Reabsorption occurs in the kidneys. Mechanisms of reabsorption and secretion in the renal tubules. What can be violations

As a result of active absorption of most of the osmotically active components of the filtrate, water is reabsorbed through the walls of the tubules, moving due to diffusion, i.e. passively.
To quantify fate various substances in the nephron, they are compared with the release of substances that are completely filtered in the glomeruli and subsequently completely excreted with secondary urine.
Clearance - the coefficient of blood purification from various substances - the concept is to a certain extent conditional. In quantitative terms, it is characterized by the volume of blood plasma, completely cleared by the kidneys from one or another substance in 1 min. Clearance is determined by the so-called "non-threshold" substances, i.e. substances completely excreted in a single passage through the kidneys. Inulin clearance determines the glomerular filtration volume and is approximately 120 ml/min. The clearance of para-aminohippuric acid is used to assess the effective renal plasma flow and is 600-650 ml / min.
In the proximal part of the nephron, mainly metabolites are secreted, in the distal part - ions K, H, NH4.

Impaired protein reabsorption

Filtered glucose is almost completely reabsorbed by proximal tubular cells and is usually excreted in the urine in small amounts. During reabsorption, glucose combines with a carrier (it is phosphorylated) and transported through the basal part of the cell into the blood. The role of sodium ions and, accordingly, the Na-pump is essential.
With hyperglycemia accompanying diabetes mellitus, the blood glucose level exceeds the “renal threshold” of 8 mmol / l, a lot of glucose is filtered through the glomeruli, and enzyme systems are not able to provide complete reabsorption, glucosuria develops. True, in advanced cases diabetes glucosuria may not be due to kidney damage (angiopathy) and decreased filtration. A hereditary defect in the enzyme systems of glucose reabsorption manifests itself in the form of renal diabetes mellitus, a dominantly inherited disease in which glucosuria develops against a background of normal or even low blood glucose levels. Glycosuria may be the result of damage to the epithelium of the tubules during renal ischemia or poisoning with mercury-containing drugs or lysol.

Impaired protein reabsorption

The protein is reabsorbed in the proximal tubules by pinocytosis, partially cleaved, and then low molecular weight components enter the blood. The mechanisms of protein reabsorption are poorly understood. It is known, in particular, the essential importance of hemodynamics. The appearance of protein in the urine is referred to as proteinuria (albuminuria more often). Temporary low proteinuria up to 1 g / l can occur in healthy individuals after intense prolonged physical work. Persistent and higher proteinuria is a sign of kidney disease. According to the mechanism of development, it is conventionally divided into glomerular and tubular (glomerular and tubular). With glomerular proteinuria, due to an increase in the permeability of the filtering membrane, the protein in large quantities enters the cavity of the Shumlyansky-Bowman capsule, which exceeds the resorption capacity of the tubular apparatus. If the glomeruli are damaged, moderate proteinuria develops. True, the degree of proteinuria does not reflect the severity of kidney disease. Tubular proteinuria is associated with a violation of protein reabsorption against the background of damage to the epithelium of the tubules (amyloidosis, sublimate necronephrosis) or in violation of lymphatic drainage. Massive proteinuria is observed in nephrotic syndrome, when both glomeruli and tubules are damaged.

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Transport of electrolytes in the nephron

The cells of the proximal nephron reabsorb most of the components of the ultrafiltrate, but the leading role in this process belongs to the reabsorption of sodium with accompanying anions. It is the reabsorption of sodium that is the most significant function of the kidneys in terms of volume and energy costs. Sodium reabsorption largely determines the total amount of urine excreted, the participation of the kidneys in the regulation of water in the body, the osmotic concentration, the ionic composition of the blood, and other vital signs. The kidneys filter 1200 g of sodium per day, and the excretion does not exceed 5-10 g. Sodium reabsorption in various parts of the nephron has pronounced features. So, in the proximal sections, where up to 75% of the filtered sodium is reabsorbed, its reabsorption is an active process, but it is carried out against a low gradient. Reabsorption of sodium in the distal sections is carried out against a high concentration gradient, which leads to the release of urine, which contains almost no sodium ions. It has been established that distal sodium reabsorption is regulated by aldosterone, a hormone of the adrenal cortex. The biochemical mechanisms of active transport of sodium ions remain largely unclear. A certain importance is attached to Mg-dependent ATPase, SDH, alpha-ketoglutarate dehydrogenase.
Violations of the reabsorption of sodium ions can develop when the production of aldosterone is reduced, either under the action of inhibitors (osmotic diuretics), or when the sensitivity of the renal epithelium to aldosterone is reduced. Under such conditions, along with sodium ions, water is also lost with the possible development of dehydration.
The release of potassium ions is about 10% filtered in the glomeruli, and potassium ions are not only reabsorbed, but also partially secreted in the distal tubules.

Osmotic dilution and concentration of urine

From 120 ml of filtrate, 119 ml are reabsorbed in 1 minute. Up to 85% of this amount is reabsorbed in the proximal tubules following osmotically active substances (Na, glucose, etc.), which is defined as "mandatory reabsorption" of water. About 15% is reabsorbed in the distal and collecting ducts - "facultative reabsorption".
The level of obligatory reabsorption may fall if there is a violation of the reabsorption of sodium or glucose ions (polyuria in diabetes mellitus, the appointment of osmotic diuretics aldoctan). Facultative reabsorption of water is suppressed with a lack of ADH or the absence of a reaction of the renal epithelium to the latter (forms of diabetes insipidus).
The kidneys are able to excrete urine 4 times more hypertonic and 6 times more hypotonic than blood plasma with fluctuations in the relative osmotic concentration of 1002-1035. A decrease in the ability of the kidneys to concentrate urine is expressed in the form of hypostenuria or isosthenuria.
There is a complete cessation of osmotic concentration. The maximum osmotic concentration is 270-330 mmol / l (relative - 1010-1012).
Daily diuresis in healthy adults is about 70% of exogenously administered water. The minimum volume of urine required to excrete waste is 500 ml. Polyuria - excretion of a daily amount of urine more than 2000 ml, oliguria - 400-500 ml, anuria - up to 200 ml.
In the pathogenesis of urinary excretion disorders, the state of nervous and humoral regulation is important. Emotional factors can change diuresis, and activation of excitatory processes in the cerebral cortex leads to polyuria, and inhibition leads to coliguria. Polyuria and oliguria can be obtained by conditioned reflex or by hypnotic suggestion.
Quite often, in conditions of pathology, reflex pain anuria occurs. Reflex inhibition of urination is possible from different reflexogenic zones. In pathogenesis, the reno-renal reflex is of particular importance, when trauma or other damage to one kidney causes temporary anuria of another, intact one. At the same time, due to the activation of the sympathoadrenal system, the tone of the renal arterioles increases, which leads to a decrease in glomerular filtration.
Hormonal influences matter - thyroxine increases glomerular filtration and, like glucocorticoids, increases diuresis.

Up to 80% of the filtered sodium is reabsorbed in the proximal segments of the tubules, while about 8-10% is absorbed in the distal segments and collecting ducts.

In the proximal segment, sodium is absorbed with an equivalent amount of water, so the contents of the tubule remain isosmotic. In the proximal sections, the permeability for both sodium and water is high. Through the apical membrane, sodium enters the cytoplasm passively along the electrochemical potential gradient. Next, sodium moves through the cytoplasm to the basal part of the cell, where sodium pumps (Na-K-ATPase dependent on Mg) are located.

Passive reabsorption of chlorine ions occurs in the zones of cell contacts, which are permeable not only to chlorine, but also to water. The permeability of intercellular spaces is not a strictly constant value, it can change under physiological and pathological conditions.

In the descending part of the loop of Henle, sodium and chloride are practically not absorbed.

In the ascending part of the loop of Henle, a different mechanism for the absorption of sodium and chlorine functions. On the apical surface, there is a system for transporting sodium, potassium, and two chloride ions into the cell. There are also Na-K pumps on the basal surface.

In the distal segment, the leading salt reabsorption mechanism is the Na-pump, which provides sodium reabsorption against a high concentration gradient. About 10% of sodium is absorbed here. Chlorine reabsorption occurs independently of sodium and passively.

In the collecting ducts, sodium transport is regulated by aldosterone. Sodium enters through the sodium channel, moves to the basement membrane and is transported into the extracellular fluid by Na-K-ATPase.

Aldosterone acts on the distal convoluted tubules and the initial sections of the collecting ducts.

Potassium transport

In the proximal segments, 90-95% of the filtered potassium is absorbed. Part of the potassium is absorbed in the loop of Henle. The excretion of potassium in the urine depends on its secretion by the cells of the distal tubule and collecting ducts. With excessive intake of potassium in the body, its reabsorption in the proximal tubules does not decrease, but secretion in the distal tubules sharply increases.

With all pathological processes accompanied by a decrease in the filtration function, there is a significant increase in potassium secretion in the tubules of the kidneys.

In the same cell of the distal tubule and collecting ducts, there are systems of reabsorption and secretion of potassium. With a deficiency of potassium, they provide the maximum extraction of potassium from the urine, and with an excess - its secretion.

The secretion of potassium through the cells into the lumen of the tubule is a passive process that occurs along a concentration gradient, and reabsorption is an active one. The increased secretion of potassium under the influence of aldosterone is associated not only with the effect of the latter on the permeability of potassium, but also with an increase in the entry of potassium into the cell due to increased work of the Na-K pump.

Other an important factor regulation of potassium transport in the tubules is insulin, which reduces the excretion of potassium. The state of acid-base balance has a great influence on the level of potassium excretion. Alkalosis is accompanied by an increase in potassium excretion by the kidney, and acidosis leads to a decrease in kaliuresis.

calcium transport

The kidneys and bones play a major role in maintaining a stable level of calcium in the blood. Calcium intake is about 1 g per day. 0.8 g is excreted by the intestines, 0.1-0.3 g / day is excreted by the kidneys. In the glomeruli, ionized calcium is filtered and is in the form of low molecular weight complexes. In the proximal tubules, 50% of the filtered calcium is reabsorbed, in the ascending knee of the loop of Henle - 20-25%, in the distal tubules - 5-10%, in the collecting ducts - 0.5-1.0%.

The secretion of calcium in humans does not occur.

Calcium enters the cell along a concentration gradient and is concentrated in the endoplasmic reticulum and mitochondria. Calcium is excreted from the cell in two ways: with the help of the calcium pump (Ca-ATPase) and the Na/Ca exchanger.

In the cell of the renal tubule there should be a particularly effective system for stabilizing the level of calcium, since it continuously enters through the apical membrane, and the weakening of transport into the blood would not only disrupt the balance of calcium in the body, but would also lead to pathological changes within the nephron cell.

Hormones that regulate calcium transport in the kidney:

Parathormone
thyrocalcitonin
growth hormone

Among the hormones that regulate calcium transport in the kidney, parathyroid hormone is of the greatest importance. It reduces the reabsorption of calcium in the proximal tubule, but at the same time its excretion by the kidney decreases due to the stimulation of calcium absorption in the distal segment of the nephron and collecting ducts.

In contrast to parathyroid hormone, thyrocalcitonin causes an increase in calcium excretion by the kidney. The active form of vitamin D3 increases calcium reabsorption in the proximal tubule. Growth hormone contributes to increased calciuresis, which is why patients with acromegaly often develop urolithiasis.

Magnesium transport

A healthy adult excretes 60-120 mg of magnesium in the urine per day. Up to 60% of filtered magnesium is reabsorbed in the proximal tubules. Large amounts of magnesium are reabsorbed in the ascending limb of the loop of Henle. Magnesium reabsorption is an active process and is limited by the maximum tubular transport. Hypermagnesemia leads to increased renal excretion of magnesium and may be accompanied by transient hypercalciuria.

With a normal level of glomerular filtration, the kidney quickly and effectively copes with an increase in the level of magnesium in the blood, preventing hypermagnesemia, so the clinician is more likely to encounter manifestations of hypomagnesemia. Magnesium, like calcium, is not secreted in the tubules of the kidneys.

The rate of magnesium excretion increases with an acute increase in the volume of extracellular fluid, with an increase in thyrocalcitonin and ADH. Parathyroid hormone reduces the release of magnesium. However, hyperparathyroidism is accompanied by hypomagnesemia. This is probably due to hypercalcemia, which increases the excretion of not only calcium, but also magnesium in the kidneys.

Phosphorus transport

The kidneys play a key role in maintaining the constancy of phosphate in the fluids of the internal environment. In blood plasma, phosphates are present in the form of free (about 80%) and protein-bound ions. About 400-800 mg of inorganic phosphorus is excreted through the kidneys per day. 60-70% of filterable phosphates are absorbed in the proximal tubules, 5-10% in the loop of Henle, and 10-25% in the distal tubules and collecting ducts. If the transport system of the proximal tubules is sharply reduced, then a large capacity of the distal segment of the nephron begins to be used, which can prevent phosphaturia.

In the regulation of tubular transport of phosphates, the main role belongs to the parathyroid hormone, which inhibits reabsorption in the proximal segments of the nephron, vitamin D3, somatotropic hormone, which stimulate the reabsorption of phosphates.

Glucose transport

Glucose that has passed through the glomerular filter is almost completely reabsorbed in the proximal segments of the tubules. Up to 150 mg of glucose can be released per day. Glucose reabsorption is carried out actively with the participation of enzymes, energy consumption and oxygen consumption. Glucose flows across the membrane along with sodium against a high concentration gradient.

Glucose is accumulated in the cell, phosphorylated to glucose-6-phosphate, and passively transferred to the peritubular fluid.

Complete reabsorption of glucose occurs only when the number of carriers and the speed of their movement through the cell membrane ensure the transfer of all glucose molecules that have entered the lumen of the proximal tubules from the renal corpuscles. The maximum amount of glucose that can be reabsorbed in the tubules when all carriers are fully loaded is normally 375 ± 80 mg/min in men and 303 ± 55 mg/min in women.

The level of glucose in the blood, at which it appears in the urine, is 8-10 mmol / l.

Protein transport

Normally, the protein filtered in the glomeruli (up to 17–20 g/day) is almost completely reabsorbed in the proximal segments of the tubules and is found in the daily urine in a small amount - from 10 to 100 mg. Tubular protein transport is an active process; proteolytic enzymes take part in it. Protein reabsorption is carried out by pinocytosis in the proximal segments of the tubules.

Under the influence of proteolytic enzymes contained in lysosomes, the protein undergoes hydrolysis with the formation of amino acids. Penetrating through the basement membrane, amino acids enter the peri-tubular extracellular fluid.

Transport of amino acids

In the glomerular filtrate, the concentration of amino acids is the same as in blood plasma - 2.5-3.5 mmol / l. Normally, about 99% of amino acids are reabsorbed, and this process occurs mainly in the initial sections of the proximal convoluted tubule. The mechanism of amino acid reabsorption is similar to that described above for glucose. There are a limited number of carriers, and when all of them combine with the appropriate amino acids, the excess of the latter remains in the tubular fluid and is excreted in the urine.

Normally, urine contains only traces of amino acids.

The causes of aminoaciduria are:

an increase in the concentration of amino acids in plasma with increased intake into the body and in violation of their metabolism, which leads to an overload of the transport system of the tubules of the kidneys and aminoaciduria
amino acid reabsorption transporter defect
a defect in the apical membrane of the tubular cells, which leads to an increase in the permeability of the brush border and the zone of intercellular contacts. As a result, there is a reverse flow of amino acids into the tubule
violation of the metabolism of cells of the proximal tubule

kidneys in human body perform a number of functions: this is the regulation of the volume of blood and intercellular fluid, and the removal of decay products, and stabilization acid-base balance, and regulation of water-salt balance and so on. All these tasks are solved thanks to urination. Tubular reabsorption is one of the steps in this process.

tubular reabsorption

During the day, the kidneys pass up to 180 liters of primary urine. This fluid is not excreted from the body: the so-called filtrate passes through the tubules, where almost all the fluid is absorbed, and the substances necessary for vital activity - amino acids, trace elements, vitamins, return to the blood. Decay and metabolic products are removed with secondary urine. Its volume is much smaller - about 1.5 liters per day.

The efficiency of the kidney as an organ is largely determined by the efficiency of tubular reabsorption. To imagine the mechanism of the process, it is necessary to understand the structure - the renal unit.

The structure of the nephron

The "working" cell of the kidney consists of the following parts.

The renal corpuscle is a glomerular capsule with capillaries inside.
Proximal convoluted tubule.
Loop of Henle - consists of a descending and ascending part. The thin descending is located in the medulla, bends 180 degrees in order to rise into the cortex to the level of the glomerulus. This part forms the ascending thin and thick parts.
Distal convoluted tubule.
The terminal section is a short fragment connected to the collecting duct.
Collecting duct - located in the medulla, diverts secondary urine into the renal pelvis.

The general principle of placement is as follows: the renal glomeruli, the proximal and distal tubules are located in the cortex, and the descending and thick ascending parts and collecting ducts are located in the medulla. In the internal medulla remain thin sections, the collecting ducts.
In the video, the structure of the nephron:

Mechanism of reabsorption

To implement tubular reabsorption, molecular mechanisms are involved that are similar to the movement of molecules through plasma membranes: diffusion, endocytosis, passive and active transport, and so on. The most significant are active and passive transport.

Active - carried out against the electrochemical gradient. Its implementation requires energy and special transport systems.

Consider 2 types of active transport:

Primary active - in progress energy is coming released during the breakdown of adenosine triphosphoric acid. In this way, for example, sodium, calcium, potassium, hydrogen ions move.
Secondary-active - no energy is spent on the transfer. The driving force is the difference in the concentration of sodium in the cytoplasm and the lumen of the tubule. The carrier necessarily includes a sodium ion. In this way, glucose and amino acids pass through the membrane. The difference in the amount of sodium - less in the cytoplasm than outside, is explained by the withdrawal of sodium into the intercellular fluid with the participation of ATP.

After overcoming the membrane, the complex is cleaved into a carrier - a special protein, a sodium ion and glucose. The carrier returns to the cell, where it is ready to attach the next metal ion. Glucose from the interstitial fluid follows into the capillaries and returns to the bloodstream. Glucose is reabsorbed only in the proximal region, since only here the required carrier is formed.

Amino acids are absorbed in a similar way. But the process of protein reabsorption is more complicated: the protein is absorbed by pinocytosis - the capture of fluid by the cell surface, decomposes into amino acids in the cell, and then follows into the intercellular fluid.

Passive transport - absorption is carried out along an electrochemical gradient and does not need support: for example, the absorption of chloride ions in the distal tubule. It is possible to move along the concentration, electrochemical, osmotic gradients.

In fact, reabsorption is carried out according to schemes that include the most different ways transportation. Moreover, depending on the site of the nephron, substances can be absorbed differently or not absorbed at all.

For example, water is absorbed in any part of the nephron, but by different methods:

about 40-45% of water is absorbed in the proximal tubules by the osmotic mechanism - following the ions;
25–28% of water is absorbed in the loop of Henle by a reverse-flow mechanism;
up to 25% of water is absorbed in the distal convoluted tubules. Moreover, if in the two previous sections the absorption of water is carried out regardless of the water load, then in the distal the process is regulated: water can be excreted with secondary urine or retained.

The volume of secondary urine reaches only 1% of the primary volume.
On the video, the reabsorption process:

Movement of reabsorbed matter

There are 2 methods of moving the reabsorbed substance into the interstitial fluid:

paracellular - the transition is made through one membrane between two tightly connected cells. This is, for example, diffusion, or transfer with a solvent, that is, passive transport;
transcellular - "through the cell." The substance overcomes 2 membranes: luminal or apical, which separates the filtrate in the lumen of the tubule from the cell cytoplasm, and basolateral, which acts as a barrier between the interstitial fluid and the cytoplasm. At least one transition is implemented by the active transport mechanism.

Kinds

In different departments of the nephron are realized different methods reabsorption. Therefore, in practice, the division according to the features of work is often used:

proximal part - convoluted part of the proximal tubule;
thin - parts of the loop of Henle: thin ascending and descending;
distal - distal convoluted tubule connecting the thick ascending part of the loop of Henle.

Proximal

Up to 2/3 of water is absorbed here, as well as glucose, amino acids, proteins, vitamins, a large amount of calcium, potassium, sodium, magnesium, and chlorine ions. The proximal tubule is the main supplier of glucose, amino acids and proteins to the blood, so this stage is mandatory and independent of the load.

Reabsorption schemes are used differently, which is determined by the type of absorbed substance.

Glucose in the proximal tubule is absorbed almost completely. From the lumen of the tubule to the cytoplasm, it follows through the luminal membrane by means of countertransport. This is a secondary active transport that needs energy. The one that is released when the sodium ion moves along the electrochemical gradient is used. Then glucose passes through the basolateral membrane by diffusion: glucose accumulates in the cell, which provides a difference in concentration.

Energy is needed when passing through the luminal membrane; transfer through the second membrane does not require energy costs. Accordingly, the main factor in the absorption of glucose is the primary active transport of sodium.

Amino acids, sulfate, inorganic calcium phosphate, nutrient organic substances are reabsorbed according to the same scheme.

Low molecular weight proteins enter the cell through pinocytosis and are decomposed into amino acids and dipeptides in the cell. This mechanism does not provide 100% absorption: part of the protein remains in the blood, and part is removed in the urine - up to 20 g per day.

Weak organic acids and weak bases are reabsorbed by the non-ionic diffusion method due to the low degree of dissociation. Substances dissolve in the lipid matrix and are absorbed along a concentration gradient. Absorption depends on the pH level: when it decreases, acid dissociation decreases, and base dissociation increases. At high level pH increases the dissociation of acids.

This feature has found application in the removal of toxic substances: in case of poisoning, drugs are injected into the blood that alkalize it, which increases the degree of dissociation of acids and helps to remove them with urine.

Loop of Henle

If in the proximal tubule metal ions and water are reabsorbed in almost equal proportions, then in the loop of Henle, mainly sodium and chlorine are absorbed. Water is absorbed from 10 to 25%.

In the loop of Henle, a turn-and-flow mechanism is implemented, based on the location of the descending and ascending parts. The descending part does not absorb sodium and chlorine, but remains permeable to water. The ascending one sucks in ions, but is impermeable to water. As a result, the absorption of sodium chloride by the ascending part determines the degree of water absorption by the descending part.

The primary filtrate enters the initial part of the descending loop, where the osmotic pressure is lower compared to the pressure of the interstitial fluid. Urine travels down the loop, releasing water but retaining sodium and chloride ions.

As water is withdrawn, the osmotic pressure in the filtrate rises and reaches its maximum value at the turning point. Urine then follows the ascending region, retaining water but losing sodium and chloride ions. Hypoosmotic urine enters the distal tubule - up to 100-200 mosm / l.

In fact, urine is concentrated in the descending loop of Henle and diluted in the ascending loop.

On the video, the structure of the Gentle loop:

Distal

The distal tubule is poorly permeable to water, and organic matter is not absorbed here at all. Further breeding is carried out in this department. About 15% of primary urine enters the distal tubule, and about 1% is excreted.

As it moves along the distal tubule, it becomes more and more hyperosmotic, since mainly ions and partially water are absorbed here - no more than 10%. Dilution continues in the collecting ducts, where the final urine is formed.

A feature of this segment is the ability to adjust the process of absorption of water and sodium ions. For water, the regulator is antidiuretic hormone, and for sodium, aldosterone.

Norm

To assess the functionality of the kidney, various parameters are used: the biochemical composition of blood and urine, the value of the concentration ability, as well as partial indicators. The latter also include indicators of tubular reabsorption.

Glomerular filtration rate - indicates the excretory ability of the organ, this is the filtration rate of primary urine that does not contain protein through the glomerular filter.

Tubular reabsorption indicates absorption capacity. Both of these values are not constant and change during the day.

The GFR norm is 90–140 ml/min. Its highest rate is during the day, decreases in the evening, and in the morning it is at its lowest level. With exercise, shock, kidney or heart failure, and other ailments, GFR falls. May increase in the initial stages of diabetes and hypertension.

Tubular reabsorption is not measured directly, but is calculated as the difference between GFR and minute urine output using the formula:

R = (GFR - D) x 100 / GFR, where,

GFR, glomerular filtration rate;
D - minute diuresis;
P - tubular reabsorption.

With a decrease in blood volume - surgery, blood loss, an increase in tubular reabsorption towards growth is observed. Against the background of taking diuretics, with some renal ailments, it decreases.

The norm for tubular reabsorption is 95-99%. Hence, there is such a big difference between the volume of primary urine - up to 180 liters, and the volume of secondary urine - 1-1.5 liters.

To obtain these values, the Rehberg test is used. With its help, clearance is calculated - the coefficient of purification of endogenous creatinine. According to this indicator, GFR and the amount of tubular reabsorption are calculated.

The patient is kept in the supine position for 1 hour. During this time, urine is collected. The analysis is carried out on an empty stomach.

Half an hour later, blood is taken from the vein.

Then the amount of creatinine in the urine and blood is found and the GFR is calculated using the formula:

GFR = M x D / P, where

M is the level of creatinine in the urine;
P - level of substance in plasma
D is the minute volume of urine. It is calculated by dividing the volume by the time of extraction.

According to the data, the degree of kidney damage can be classified:

A decrease in filtration rate to 40 ml / min is a sign of renal failure.
A decrease in GFR to 5–15 ml/min indicates the terminal stage of the disease.
A decrease in CR usually follows after water loading.
The increase in CR is associated with a decrease in blood volume. The cause may be blood loss, as well as nephritis - with such an ailment, the glomerular apparatus is damaged.

violation of tubular reabsorption

regulation of tubular reabsorption

Blood circulation in the kidneys is a relatively autonomous process. With changes in blood pressure from 90 to 190 mm. rt. Art. the pressure in the renal capillaries is kept at a normal level. This stability is explained by the difference in diameter between the afferent and efferent blood vessels.

There are two most significant methods: myogenic autoregulation and humoral.

Myogenic - with an increase in blood pressure, the walls of the bringing arterioles are reduced, that is, a smaller volume of blood enters the organ and the pressure drops. Narrowing is most often caused by angiotensin II, in the same way thromboxanes and leukotrienes act. Vasodilators are acetylcholine, dopamine, and so on. As a result of their action, the pressure in the glomerular capillaries is normalized in order to maintain a normal level of GFR.

Humoral - that is, with the help of hormones. In fact, the main indicator of tubular reabsorption is the level of water absorption. This process can be divided into 2 stages: mandatory - the one that takes place in the proximal tubules and is independent of water load, and the dependent one - is realized in the distal tubules and collecting ducts. This stage is regulated by hormones.

Chief among them is vasopressin, an antidiuretic hormone. It retains water, that is, it promotes fluid retention. The hormone is synthesized in the nuclei of the hypothalamus, moves to the neurohypophysis, and from there enters the bloodstream. In the distal regions there are receptors for ADH. The interaction of vasopressin with receptors leads to an improvement in the permeability of membranes for water, due to which it is absorbed better. At the same time, ADH not only increases the permeability, but also determines the level of permeability.

Due to the pressure difference in the parenchyma and the distal tubule, water from the filtrate remains in the body. But against the background of low absorption of sodium ions, diuresis can remain high.

The absorption of sodium ions is regulated by aldosterone -, as well as natriuretic hormone.

Aldesterone promotes tubular reabsorption of ions and is formed when the level of sodium ions in plasma decreases. The hormone regulates the creation of all the mechanisms required for sodium transfer: the apical membrane channel, the carrier, the components of the sodium-potassium pump.

Its effect is especially strong in the area of the collecting ducts. The hormone “works” both in the kidneys, and in the glands, and in the gastrointestinal tract, improving the absorption of sodium. Aldosterone also regulates the sensitivity of receptors to ADH.

Aldosterone appears for another reason. With a decrease in blood pressure, renin is synthesized - a substance that controls vascular tone. Under the influence of renin, ag-globulin from the blood is transformed into angiotensin I, and then into angiotensin II. The latter is the strongest vasoconstrictor. In addition, it triggers the production of aldosterone, which causes the reabsorption of sodium ions, which causes water retention. This mechanism - water retention and vasoconstriction, creates optimal blood pressure and normalizes blood flow.

Natriuretic hormone is produced in the atrium when it is stretched. Once in the kidneys, the substance reduces the reabsorption of sodium and water ions. At the same time, the amount of water that enters the secondary urine increases, which reduces the total blood volume, that is, the atrial distension disappears.

In addition, other hormones also affect the level of tubular reabsorption:

parathyroid hormone - improves calcium absorption;
thyrocalcitonin - reduces the level of reabsorption of ions of this metal;
adrenaline - its effect depends on the dose: at a small amount, adrenaline reduces GFR filtration, at a large dose, tubular reabsorption is increased here;
thyroxine and somatropic hormone - increase diuresis;
insulin - improves the absorption of potassium ions.

The mechanism of influence is different. Thus, prolactin increases the permeability of the cell membrane for water, and parathyrin changes the osmotic gradient of the interstitium, thereby affecting the osmotic transport of water.

Tubular reabsorption is a mechanism that causes the return of water, trace elements and nutrients into the blood. There is a return - reabsorption, in all parts of the nephron, but according to different schemes.

The main function of the kidneys is the processing and excretion of metabolic products, toxic, drug compounds from the body.

The normal functioning of the kidneys contributes to the normalization of blood pressure, the process of homeostasis, the formation of the hormone erythropoietin.

As a result of the normal functioning of the renal system, urine is formed. The mechanism of urine formation consists of three interrelated stages: filtration, reabsorption, secretion. The appearance of failures in the work of the body leads to the development of undesirable consequences.

General concepts

Reabsorption is the absorption by the body from the urinary fluid of substances of various origins.

The process of reverse absorption of chemical elements occurs through the renal channels with the participation of epithelial cells. They act as an absorbent. They distribute the elements that are contained in the filtration products.

Water, glucose, sodium, amino acids, and other ions are also absorbed, which are transported to the circulatory system. Chemical constituents, which are decay products, are in excess in the body and are filtered out by these cells.

The process of absorption occurs in the proximal tubules. Then the filtering mechanism chemical compounds passes into the loop of Henle, distal convoluted tubules, collecting ducts.

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Process mechanics

At the stage of reabsorption, the maximum absorption of chemical elements and ions necessary for the normal functioning of the body occurs. There are several ways of absorbing organic components.

Active. Transportation of substances occurs against an electrochemical, concentration gradient: glucose, sodium, potassium, magnesium, amino acids.
Passive. It is characterized by the transfer of the necessary components along the concentration, osmotic, electrochemical gradient: water, urea, bicarbonates.
Transport by pinocytosis: protein.

The speed and level of filtration, transportation of the necessary chemical elements and components depends on the nature of the food consumed, lifestyle, and chronic diseases.

Types of reabsorption

Depending on the area of the tubules through which the distribution of nutrients occurs, there are several types of reabsorption:

proximal;
distal.

The proximal one is distinguished by the ability of these channels to secrete and transfer amino acids, protein, dextrose, vitamins, water, sodium ions, calcium, chlorine, microelements from the primary urine.

The release of water is a passive transport mechanism. The speed and quality of the process depends on the presence of hydrochloride and alkali in the filtration products.
The movement of bicarbonate occurs with the help of an active and passive mechanism. The absorption rate depends on the area of the organ through which the primary urine passes. Its passage through the tubules is dynamic. The absorption of components through the membrane requires a certain time. The passive mechanism of transport is characterized by a decrease in the volume of urine, an increase in the concentration of bicarbonate.
Transportation of amino acids and dextrose takes place with the participation epithelial tissue. They are located in the brush border of the apical membrane. The process of absorption of these components is characterized by the simultaneous formation of hydrochloride. At the same time, a low concentration of bicarbonate is observed.
The release of glucose is characterized by maximum connection with the transporting cells. High glucose concentrations increase the load on transport cells. As a result, glucose does not move into the circulatory system.

With the proximal mechanism, the maximum absorption of peptides and proteins is observed.

Distal reabsorption affects the final composition, the concentration of organic components in the urinary substance. With distal absorption, active absorption of alkali is observed. Potassium, calcium ions, phosphates, chloride are transported passively.

The concentration of urine, the activation of absorption is due to the peculiarities of the structure of the renal system.

Possible problems

Dysfunctions of the filtering organ can lead to the development of various pathologies and disorders. The main pathologies include:

Disorders of tubular reabsorption are characterized by an increase and decrease in the absorption of water, ions, organic components from the lumen of the tubules. Dysfunction occurs as a result of a decrease in the activity of transport enzymes, a lack of carriers, macroergs, trauma to the epithelium.
Violations of excretion, secretion by the epithelial cells of the renal tubules of potassium ions, hydrogen, metabolic products: paraaminohippuric acid, diodrast, penicillin, ammonia. Dysfunctions arise as a result of trauma to the distal nephron tubules, damage to cells and tissues of the cortical and medulla of the organ. These dysfunctions lead to the development of renal, extrarenal syndromes.
Renal syndromes are distinguished by the development of diuresis, worsening of the urination rhythm, changes in chemical composition and specific gravity of the urinary substance. Dysfunctions lead to the development of renal failure, nephritic syndrome, tubulopathy.
Polyuria is characterized by increased diuresis, decreased specific gravity urine. The causes of pathology are:

excess fluid;
activation of blood flow through the cortical substance of the kidneys;
increase in hydrostatic pressure in vessels;
reduction of oncotic pressure of the circulatory system;
violations of colloid osmotic pressure;
deterioration of tubular reabsorption of water, sodium ions.

Oliguria. With this pathology, there is a decrease in daily diuresis, an increase in the specific gravity of the urinary fluid. The main reasons for the violation are:

lack of fluid in the body. Occurs as a result of increased sweating, with diarrhea;
spasm of the afferent arterioles of the kidneys. The main symptom of a violation is edema;
arterial hypotension;
blockage, traumatization of capillaries;
activation of the process of transporting water, sodium ions in the distal tubules.

Hormonal disruptions. Activation of the production of aldosterone increases the absorption of sodium into the circulatory system. As a result, there is an accumulation of fluid, which leads to swelling, a decrease in the concentration of potassium in the body.
Pathological changes in epithelial cells. They are the main cause of urinary concentration control dysfunction.

The cause of the pathology can be determined with the help of laboratory tests of urine.

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The normal functioning of the kidneys contributes to the timely removal of decay products of chemical compounds, metabolism, and toxic elements from the body.

When the first signs of a violation of the normal functioning of the body appear, it is necessary to consult a specialist. Untimely treatment or its absence can lead to the development of complications, chronic diseases.

Reabsorption literally means reabsorption of fluid. This refers to the function of absorbing various elements from the urine and transporting them back to the lymph and blood. Such substances can be protein, dextrose, sodium, amino acids, water and other organic and inorganic compounds.

General information

The reverse absorption of organic substances occurs through the renal tubules with the help of special cells - "carriers". They play the role of a kind of filter and they filter out those elements that are overabundant in the body or that are not needed (decay products). For example, in diabetes, the body does not need sugar and it will automatically remain in the ion channels.

The so-called filtration apparatus is surrounded by an apical membrane, in which the "transporters" are concentrated, they are responsible for the delivery of substances to other cells. They function as pumps and work on the energy that the mitochondria produce. Thus, the necessary compounds enter the intercellular fluid, and then into the vascular bed.

Types of reabsorption

Scheme of the process of reabsorption in the tubules of the kidneys.

Reception of nutrients occurs through different sections of the channels, in this dependence, two types of reabsorption are distinguished:

Proximal

It causes the transport of amino acids, protein, dextrose and vitamins into the body from the primary urine. Absorption in this case occurs almost in full, only 1/3 of the total volume is filtered out. The mechanism of water reabsorption is passive and depends on the content of hydrochloride and alkali in the urine. Bicarbonate can be absorbed in both a fast and a slow way - when entering and leaving the tubules, the element behaves dynamically, and when passing through the membrane, the behavior can be characterized as inhibited. Bicarbonate acts as a carrier here.

As the urine passes through the tubule, the volume of urine decreases as the fluid is passively reabsorbed and this results in a high concentration of bicarbonate. They will be absorbed along with the liquid. This inhibition in the tubules provides a consistency of urine similar to blood plasma. In addition, phosphates, cations, potassium, hydrochloride, urea and uric acid ions are absorbed in the proximal sections.

Amino acids and dextrose are transported into the blood by epithelial cells located in the brush border of the apical membrane. The absorption of these substances is possible only if there is a simultaneous connection with the hydrochloride. To do this, the concentration must be low. Therefore, in the process of transportation, bicarbonate is actively removed from the cell - this process is called symport.

Proximal reabsorption of glucose requires the connection of its molecule with a transporting cell. But in the case when its content in the primary urine is too high, the capabilities of the carriers are overloaded. This leads to the fact that this element can no longer get back into the blood. And accordingly, the concentration of this substance in the final urine is increased. From this it can be concluded that the renal excretion threshold has been reached or the value of the maximum flowing transport of the substance has been reached.

Allowable blood sugar levels are different for men and women. For the former, this figure is 375 mg / min, and for the latter - 303 mg / min. Glucose is an example of threshold substances, that is, those that have a limiting concentration. An example of compounds that are not absorbed into the blood or are poorly absorbed are inulin, manitol, sulfates, and urea. They are also called non-threshold. It is understood that they have no elimination threshold. In the process of proximal absorption, peptides and proteins are almost completely returned to the blood and lymph. Only a small proportion is found in the final urine.

Distal

This type of reabsorption is much less proximal. But it is the distal absorption of substances that affects the final composition of urine and its concentration. In these parts of the tubules, alkali is actively reabsorbed, while chloride, on the contrary, is passively reabsorbed. Potassium, calcium ions and phosphates are actively transported. In addition, thanks to such an element as vasopressin, the absorption of urea increases and it enters the intercellular fluid.

Diagram of the urinary system.

The renal system consists of the collecting ducts and the loop of Gentle. This structure gives the kidneys the ability to form urine of various concentrations and causes increased reabsorption. In the kidneys, it moves in different directions, and filtration occurs in the nephron. Filtration in the nephron causes the formation of a more saturated solution in the region of the descending knee and less saturated due to the amount of bicarbonate in the region of the ascending knee of the loop of Gentle. The collecting duct is watertight and the possibility of reabsorption exists only in the presence of vasepressin. Because of this, little water accumulates and the saturation of the final urine increases.