The kidneys perform very important functions in the human body. This is the main one that produces urine. This is a paired organ, but with one kidney a person can lead a completely normal life. It even happened that a person from birth had one or even three kidneys. But the life of a person with one kidney can be quite difficult, since in this case there is a threat of acquiring infections.

The structure of the kidneys

A healthy person should have two kidneys - right and left. In its shape, this organ resembles a bean. Its main function is urinary. But besides it, the kidneys perform a lot of other actions.

The kidneys are located in the lumbar region. But they are not on the same level, since the right kidney is located lower than the left. The thing is that on the other side there is a liver, which does not let the kidney go up.

But in size, both kidneys are approximately equal, about 12 centimeters long and 3-4 centimeters thick. The width can be about 5 centimeters, and the weight can be from 125 to 200 grams. The right kidney may be slightly smaller than the left.

The structure of the kidney is the nephron. If a person is healthy, then in his kidney there can be more than a million nephrons. It is in these units that a liquid such as is formed. The structure of the nephron is as follows:

• Inside each nephron is a renal corpuscle;
• Inside the renal corpuscle are tangles of capillaries;
• The capillaries are surrounded by a capsule of two layers;
• Inside the capsule is lined with epithelium;
• Outside, the capsule is covered by a membrane and tubules.

Nephrons are divided into three types. Their varieties depend on the location of the tubules and their structure. Nephrons are of the following types:

• intracortical
• Surface
• Juxtamedullary.

In the kidney, blood circulation continues all the time. Blood is supplied to this organ by an artery, which in the organ itself is divided into arterioles. They bring blood to each glomerulus.
Urine is formed during the following actions of the body:

• At the first stage, fluid and blood plasma are filtered in the glomeruli.
• The urine that was formed (primary) is collected in special barrels, where the body absorbs all the nutrients from it.

Tubular secretion occurs, in which all excess substances move into the urine.

Basic functions of the kidneys

The function of the kidneys in the human body is not unique. This body performs the following functions:

• excretory
• Ion regulating
• Endocrine
• Osmoregulatory
• metabolic
• Blood Creation Function
• concentration.

For 24 hours, the kidneys pump all the blood that is in the body. This process is repeated an unlimited number of times. For 60 seconds, the body pumps about a liter of blood. But the kidneys are not limited to one pumping. During this time, they manage to select from the composition of the blood all substances harmful to the human body, including toxins, microbes and other slags.

After that, the decay products enter the plasma. After that, they go into the ureters, from where they enter the bladder. Together with urine, all harmful substances leave the human body.

The ureters have a special valve that prevents toxins from entering the body a second time. This is due to the fact that the valve is designed in such a way that it opens only in one direction.

The kidneys perform a huge amount of work per day. They pump over 1000 liters of blood and, moreover, manage to completely purify it. And this is very important, because the blood reaches every cell of the human body and it is extremely necessary that it be clean and not contain harmful agents.

excretory function

The essence of the excretion function is that the structure of the kidneys allows you to remove decay products and other harmful substances from the blood, the use of which in the body is no longer possible. The body also removes the following substances from the body:

• Toxins (mainly ammonia)
• Excess liquid
• mineral salts
• Too much glucose or amino acids.

If this function is given to changes, then various pathological deviations can occur in the body, which is very dangerous for the health and full life of a person.

Homeostatic and metabolic functions

The kidneys are very efficient in regulating the volume of blood and interstitial fluid. This is their homeostatic function. They are involved in the regulation of the balance of ions. The kidneys influence the volume of fluid between cells by regulating its ionic state.

The metabolic function of the kidneys is manifested in the metabolism, namely, carbohydrates and lipids. There is also their direct participation in processes such as gluconeogenesis (if a person is starving) or the breakdown of peptides and amino acids.

It is only in the kidneys that vitamin D is converted to its effective D3 form. Such a vitamin at the initial stage enters the body through skin cholesterol, which is produced under the influence of sunlight.

It is in the kidneys that active protein synthesis occurs. And already this element is needed by the whole body for the construction of new cells.

Protective and endocrine functions

The kidneys are also the last line of defense of the body. Their protective function helps to remove from the body those substances that can harm it (alcohol, drugs, including nicotine, medicines).

The kidneys synthesize the following substances:

• Renin is an enzyme that regulates the amount of blood in the body.
• Calcitriol is a hormone that controls calcium levels.
• Erythropoietin is a hormone that causes blood synthesis in the bone marrow.
• Prostaglandins are a substance that controls blood pressure.

Health impact

If there is a drop in the efficiency of the kidneys, then this may mean that some kind of pathology has arisen. This condition will become very dangerous for the body. In some cases, there may be a slowdown in the process of urination, which entails the problematic removal of toxic substances and decay products from the body.

Kidney failure can lead to violations of the water-salt or acid-base balance.
There can be many reasons for this. Here are just a few of them:

• Pathologies in the work of the urinary system.
• The appearance of inflammation.
• The existence of diseases that affect the immune system.
• metabolic dysfunction.
• that are chronic.
• Vascular diseases.
• A blockage in the urinary tract.

Damage to the tissues of the kidneys of various kinds of toxins (alcohol, drugs, long-term medication).

The most severe cases are accompanied by possible blockages in the urinary tract, which does not allow urine to leave the body naturally. At the next stage, damage to organs can be observed.

What's happening

If about 80% of the nephrons in the kidneys are damaged, symptoms of kidney failure may occur. And they can be quite unpredictable and varied.

In the first stages, polyuria appears (high sensitivity to changes in food).

At the next stages of the disease, the exchange of calcium and phosphorus is disturbed, which practically paralyzes the effective functioning of the parathyroid glands, leading to the formation of diseases such as osteofibrosis and osteoporosis.

If a lot of nephrons succumbed to damage, then a protein deficiency occurs. And because of this, dystrophy occurs.
The metabolism of fats and carbohydrates also suffers.

There are failures in the metabolism of fats, which leads to an excess of atherogenic fats in the body (and atherosclerosis, as a result).
Reduces the efficiency of the blood circulation process.

Dysfunction in the work of the heart and vascular system begins to manifest itself only when a large amount of protein metabolism products, which are toxic, accumulate in the blood.

The nervous system is also affected, but its symptoms develop gradually. First, a person is haunted by fatigue, fatigue from work. Then even stupor or coma can be observed, as a result of a decrease in cognitive function.

Very often, due to disturbances in the functioning of the kidneys, arterial hypertension, or rather, its malignant form, manifests itself. You can also observe swelling, which first appear on the face near the eyes, and then move to the trunk.

If the protective and excretory functions are impaired, a lot of toxic substances accumulate in the body, which also affect the functioning of the digestive system. This is manifested in the lack of appetite, a decrease in the stress resistance of the digestive system.

Preventive actions

Kidneys suffer due to chronic diseases, high blood pressure, extra pounds in weight. They do not tolerate drugs that are made on an unnatural basis and hormonal contraceptives. The functions of this organ are disrupted due to a sedentary lifestyle (because of this, disturbances occur in salt and water metabolism), as a result, stones can form in.

The kidneys react very sharply to poisoning, traumatic shocks, various infections and diseases that are associated with obstruction of the urinary tract.

For the kidneys to perform their functions well, at least 2 liters of water (or liquid in its various forms) must enter the body per day. To maintain the tone of this body, you can drink green tea, boil parsley leaves, use cranberry or lingonberry fruit drinks. You can drink just pure water with lemon or honey and this will already become a good medicine for the kidneys.

The above drinks prevent the formation of stones and remove urine faster.

Conversely, alcohol and coffee negatively affect kidney function. They destroy its cells and tissues, dehydrate the body. And if you drink a lot of mineral water, then stones can form in the kidneys. Mineral water can be used for a long time only for medicinal purposes and with the permission of a doctor.

It is important to be careful with salty foods. Too much salt in food is dangerous for humans. Its maximum possible amount reaches 5 grams, while some people can eat up to 10 grams.

While watching the video, you will learn about the function of the kidneys.

The functionality of the kidneys is very important for the proper functioning of everything. Violation of only one of the functions of this organ leads to pathological changes in all human systems.

(Fig. 1). They are bean-shaped and located in the retroperitoneal space on the inner surface of the posterior abdominal wall on both sides of the spinal column. Weight of each kidney an adult is about 150 g, and its size roughly corresponds to a clenched fist. Outside, the kidney is covered with a dense connective tissue capsule that protects the delicate internal structures of the organ. The renal artery enters the gate of the kidney, the renal vein, lymphatic vessels and the ureter emerge from them, originating from the pelvis and removing the final urine from it into the bladder. On a longitudinal section in the tissue of the kidney, two layers are clearly distinguished.

Rice. 1. Structure of the urinary system: words: kidney and ureters (paired organs), bladder, urethra (indicating the microscopic structure of their walls; SMC - smooth muscle cells). The composition of the right kidney shows the renal pelvis (1), the medulla (2) with pyramids opening into the cups of the cups of the pelvis; cortical substance of the kidneys (3); right: main functional elements of the nephron; A - juxtamedullary nephron; B - cortical (intracortical) nephron; 1 - renal body; 2 - proximal convoluted tubule; 3 — a loop of Henle (consisting of three departments: a thin descending part; a thin ascending part; a thick ascending part); 4 — a dense spot of a distal tubule; 5 - distal convoluted tubule; 6 connecting tubule; 7-collective duct of the medulla of the kidney.

outer layer, or cortical gray-red substance, kidneys has a granular appearance, as it is formed by numerous microscopic structures of red color - renal corpuscles. The inner layer, or medulla, kidneys consists of 15-16 renal pyramids, the tops of which (renal papillae) open into small renal calyces (large calyces of the pelvis). In the medulla, the kidneys secrete the outer and inner medulla. The parenchyma of the kidney is made up of the renal tubules, and the stroma is made up of thin layers of connective tissue through which the vessels and nerves of the kidneys pass. The walls of the cups, cups, pelvis and ureters have contractile elements that help move urine into the bladder, where it accumulates until it is emptied.

The value of the kidneys in the human body

The kidneys perform a number of homeostatic functions, and the idea of ​​them only as an organ of excretion does not reflect their true significance.

TO kidney function their involvement in the regulation

• the volume of blood and other fluids of the internal environment;
• constancy of the osmotic pressure of the blood;
• constancy of the ionic composition of the liquids of the internal environment and the ionic balance of the body;
• acid-base balance;
• excretion (excretion) of the end products of nitrogen metabolism (urea) and foreign substances (antibiotics);
• excretion of excess organic substances received with food or formed during metabolism (glucose, amino acids);
• blood pressure;
• blood clotting;
• stimulation of the process of formation of red blood cells (erythropoiesis);
• secretion of enzymes and biologically active substances (renin, bradykinin, urokinase)
• metabolism of proteins, lipids and carbohydrates.

Kidney Functions

The functions of the kidneys are diverse and important for the life of the body.

Excretory (excretory) function- the main and most well-known function of the kidneys. It consists in the formation of urine and the removal with it from the body of metabolic products of proteins (urea, ammonium salts, creaginine, sulfuric and phosphoric acids), nucleic acids (uric acid); excess water, salts, nutrients (micro and macro elements, vitamins, glucose); hormones and their metabolites; medicinal and other exogenous substances.

However, in addition to excretion, the kidneys perform a number of other important (non-excretory) functions in the body.

homeostatic function kidney is closely related to the excretory and is to maintain the constancy of the composition and properties of the internal environment of the body - homeostasis. The kidneys are involved in the regulation of water and electrolyte balance. They maintain an approximate balance between the amount of many substances excreted from the body and their entry into the body, or between the amount of the resulting metabolite and its excretion (for example, water in and out of the body; incoming and outgoing electrolytes of sodium, potassium, chlorine, phosphates, etc.) . Thus, the body maintains water, ionic and osmotic homeostasis, a state of isovolumy (relative constancy of the volumes of circulating blood, extracellular and intracellular fluid).

By excreting acidic or basic products and regulating the buffer capacities of body fluids, the kidneys, together with the respiratory system, ensure the maintenance of the acid-base state and isohydria. The kidneys are the only organ that secretes sulfuric and phosphoric acids, which are formed during protein metabolism.

Participation in the regulation of systemic blood pressure - The kidneys play the main role in the mechanisms of long-term regulation of blood pressure through changes in the excretion of water and sodium chloride from the body. Through the synthesis and secretion of various amounts of renin and other factors (prostaglandins, bradykinin), the kidneys are involved in the mechanisms of rapid regulation of blood pressure.

Endocrine function of the kidneys - this is their ability to synthesize and release into the blood a number of biologically active substances necessary for the life of the body.

With a decrease in renal blood flow and hyponatremia, renin is formed in the kidneys - an enzyme, under the action of which the angiotensin I peptide, a precursor of the powerful vasoconstrictor substance angiotensin II, is cleaved from the a 2 -globulin (angiotensinogen) of the blood plasma.

In the kidneys, bradykinin and prostaglandins (A 2 , E 2) are formed, which dilate blood vessels and lower blood pressure, the enzyme urokinase, which is an important part of the fibrinolytic system. It activates plasminogen, which causes fibrinolysis.

With a decrease in arterial oxygen tension in the kidneys, erythropoietin is formed - a hormone that stimulates erythropoiesis in the red bone marrow.

With insufficient formation of erythropoietin in patients with severe nephrological diseases, with removed kidneys or for a long time undergoing hemodialysis procedures, severe anemia often develops.

The kidneys complete the formation of the active form of vitamin D 3 - calcitriol, which is necessary for the absorption of calcium and phosphates from the intestines and their reabsorption from the primary urine, which ensures an adequate level of these substances in the blood and their deposition in the bones. Thus, through the synthesis and excretion of calcitriol, the kidneys regulate the supply of calcium and phosphates to the body and bone tissue.

Metabolic function of the kidneys is their active participation in the metabolism of nutrients and, above all, carbohydrates. The kidneys, along with the liver, are an organ capable of synthesizing glucose from other organic substances (gluconeogenesis) and releasing it into the blood for the needs of the whole organism. Under fasting conditions, up to 50% of glucose can enter the blood from the kidneys.

The kidneys take part in the metabolism of proteins - the breakdown of proteins reabsorbed from secondary urine, the formation of amino acids (arginine, alanine, serine, etc.), enzymes (urokinase, renin) and hormones (erythropoietin, bradykinin) with their secretion into the blood. In the kidneys, important components of cell membranes of a lipid and glycolipid nature are formed - phospholipids, phosphatidylinositol, triacylglycerols, glucuronic acid and other substances that enter the blood.

Features of blood supply and blood flow in the kidneys

The blood supply to the kidneys is unique compared to other organs.

• Large specific value of blood flow (by 0.4% of body weight, 25% of the IOC)
• High pressure in the glomerular capillaries (50-70 mm Hg. Art.)
• Constancy of blood flow, regardless of fluctuations in systemic blood pressure (Ostroumov-Beilis phenomenon)
• The principle of a double capillary network (2 systems of capillaries - glomerular and peritubular)
• Regional features in the organ: the ratio of the cortical substance: the outer layer of the medulla: the inner layer -> 1: 0.25: 0.06
• The arteriovenous difference in O 2 is small, but its consumption is quite large (55 µmol / min. g)

Rice. Ostroumov-Beilis phenomenon

Ostroumov-Beilis phenomenon- the mechanism of myogenic autoregulation, which ensures the constancy of renal blood flow, regardless of changes in systemic arterial pressure, due to which the value of renal blood flow is maintained at a constant level.

Selection. kidney physiology

Mechanisms of urination

Tubular secretion and its regulation

Mechanisms of urinary excretion and urination

Other kidney functions

The role of the kidneys in the regulation of blood pressure

Selection. kidney physiology

Isolation is the process of freeing the body from metabolic products that cannot be used by the body, foreign and toxic substances, excess water, salts, organic compounds.

Excretory organs include the kidneys, lungs, sweat glands, and the gastrointestinal tract. The lungs emit carbon dioxide, water vapor, some volatile substances: ether vapor, alcohol. The salivary glands, glands of the stomach and intestines are capable of releasing heavy metals when they enter the body, medicinal substances, for example, salicylates, foreign organic compounds; the role of these glands increases with a decrease in kidney function.

The kidney occupies a special place among the excretory organs.

The kidney is a true excretory organ - thanks to its activity, the end products of nitrogen metabolism and foreign substances are excreted: urea, uric acid, creatinine, ammonia.

The kidney carries out the excretion of drugs and excess organic substances that come with food or formed during metabolism, for example, glucose, amino acids.

The kidney is also a regulatory organ - due to the mechanisms of urination, the volumes of circulating blood, intra- and extracellular water, the constancy of osmotic pressure and the ionic composition of plasma and other body fluids are regulated, acid-base balance (ABR) is regulated.

Due to the production of biologically active substances and hormones, the kidney is involved in the regulation of systemic arterial pressure, erythropoiesis, and hemocoagulation.

Mechanisms of urination

Urine It is formed in the kidneys from the blood, and the kidney is one of the most intensively supplied organs - every minute 1/4 of the total volume of blood ejected by the heart passes through the kidney. The main structural and functional unit of the kidney, which provides the formation of urine, is nephron. The kidney of humans and many mammals contains about 1.2 million nephrons. However, not all nephrons work in the kidney at the same time, there is a certain periodicity in the functioning of individual nephrons, when some of them function, while others do not. This periodicity ensures the reliability of the kidney due to functional duplication. In this regard, an important indicator of the functional activity of the kidney is the mass of active nephrons at a particular point in time.

Diagram of the structure of the nephron. - interlobar artery, 2 - interlobar vein, 3 - arcuate venule, 5 - interlobular arteriole, 6 - interlobular venule, 7 - afferent arteriole, 8 - efferent arteriole, 9 - vascular glomerulus, 10 - proximal convoluted tubule, 11 - direct descending vessel , 12 - direct ascending vessel, 13 - loop of Henle, 14 - distal convoluted tubule, 15 - collecting duct.

The nephron consists of several series-connected departments located in the cortex and medulla of the kidney.

1) vascular glomerulus. Outside, the glomeruli are covered with a two-layer Bowman-Shumlyansky capsule.

2) Main or proximal tubule, starting from the cavity of the capsule with a convoluted part, which then passes into the straight part of the tubule. The proximal cells on the apical membrane have a brush border of microvilli covered with glycocalyx. The proximal part is located in the cortex, where it passes into the loop of Henle.

3) Thin descending loop of Henle, descending into the medulla of the kidney, where it turns 180 ° and passes into the ascending part, which is the beginning of the distal tubules.

4) Distal tubule, consisting of an ascending part, a loop of Henle or a straight section and a convoluted part. The distal convoluted tubules, through a short connecting section, flow into the renal cortex into the next section of the nephron - the collecting ducts.

5) Collecting tubes descend from the renal cortex deep into the medulla, merge into the excretory ducts that open into the cavity of the pelvis.

According to the peculiarities of the localization of the glomeruli in the renal cortex, the structure of the tubules and the characteristics of the blood supply, three types of nephrons are distinguished: superficial, intracortical and juxtamedullary .

Superficial nephrons have glomeruli superficially located in the cortex, the shortest loop of Henle, 20-30% of them. Intracortical nephrons, the glomeruli of which are located in the middle part of the renal cortex, are the most numerous (60-70%) and play the main role in the processes of urine ultrafiltration. There are much fewer juxtamedullary nephrons (10-15%), their glomeruli are located at the border of the cortical and medulla of the kidney, the efferent arterioles are wider than the afferent ones, the loops of Henle are the longest and descend almost to the top of the papilla of the pyramids.

Mechanism of urination consists of three main processes:

1) glomerular ultrafiltration from blood plasma of water and low molecular weight components with the formation primary urine;

2) tubular reabsorption (reabsorption into the blood) of water and substances necessary for the body from primary urine;

3) tubular secretion of ions, organic substances of endogenous and exogenous nature.

Filtration is the initial and main stage in the formation of urine. Filtration is determined, on the one hand, by the magnitude of the hydrostatic pressure that promotes the release of fluid from the capillary, and, on the other hand, by the magnitude of the oncotic pressure created by large molecular proteins dissolved in the plasma, which prevent the exit of fluid from the capillaries.

The endothelial cells of the capillaries of the glomeruli are adapted for the filtration process - there are huge pores up to 40-100 nm in diameter, which let through almost all large blood particles, including proteins, with the exception of blood cells - erythrocytes, leukocytes, platelets. The main barrier to filtration is the basement membrane, which separates capillary endothelial cells from podocytes.

An additional filter is podocytes - epithelial cells of the visceral layer of the capsule. Between the legs of these cells there are diaphragms pierced by pores. Probably, the diameter of these pores also does not exceed 8 nm, and the pores contain anions. All this together leads to the fact that, under normal blood flow, protein permeability is sharply limited. Large protein molecules clog pores and, due to the presence of anionic charges on proteins, do not allow smaller protein molecules to reach the pores.

So, in the process of filtration, together with 120-110 ml of water, all low-molecular substances that freely pass through the filtration surface are filtered, with the exception of most of the proteins and blood cells. Therefore, the ultrafiltrate resembles plasma in terms of the concentration of substances.

Tubular reabsorption and its regulation. All valuable, necessary substances are reabsorbed in the renal tubules. So, sodium is reabsorbed by 99%, potassium - by 90%, calcium - by 99%, magnesium - by 94%, chlorine - by 99%, bicarbonates - by 99%, phosphates - by 90%, sulfates - by 69%, glucose (if its content does not exceed the norm) - by 100%, amino acids - by 90%, water - by 99%, urea - by 53%. As a result, the volume of the final urine reaches 1.0-1.5 liters per day. Most of the molecules are reabsorbed in the proximal convoluted tubule, and less in the loop of Henle, in the distal convoluted tubule and collecting ducts. Reabsorption of substances is carried out with the participation of various mechanisms, the main of which is active transport.

proximal reabsorption provides complete absorption of a number of primary urine substances - glucose, protein, amino acids and vitamins. In the proximal sections, 2/3 of the filtered water and sodium are absorbed, large amounts of potassium, chlorine, bicarbonate, phosphate, as well as uric acid and urea. By the end of the proximal section, only 1/3 of the ultrafiltrate volume remains in its lumen.

Suction water occurs passively, along the osmotic pressure gradient and depends on the reabsorption of sodium and chloride. Reabsorption sodium in the proximal section is carried out by both active and passive transport. In the initial section of the tubules, this is an active process.

proximal reabsorption glucose and amino acids carried out with the help of special carriers.

Small amounts of filtered squirrel almost completely reabsorbed in the proximal tubules by pinocytosis.

Distal reabsorption ions and water in volume is much less than the proximal one. However, changing significantly under the influence of regulatory influences, it determines the composition of the final urine and the ability of the kidney to excrete either concentrated or diluted urine (depending on the body's water balance). Active reabsorption occurs in the distal nephron sodium, chlorine, potassium, calcium, phosphates. In the collecting ducts, mainly juxtamedullary nephrons, under the influence of vasopressin, the permeability of the wall to urea and it, due to the high concentration in the lumen of the tubule, passively diffuses into the surrounding interstitial space. Under the influence of vasopressin, the wall of the distal convoluted tubules and collecting ducts becomes permeable to water .

The ability of the kidney to form concentrated or diluted urine is provided by the activity countercurrent multiplying tubular system kidney, which is represented by parallel knees of the loop of Henle and collecting ducts. Urine moves in these tubules in opposite directions (why the system was called countercurrent), and the processes of transport of substances in one knee of the system are enhanced (“multiplied”) due to the activity of the other knee. A decisive role in the operation of the countercurrent mechanism is played by the ascending knee of the loop of Henle, the wall of which is impermeable to water, but actively reabsorbs sodium ions into the surrounding interstitial space. As a result, the interstitial fluid becomes hyperosmotic with respect to the contents of the descending limb of the loop, and towards the top of the loop, the osmotic pressure in the surrounding tissue increases. The wall of the descending knee is permeable to water, which passively moves out of the lumen into the hyperosmotic interstitium. Thus, in the descending knee, the urine becomes more and more hyperosmotic due to the absorption of water, i.e. osmotic equilibrium is established with the interstitial fluid. In the ascending knee, due to the absorption of sodium, the urine becomes less and less osmotic and already hypotonic urine ascends into the cortical section of the distal tubule. However, its amount, due to the absorption of water and salts in the loop of Henle, has significantly decreased.

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.

Some substances are formed in the kidney that are excreted in the urine (hippuric acid, ammonia, etc.), as well as absorbed into the blood (renin, prostaglandins, glucose formed in the kidney, etc.). Hippuric acid is synthesized in tubular cells from benzoic acid and glycocol. In experiments on an isolated kidney, it was shown that when a solution of benzoic acid and glycocol are injected into the renal artery, hippuric acid appears in the urine. In the cells of the tubules, when amino acids, mainly glutamine, are deaminated, ammonia is formed from amino groups. It enters mainly into the urine, but partially penetrates through the basement plasma membrane into the blood, and there is more ammonia in the renal vein than in the renal artery.

OSMOTIC DILUTION AND URINE CONCENTRATION

Only the kidneys of warm-blooded animals have the ability to form urine with a higher osmotic concentration than blood. Many researchers tried to unravel the physiological mechanism of this process, but only in the early 1950s was the hypothesis substantiated that the formation of osmotically concentrated urine is associated with the mechanism of the countercurrent-turning multiplying system of some parts of the nephron.

The principle of countercurrent exchange is quite widespread in nature and is used in technology. Let us consider the mechanism of operation of such a system using the example of blood vessels in the limbs of arctic animals. To avoid large heat losses, the blood in the parallel arteries and veins of the limbs flows in such a way that the warm arterial blood warms the cooled venous blood moving towards the heart (Fig. 204). Arterial blood with a low temperature flows into the foot, which sharply reduces heat transfer. Here such a system functions only as a countercurrent exchanger: in the kidney it has a multiplier effect. For a better understanding of its operation, consider a system consisting of three parallel tubes. Tubes I and II are arcuately connected at one end (Fig. 204, B). The wall common to both tubes has the ability to carry salts, but it does not allow water to pass through. When a liquid with a concentration of 300 mosmol / l is poured into such a system through inlet 1 and it does not flow, then after some time, as a result of the transport of salts in tube I, the liquid will become hypotonic, and in tube II - hypertonic. In the case when the liquid flows continuously through the tubes, the concentration of salts begins. At each horizontal level, the difference in their concentrations due to a single effect of salt transport cannot exceed 200 mosmol/l, however, along the length of the tube, single effects multiply and the system begins to work as a countercurrent multiplier system. Since not only salt but also a certain amount of water is extracted from the liquid in the course of its movement, the concentration of the solution increases more and more as it approaches the bend of the loop. In tube III, regulation

300 300 300 300 zoo 300

200- 250" 300" 350" 400-

Rice. 205. Increased concentration (shown by hatching with increased frequency) of osmotically active substances in various parts of the kidney.

a - the state of antidiuresis; b - the state of water diuresis. Broad arrows indicate the direction of transport of the main substances involved in osmotic concentration; thin arrows - the movement of primary and secondary urine.

the permeability of the walls for water is increased; when the wall begins to pass water, the volume of liquid in it decreases. In this case, water moves towards a higher osmotic concentration. As a result, the concentration of liquid in tube III increases and the volume of liquid contained in it decreases. The concentration of substances in it will depend on a number of conditions, including the operation of the countercurrent multiplier system of tubes I and II. As will be clear from the following presentation, the work of the renal tubules in the process of osmotic concentration of urine is similar to the described model.

Depending on the state of the water balance of the body, the kidneys excrete diluted or concentrated urine. In the process of osmotic concentration of urine in the kidney, all parts of the tubules, vessels of the medulla, and interstitial tissue take part. Of the 100 ml of filtrate formed in the glomeruli, 2/3 of it is reabsorbed towards the end of the proximal segment. The liquid remaining in the tubules contains osmotically active substances in the same concentration as the blood plasma ultrafiltrate, although it differs from it in composition due to the reabsorption of a number of substances in the previous parts of the nephron. Next, the tubular fluid passes from the cortical layer of the kidney into the medulla - into the descending (thin) section of the nephron loop (the loop of Henle) and moves to the top of the renal papilla, where the tubule bends 180 °, and urine passes into the ascending section of the loop, located parallel to its descending department.

The functional significance of the various departments of the loop is ambiguous. When fluid from the proximal tubule enters the thin descending nephron loop, it enters the kidney zone, in the interstitial tissue of which the concentration of osmotically active substances is higher than in the kidney cortex. This increase in osmolar concentration in the outer medulla is due to the activity of the thick ascending nephron loop. Its wall is impermeable to water, and the cells transport C1 "and Na + ions into the interstitial tissue. The wall of the descending loop is permeable to water, and therefore water is absorbed from the lumen of the tubule into the surrounding interstitial tissue of the kidney along the osmotic gradient, and osmotically active substances remain in the lumen this section of the tubule.

The further away from the cortex along the longitudinal axis is the liquid in the descending limb of the loop, the higher its osmolar concentration. In each adjacent section of the descending part of the loop, there is only a slight increase in osmotic pressure, but along the length of the loop, the osmolar concentration gradually increases from 300 mosmol/l to almost 1450 mosmol/l. In other words, at the top of the nephron loop, the osmolar concentration of the fluid increases several times and, at the same time, its volume decreases. With further movement of the fluid along the ascending section of the nephron loop, reabsorption of C1 "and Na + ions occurs, water remains in the lumen of the tubule, therefore, hypotonic fluid always enters the initial parts of the distal convoluted tubule, the concentration of osmotically active substances in which is less than 200 mosmol / l.

Water is reabsorbed from the hypotonic fluid along the osmotic gradient, the osmolar concentration of the fluid in this section increases, i.e., the fluid in the lumen of the tubule becomes isoosmotic. The final concentration of urine occurs in the collecting ducts; they are located parallel to the tubules of the nephron loop, in the medulla of the kidney. As noted above, osmolar concentration increases in the interstitial fluid of the renal medulla. As a result, water is reabsorbed from the fluid of the collecting ducts and the concentration of urine in them increases, balancing with the ever-increasing osmolar concentration of the inner medulla of the kidney. Ultimately, hyperosmotic urine is released, in which the maximum concentration of osmotically active substances can be equal to the osmolar concentration of interstitial fluid at the top of the renal papilla (Fig. 205).

In conditions of water deficiency in the body, the secretion of pituitary antidiuretic hormone (ADH) increases, which increases the permeability of the walls of the end parts of the distal segment and collecting ducts for water.

14 - Human Physiology

In contrast to the outer zone of the renal medulla, where the increase in osmolarity is based mainly on chloride transport, the increase in osmolar concentration in the inner zone of the renal medulla depends on several mechanisms. The accumulation of urea plays a special role in osmotic concentration. The walls of the proximal tubule are permeable to urea. In this section of the nephron, up to 50% of the filtered urea is reabsorbed. However, when extracting fluid from the convoluted distal tubule, it turned out that the urea content even slightly exceeds its amount that came with the filtrate, and is about 110%. It has been shown that there is an intrarenal urea circulation system that is involved in the osmotic concentration of urine. In the lumen of the collecting ducts, due to water reabsorption, the concentration of urea increases, ADH increases the permeability of the collecting ducts in the medulla not only for water, but also for urea. When the permeability of the tubular wall for urea increases, it diffuses into the renal medulla. The constant supply of urea, C1 "and Na + ions, reabsorbed by the cells of the thin ascending nephron loop and collecting ducts, into the inner medulla ensures an increase in the osmotic concentration in the medulla of the kidney. Following an increase in the osmolarity of the interstitial tissue surrounding the collecting ducts, the reabsorption of water from them also increases. and the effectiveness of the osmoregulatory function of the kidney is increased.The change in the permeability of the tubular wall to urea makes it possible to understand why the clearance of urea decreases with a decrease in urination.

The direct blood vessels of the renal medulla, like the tubules of the nephron loop, also form a countercurrent system that plays a very important role in osmotic concentration. Due to the peculiarities of the location of the direct vessels, an effective blood supply to the medulla of the kidney is ensured, but osmotically active substances are not washed out, since the same changes in osmotic concentration are observed in the blood of the direct vessels, as in the thin descending section of the nephron loop. As blood moves, the osmotic concentration gradually increases in it, and during its reverse movement to the kidney cortex, salts and other dissolved substances diffusing through the vascular wall pass into the interstitial tissue. Thus, the concentration gradient of osmotically active substances is preserved, i.e., direct vessels function as a countercurrent system. The speed of blood movement through the direct vessels affects the amount of Na + , SG and urea ions removed from the medulla involved in the creation of the osmotic gradient, and the outflow of reabsorbed water.

Under water load, the relative proximal reabsorption of ions and water does not change, and the same amount of fluid enters the distal nephron as without load. At the same time, the wall of the distal parts of the renal tubules remains waterproof, and the cells continue to reabsorb sodium salts from the flowing urine; in this case, hypotonic urine is released, the concentration of osmotically active substances in which is below 50 mosmol / l. The tubular permeability for urea is low and it is excreted in the urine without accumulating in the renal medulla. The collecting ducts also provide for the reabsorption of sodium, chloride, and other ions. Their main functional feature is that the reabsorption of substances occurs in small amounts, but against the most significant gradient, which causes significant differences in the concentration of a number of inorganic substances in the urine compared to the blood.

■ Thus, the activity of the nephron loop, the end parts of the distal collecting ducts, determines the ability of the human kidneys to excrete large volumes (up to 900 ml / h) of diluted, hypotonic urine during water stress, and with a lack of water in the body, excrete only 10-12 ml / h, 4 "/g times osmotically more concentrated than blood. The ability of the kidney to osmotically concentrate urine is exceptionally developed in some desert rodents, which allows them to not drink water for a long time.

Date added: 2015-05-19 | Views: 1017 | Copyright infringement

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