The mechanism of formation of primary and secondary urine. Primary and secondary urine. Regulation of kidney function. The influence of the concentration of substances circulating in the blood on the degree of filtration in the kidneys

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.

1) mechanism of formation of primary urine

2) mechanism of final urine formation

Composition and properties of urine
Urine excretion
Regulation of urine formation

Selection- this is liberation from excreta, excess water, salts, and foreign substances coming from food.

Stages of the extraction process:

· Formation of excreta and their entry from tissues into the blood

Transport of excreta by blood to organs that neutralize them, to excretory organs, to nutrient depots

· Removing excreta from the body, foreign substances that have entered the blood (penicillin, iodides, paints, etc.)

Education process and the release of urine is called diuresis. Urine is formed from blood plasma flowing through the kidneys. The process of urine formation occurs in 3 phases:

glomerular filtration

tubular reabsorption

tubular secretion

Glomerular filtration

Blood filtration occurs in the Bowman-Shumlyansky capsule, where arterial blood enters the capillaries of the Malpighian glomerulus through the afferent arteriole. High blood pressure is created in the capillaries of the glomerulus due to the difference in diameters of the afferent and efferent arterioles. In addition, blood enters here already under the pressure provided by the heart. Due to high pressure and due to the high permeability of the capsule walls, blood plasma devoid of protein enters the lumen of the capsule. Primary urine is formed. During the day, 150-170 liters are formed. Primary urine, in addition to metabolic products, also contains nutrients necessary for the body: amino acids, glucose, vitamins, salts. A prerequisite for the filtration of primary urine is high hydrostatic blood pressure in the capillaries of the glomeruli - 70-90 mm Hg. It is counteracted by oncotic blood pressure = 25-30 mm Hg. and the pressure of the fluid located in the cavity of the nephron capsule is equal to 10-15 mm Hg. The value of the blood pressure difference that ensures glomerular filtration is 30 mmHg, i.e. 75 mmHg – (30 mmHg+15 mmHg) = 30 mmHg. Urine filtration stops if glomerular blood pressure is below 30 mmHg.

The final urine produced per day is 1.5 liters. This means that the nephron must ensure the reabsorption of these substances. This process is called tubular reabsorption.

Tubular reabsorption

Tubular reabsorption is the process of transporting substances from primary urine into the blood. Primary urine, passing through the urinary tubule system, changes its composition. H2O, glucose, amino acids, vitamins, Na +, K +, Ca +2 ions are absorbed back into the blood. CI¯. The latter are excreted in the urine only if their concentration in the blood is higher than normal. Metabolic products (urea, creatinine, sulfates, etc.) are excreted in the urine at any concentration in the blood and are not reabsorbed. Reabsorption occurs actively and passively. Active reabsorption occurs due to the activity of the renal tubular epithelium with the participation of enzymes and energy expenditure. Glucose, amino acids, phosphates, and sodium salts are actively absorbed. They are completely absorbed in the tubules and are absent in the final urine. Passive reabsorption occurs due to diffusion and osmosis without energy expenditure. H 2 O, chlorides, etc. are reabsorbed. A special place in the mechanism of reabsorption of water and sodium ions from primary urine is occupied by the loop of Henle of the nephron due to the rotary-countercurrent system. The loop of Henle has 2 limbs: descending and ascending. The epithelium of the descending part allows water to pass through, while the epithelium of the ascending part is impermeable to water, but actively absorbs Na + back into the blood. Passing through the descending part of the loop of Henle, urine releases water, thickens, and becomes more concentrated. The release of water occurs passively, since Na + ions are actively reabsorbed in the ascending part of the loop of Henle. Entering the tissue fluid, Na + ions increase the osmotic pressure in it and thereby contribute to the attraction of water into the tissue fluid from the descending part of the loop of Henle. Thus, a large amount of water and Na + ions are reabsorbed in the loop of Henle.

Tubular secretion

Secretion is the active transport of certain substances by epithelial cells with the expenditure of ATP energy.

Thanks to secretion, substances are released from the body that are not amenable to glomerular filtration or are contained in the blood in large quantities: xenobiotics (dyes, antibiotics and other drugs), organic acids and bases, ammonia, K +, H + ions.

SCHEME OF URINE FORMATION

For the normal functioning of the body, the coordinated work of all systems is necessary. Then the constancy of the internal environment – homeostasis – is maintained. One important system taking part in this process is the urinary system. It consists of two kidneys, ureters, bladder and urethra. The kidney takes part not only in the formation and excretion of urine, but also performs the following functions: regulation of osmosis, metabolic, secretory, participates in hematopoiesis, and maintains the constancy of buffer systems.

The buds are bean-shaped, weighing about 150-250 grams. They are located retroperitoneally, in the lumbar region. Consist of cortex and medulla. The process of urine formation occurs primarily in the brain. In addition, they perform an important endocrine function, secreting hormones (renin, erythropoietin and prostaglandins), as well as biologically active substances.

Primary urine is formed in the renal corpuscle. This formation is a glomerulus, enveloped in an abundant network of capillaries. The process of urine formation occurs due to the pressure difference in the nephron (the structural and functional unit of the kidney). In the network of capillaries, blood is filtered and the output is primary urine. At the same time, the formed elements of blood (red blood cells, platelets, leukocytes) and large protein molecules remain in the bloodstream, and a liquid is formed at the outlet, which is similar in composition to plasma.

The composition of primary urine includes glucose, electrolytes (sodium, potassium, calcium, magnesium, chlorine), some hormones, biologically active substances and a small amount of hemoglobin and albumin. All these substances are necessary for the body, so losing them can cause life-threatening situations. Therefore, the process of urine formation does not end there and consists of stages such as glomerular filtration, tubular reabsorption, and secretion.

The process of urine formation

It is at the first stage of glomerular filtration that blood turns into primary urine. Since the kidneys have a huge network of capillaries, about 1500-2000 liters of blood pass through their parenchyma per day. From it, 130-170 liters of primary urine are further formed. Naturally, a person does not excrete such an amount of liquid per day, so the second phase of urine formation begins.

Where is secondary urine formed? Since the nephron consists of several parts, the second phase of urine formation begins in the area of the proximal tubules. During tubular reabsorption, secondary urine is formed. About 90% of water and other substances are reabsorbed from primary urine: glucose, albumin, hemoglobin, proteins. At the exit, the amount of secondary urine in an adult is about 1.2 - 2.0 liters. Further, substances that need to be removed from the body are excreted into secondary urine.

This begins the secretion phase, which takes place through active diffusion using two options:

With the help of special transport systems, it is pumped from the bloodstream into the lumen of the tubules, where secondary urine is collected.
Substances are synthesized directly in the tubular system.

Next, through the collecting duct system, the formed secondary substrate enters the renal pelvis. Then, it descends through the ureters into the bladder cavity. This is where she gathers. If its level reaches 200 ml, the receptors on the walls of the organ are excited. The impulse is transmitted to the central nervous system and then through descending pathways back to the bladder.

They give a signal to the organ to relax the sphincters, after which the process of urination occurs.

Video: The process of urine formation

Causes of urinary disorders

The formation of primary and secondary urine is a very important process. Because, along with urine, the body gets rid of substances it does not need. These are products of nitrogen metabolism, final metabolites of drugs, and various toxins. If they are not eliminated, the body is poisoned by its own waste products. And, first of all, the kidneys themselves will suffer. Acute or chronic renal failure may develop.

An indicator of the normal functioning of the excretory system is the glomerular filtration rate. This value determines the rate at which a certain amount of primary urine is produced per unit of time.

The norm is 125 ml/min in males and 110 ml/min in women.

The cause of organ dysfunction may be:

poisoning with mushrooms, heavy metals, toxic substances;
when transfusion of incompatible blood;
acute blood loss;
overdose of certain medications;
poisoning with aniline dyes;
entry into the bloodstream of tissue necrosis products;
crash syndrome;
injuries;
hepatorenal syndrome;
diabetes;
systemic lupus erythematosus;
systemic scleroderma;
rheumatism;
diabetes;
kidney amyloidosis;
glomerulonephritis;
neoplasms;
hydronephrosis;
heart diseases.

The glomerular filtration rate is determined by several formulas: Schwartz, MDRD, Cockroft-Gault, when performing the Rehberg test. The further tactics of patient management depend on the value of this indicator. If the GFR is more than 90 ml/min, the kidneys are working normally or there is minor nephropathy. At a level of 89-60 ml/min, nephropathy and a slight decrease in GFR appear, 59-45 ml/min corresponds to a moderate decrease in GFR, 44-30 ml/min – pronounced, 29-15 ml/min – severe, less than 15 ml/ min – terminal state, uremia, blood stops being filtered. A significant decrease in filtration function is an indication for hemodialysis.

The most characteristic symptoms of kidney failure are the following:

Smell of urine from the patient's skin and mouth.
Tissue swelling.
Cardiac dysfunction - arrhythmia, tachycardia.
Rapid breathing.
In the blood - increased creatinine and urea.
Fever.
Loss of consciousness.
Lower blood pressure.

Therapy depends on the cause of kidney damage. If the condition threatens the patient’s life, first of all, measures are taken aimed at restoring homeostasis: restoring acid-base balance, heart function, preventing cerebral edema. Acute renal failure, unlike chronic, can be reversible. Dialysis therapy is carried out. After which, the patient is prescribed renoprotective drugs for a long time - angiotensin-converting enzyme blockers (Lisinopril, Enalapril, Perindopril).

In the presence of a chronic disease that has led to kidney damage, the treatment of this disease should be corrected: insulin therapy for diabetes mellitus, antihypertensive therapy for hypertension, hormonal and cytostatic therapy for systemic lupus erythematosus.

To prevent diseases leading to defects in the formation of primary and secondary urine from occurring, it is necessary to adhere to the following recommendations:

contact medical institutions in a timely manner;
adhere to prescribed therapy;
control over food intake;
avoiding eating mushrooms of unknown origin;
Avoid prolonged contact with harmful substances.

Video: Filtration of primary and secondary urine

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.

urinary system

Bud

Bud- a paired organ that produces and removes urine. The kidneys are located in the lumbar region, in the retroperitoneal space in the so-called renal bed formed by the abdominal muscles. They are located at the level of the 12th thoracic and three upper lumbar vertebrae. At the same time, the right kidney is 2 - 3 cm lower than the left. The adrenal glands are adjacent to the upper pole of each kidney; in front and on the sides they are surrounded by loops of the small intestine, the liver is adjacent to the right kidney, and the spleen is adjacent to the left.

The bud has a bean-shaped shape, red-brown color, smooth surface, and dense consistency. The concave inner edge is called the gate. The portal enters the renal artery and nerve, and exits the renal vein, lymphatic vessels and ureter. The average weight of a kidney is 120 g, length - 10 - 12 cm, width about 6 cm, thickness 3 - 4 cm.

The kidney is covered by a fibrous capsule that is connected to its parenchyma. Outside the kidney capsule is a thick layer of fatty tissue called the fat capsule. The latter is covered in front by intraperitoneal fascia and protects the kidney from shocks and fixes it in the retroperitoneal space.

The kidney parenchyma consists of two layers: the outer (dark red) cortex and the inner, lighter, medulla. The medulla is represented by renal pyramids (about 12), the base of which faces the cortex of the kidney, and the apex faces the center. The renal tubules pass through the medulla. The cortex on a kidney section occupies the narrow outer layer of the renal parenchyma, as well as areas of the substance between the renal pyramids, which are called renal columns. The nephrons, which are the structural and functional units of the kidney, are located in the renal cortex. There are more than 1 million nephrons in the kidney.

The beginning of the urinary tract of the kidney are the collecting ducts, into which convoluted tubules of the second order open. The collecting ducts fuse to form papillary ducts that pass through the medulla and open at the tops of the pyramids into small calyces. The latter, uniting, form two or three large calyces, which open into an expanded cavity called the renal pelvis. The walls of the renal pelvis, small and large calyces consist of mucous, muscular and outer adventitia. The muscular layer of all urinary tracts, through its peristalsis, ensures the active movement of urine into the underlying urinary tract. The renal pelvis opens into the ureter

The renal artery entering the portal of the kidney branches into a large number of arterioles, the terminal branches of which are called afferent arterioles. Each of these arterioles enters the Shumlyansky-Bowman capsule, breaks up into capillaries and forms a vascular glomerulus - the primary capillary network of the kidney. Numerous capillaries of the primary network, in turn, are collected into the efferent arteriole, the diameter of which is half the diameter of the afferent arteriole. The efferent arteriole again breaks up into a network of capillaries that intertwine the tubules of all parts of the nephron, thereby forming a secondary capillary network of the kidney. Consequently, the kidney has two capillary systems, which is associated with the function of urine formation. After this, the capillaries finally merge and form veins that flow into the renal vein.

The kidneys consume 9% of the total amount of oxygen used by the body. The high intensity of oxygen consumption, biochemical processes of ATP breakdown and subsequent oxidative phosphorylation in the kidneys is due to the energy intensity of the processes of urine formation from the blood entering these organs. During the day, all five liters of blood circulate through the kidneys up to 300 times.

Blood enters the vascular glomerulus of the renal corpuscle from the afferent arteriole, the terminal branch of the renal artery (up to 1500 liters of blood passes through this section of the kidney per day). The renal arteries are large and wide, and the kidneys themselves are located quite close to the heart, therefore the hydrostatic blood pressure in the afferent arterioles, and therefore in the vascular glomerulus, is quite high - up to 70 mm Hg, while in the lumen of the Shumlyansky-Bowman capsule it reaches only 30 mm Hg. This level of pressure is also maintained by the fact that the afferent blood vessel has a wider lumen than the efferent one and arterial blood flows through the capillaries of the glomerulus slowly. The inner wall of the capsule is tightly fused with the capillaries of the vascular glomerulus. However, between the fused capillaries and the inner wall of the capsule there are gaps, which, under certain conditions, are pathways for the passage of blood plasma into the lumen of the capsule.

Nephron

Nephron It is a long, non-branching tubule, the initial section of which, in the form of a double-walled cup, surrounds the capillary glomerulus, and the final section flows into the collecting duct.

There are two types of nephrons in the human kidney cortical(80%), the Malpighian (renal) corpuscle of which is located in the outer zone of the cortex, and juxtamedullary(20%), the Malpighian corpuscle is located in the inner zone of the cortex at the border with the medulla. The latter type of nephrons, due to the peculiarities of their structure (the afferent arteriole is equal in diameter to the efferent arteriole), functions only in extreme situations associated with a decrease in the flow of arterial blood into the renal cortex (blood loss).

The nephron has four sections:

1. renal, or Malpigean corpuscle (glomerulus + Shumlyansky-Bowman capsule)

2. convoluted tubule of the first order - proximal convoluted tubule;

3. straight tubule - loop of Henle;

4. convoluted tubule of the second order - distal convoluted tubule.

Renal corpuscle is located in the renal cortex and consists of a vascular glomerulus surrounded by the Shumlyansky-Bowman capsule. This capsule is a cup consisting of two walls - outer and inner, between which there is a slit-like space that communicates with the next section of the nephron.

vascular glomerulus, in turn, is a narrow-loop network of interconnected capillaries. The total surface of all capillary glomeruli in both kidneys is about 1.5 square meters. m. Blood enters the glomerulus through the afferent arteriole, and flows into the efferent arteriole, which is smaller in diameter.

The part of the nephron following the renal corpuscle is called convoluted tubule of the first order. This tubule descends into the medulla, where it gradually passes into the next section of the nephron - loop of Henle.

The loop of Henle consists of a descending and ascending part of the loop. The ascending part upon returning to the cortex is called convoluted tubule of the second order.

The last section of the nephron flows into the initial section of the urinary tract of the kidney - collecting duct. The total length of the nephron tubules from the Shumlyansky-Bowman capsule to the beginning of the collecting ducts is 35-50 mm, the total length of all tubules of both

Urine formation

Primary urine from the capsule enters the loop of Henle, where the second phase of urine formation takes place - the process of reabsorption, as a result of which secondary or final urine is formed, which is excreted from the body. The formation of final urine occurs as the filtrate passes through the remaining parts of the nephron. The cells lining the walls of the convoluted and straight nephron tubules absorb almost 99% of water, sugar, amino acids, vitamins and some salts back into the secondary capillary network of the kidney (reabsorption). Reabsorption can occur passively, according to the principle of diffusion and osmosis, and actively - due to the activity of the epithelium of the renal tubules with the participation of enzyme systems with energy consumption. In addition to reabsorption, the process of secretion occurs in the tubules, i.e. active transport of certain substances from the blood into the lumen of the tubule (creatinine, drugs) occurs. Thus, from 180 liters of primary urine per day, only about 1.5 liters of secondary urine are formed and excreted from the body.

Secondary urine is a clear, light yellow liquid containing 95% water and 5% solids. Solids are represented by protein breakdown products (nitrogen-containing substances) - urea, uric acid, creatinine; salts of potassium, sodium, etc.

The urine reaction is not constant: during muscular work, due to the accumulation of phosphoric, lactic and carbonic acids in the blood, when eating protein foods, its reaction is acidic, and when consuming plant foods, the urine reaction is neutral or even alkaline. Normally, urine contains pigment - urobilin, giving urine a characteristic yellowish color. Urine pigments are formed in the intestines and kidneys from bile pigments, which in turn are formed from the breakdown products of hemoglobin. Specific gravity of urine on average equal to 1.012-1.025 g/cm2.

Normal urine does not contain protein; if protein appears in the urine, this indicates kidney disease. Protein can also appear in the urine of healthy people during heavy physical activity. The appearance of protein in the urine is called albuminuria. Sugar (glucose) is usually not found in the urine of a healthy person and appears temporarily when there is excess sugar in the blood. The appearance of glucose in the urine is referred to as food glucosuria. Increased blood sugar levels are also observed in patients with diabetes mellitus.

PROCESSES OF URINE FORMATION.

Body water balance

The body receives on average about 2500 ml of water per day in the form of drinking and solid food. About 150 ml of water is formed during the metabolism process. To keep the amount of water in the body constant, its inflow must correspond to its outflow. The kidneys play the main role in excreting water. Daily diuresis (urination) is on average 1500 ml. The rest of the water is excreted by the lungs (about 500 ml), the skin (about 400 ml) and a small amount in the feces.

Blood supply to the kidneys

Every minute, about 1.2 liters of blood passes through the kidneys, which is up to 25% of the blood entering the aorta. The mass of the kidneys in humans is 0.43% of the body weight, so an exceptionally high level of blood supply to the kidneys is obvious (for comparison: in terms of 100 g of tissue, the blood flow for the kidney is 430 ml/min, for the coronary system of the heart - 66, for the brain - 53). 91 - 93% of the blood entering the kidneys passes through the cortex. An important feature of the renal blood supply is that the blood flow in them remains constant when blood pressure changes more than twice (for example, from 90 to 190 mm Hg). Since the renal arteries arise from the abdominal aorta, their blood pressure level is high.

Glomerular filtration (formation of primary urine)

Formation of primary urine

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.