as well as stopping hemorrhage as a result of damage to the membranes of blood vessels.
• Blood pressure in the vascular system.
• The strength of blood flow, which depends on the lumen of the vessels. This is influenced by hormonal levels, the state of the nervous system and metabolic products that undergo the filtration process.

Metabolic products

The formation of secondary and primary urine is influenced by organic and inorganic compounds involved in metabolic processes, namely:

• Threshold substances that are not excreted in urine during the filtration process until their level exceeds the limit limits. These are amino acids, vitamins, sugar, ions.
• Non-threshold - compounds excreted by the kidneys during filtration that do not undergo reabsorption. This group includes urea and sulfates.

An increase in the concentration of threshold substances in the secondary biological fluid indicates dysfunction of the glomerular apparatus of the paired organs of the urinary system, which led to resorption failures.

Hormonal background

Hormones that affect kidney function include:

• Cortisone, hydrocortisone, aldosterone, synthesized by the adrenal cortex, thyroxine, produced by the thyroid gland, as well as androgens, which suppress the absorption of water, which leads to increased urination.

It would seem like a childish question: why does a person need urine? But everything is much more complicated. In addition to getting rid of dangerous and harmful metabolic products, urination is necessary to maintain electrolyte balance, control the amount of fluid in the body and regulate blood pressure, as well as the functioning of the heart and blood vessels. To understand how all this happens, you need to understand a little about urine formation.

The formation of primary urine begins with the entry of blood into the kidneys and its movement through the vessels. At this time, the kidney plays the role of a filter, passing through the pores all substances that enter the kidney. Most of the processes of primary urine formation occur in the Malpighian glomeruli of the kidney. Blood is delivered to the kidneys through the renal arteries. In 24 hours, all the blood in the kidneys is filtered approximately 20 times.

It is important to understand that the fibrous capsule of the kidney consists of three layers:

1. In the first layer, consisting of capillaries, there are large pores through which all blood passes, with the exception of some proteins and formed particles.
2. In the second layer consists of collagen threads and is a membrane that does not allow proteins to pass through.
3. And finally, in the third layer is epithelial, its cells have a negative charge and do not allow blood albumin to pass into the primary urine. All filtered blood enters the kidney tubules. This is primary urine.

Thanks to this, there are no proteins in the resulting primary urine, and the kidneys filter and restore negative elements, bringing them to a normal state. Thus, primary urine is a protein-free filtrate of blood plasma. Thanks to all these processes, pressure is formed in the body.

The normal state of filtration of the primary composition per day is almost one and a half thousand liters of blood (more precisely, 1400). This is followed by the formation of the primary liquid (up to 180 liters). But no one produces such an amount of urine in 24 hours.

Reabsorption

This is the formation of secondary urine. Now all elements move into the blood from the tubules. All proteins caught in the filtrate, as well as other particles and components present in the ultrafiltrate, are subject to reabsorption; it occurs through disfusion or active transportation.

As a result of active transportation, a very large consumption of oxygen occurs. During reabsorption, substances and elements from the kidney channels are returned to the blood. Thus, almost all of the primary urine is returned to the bloodstream. 160 liters turns into 1.5 liters of concentrate called secondary urine. The composition of secondary urine includes:

• ammonium salts;
• urea;
• creatinine;
• acids;
• other salts.

The result of this whole process is that secondary fluid enters the bladder. It gets here through the ureters.

Comparison of secondary and primary urine

Secretion

The third and no less important stage of urine formation. This process is similar to reabsorption occurring in the opposite direction. The secretion process is quite active, and reabsorption occurs in parallel with it. Secretion occurs in the renal tubules and capillaries of the kidneys. With the help of distal and collecting ducts, ammonia, salts and hydrogen (all in ions) are secreted into the urine. Thanks to this process, unnecessary substances are released from the body through the urethra, which are partially absorbed into the blood. Daily dose of urine. The amount released due to secretion can be from a liter to two.

Features of urine formation in children

In the youngest children, at the time of birth, many functional and structural changes in the kidneys have not yet been completed, which affects the formation of urine. Here are a few main features:

• The weight of organs in a child is greater than in an adult: for example, the kidney weighs 1 percent of the total body weight. But there are the same number of nephrons as in an adult, but they are much smaller. As for the epithelial layer on the basement membrane of the glomerulus, it consists of tall cylindrical cells. Their filtration surface is reduced and the resistance is strong.
• In an infant, the epithelium of the kidneys is not fully prepared for secretion, and the tubules are short and narrow. The renal apparatus (its morphological structure) matures in children only by the age of three, and sometimes much later. So, the urine of a small child differs from that of an adult in both composition and quantity.
• During the first months of a child’s life, smaller volumes of fluid are filtered in his kidneys, but urine (if calculated per kilogram of body weight) is produced in larger volumes than in adults. At the same time, the kidneys cannot yet rid the body of excess fluid.
• A one-year-old child excretes 0.75 liters of urine per day, a five-year-old - about one liter, a ten-year-old - almost the same as adults. Reabsorption processes in children are not as smooth and complete as in adults: in order to remove toxins, the child needs much more fluid. Secretion is also poorly developed in the child. Since the tubules have not yet formed, they do not fully cope with the conversion of phosphates from primary urine into acid salts.
• The synthesis of ammonia, as well as the reabsorption of bicarbonates and the release of acid residues, is also inferior to adults, which can lead to acidosis. In addition, children usually have low urine specific gravity.

Urine formation is a complex and intensive process. All parts of the kidney take part in it, as well as the ureters, aorta and arteries. As a result, the body gets rid of unnecessary substances, and pressure is also formed. All its mechanisms are finally formed only at the age of six.

You can also watch this video, which talks about the process of urine formation.

Formation of primary urine

First stage The formation of urine in the kidneys begins with the filtration of blood plasma in the renal glomeruli. In this case, the liquid part of the blood passes through the wall of the capillaries into the cavity of the capsule of the renal corpuscle. The ability to filter is provided by a number of anatomical features:

capillary endothelial cells are flat, they are especially thin along their periphery and have pores in these parts, through which, however, protein molecules do not pass due to their large size

The inner wall of the Shumlyansky-Bowman capsule is formed by flat epithelial cells, which also do not allow only large molecules to pass through.

The main force that ensures the possibility of filtration in the renal glomeruli is the high pressure in them due to:

high pressure in the renal artery

difference in diameter of the afferent and efferent arterioles of the renal corpuscle. The pressure in the capillaries of the body is about 60 - 70 mm Hg. Art., and in the capillaries of other tissues it is 15-30 mm Hg. Art. The filtered plasma easily enters the nephron capsule, since the pressure in the capsule is low - about 30 mm Hg. Art.

Water and all substances dissolved in the plasma, with the exception of large molecular compounds, are filtered into the capsule cavity from the capillaries. Inorganic salts, organic compounds, such as urea, uric acid, glucose, amino acids, etc. freely pass into the capsule cavity. Proteins with high molecular weight normally do not pass into the capsule cavity and remain in the blood. The liquid filtered into the capsule cavity is called primary urine. Human kidneys form in a day 150 - 180 liters of primary urine.

Formation of secondary urine

Second phase urine formation is reverse absorption (reabsorption), occurs in convoluted tubules and the loop of Gnele. Primary urine, passing through them, undergoes a process of reverse absorption (reabsorption). Reabsorption is carried out passively on the principle of osmosis and diffusion and actively the cells of the nephron wall themselves. The significance of this process is to return all vital substances to the blood in the required quantities and remove the end products of metabolism, toxic and foreign substances. In the initial section of the nephron, organic substances are absorbed: amino acids, glucose, low molecular weight proteins, vitamins, Na +, K +, Ca ++, Mg ++ ions, water and many other substances. In subsequent sections of the nephron, only water and ions are absorbed.

The third stage is secretion: In addition to reabsorption, an active secretion process occurs in the nephron tubules, i.e. the release of certain substances from the blood into the lumen of the nephron, carried out by the cells of the nephron walls. As a result of secretion from the blood, creatinine and medicinal substances enter the urine.

The result of reabsorption and secretion is the formation secondary urine, the composition of which is very different from primary urine. Secondary urine contains a high concentration of urea, uric acid, chlorine, magnesium, sodium, potassium, sulfates, phosphates, and creatinine ions. About 95% of secondary urine is water, 5% is dry residue. Approximately 1,5 liters of secondary urine.