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Sodium reabsorption. Reabsorption and secretion of protein, sodium and chlorine in the renal tubules. Mechanism of glucose reabsorption

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Reabsorption is the transport of substances from the lumen of the renal tubules into the blood flowing through the peri-tubular capillaries. Is reabsorbed 65% of the volume of primary urine(about 120 l / day. It was 170 l, 1.5 was released): water, mineral salts, all the necessary organic components (glucose, amino acids). Transport passive(osmosis, electrochemical gradient diffusion) and active(primary-active and secondary-active with the participation of protein carrier molecules). The transport systems are the same as in the small intestine.

Threshold substances - usually completely reabsorbed(glucose, amino acids) and are excreted in the urine only if their concentration in the blood plasma exceeds a threshold value (the so-called "excretion threshold"). For glucose, the excretion threshold is 10 mmol / l (at a normal blood glucose concentration of 4.4-6.6 mmol / l).

Thresholdless substances - are always excreted regardless of their concentration in blood plasma... They are not reabsorbed or are partially reabsorbed, for example, urea and other metabolites.

The mechanism of work of various parts of the renal filter.

1. In the proximal tubule the process of concentration of the glomerular filtrate originates, and the most important point here is the active absorption of salts. With the help of active transport, about 67% Na + is reabsorbed from this section of the tubule. An almost proportional amount of water and some other solutes, such as chlorine ions, follows sodium ions passively. Thus, before the filtrate reaches the Henle loop, about 75% of the substances are reabsorbed from it. As a result, the tubular fluid becomes isoosmotic with respect to blood plasma and tissue fluids.

The proximal tubule is ideally suited for intensive reabsorption of salt and water... Numerous microvilli of the epithelium form the so-called brush border, which covers the inner surface of the renal tubule lumen. With this arrangement of the absorbing surface, the area of ​​the cell membrane is greatly increased and as a result, the diffusion of salt and water from the lumen of the tubule into the epithelial cells is facilitated.

2. The descending knee of the loop of Henle and part of the ascending knee located in the inner layer medulla, consist of very thin cells that do not have a brush border, and the number of mitochondria is small. The morphology of thin sections of the nephron indicates the absence of active transport of solutes through the tubule wall. In this area of ​​the nephron, NaCl very poorly penetrates the tubule wall, urea is somewhat better, and water passes without difficulty.

3. The wall of the thin section of the ascending knee of the loop of Henle also inactive with respect to salt transport. Nevertheless, it has high permeability to Na + and Cl-, but low permeability to urea and almost impermeable to water.

4. Thick section of the ascending knee of the loop of Henle located in the medulla of the kidney, differs from the rest of the specified loop. It actively transfers Na + and Cl- from the loop lumen to the interstitial space. This section of the nephron, together with the rest of the ascending knee, is extremely water-tight. Due to the reabsorption of NaCl, the liquid enters the distal tubule somewhat hypoosmotic compared to the tissue fluid.

5. Water movement through the distal tubule wall- the process is complex. The distal tubule is of particular importance for the transport of K +, H + and NH3 from the tissue fluid into the nephron lumen and for the transport of Na +, Cl- and H2O from the nephron lumen into the tissue fluid. Since salts are actively "pumped out" from the lumen of the tubule, water follows them passively.

6. Collecting duct permeable to water, which allows it to pass from diluted urine to a more concentrated tissue fluid of the medulla of the kidney. This is the final stage of the formation of hyperosmotic urine. Reabsorption of NaCl also occurs in the duct, but due to the active transport of Na + through the wall. The collecting duct is impermeable for salts, and its permeability changes with respect to water. An important feature of the distal collecting duct, located in the inner medulla of the kidneys, is its high permeability to urea.

Mechanism of glucose reabsorption.

Proximal(1/3) glucose reabsorption is carried out using special carriers of the brush border of the apical membrane of epithelial cells... These transporters transport glucose only if they simultaneously bind and transport sodium. Passive movement of sodium along the concentration gradient into cells leads to transport across the membrane and transporter with glucose.

To implement this process, a low sodium concentration in the epithelial cell is required, which creates a concentration gradient between the external and intracellular environment, which is ensured by energy-dependent work basement membrane sodium potassium pump.

This type of transport is called secondarily active, or symptom, i.e., joint passive transport of one substance (glucose) due to the active transport of another (sodium) with the help of one carrier. With an excess of glucose in the primary urine, a full load of all carrier molecules can occur and glucose can no longer be absorbed into the blood.

This situation is characterized by the concept " maximum tubular transport of matter»(Tm of glucose), which reflects the maximum loading of tubular carriers at a certain concentration of the substance in the primary urine and, accordingly, in the blood. This value ranges from 303 mg / min in women to 375 mg / min in men. The value of the maximum tubular transport corresponds to the concept of "renal excretion threshold".

Renal elimination threshold call that concentration of a substance in the blood and, accordingly, in the primary urine, at which it can no longer be completely reabsorbed in the tubules and appears in the final urine. Such substances for which the excretion threshold can be found, that is, reabsorbed at low concentrations in the blood completely, and at high concentrations - not completely, are called threshold. An example is glucose, which is completely absorbed from the primary urine at plasma concentrations below 10 mmol / L, but appears in the final urine, i.e., is not completely reabsorbed, when its content in the blood plasma is above 10 mmol / L. Hence, for glucose, the excretion threshold is 10 mmol / l.

Secretion mechanisms in the renal filter.

Secretion is the transport of substances from the blood flowing through the peri-tubular capillaries into the lumen of the renal tubules. Transport is passive and active. Ions H +, K +, ammonia, organic acids and bases (for example, foreign substances, in particular, drugs: penicillin, etc.) are secreted. The secretion of organic acids and bases occurs via a secondary-active sodium-dependent mechanism.

Potassium ion secretion.

Most of the potassium ions that are easily filtered in the glomeruli are usually reabsorbed from filtrate in the proximal tubules and loops of Henle... The rate of active reabsorption in the tubule and loop does not decrease even when the concentration of K + in the blood and filtrate increases strongly in response to excessive consumption of this ion by the body.

However, the distal tubules and collecting ducts are capable of not only reabsorbing, but also secreting potassium ions. By secreting potassium, these structures strive to achieve ionic homeostasis in the event of an unusually large amount of this metal entering the body. The transport of K +, apparently, depends on its postulation into the tubular cells from the tissue fluid, due to the activity of the usual Nar + - Ka + -pump, with the leakage of K + from the cytoplasm into the tubular fluid. Potassium can simply diffuse along an electrochemical gradient from the cells of the renal tubules into the lumen, because the tubular fluid is electronegative to the cytoplasm. The secretion of K + by these mechanisms is stimulated by the adrenocortical hormone aldosterone, which is released in response to an increase in the K + content in the blood plasma.

Substance to be reabsorbed, should (1) move through the epithelial lining of the tubule into the intercellular fluid, and then (2) through the membranes of the peritubular capillaries - back into the blood. Consequently, the reabsorption of water and solutes is a multi-stage process. The transfer of substances through the epithelium of the tubules into the intercellular fluid is carried out using the mechanisms of active and passive transport. For example, water and substances dissolved in it are able to penetrate into cells either directly through the membrane (transcellularly), or using the gaps between cells (paracellularly).

Then after getting into the intercellular fluid the rest of the way, the solutions are made by ultrafiltration (mass movement), mediated by hydrostatic and colloidal-osmotic forces. Under the influence of the resulting force directed to the reabsorption of water and substances dissolved in it from the intercellular fluid into the blood, the peritubular capillaries perform a function similar to the venous ends of most capillaries.

Using energy generated in the process of exchange, active transport is able to move solutes against an electrochemical gradient. The mode of transport, which depends on the energy consumption, obtained, for example, during the hydrolysis of adenosine triphosphate, is called primary active transport. An example of such a transport is sodium-potassium ATPase, the activity of which is carried out in many parts of the tubular system.

View transport, which does not directly depend on the energy source, for example, due to the concentration gradient, is called secondary active transport. An example of this mode of transport is the reabsorption of glucose in the proximal tubule. Water is always reabsorbed passively by a mechanism called osmosis. This term refers to the diffusion of water from an area with a low concentration of a substance (high water content) to an area with a high concentration of a substance (low water content).
Dissolved substances can move through the membrane of epithelial cells or through the intercellular spaces.

Renal tubular cells, like other epithelial cells, are held together by tight contacts. On the sides of the cells in contact with each other, behind these junctions, there are intercellular spaces. Dissolved substances can be reabsorbed through the cell using the transcellular pathway, or they can penetrate the area of ​​tight contact and intercellular spaces through the paracellular pathway. This mode of transport is also used in some segments of the nephron, especially in the proximal tubule, where water and substances such as potassium, magnesium, and chlorine ions are reabsorbed.

Primarily active transport through the membrane associated with hydrolysis of ATP. The special significance of the primary active transport is that, with its help, solutes can move against the electrochemical gradient. The energy required for this type of transport is provided by ATP, the hydrolysis of the molecule of which is provided by ATP-ase bound to the membrane. The enzyme ATP-ase is also an integral part of the transport system that attaches and moves solutes across the membrane. Known primarily active systems for the transfer of substances include the following ATP-ases: sodium-potassium, transporting hydrogen ions, hydrogen-potassium and calcium.

A striking example of how the system works primary active transport is the process of sodium reabsorption through the membrane of the proximal convoluted tubule. It is located on the lateral surfaces of epithelial cells closer to the basement membrane and is a powerful Na + / K + pump. Its ATP-ase supplies the system with the energy released with the hydrolysis of ATP and used to transport Na + ions from the cell to the extracellular space. At the same time, potassium is transferred from the intercellular fluid into the cell. The activity of this ion pump is aimed at maintaining a high concentration of potassium in the cell and a low concentration of sodium.

In addition, a relative potential difference with a charge inside the cell of about -70 mV. The excretion of sodium by means of a pump located on the membrane of the basolateral region of the cell promotes its diffusion back into the cell through the area facing the canal lumen for the following reasons: (1) the presence of a concentration gradient for sodium directed from the canal lumen into the cell, because ... its concentration in the cell is low (12 meq / l), in the lumen it is high (140 meq / l); (2) the negative charge inside the cell (-70 mV) attracts positively charged Na ions.

Active sodium reabsorption with the help of sodium-potassium ATPase occurs in many parts of the tubular system of the nephron. In certain parts of it, there are additional mechanisms that ensure the reabsorption of large amounts of sodium into the cell. In the proximal tubule, the side of the cell facing the lumen of the tubule is represented by a brush border, which increases the surface area by about 20 times. This membrane also contains carrier proteins that attach and transport sodium from the lumen of the tubules into the cell, making it easier for them to diffuse. These carrier proteins also play an important role in the reactive transport of other substances such as glucose and amino acids. This process is detailed below.
In this way, reabsorption of Na + ions from the lumen of the tubules back into the blood consists of at least three stages.

1. Diffusion of Na + ions through the membrane of the tubular epithelial cells (also called the apical membrane) into the cells along an electrochemical gradient supported by a Na + / K + pump, which is located on the basolateral side of the membrane.

2. Sodium transport across the basolateral membrane into the intercellular fluid... It is carried out against an electrochemical gradient using a Na + / K + pump with ATP-ase activity.

3. Sodium reabsorption, water and other substances from the intercellular fluid into the peritubular capillaries by ultrafiltration - a passive process provided by gradients of hydrostatic and colloidal-osmotic pressure.

Up to 80% of filtered sodium is reabsorbed in the proximal tubule segments, 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 tubule content remains isoosmotic. The proximal regions are highly permeable to both sodium and water. Through the apical membrane, sodium enters the cytoplasm passively along the gradient of the electrochemical potential. Further, sodium moves through the cytoplasm to the basal part of the cell, where sodium pumps are located (Na-K-ATPase, dependent on Mg).

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

In the descending part of Henle's loop, sodium and chlorine are practically not absorbed.

In the ascending part of Henle's loop, a different mechanism for the absorption of sodium and chlorine functions. On the apical surface there is a system for transferring 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 mechanism of salt reabsorption is the Na-pump, which ensures 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. Some of the potassium is absorbed in the Henle loop. The excretion of potassium in the urine depends on its secretion by cells of the distal tubule and collecting ducts. With an excessive intake of potassium into the body, its reabsorption in the proximal tubules does not decrease, but the secretion in the distal tubules increases sharply.

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

Systems for reabsorption and secretion of potassium exist in the same cell of the distal tubule and collecting ducts. In case of potassium deficiency, they ensure maximum extraction of potassium from urine, and in 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 active. The increase in potassium secretion 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 supply of potassium into the cell due to the increased work of the Na-K pump.

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

Calcium transport

The kidneys and bones play a major role in maintaining stable blood calcium levels. Calcium consumption per day is about 1 g. The intestines are excreted by 0.8, by the kidneys - 0.1-0.3 g / day. Ionized calcium is filtered in the glomeruli 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%.

Calcium secretion in humans does not occur.

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

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

    Hormones that regulate calcium transport in the kidney:

  • Parathyroid hormone
  • 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, however, 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 renal calcium excretion. The active form of vitamin D3 increases the reabsorption of calcium in the proximal segment of the tubule. Growth hormone enhances calciuresis, which is why patients with acromegaly often develop urolithiasis.

Magnesium transport

A healthy adult excretes 60-120 mg of magnesium in urine per day. Up to 60% of the filtered magnesium is reabsorbed in the proximal tubules. Large amounts of magnesium are reabsorbed in the ascending knee of Henle's loop. The reabsorption of magnesium is an active process and is limited by the magnitude of 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 often has to meet with manifestations of hypomagnesemia. Magnesium, like calcium, is not secreted in the kidney tubules.

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 excretion 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 consistency of phosphate in the fluids of the internal environment. In blood plasma, phosphates are presented 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 filtered 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 the greater power 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 hormone of the parathyroid glands, which inhibits reabsorption in the proximal segments of the nephron, vitamin D3, growth hormone, which stimulate the reabsorption of phosphates.

Glucose transport

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

In the cell, the accumulation of glucose occurs, its phosphorylation to glucose-6-phosphate and passive transfer to the peri-tubular 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 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, protein filtered in the glomeruli (up to 17-20 g / day) is practically all reabsorbed in the proximal tubule segments and is found in a small amount in daily urine - 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 tubule segments.

Under the influence of proteolytic enzymes contained in lysosomes, the protein undergoes hydrolysis to form 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 undergo reabsorption, and this process occurs mainly in the initial sections of the proximal convoluted tubule. The amino acid reabsorption mechanism is similar to that described above for glucose. There are a limited number of carriers, and when all of them combine with the corresponding amino acids, the excess of the latter remains in the tubular fluid and is excreted in the urine.

Normal urine contains only traces of amino acids.

    The causes of aminoaciduria are:

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

Back in 1842, the German physiologist K. Ludwig assumed that urine formation consists of 3 processes. In the 1920s, the American physiologist A. Richards confirmed this assumption.

Final urine production is the result of three successive processes:

I. In the renal glomeruli, the initial stage of urination occurs - glomerular or glomerular ultrafiltration protein-free fluid from blood plasma into the capsule of the renal glomerulus, resulting in the formation of primary urine.

II. Tubular reabsorption - the process of re-suction of the filtered substances and water.

III ... Secretion ... Cells of some parts of the tubule transfer from the extracellular fluid into the lumen of the nephron (secrete) a number of organic and inorganic substances, or secrete molecules synthesized in the tubule cell into the lumen of the tubule.

I. GLOMERULAR FILTRATION

Urine production begins with glomerular filtration, i.e. transfer of fluid from the glomerular capillaries into the Bowman's capsule, while the fluid passes through the glomerular filter.

Filter membrane... The filtration barrier in the renal corpuscle consists of three layers: endothelium of glomerular capillaries, basement membrane and single-row layer of epithelial cells, lining Bowman's capsule. The first layer, capillary endothelial cells, is perforated with many openings ("windows" or "fenestres") (d pores 40 - 100 nm). The basement membrane is a gel-like, acellular, cellular structure composed of glycoproteins and proteoglycans. The epithelial cells of the capsule, which rest on the basement membrane, are called podocytes. Podocytes have an unusual octopus-like structure, as a result of which they have many finger-like processes that are depressed into the basement membrane. The slit spaces between the adjacent finger-like processes are the passages through which the filtrate, passing through the endothelial cells and the basement membrane, enters the Bowman space (d gaps between the pedicles of podocytes 24-30 nm)

There are pores in the basement membrane (d pores 2.9 - 3.7 nm), which restrict the passage of blood cells, as well as large molecules more than 5-6 mm (molecular weight more than 70,000 Da: molecules with a molecular weight of less than 70,000 Da are filtered: all minerals, organic compounds (with the exception of large-molecular proteins, lipoids)

Therefore, large proteins such as globulins (molecular weight 160,000) and caseins (molecular weight 100,000) do not enter the filtrate. Plasma albumin (molecular weight about 70,000) pass into the filtrate in an insignificant amount. Inulin penetrates into the lumen of the nephron capsule about 22% of egg albumin, 3% of hemoglobin and less than 0.01% of serum albumin (in the case of hemolysis), thus filtration occurs. The free passage of proteins through the glomerular filter is impeded by negatively charged molecules in the basement membrane substance and the lining lying on the surface of podocytes, since the overwhelming majority of plasma proteins carry almost only negative electrical charges. In a certain form of pathology, the kidneys, when the negative charge disappears on the membranes, becomes "permeable" in relation to proteins.

Glomerular filter permeability is determined by the minimum size of molecules that are able to filter and depends on: 1) pore size; 2) pore charge (basement membrane - anionite); 3) hemodynamic conditions; 4) the work of podocyte pedicles (they contain actomyosin filaments) and mesangial cells.

According to its composition, ultrafiltrate - primary urine is isotonic to blood plasma. Inorganic salts and low molecular weight organic compounds (urea, uric acid, glucose, amino acids, creatinine) - freely pass through the glomerular filter and enter the cavity of Bowman's capsule. The main force providing the possibility of ultrafiltration in the renal glomeruli, is the hydrostatic pressure of blood in the vessels. Its value is due to the fact that the bringing arteriole is larger in diameter than the outgoing one, and also by the fact that the renal arteries extend from the abdominal aorta.

The filtration area in two kidneys is 1.5 m 2 per 100 g of tissue(i.e. almost equal to the surface of the body - S of the body 1.73 m 2). Depends on : 1) the surface area of ​​the capillaries; 2) the number of pores (more than in any other organ; they account for up to 30% of the surface of endothelial cells); 3) the number of functioning nephrons.

Effective filtration pressure (EFD), on which the glomerular filtration rate depends, is determined by the difference between the HDC (hydrostatic blood pressure) in the capillaries of the glomerulus (in humans, from 60-90 mm Hg) and the factors opposing it - the oncotic pressure of blood plasma proteins (ODC is 30 mm. Hg) and the hydrostatic pressure of the liquid (or ultrafiltrate) or in the capsule of the glomerulus is about 20 mm Hg.

EFD = GDK- (UEC + GDU)

EFD = 70 mm Hg - (30 mm Hg + 20 mm Hg) = 20 mm Hg.

EFD can vary from 20 to 30 mm Hg. Filtration occurs only if the blood pressure in the capillaries of the glomeruli exceeds the sum of the oncotic pressure of proteins in the plasma and the pressure of the fluid in the capsule of the glomerulus. With an increase in filtration pressure, diuresis increases, with a decrease, it decreases. The blood pressure in the capillaries of the glomeruli and the blood flow through them almost do not change, since with an increase in systemic arterial pressure, the tone of the bringing arteriole increases, and with a decrease in systemic pressure, its tone decreases (the Ostroumov-Beilis effect).

Factors determining filtration

Renal factors

Number of functioning glomeruli

Diameter of the inflowing and outflowing vessels

Capsule filtrate pressure

Extrarenal factors

General functional state of the circulatory system, number of circulating blood, blood pressure and blood flow velocity

The degree of hydration of the body. Osmotic and oncotic pressure.

Functioning of other urinary excretion mechanisms (sweat glands )

The amount of primary urine - 150-180 l / day... 1700 liters of blood flow through the kidneys per day. Glomerular filtration rate is 125 ml / min in men and 110 ml / min in women. Thus, about 180 liters per day. The average total volume of plasma in the human body is approximately 3 liters, which means that all plasma is filtered in the kidneys about 60 times a day. The ability of the kidneys to filter such a huge volume of plasma enables them to excrete a significant amount of metabolic end products and very accurately regulate the elemental composition of fluids in the internal environment of the body.

II. TUBULE REABSORPTION

In the kidneys of a person, up to 170 liters of filtrate is formed in one day, and 1-1.5 liters of final urine are released, the rest of the liquid is absorbed in the tubules. Primary urine is isotonic to blood plasma (i.e. it is blood plasma without proteins) Reabsorption of substances in the tubules is to return all vital substances and in the required quantities from the primary urine.

Reabsorption volume = ultrafiltrate volume - final urine volume.

The molecular mechanisms involved in the implementation of reabsorption processes are the same as the mechanisms that act during the transfer of molecules across plasma membranes in other parts of the body, such as diffusion, active and passive transport, endocytosis, etc.

There are two routes for the movement of the reabsorbed substance from the lumen to the interstitial space.

The first is movement between cells, i.e. through a tight connection of two adjacent cells - it is a paracellular pathway ... Paracellular reabsorption can be carried out by diffusion or due to the transfer of a substance together with a solvent. The second way of reabsorption - transcellular ("through" the cell). In this case, the reabsorbable substance must overcome two plasma membranes on its way from the lumen of the tubule to the interstitial fluid - the luminal (or apical) membrane, which separates the fluid in the lumen of the tubule from the cytoplasm of cells, and the basolateral (or counterluminal) membrane, which separates the cytoplasm from the interstitial fluid. Transcellular transport defined by the term active , for brevity, although crossing at least one of the two membranes is accomplished by a primary or secondary active process. If a substance is reabsorbed against electrochemical and concentration gradients, the process is called active transport. There are two types of transport - primary-active and secondary-active ... Primary active transport is called when a substance is transferred against an electrochemical gradient due to the energy of cellular metabolism. This transport is provided by the energy obtained directly from the cleavage of ATP molecules. An example is the transport of Na ions, which occurs with the participation of Na +, K + ATPase, which uses the energy of ATP. Currently, the following systems of primary active transport are known: Na +, K + - ATPase; H + -ATPase; H +, K + -ATPase and Ca + ATPase.

Secondary-active the transfer of a substance against a concentration gradient is called, but without the expenditure of the cell's energy directly for this process, this is how glucose and amino acids are reabsorbed. From the lumen of the tubule, these organic substances enter the cells of the proximal tubule using a special carrier, which must necessarily attach the Na + ion. This complex (carrier + organic matter + Na +) promotes the movement of the substance through the membrane of the brush border and its entry into the cell. The driving force of the transfer of these substances through the apical plasma membrane is the lower concentration of sodium in the cytoplasm of the cell compared to the lumen of the tubule. The sodium concentration gradient is due to the direct active excretion of sodium from the cell into the extracellular fluid using Na +, K + -ATPase, localized in the lateral and basement membranes of the cell. Reabsorption of Na + Cl - is the most significant process in terms of volume and energy consumption.

Different parts of the renal tubules differ in their ability to absorb substances. By analyzing fluids from different parts of the nephron, the composition of the fluid and the features of the work of all parts of the nephron were established.

Proximal tubule. Reabsorption in the proximal segment is obligatory. In the proximal convoluted tubules, most of the components of the primary urine are reabsorbed with an equivalent amount of water (the volume of primary urine is reduced by about 2/3). In the proximal nephron, amino acids, glucose, vitamins, the required amount of protein, trace elements, a significant amount of Na +, K +, Ca +, Mg +, Cl _, HCO 2 are completely reabsorbed. The proximal tubule plays a major role in the return of all these filtered substances to the blood through efficient reabsorption. Filtered glucose is almost completely reabsorbed by the cells of the proximal tubule, and normally a small amount (no more than 130 mg) can be excreted in the urine per day. Glucose moves against the gradient from the lumen of the tubule through the luminal membrane into the cytoplasm via a sodium cotransport system. This movement of glucose is mediated by the participation of the carrier and is a secondarily active transport, since the energy required for the movement of glucose through the luminal membrane is generated due to the movement of sodium along its electrochemical gradient, i.e. by means of cotransport. This cotransport mechanism is so powerful that it allows you to completely absorb all glucose from the lumen of the tubule. After penetration into the cell, glucose must overcome the basolateral membrane, which occurs through facilitated diffusion, independent of sodium participation, this movement along the gradient is maintained due to the high concentration of glucose accumulating in the cell, due to the activity of the luminal cotransport process. To ensure active transcellular reabsorption, the system functions: with the presence of 2 membranes that are asymmetric with respect to the presence of glucose transporters; energy is released only when overcoming one membrane, in this case the luminal. The decisive factor is that the entire process of glucose reabsorption ultimately depends on the primary active transport of sodium. Secondary active reabsorption when co-transported with sodium through the luminal membrane, in the same way as glucose amino acids are reabsorbed, inorganic phosphate, sulfate and some organic nutrients. Low molecular weight proteins are reabsorbed by pinocytosis in the proximal segment. Protein reabsorption begins with endocytosis (pinocytosis) at the luminal membrane. This energy-dependent process is initiated by the binding of filtered protein molecules to specific receptors on the luminal membrane. Separate intracellular vesicles, which appeared during endocytosis, fuse inside the cell with lysosomes, whose enzymes break down proteins to low molecular weight fragments - dipeptides and amino acids, which are removed into the blood through the basolateral membrane. The excretion of proteins in the urine is normally no more than 20 - 75 mg per day, and with kidney disease, it can increase up to 50 g per day (proteinuria ).

An increase in the excretion of proteins in urine (proteinuria) may be due to a violation of their reabsorption or filtration.

Non-ionic diffusion- weak organic acids and bases dissociate poorly. They dissolve in the lipid matrix of membranes and are reabsorbed along a concentration gradient. The degree of their dissociation depends on the pH in the tubules: when it decreases, acid dissociation decreases, of grounds is rising. Acid reabsorption increases, grounds - decreases... As the pH rises, the opposite is true. This is used in the clinic to accelerate the excretion of toxic substances - in case of poisoning with barbiturates, the blood is alkalized. This increases their content in the urine.

Loop of Henle... In the Henle loop, in general, more sodium and chlorine (about 25% of the filtered amount) is always reabsorbed than water (10% of the filtered water volume). This is an important difference between the loop of Henle and the proximal tubule, where water and sodium are reabsorbed in almost equal proportions. The descending part of the loop does not reabsorb sodium or chlorine, but it has a very high permeability to water and reabsorbs it. The ascending part (both its thin and thick part) reabsorbs sodium and chlorine and practically does not reabsorb water, since it is completely impermeable to it. The reabsorption of sodium chloride by the ascending part of the loop is responsible for the reabsorption of water in its descending part, i.e. the transition of sodium chloride from the ascending part of the loop to the interstitial fluid increases the osmolarity of this fluid, and this entails a large reabsorption of water through diffusion from the water-permeable descending part of the loop. Therefore, this section of the canaliculus was called the rearing segment. As a result, the liquid, being already hypoosmotic in the ascending thick part of Henle's loop (due to the release of sodium), enters the distal convoluted tubule, where the dilution process continues and it becomes even more hypoosmotic, since in the subsequent parts of the nephron, organic substances are not absorbed into them, only ions are reabsorbed. and H 2 O. Thus, it can be argued that the distal convoluted tubule and the ascending part of Henle's loop function as segments where urine is diluted. As it moves along the collecting tube of the medulla, the tubular fluid becomes more and more hyperosmotic, because The reabsorption of sodium and water continues in the collecting tubes, in which the final urine is formed (concentrated, due to the regulated reabsorption of water and urea. H 2 O passes into the interstitial substance according to the laws of osmosis, since there is a higher concentration of substances. Percentage of reabsorption water can vary widely depending on the water balance of a given organism.

Distal reabsorption. Optional, adjustable.

Peculiarities:

1. The walls of the distal segment are poorly permeable to water.

2. Here sodium is actively reabsorbed.

3. Wall permeability regulated : for water- antidiuretic hormone, for sodium- aldosterone.

4. There is a process of secretion of inorganic substances.

The role of the kidneys in the human body is invaluable. These vital organs perform many functions, they regulate blood volume, eliminate waste products from the body, normalize acid-base and water-salt balance, etc. These processes are carried out due to the fact that urine formation occurs in the body. Tubular reabsorption refers to one of the stages of this important process that affects the activity of the whole organism as a whole.

The importance of the body's excretory system

Excretion of the end products of tissue metabolism from the body is a very important process, since these products are already unable to benefit, but can have a toxic effect on humans.

The excretory organs include:

  • leather;
  • intestines;
  • kidneys;
  • lungs.

The formation of atrial natriuretic hormone is carried out in the atria when they are stretched, caused by excess blood. This hormonal substance, on the contrary, reduces the absorption of water in the distal tubules, enhancing the process of urination and facilitating the removal of excess fluid from the body.

What violations can there be?

Kidney disease can be caused by various reasons, among which pathological changes in reabsorption are not the last. With impaired absorption of water, polyuria, or a pathological increase in urine formation, can develop, as well as oliguria, in which the daily urine content is less than one liter.

Disorders of glucose absorption lead to glucosuria, in which this substance is not reabsorbed at all, and is fully excreted from the body along with urine.

A very dangerous state of acute renal failure, when the functions of the kidneys are impaired, and the organs cease to function normally.