Metabolic kidney function. What do the kidneys provide? Transport of gases by blood

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Nephropathy is a pathological condition of both kidneys in which they cannot fully perform their functions. The processes of blood filtration and urine excretion are disrupted for various reasons: endocrine diseases, tumors, congenital anomalies, metabolic changes. Metabolic nephropathy is diagnosed more often in children than in adults, although the disorder may go unnoticed. The danger of developing metabolic nephropathy lies in the negative impact of the disease on the entire body.

Metabolic nephropathy: what is it?

A key factor in the development of pathology is a violation of metabolic processes in the body. There is also dysmetabolic nephropathy, which is understood as a number of metabolic disorders accompanied by crystalluria (the formation of salt crystals detected during a urine test).

Depending on the cause of development, there are 2 forms of kidney disease:

Primary - occurs against the background of progression of hereditary diseases. It promotes the formation of kidney stones and the development of chronic renal failure.
Secondary - manifests itself with the development of diseases of other body systems, and may occur during the use of drug therapy.

Important! Most often, metabolic nephropathy is a consequence of calcium metabolism disorders, oversaturation of the body with phosphate, calcium oxalate and oxalic acid.

Development factors

The following pathologies are predisposing factors to the development of metabolic nephropathy:

Among metabolic nephropathies, there are subtypes that are characterized by the presence of salt crystals in the urine. Children more often have calcium oxalate nephropathy, where a hereditary factor influences the development of the disease in 70-75% of cases. In the presence of chronic infections in the urinary system, phosphate nephropathy is observed, and in the case of impaired uric acid metabolism, urate nephropathy is diagnosed.

Congenital metabolic disorders occur in children experiencing hypoxia during intrauterine development. In adulthood, the pathology is acquired. The disease can be recognized in time by its characteristic signs.

Symptoms and types of disease

Impaired kidney function due to metabolic failure leads to the following manifestations:

development of inflammatory processes in the kidneys and bladder;
polyuria - an increase in the volume of urine output by 300-1500 ml above normal;
the occurrence of kidney stones (urolithiasis);
the appearance of edema;
disturbance of urination (delay or frequency);
the appearance of pain in the abdomen, lower back;
redness and swelling of the genitals, accompanied by itching;
deviations from the norm in urine analysis parameters: detection of phosphates, urates, oxalates, leukocytes, protein and blood in it;
decreased vitality, increased fatigue.

As the disease develops, a child may experience signs of vegetative-vascular dystonia - vagotonia (apathy, depression, sleep disturbances, poor appetite, feeling of lack of air, lump in the throat, dizziness, swelling, constipation, tendency to allergies) or sympathicotonia (hot temper, absent-mindedness, increased appetite, numbness of the limbs in the morning and heat intolerance, tendency to tachycardia and increased blood pressure).

Diagnostics

One of the main tests indicating the development of metabolic nephropathy is a biochemical urine test. It allows you to determine whether there are abnormalities in the functioning of the kidneys, thanks to the ability to detect and determine the amount of potassium, chlorine, calcium, sodium, protein, glucose, uric acid, cholinesterase.

Important! To carry out a biochemical analysis, you will need 24-hour urine, and to ensure the reliability of the result, you need to refrain from drinking alcohol, spicy, fatty, sweet foods, and foods that color the urine. The day before the test, you should stop taking uroseptics and antibiotics and warn your doctor about this.

The degree of change in the kidneys, the presence of an inflammatory process or sand in them will help to identify diagnostic methods: ultrasound, radiography.

The condition of the body as a whole can be judged by a blood test. Depending on the results of diagnosis of kidney disease, treatment is prescribed. Therapy will also be aimed at the organs that are the root cause of the metabolic failure.

Treatment and prevention

Since nephropathy can occur in various diseases, each specific case requires separate consideration and treatment.

The selection of medications is carried out only by a doctor. If, for example, nephropathy is caused by inflammation, the need to take antibiotics cannot be ruled out, and if there is an increased radioactive background, eliminating the negative factor or, if radiation therapy is necessary, introducing radioprotectors will help.

Drugs

Vitamin B6 is prescribed as a drug that corrects metabolism. With its deficiency, the production of the transaminase enzyme is blocked, and oxalic acid ceases to be converted into soluble compounds, forming kidney stones.

Calcium metabolism is normalized by the drug Xidifon. It prevents the formation of insoluble calcium compounds with phosphates, oxalates, and promotes the removal of heavy metals.

Cyston is a drug based on herbal components that improves blood supply to the kidneys, promotes urine excretion, relieves inflammation, and promotes the destruction of kidney stones.

Dimephosphone normalizes the acid-base balance in cases of kidney dysfunction due to the development of acute respiratory infections, lung diseases, diabetes mellitus, and rickets.

Diet

The generalizing factor of therapy is:

the need to follow a diet and drinking regime;
rejection of bad habits.

The basis of dietary nutrition for metabolic nephropathy is a sharp limitation of sodium chloride, foods containing oxalic acid, and cholesterol. As a result, swelling is reduced, proteinuria and other manifestations of impaired metabolism are eliminated. Portions should be small and meals should be regular, at least 5-6 times a day.

Allowed for use:

cereal, vegetarian, dairy soups;
bran bread without adding salt and raising agents;
boiled meat with the possibility of further frying: veal, lamb, rabbit, chicken;
low-fat fish: cod, pollock, perch, bream, pike, flounder;
dairy products (except salted cheeses);
eggs (no more than 1 per day);
cereals;
vegetable salads without adding radish, spinach, sorrel, garlic;
berries, fruit desserts;
tea, coffee (weak and no more than 2 cups per day), juices, rosehip decoction.

It is necessary to eliminate from the diet:

soups based on fatty meats, mushrooms;
baked goods; regular bread; puff pastry, shortbread;
pork, offal, sausages, smoked meat products, canned food;
fatty fish (sturgeon, halibut, saury, mackerel, eel, herring);
cocoa-containing foods and drinks;
hot sauces;
water rich in sodium.

You can prepare many dishes from the permitted foods, so sticking to the diet is not difficult.

An important condition for treatment is compliance with the drinking regime. A large amount of fluid helps eliminate stagnation of urine and removes salts from the body. Constantly exercising moderation in food and giving up bad habits will help normalize kidney function and prevent the onset of disease for people with metabolic disorders.

If symptoms of pathology occur, you should visit a specialist. The doctor will examine the patient and select the optimal method of therapy. Any attempts at self-medication can lead to negative consequences.

The kidneys are involved in the metabolism of proteins, lipids and carbohydrates. This function is due to the participation of the kidneys in ensuring the constant concentration of a number of physiologically significant organic substances in the blood. Low molecular weight proteins and peptides are filtered in the renal glomeruli. In the proximal nephron they are broken down into amino acids or dipeptides and transported across the basal plasma membrane into the blood. With kidney disease, this function may be impaired. The kidneys are capable of synthesizing glucose (gluconeogenesis). During prolonged fasting, the kidneys can synthesize up to 50% of the total amount of glucose produced in the body and entering the blood. The kidneys can use glucose or free fatty acids for energy expenditure. When the level of glucose in the blood is low, kidney cells consume fatty acids to a greater extent; with hyperglycemia, glucose is predominantly broken down. The importance of the kidneys in lipid metabolism is that free fatty acids can be included in the composition of triacylglycerol and phospholipids in the kidney cells and enter the blood in the form of these compounds.

Regulation of kidney activity

From a historical perspective, experiments carried out with irritation or transection of the efferent nerves innervating the kidneys are of interest. Under these influences, diuresis changed slightly. It changed little if the kidneys were transplanted to the neck and the kidney artery was sutured to the carotid artery. However, even under these conditions it was possible to develop conditioned reflexes to painful stimulation or to water load, and diuresis also changed under unconditioned reflex influences. These experiments gave reason to assume that reflex influences on the kidneys are carried out not so much through the efferent nerves of the kidneys (they have relatively little effect on diuresis), but that a reflex release of hormones (ADH, aldosterone) occurs and they have a direct effect on the process of diuresis in the kidneys. Therefore, there is every reason to distinguish the following types in the mechanisms of regulation of urine formation: conditioned reflex, unconditioned reflex and humoral.

The kidney serves as an executive organ in a chain of various reflexes that ensure the constancy of the composition and volume of fluids in the internal environment. The central nervous system receives information about the state of the internal environment, signals are integrated and the regulation of kidney activity is ensured. Anuria that occurs with painful stimulation can be reproduced by a conditioned reflex. The mechanism of pain anuria is based on irritation of the hypothalamic centers that stimulate the secretion of vasopressin by the neurohypophysis. Along with this, the activity of the sympathetic part of the nervous system and the secretion of catecholamines by the adrenal glands increase, which causes a sharp decrease in urination due to both a decrease in glomerular filtration and an increase in tubular reabsorption of water.

Not only a decrease, but also an increase in diuresis can be caused by a conditioned reflex. Repeated introduction of water into the dog's body in combination with the action of a conditioned stimulus leads to the formation of a conditioned reflex, accompanied by an increase in urination. The mechanism of conditioned reflex polyuria in this case is based on the fact that impulses are sent from the cerebral cortex to the hypothalamus and the secretion of ADH decreases. Impulses arriving through adrenergic fibers stimulate sodium transport, and through cholinergic fibers they activate the reabsorption of glucose and the secretion of organic acids. The mechanism of changes in urine formation with the participation of adrenergic nerves is due to the activation of adenylate cyclase and the formation of cAMP in tubular cells. Catecholamine-sensitive adenylate cyclase is present in the basolateral membranes of the cells of the distal convoluted tubule and the initial sections of the collecting ducts. The afferent nerves of the kidney play a significant role as an information link in the ionic regulation system and ensure the implementation of reno-renal reflexes. As for the humoral-hormonal regulation of urine formation, this was described in detail above.

1. Formation of the active form of vitamin D 3. In the kidneys, as a result of microsomal oxidation, the final stage of maturation of the active form of vitamin D 3 occurs - 1,25-dihydroxycholecalciferol, which is synthesized in the skin under the influence of ultraviolet rays from cholesterol, and then hydroxylated: first in the liver (at position 25) and then in the kidneys (at position 1). Thus, by participating in the formation of the active form of vitamin D 3, the kidneys influence phosphorus-calcium metabolism in the body. Therefore, in kidney diseases, when the processes of hydroxylation of vitamin D 3 are disrupted, osteodystrophy may develop.

2. Regulation of erythropoiesis. The kidneys produce a glycoprotein called renal erythropoietic factor (PEF or erythropoietin). This is a hormone that is capable of influencing red bone marrow stem cells, which are target cells for PEF. PEF directs the development of these cells along the path of erythropoiesis, i.e. stimulates the formation of red blood cells. The rate of PEF release depends on the supply of oxygen to the kidneys. If the amount of incoming oxygen decreases, the production of PEF increases - this leads to an increase in the number of red blood cells in the blood and an improvement in oxygen supply. Therefore, in kidney diseases, renal anemia is sometimes observed.

3. Biosynthesis of proteins. In the kidneys, the processes of biosynthesis of proteins that are necessary for other tissues are actively taking place. Components of the blood coagulation system, complement system and fibrinolysis system are also synthesized here.

The kidneys synthesize the enzyme renin and the protein kininogen, which are involved in the regulation of vascular tone and blood pressure.

4. Protein catabolism. The kidneys are involved in the catabolism of some low molecular weight proteins (5-6 kDa) and peptides that are filtered into the primary urine. Among them are hormones and some other biologically active substances. In tubule cells, under the action of lysosomal proteolytic enzymes, these proteins and peptides are hydrolyzed to amino acids, which then enter the blood and are reutilized by cells of other tissues.

Large expenditures of ATP by the kidneys are associated with the processes of active transport during reabsorption, secretion, as well as with protein biosynthesis. The main pathway for producing ATP is oxidative phosphorylation. Therefore, kidney tissue needs significant amounts of oxygen. The mass of the kidneys is 0.5% of the total body weight, and the oxygen consumption of the kidneys is 10% of the total oxygen intake.

7.4. REGULATION OF WATER-SALT METABOLISM
AND URINARY

The volume of urine and the content of ions in it are regulated due to the combined action of hormones and the structural features of the kidney.

Renin-angiotensin-aldosterone system. In the kidneys, in the cells of the juxtaglomerular apparatus (JGA), renin is synthesized, a proteolytic enzyme that is involved in the regulation of vascular tone, converting angiotensinogen into the decapeptide angiotensin I through partial proteolysis. From angiotensin I, under the action of the enzyme carboxycathepsin, the octapeptide angiotensin II is formed (also by partial proteolysis). It has a vasoconstrictor effect and also stimulates the production of the adrenal cortex hormone - aldosterone.

Aldosterone is a steroid hormone of the adrenal cortex from the group of mineralocorticoids, which enhances the reabsorption of sodium from the distal part of the renal tubule due to active transport. It begins to be actively secreted when the sodium concentration in the blood plasma decreases significantly. In the case of very low sodium concentrations in the blood plasma, almost complete removal of sodium from the urine can occur under the influence of aldosterone. Aldosterone enhances the reabsorption of sodium and water in the renal tubules - this leads to an increase in the volume of blood circulating in the vessels. As a result, blood pressure (BP) increases (Fig. 19).

Rice. 19. Renin-angiotensin-aldosterone system

When the angiotensin-II molecule fulfills its function, it undergoes total proteolysis under the action of a group of special prosthetics - angiotensinases.

Renin production depends on the blood supply to the kidneys. Therefore, when blood pressure decreases, renin production increases, and when blood pressure increases, it decreases. With kidney pathology, increased production of renin is sometimes observed and persistent hypertension (increased blood pressure) may develop.

Hypersecretion of aldosterone leads to sodium and water retention - then edema and hypertension develop, including heart failure. Aldosterone deficiency leads to significant loss of sodium, chloride and water and a decrease in blood plasma volume. In the kidneys, the processes of secretion of H + and NH 4 + are simultaneously disrupted, which can lead to acidosis.

The renin-angiotensin-aldosterone system works in close contact with another system regulating vascular tone kallikrein-kinin system, the action of which leads to a decrease in blood pressure (Fig. 20).

Rice. 20. Kallikrein-kinin system

The protein kininogen is synthesized in the kidneys. Once in the blood, kininogen, under the action of serine proteinases - kallikreins, is converted into vasoactin peptides - kinins: bradykinin and kallidin. Bradykinin and kallidin have a vasodilating effect - they lower blood pressure.

Inactivation of kinins occurs with the participation of carboxycathepsin - this enzyme simultaneously affects both systems of regulation of vascular tone, which leads to an increase in blood pressure (Fig. 21). Carboxycathepsin inhibitors are used for medicinal purposes in the treatment of certain forms of arterial hypertension. The participation of the kidneys in the regulation of blood pressure is also associated with the production of prostaglandins, which have a hypotensive effect.

Rice. 21. Renin-angiotensin-aldosterone relationship
and kallikrein-kinin systems

Vasopressin– a peptide hormone synthesized in the hypothalamus and secreted from the neurohypophysis, has a membrane mechanism of action. This mechanism in target cells is realized through the adenylate cyclase system. Vasopressin causes constriction of peripheral blood vessels (arterioles), resulting in an increase in blood pressure. In the kidneys, vasopressin increases the rate of water reabsorption from the initial part of the distal convoluted tubules and collecting ducts. As a result, the relative concentrations of Na, C1, P and total N increase. Vasopressin secretion increases when plasma osmotic pressure increases, for example, with increased salt intake or dehydration. It is believed that the action of vasopressin is associated with phosphorylation of proteins in the apical membrane of the kidney, resulting in an increase in its permeability. If the pituitary gland is damaged, if the secretion of vasopressin is impaired, diabetes insipidus is observed - a sharp increase in the volume of urine (up to 4-5 l) with a low specific gravity.

Natriuretic factor(NUF) is a peptide that is formed in the cells of the atrium in the hypothalamus. This is a hormone-like substance. Its targets are cells of the distal renal tubules. NUF acts through the guanylate cyclase system, i.e. its intracellular mediator is cGMP. The result of the influence of NUF on tubular cells is a decrease in Na + reabsorption, i.e. Natriuria develops.

Parathyroid hormone– parathyroid hormone of protein-peptide nature. It has a membrane mechanism of action through cAMP. Affects the removal of salts from the body. In the kidneys, parathyroid hormone enhances the tubular reabsorption of Ca 2+ and Mg 2+, increases the excretion of K +, phosphate, HCO 3 - and reduces the excretion of H + and NH 4 +. This is mainly due to a decrease in tubular reabsorption of phosphate. At the same time, the concentration of calcium in the plasma increases. Hyposecretion of parathyroid hormone leads to the opposite phenomena - an increase in the phosphate content in the blood plasma and a decrease in the Ca 2+ content in the plasma.

Estradiol– female sex hormone. Stimulates synthesis
1,25-dioxycalciferol, enhances the reabsorption of calcium and phosphorus in the renal tubules.

Adrenal hormone influences the retention of a certain amount of water in the body. cortisone. In this case, there is a delay in the release of Na ions from the body and, as a result, water retention. Hormone thyroxine leads to a drop in body weight due to increased release of water, mainly through the skin.

These mechanisms are under the control of the central nervous system. The diencephalon and gray tubercle of the brain are involved in the regulation of water metabolism. Excitation of the cerebral cortex leads to changes in the functioning of the kidneys as a result of either the direct transmission of corresponding impulses along the nerve pathways, or by excitation of certain endocrine glands, in particular the pituitary gland.

Disturbances in water balance in various pathological conditions can lead to either water retention in the body or partial dehydration of tissues. If water retention in tissues is chronic, various forms of edema usually develop (inflammatory, salt, starvation).

Pathological tissue dehydration is usually a consequence of the excretion of an increased amount of water through the kidneys (up to 15-20 liters of urine per day). Such increased urination, accompanied by extreme thirst, is observed in diabetes insipidus (diabetes insipidus). In patients suffering from diabetes insipidus due to a lack of the hormone vasopressin, the kidneys lose the ability to concentrate primary urine; the urine becomes very dilute and has a low specific gravity. However, limiting drinking during this disease can lead to tissue dehydration incompatible with life.

Control questions

1. Describe the excretory function of the kidneys.

2. What is the homeostatic function of the kidneys?

3. What metabolic function do the kidneys perform?

4. What hormones are involved in the regulation of osmotic pressure and extracellular fluid volume?

5. Describe the mechanism of action of the renin-angiotensin system.

6. What is the relationship between the renin-aldosterone-angiotensin and kallikrein-kinin systems?

7. What hormonal regulation disorders can cause hypertension?

8. Specify the reasons for water retention in the body.

9. What causes diabetes insipidus?

The kidneys serve as a natural “filter” of the blood, which, when working properly, remove harmful substances from the body. Regulating kidney function in the body is vital for the stable functioning of the body and immune system. For a comfortable life you need two organs. There are cases that a person remains with one of them - it is possible to live, but he will have to depend on hospitals for the rest of his life, and the protection against infections will decrease several times. What are the kidneys responsible for, why are they needed in the human body? To do this, you should study their functions.

Kidney structure

Let's delve a little into the anatomy: the excretory organs include the kidneys - this is a paired bean-shaped organ. They are located in the lumbar region, with the left kidney being higher. This is nature: above the right kidney there is a liver, which prevents it from moving anywhere. Regarding the size, the organs are almost the same, but note that the right one is slightly smaller.

What is their anatomy? Externally, the organ is covered with a protective shell, and inside it organizes a system capable of accumulating and removing fluid. In addition, the system includes parenchyma, which creates the medulla and cortex and provides the outer and inner layers. Parenchyma is a set of basic elements that are limited to the connective base and the membrane. The storage system is represented by a small renal calyx, which forms a large one in the system. The union of the latter forms the pelvis. In turn, the pelvis is connected to the bladder through the ureters.

Main activities

During the day, the kidneys pump all the blood in the body, while cleansing it of waste, toxins, microbes and other harmful substances.

Throughout the day, the kidneys and liver process and cleanse the blood of impurities and toxins, and remove decay products. More than 200 liters of blood are pumped through the kidneys per day, ensuring its purity. Negative microorganisms penetrate the blood plasma and are sent to the bladder. So what do the kidneys do? Considering the amount of work that the kidneys provide, a person could not exist without them. The main functions of the kidneys are:

excretory (excretory);
homeostatic;
metabolic;
endocrine;
secretory;
hematopoietic function.

Excretory function - as the main responsibility of the kidneys

The formation and excretion of urine is the main function of the kidneys in the body's excretory system.

The excretory function is to remove harmful substances from the internal environment. In other words, this is the ability of the kidneys to correct the acid state, stabilize water-salt metabolism, and participate in maintaining blood pressure. The main task lies with this function of the kidneys. In addition, they regulate the amount of salts and proteins in the liquid and ensure metabolism. Violation of the excretory function of the kidneys leads to a terrible result: coma, disruption of homeostasis and even death. In this case, a violation of the excretory function of the kidneys is manifested by an increased level of toxins in the blood.

The excretory function of the kidneys is carried out through nephrons - functional units in the kidneys. From a physiological point of view, a nephron is a renal corpuscle in a capsule, with proximal tubules and a storage tube. Nephrons perform important work - they control the correct execution of internal mechanisms in humans.

Excretory function. Stages of work

The excretory function of the kidneys goes through the following stages:

secretion;
filtration;
reabsorption.

Violation of the excretory function of the kidneys leads to the development of a toxic condition of the kidney.

During secretion, the metabolic product, the remainder of electrolytes, is removed from the blood. Filtration is the process of a substance entering the urine. In this case, the fluid that has passed through the kidneys resembles blood plasma. Filtration has an indicator that characterizes the functional potential of the organ. This indicator is called the glomerular filtration rate. This value is needed to determine the rate of urine excretion for a specific time. The ability to absorb important elements from urine into the blood is called reabsorption. These elements are proteins, amino acids, urea, electrolytes. The reabsorption rate depends on the amount of fluid in food and the health of the organ.

What is the secretory function?

Let us note once again that our homeostatic organs control the internal mechanism of work and metabolic rates. They filter blood, monitor blood pressure, and synthesize biological active substances. The appearance of these substances is directly related to secretory activity. The process reflects the secretion of substances. Unlike the excretory function, the secretory function of the kidneys takes part in the formation of secondary urine - a liquid without glucose, amino acids and other substances useful to the body. Let us consider the term “secretion” in detail, since in medicine there are several interpretations:

synthesis of substances that will subsequently be returned to the body;
synthesis of chemicals that saturate the blood;
removal of unnecessary elements from the blood by nephron cells.

Homeostatic work

The homeostatic function serves to regulate the water-salt and acid-base balance of the body.

The kidneys regulate the water-salt balance of the entire body.

Water-salt balance can be described as follows: maintaining a constant amount of fluid in the human body, where homeostatic organs influence the ionic composition of intracellular and extracellular waters. Thanks to this process, 75% of sodium and chloride ions are reabsorbed from the glomerular filter, while anions move freely and water is reabsorbed passively.

Regulation of acid-base balance by the body is a complex and confusing phenomenon. Maintaining a stable pH value in the blood occurs thanks to the “filter” and buffer systems. They remove acid-base components, which normalizes their natural amount. When the pH value of the blood changes (this phenomenon is called tubular acidosis), alkaline urine is formed. Tubular acidoses pose a threat to health, but special mechanisms in the form of h+ secretion, ammoniogenesis and gluconeogenesis stop urine oxidation, reduce enzyme activity and are involved in the conversion of acid-reacting substances into glucose.

Role of metabolic function

The metabolic function of the kidneys in the body occurs through the synthesis of biological active substances (renin, erythropoietin and others), as they affect blood clotting, calcium metabolism, and the appearance of red blood cells. This activity determines the role of the kidneys in metabolism. Participation in protein metabolism is ensured by the reabsorption of the amino acid and its further excretion by body tissues. Where do amino acids come from? They appear after the catalytic breakdown of biologically active substances such as insulin, gastrin, parathyroid hormone. In addition to the processes of glucose catabolism, tissues can produce glucose. Gluconeogenesis occurs within the cortex, and glycolysis occurs in the medulla. It turns out that the conversion of acidic metabolites into glucose regulates blood pH levels.

Endocrine function of the kidneys

The kidneys produce several biologically active substances, which make it possible to consider it as an endocrine organ. Granular cells of the juxtaglomerular apparatus release renin into the blood when blood pressure in the kidney decreases, sodium content in the body decreases, and when a person moves from a horizontal to a vertical position. The level of renin release from cells into the blood also varies depending on the concentration of Na+ and C1- in the area of the macula densa of the distal tubule, providing regulation of electrolyte and glomerular-tubular balance. Renin is synthesized in granular cells of the juxtaglomerular apparatus and is a proteolytic enzyme. In the blood plasma, it splits off from angiotensinogen, located mainly in the α2-globulin fraction, a physiologically inactive peptide consisting of 10 amino acids, angiotensin I. In the blood plasma, under the influence of the angiotensin-converting enzyme, 2 amino acids are split off from angiotensin I, and it turns into an active vasoconstrictor substance angiotensin II. It increases blood pressure due to the constriction of arterial vessels, increases the secretion of aldosterone, increases the feeling of thirst, and regulates sodium reabsorption in the distal tubules and collecting ducts. All of these effects help normalize blood volume and blood pressure.

The kidney synthesizes plasminogen activator - urokinase. Prostaglandins are produced in the renal medulla. They participate, in particular, in the regulation of renal and general blood flow, increase the excretion of sodium in the urine, and reduce the sensitivity of tubular cells to ADH. Kidney cells extract from the blood plasma the prohormone formed in the liver - vitamin D3 - and convert it into a physiologically active hormone - active forms of vitamin D3. This steroid stimulates the formation of calcium-binding protein in the intestines, promotes the release of calcium from bones, and regulates its reabsorption in the renal tubules. The kidney is the site of production of erythropoietin, which stimulates erythropoiesis in the bone marrow. The kidney produces bradykinin, which is a strong vasodilator.

Metabolic kidney function

The kidneys are involved in the metabolism of proteins, lipids and carbohydrates. The concepts of “kidney metabolism,” i.e., the metabolic process in their parenchyma, through which all forms of kidney activity are carried out, and “metabolic function of the kidneys,” should not be confused. This function is due to the participation of the kidneys in ensuring the constant concentration of a number of physiologically significant organic substances in the blood. Low molecular weight proteins and peptides are filtered in the renal glomeruli. Cells in the proximal nephron break them down into amino acids or dipeptides and transport them across the basal plasma membrane into the blood. This helps restore the amino acid pool in the body, which is important when there is a deficiency of proteins in the diet. With kidney disease, this function may be impaired. The kidneys are capable of synthesizing glucose (gluconeogenesis). During prolonged fasting, the kidneys can synthesize up to 50% of the total amount of glucose produced in the body and entering the blood. The kidneys are the site of synthesis of phosphatidylinositol, an essential component of plasma membranes. The kidneys can use glucose or free fatty acids for energy expenditure. When the level of glucose in the blood is low, kidney cells consume fatty acids to a greater extent; with hyperglycemia, glucose is predominantly broken down. The importance of the kidneys in lipid metabolism is that free fatty acids can be included in the composition of triacylglycerol and phospholipids in the kidney cells and enter the blood in the form of these compounds.

Principles of regulation of reabsorption and secretion of substances in renal tubular cells

One of the features of the kidneys is their ability to change the intensity of transport of various substances over a wide range: water, electrolytes and non-electrolytes. This is an indispensable condition for the kidney to fulfill its main purpose - to stabilize the basic physical and chemical indicators of internal fluids. The wide range of changes in the rate of reabsorption of each of the substances necessary for the body filtered into the lumen of the tubule requires the existence of appropriate mechanisms for regulating cell functions. The action of hormones and mediators that affect the transport of ions and water is determined by changes in the functions of ion or water channels, carriers, and ion pumps. There are several known variants of the biochemical mechanisms by which hormones and mediators regulate the transport of substances by the nephron cell. In one case, the genome is activated and the synthesis of specific proteins responsible for the implementation of the hormonal effect is enhanced; in the other case, changes in permeability and pump operation occur without the direct participation of the genome.

Comparison of the features of the action of aldosterone and vasopressin allows us to reveal the essence of both variants of regulatory influences. Aldosterone increases Na+ reabsorption in renal tubular cells. From the extracellular fluid, aldosterone penetrates through the basal plasma membrane into the cell cytoplasm, connects with the receptor, and the resulting complex enters the nucleus (Fig. 12.11). In the nucleus, DNA-dependent tRNA synthesis is stimulated and the formation of proteins necessary to increase Na+ transport is activated. Aldosterone stimulates the synthesis of components of the sodium pump (Na +, K + -ATPase), enzymes of the tricarboxylic acid cycle (Krebs) and sodium channels through which Na + enters the cell through the apical membrane from the lumen of the tubule. Under normal physiological conditions, one of the factors limiting Na+ reabsorption is the permeability of the apical plasma membrane to Na+. An increase in the number of sodium channels or the time of their open state increases the entry of Na into the cell, increases the Na+ content in its cytoplasm and stimulates active Na+ transport and cellular respiration.

The increase in K+ secretion under the influence of aldosterone is due to an increase in potassium permeability of the apical membrane and the entry of K from the cell into the lumen of the tubule. Enhanced synthesis of Na+, K+-ATPase under the action of aldosterone ensures increased entry of K+ into the cell from the extracellular fluid and favors the secretion of K+.

Let us consider another version of the mechanism of cellular action of hormones using the example of ADH (vasopressin). It interacts from the side of the extracellular fluid with the V2 receptor, localized in the basal plasma membrane of the cells of the terminal parts of the distal segment and collecting ducts. With the participation of G-proteins, the enzyme adenylate cyclase is activated and 3,5"-AMP (cAMP) is formed from ATP, which stimulates protein kinase A and the insertion of water channels (aquaporins) into the apical membrane. This leads to increased water permeability. Subsequently, cAMP is destroyed by phosphodiesterase and converted into 3"5"-AMP.