Metabolic function of the kidneys. What do the kidneys provide? Gas transport by blood

Hundreds of suppliers bring hepatitis C medicines from India to Russia, but only M-PHARMA will help you buy sofosbuvir and daclatasvir, while professional consultants will answer any of your questions throughout the therapy.

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 disturbed for various reasons: endocrine diseases, tumors, congenital anomalies, metabolic shifts. Metabolic nephropathy in children is diagnosed more often 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 are also dysmetabolic nephropathy, which is understood as a number of metabolic disorders, accompanied by crystalluria (the formation of salt crystals detected during urinalysis).

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

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

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

Development factors

Predisposing factors for the development of metabolic nephropathy are the following pathologies:

Among metabolic nephropathies, subspecies are distinguished, which are characterized by the presence of salt crystals in the urine. Children often have calcium oxalate nephropathy, where the hereditary factor affects 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 violation of the metabolism of uric acid, urate nephropathy is diagnosed.

Congenital metabolic disorders occur in children experiencing hypoxia during fetal development. In adulthood, the pathology has an acquired character. In time, the disease can be recognized by its characteristic signs.

Symptoms and types of disease

Violation of the kidneys in case of failure in metabolism entails the following manifestations:

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

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

Diagnostics

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

Important! To conduct a biochemical analysis, daily urine is required, and for the reliability of the result, you need to refrain from taking alcohol, spicy, fatty, sweet foods, and products that stain urine. A day before the test, you should stop taking uroseptics and antibiotics and warn the 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 state of the body as a whole can be judged by a blood test. Depending on the results of the diagnosis of kidney disease, treatment is prescribed. Therapy will also be directed to the organs that have become the root cause of the metabolic failure.

Treatment and prevention

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

The selection of medicines is carried out only by a doctor. If, for example, nephropathy is caused by inflammation, the need to take antibiotics is not ruled out, and if the increased radioactive background will help eliminate the negative factor or, if necessary, radiation therapy, the introduction of radioprotectors.

Preparations

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

Calcium metabolism normalizes the drug Ksidifon. It prevents the formation of insoluble calcium compounds with phosphates, oxalates, promotes the removal of heavy metals.

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

Dimephosphone normalizes the acid-base balance in case of impaired renal function due to the development of acute respiratory infections, lung diseases, diabetes mellitus, rickets.

Diet

The generalizing factor of therapy is:

the need to comply with the diet and drinking regimen;
rejection of bad habits.

The basis of dietary nutrition in metabolic nephropathy is a sharp restriction of sodium chloride, products containing oxalic acid, and cholesterol. As a result, a decrease in puffiness is achieved, 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 the addition of salt and baking powder;
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 the addition of radish, spinach, sorrel, garlic;
berries, fruit desserts;
tea, coffee (weak and no more than 2 cups a day), juices, rosehip broth.

From the diet it is necessary to eliminate:

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

Many dishes can be prepared from the number of allowed foods, so sticking to a diet is easy.

An important condition for treatment is compliance with the drinking regimen. A large amount of fluid helps to eliminate stagnation of urine and removes salt from the body. The constant manifestation of moderation in eating and the rejection of bad habits will help normalize kidney function, prevent the onset of the 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 best method of therapy. Any attempt at self-treatment 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 constancy of the concentration in the blood of a number of physiologically significant organic substances. In the renal glomeruli, low molecular weight proteins and peptides are filtered. In the proximal nephron, they are cleaved to amino acids or dipeptides and transported through the basement plasma membrane into the blood. With kidney disease, this function may be impaired. The kidneys are able to synthesize glucose (gluconeogenesis). With prolonged starvation, the kidneys can synthesize up to 50% of the total amount of glucose formed in the body and entering the blood. For energy expenditure, the kidneys can use glucose or free fatty acids. With a low level of glucose in the blood, kidney cells consume fatty acids to a greater extent, with hyperglycemia, glucose is predominantly broken down. The significance of the kidneys in lipid metabolism lies in the fact that free fatty acids can be included in the composition of triacylglycerol and phospholipids in the cells of the kidneys and enter the blood in the form of these compounds.

Regulation of kidney activity

Historically, of interest are experiments carried out with irritation or cutting of the efferent nerves that innervate the kidneys. Under these influences, diuresis changed insignificantly. 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 pain stimulation or to a water load, and diuresis also changed under unconditioned reflex effects. These experiments gave reason to assume that the reflex effects on the kidneys are carried out not so much through the efferent nerves of the kidneys (they have relatively little effect on diuresis), but there is a reflex release of hormones (ADH, aldosterone) 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 urination: 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 the liquids of the internal environment. The central nervous system receives information about the state of the internal environment, the integration of signals occurs and the regulation of the activity of the kidneys is ensured. Anuria, which occurs with pain irritation, 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 water reabsorption.

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 ADH secretion decreases. Impulses coming along adrenergic fibers stimulate sodium transport, and along cholinergic fibers they activate glucose reabsorption and secretion of organic acids. The mechanism of change in urination with the participation of adrenergic nerves is due to the activation of adenylate cyclase and the formation of cAMP in the cells of the tubules. 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 an important role as an informational link in the ionic regulation system and ensure the implementation of reno-renal reflexes. As for the humoral-hormonal regulation of urination, 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-dioxycholecalciferol, which is synthesized in the skin under the action 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 affect the phosphorus-calcium metabolism in the body. Therefore, in diseases of the kidneys, when the processes of hydroxylation of vitamin D 3 are disturbed, 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 able to affect 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 release of PEF depends on the supply of oxygen to the kidneys. If the amount of incoming oxygen decreases, then 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, renal anemia is sometimes observed in kidney diseases.

3. Biosynthesis of proteins. In the kidneys, the processes of biosynthesis of proteins that are necessary for other tissues are actively going on. The components of the blood coagulation system, the complement system and the fibrinolysis system are also synthesized here.

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

4. Protein catabolism. The kidneys are involved in the catabolism of several low molecular weight (5-6 kDa) proteins 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 bloodstream 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, and also with protein biosynthesis. The main way to obtain ATP is oxidative phosphorylation. Therefore, the kidney tissue needs significant amounts of oxygen. The mass of the kidneys is 0.5% of the total body weight, and the oxygen consumption by the kidneys is 10% of the total oxygen supplied.

7.4. REGULATION OF WATER-SALT METABOLISM
AND URINATION

The volume of urine and the content of ions in it is regulated due to the combined action of hormones and 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 decapeptide angiotensin I by partial proteolysis. From angiotensin I, under the action of the enzyme carboxycatepsin, an octapeptide angiotensin II is formed (also by partial proteolysis). It has a vasoconstrictive effect, and also stimulates the production of the hormone of the adrenal cortex - aldosterone.

Aldosterone is a steroid hormone of the adrenal cortex from the group of mineralcorticoids, which provides increased sodium reabsorption from the distal part of the renal tubule due to active transport. It begins to be actively secreted with a significant decrease in the concentration of sodium in the blood plasma. In the case of very low concentrations of sodium in the blood plasma under the action of aldosterone, almost complete removal of sodium from the urine can occur. Aldosterone increases 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) rises (Fig. 19).

Rice. 19. Renin-angiotensin-aldosterone system

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

The production of renin depends on the blood supply to the kidneys. Therefore, with a decrease in blood pressure, renin production increases, and with an increase, it decreases. In kidney pathology, increased renin production 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, up to heart failure. Insufficiency of aldosterone leads to a significant loss of sodium, chlorides and water and a decrease in blood plasma volume. In the kidneys, the secretion of H + and NH 4 + is simultaneously disrupted, which can lead to acidosis.

The renin-angiotensin-aldosterone system works in close contact with another system for 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 carboxycatepsin - this enzyme simultaneously affects both systems of regulation of vascular tone, which leads to an increase in blood pressure (Fig. 21). Carboxythepsin inhibitors are used medicinally 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. The relationship of renin-angiotensin-aldosterone
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 narrowing of peripheral vessels (arterioles), resulting in increased blood pressure. In the kidneys, vasopressin increases the rate of water reabsorption from the anterior part of the distal convoluted tubules and collecting ducts. As a result, the relative concentration of Na, C1, P and total N increases. Vasopressin secretion increases with an increase in the osmotic pressure of the blood plasma, for example, with an increase in salt intake or dehydration of the body. It is believed that the action of vasopressin is associated with the phosphorylation of proteins in the apical membrane of the kidney, resulting in an increase in its permeability. With damage to the pituitary gland, in case of impaired secretion of vasopressin, diabetes insipidus is observed - a sharp increase in urine volume (up to 4-5 liters) with a low specific gravity.

Natriuretic factor(NUF) is a peptide that is produced in atrial cells in the hypothalamus. It is a hormone-like substance. Its targets are the 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 NHF on tubule cells is a decrease in Na + reabsorption, i.e. natriuria develops.

Parathormone- a hormone of the parathyroid gland of a protein-peptide nature. It has a membrane mechanism of action via cAMP. Affects the removal of salts from the body. In the kidneys, parathyroid hormone enhances 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 content of phosphates in the blood plasma and a decrease in the content of Ca 2+ in the plasma.

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

The hormone of the adrenal glands affects 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 excretion of water, mainly through the skin.

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

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

Pathological dehydration of tissues 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 intense 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; urine becomes very dilute and has a low specific gravity. However, the restriction of drinking in 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 disorders of hormonal regulation can cause hypertension?

8. Specify the causes of water retention in the body.

9. What causes diabetes insipidus?

The kidneys serve as a natural "filter" of the blood, which, when properly functioning, remove harmful substances from the body. The regulation of kidney function in the body is vital for the stable functioning of the body and the immune system. For a comfortable life, two organs are needed. There are times when a person stays with one of them - it is possible to live, but you will have to depend on hospitals all your life, and 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.

The structure of the kidneys

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, while the left kidney is higher. Such is nature: above the right kidney is the liver, which does not allow it to move 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 create the medulla and cortex and provide the outer and inner layers. Parenchyma - a set of basic elements that are limited to the connective base and shell. The accumulation system is represented by a small renal calyx, which forms a large one in the system. The connection of the latter forms a 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 clearing toxins, microbes and other harmful substances from toxins.

During the day, the kidneys and liver process and purify the blood from slagging, toxins, remove decay products. More than 200 liters of blood per day are pumped through the kidneys, which ensures its purity. Negative microorganisms penetrate the blood plasma and go to the bladder. So what do the kidneys do? Given the amount of work that the kidneys provide, a person could not exist without them. The main functions of the kidneys perform the following work:

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

Excretory function - as the main duty of the kidneys

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

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 the water-salt metabolism, and participate in the maintenance of blood pressure. The main task lies precisely on this function of the kidneys. In addition, they regulate the amount of salts, proteins in the liquid and provide 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 collection tube. Nephrons perform responsible 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 state of the kidney.

During secretion, the metabolic product, the balance of electrolytes, is removed from the blood. Filtration is the process by which a substance enters the urine. In this case, the fluid that has passed through the kidneys resembles blood plasma. In filtration, an indicator is distinguished 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 output for a specific time. The ability to absorb important elements from the urine into the blood is called reabsorption. These elements are proteins, amino acids, urea, electrolytes. The reabsorption rate changes indicators from the amount of liquid in food and the health of the organ.

What is the secretory function?

Once again, we note that our homeostatic organs control the internal mechanism of work and metabolic indicators. They filter the blood, monitor blood pressure, and synthesize biologically active substances. The appearance of these substances is directly related to secretory activity. The process reflects the secretion of substances. Unlike excretory, 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. Consider the term "secretion" in detail, since there are several interpretations in medicine:

synthesis of substances that will subsequently return to the body;
synthesizing 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 whole body.

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

The regulation of the body's acid-base balance is a complex and confusing phenomenon. Maintaining a stable pH in the blood is due to the "filter" and buffer systems. They remove acid-base components, which normalizes their natural amount. When the pH of the blood changes (this phenomenon is called tubular acidosis), alkaline urine is formed. Tubular acidosis poses a threat to health, but special mechanisms in the form of secretion of h +, ammoniogenesis and gluconeogenesis, stop the oxidation of urine, reduce the activity of enzymes and are involved in the conversion of acid-reactive substances into glucose.

Role of metabolic function

The metabolic function of the kidneys in the body occurs through the synthesis of biologically active substances (renin, erythropoietin and others), since 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 the metabolism of proteins is provided by the reabsorption of amino acids and its further excretion by body tissues. Where do amino acids come from? Appear after catalytic cleavage 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, while glycolysis occurs in the medulla. It turns out that the conversion of acidic metabolites into glucose regulates blood pH.

Endocrine function of the kidneys

The kidneys produce several biologically active substances that allow it to be considered as an endocrine organ. Granular cells of the juxtaglomerular apparatus secrete renin into the blood with a decrease in blood pressure in the kidney, a decrease in the sodium content in the body, when a person moves from a horizontal to a vertical position. The level of renin release from cells into the blood also changes depending on the concentration of Na + and C1- in the area of the dense spot of the distal tubule, providing regulation of electrolyte and glomerular-tubular balance. Renin is synthesized in the granular cells of the juxtaglomerular apparatus and is a proteolytic enzyme. In blood plasma, it cleaves from angiotensinogen, which is mainly in the α2-globulin fraction, a physiologically inactive peptide consisting of 10 amino acids, angiotensin I. In blood plasma, under the influence of angiotensin-converting enzyme, 2 amino acids are cleaved from angiotensin I, and it turns into an active vasoconstrictor substance angiotensin II. It increases blood pressure due to vasoconstriction, increases aldosterone secretion, increases thirst, and regulates sodium reabsorption in the distal tubules and collecting ducts. All of these effects contribute to the normalization of blood volume and blood pressure.

The plasminogen activator, urokinase, is synthesized in the kidney. Prostaglandins are produced in the renal medulla. They are involved, 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 the prohormone formed in the liver - vitamin D3 - from the blood plasma and convert it into a physiologically active hormone - active forms of vitamin D3. This steroid stimulates the formation of calcium-binding protein in the intestine, promotes the release of calcium from the 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 powerful vasodilator.

Metabolic function of the kidneys

The kidneys are involved in the metabolism of proteins, lipids and carbohydrates. The concepts of "kidney metabolism", i.e., the process of metabolism in their parenchyma, due to 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 constancy of the concentration in the blood of a number of physiologically significant organic substances. In the renal glomeruli, low molecular weight proteins and peptides are filtered. The cells of the proximal nephron break them down to amino acids or dipeptides and transport them through the basement plasma membrane into the blood. This contributes to the restoration of the amino acid fund 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 able to synthesize glucose (gluconeogenesis). With prolonged starvation, the kidneys can synthesize up to 50% of the total amount of glucose formed in the body and entering the blood. The kidneys are the site of the synthesis of phosphatidylinositol, an essential component of plasma membranes. For energy expenditure, the kidneys can use glucose or free fatty acids. With a low level of glucose in the blood, kidney cells consume fatty acids to a greater extent, with hyperglycemia, glucose is predominantly broken down. The significance of the kidneys in lipid metabolism lies in the fact that free fatty acids can be included in the composition of triacylglycerol and phospholipids in the cells of the kidneys and enter the blood in the form of these compounds.

Principles of regulation of reabsorption and secretion of substances in the cells of the renal tubules

One of the features of the work of the kidneys is their ability to change in a wide range of intensity of transport of various substances: water, electrolytes and non-electrolytes. This is an indispensable condition for the kidney to fulfill its main purpose - the stabilization of the main physical and chemical indicators of the liquids of the internal environment. A 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 variants of 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 the reabsorption of Na + in the cells of the renal tubules. From the extracellular fluid, aldosterone penetrates through the basal plasma membrane into the cytoplasm of the cell, connects to 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 sodium pump components (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 Na+ permeability of the apical plasma membrane. 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 content of Na+ in its cytoplasm, and stimulates active transfer of Na+ and cellular respiration.

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

Let us consider another variant of the mechanism of the cellular action of hormones using the example of ADH (vasopressin). It interacts from 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 adenylate cyclase enzyme is activated and 3",5"-AMP (cAMP) is formed from ATP, which stimulates protein kinase A and the incorporation of water channels (aquaporins) into the apical membrane. This leads to an increase in water permeability. Subsequently, cAMP is destroyed by phosphodiesterase and converted into 3"5"-AMP.