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The structure and functions of the nephron briefly. Structure and function of the nephron: choroid glomerulus

The nephron is the structural unit of the kidney responsible for the formation of urine. Working 24 hours, the organs pass up to 1700 liters of plasma, forming a little more than a liter of urine.

Nephron

The work of the nephron, which is the structural and functional unit of the kidney, determines how successfully the balance is maintained and waste products are eliminated. During the day, two million nephrons of the kidneys, as many as there are in the body, produce 170 liters of primary urine, condensed to a daily amount of up to one and a half liters. The total area of the excretory surface of the nephrons is almost 8 m2, which is 3 times the area of the skin.

The excretory system has a high reserve of strength. It is created due to the fact that only a third of the nephrons work at the same time, which allows them to survive when the kidney is removed.

Arterial blood flowing through the afferent arteriole is cleansed in the kidneys. Purified blood comes out through the exiting arteriole. The diameter of the afferent arteriole is larger than that of the arteriole, due to which a pressure difference is created.

Structure

The divisions of the nephron of the kidney are:

They begin in the cortex of the kidney with Bowman's capsule, which is located above the glomerulus of capillaries of the arteriole. The nephron capsule of the kidney communicates with the proximal (closest) tubule, directed to the medulla - this is the answer to the question in which part of the kidney the nephron capsules are located. The tubule passes into the loop of Henle - first into the proximal segment, then into the distal one. The end of the nephron is considered to be the place where the collecting duct begins, where secondary urine from many nephrons enters. Nephron diagram

Capsule

Podocyte cells surround the glomerulus of capillaries like a cap. The formation is called a renal corpuscle. Liquid penetrates its pores and ends up in Bowman's space. Infiltrate, a product of blood plasma filtration, collects here.

Proximal tubule

This species consists of cells covered on the outside with a basement membrane. The inner part of the epithelium is equipped with outgrowths - microvilli, like a brush, lining the tubule along the entire length.

Outside there is a basement membrane, assembled into numerous folds, which straighten when the tubules are filled. In this case, the tubule acquires a rounded shape in diameter, and the epithelium becomes flattened. In the absence of fluid, the diameter of the tubule becomes narrow, the cells acquire a prismatic appearance.

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Functions include reabsorption:

H2O; Na – 85%; ions Ca, Mg, K, Cl; salts - phosphates, sulfates, bicarbonate; compounds - proteins, creatinine, vitamins, glucose.

From the tubule, reabsorbents enter the blood vessels, which encircle the tubule in a dense network. In this area, bile acid is absorbed into the cavity of the tubule, oxalic, para-aminohippuric, and uric acids are absorbed, adrenaline, acetylcholine, thiamine, histamine are absorbed, and drugs are transported - penicillin, furosemide, atropine, etc.

Here, the breakdown of hormones coming from the filtrate occurs with the help of enzymes in the epithelial border. Insulin, gastrin, prolactin, bradykinin are destroyed, their concentration in plasma decreases.

Loop of Henle

After entering the medullary ray, the proximal tubule passes into the initial part of the loop of Henle. The tubule passes into the descending segment of the loop, which descends into the medulla. The ascending portion then ascends into the cortex, approaching Bowman's capsule.

The internal structure of the loop initially does not differ from the structure of the proximal tubule. Then the lumen of the loop narrows, through which Na is filtered into the interstitial fluid, which becomes hypertonic. This is important for the operation of the collecting ducts: due to the high concentration of salt in the washer fluid, water is absorbed into them. The ascending section expands and passes into the distal tubule.

Gentle's loop

Distal tubule

This area is already, in short, composed of low epithelial cells. There are no villi inside the canal; on the outside, the folding of the basement membrane is well expressed. Here sodium reabsorption occurs, water reabsorption continues, and hydrogen and ammonia ions are secreted into the lumen of the tubule.

The video shows a diagram of the structure of the kidney and nephron:

Types of nephrons

Based on their structural features and functional purpose, the following types of nephrons that function in the kidney are distinguished:

cortical - superficial, intracortical; juxtamedullary.

Cortical

There are two types of nephrons in the cortex. Superficial ones make up about 1% of the total number of nephrons. They are distinguished by the superficial location of the glomeruli in the cortex, the shortest loop of Henle, and a small volume of filtration.

The number of intracortical - more than 80% of the nephrons of the kidney, are located in the middle of the cortical layer, play a major role in filtering urine. Blood in the glomerulus of the intracortical nephron passes under pressure, since the afferent arteriole is much wider than the efferent arteriole.

Juxtamedullary

Juxtamedullary - a small part of the nephrons of the kidney. Their number does not exceed 20% of the number of nephrons. The capsule is located on the border of the cortex and medulla, the rest of it is located in the medulla, the loop of Henle descends almost to the renal pelvis.

This type of nephron is critical to the ability to concentrate urine. The peculiarity of the juxtamedullary nephron is that the efferent arteriole of this type of nephron has the same diameter as the afferent one, and the loop of Henle is the longest of all.

The efferent arterioles form loops that move into the medulla parallel to the loop of Henle and flow into the venous network.

Functions

The functions of the nephron of the kidney include:

concentration of urine; regulation of vascular tone; blood pressure control.

Urine is formed in several stages:

in the glomeruli, blood plasma entering through the arteriole is filtered, primary urine is formed; reabsorption of useful substances from the filtrate; urine concentration.

Cortical nephrons

The main function is the formation of urine, reabsorption of useful compounds, proteins, amino acids, glucose, hormones, minerals. Cortical nephrons participate in the processes of filtration and reabsorption due to the characteristics of the blood supply, and the reabsorbed compounds immediately penetrate into the blood through the nearby capillary network of the efferent arteriole.

Juxtamedullary nephrons

The main job of the juxtamedullary nephron is to concentrate urine, which is possible due to the peculiarities of blood movement in the exiting arteriole. The arteriole does not pass into the capillary network, but passes into venules that flow into veins.

Nephrons of this type are involved in the formation of a structural formation that regulates blood pressure. This complex secretes renin, which is necessary for the production of angiotensin 2, a vasoconstrictor compound.

Nephron dysfunction and how to restore it

Disruption of the nephron leads to changes that affect all body systems.

Disorders caused by nephron dysfunction include:

acidity; water-salt balance; metabolism.

Diseases that are caused by disruption of the transport functions of nephrons are called tubulopathies, among which are:

primary tubulopathy – congenital dysfunctions; secondary – acquired disorders of transport function.

The causes of secondary tubulopathy are damage to the nephron caused by the action of toxins, including drugs, malignant tumors, heavy metals, and myeloma.

According to the location of tubulopathy:

proximal – damage to the proximal tubules; distal – damage to the functions of the distal convoluted tubules. Types of tubulopathy

Proximal tubulopathy

Damage to the proximal areas of the nephron leads to the formation of:

phosphaturia; hyperaminoaciduria; renal acidosis; glucosuria.

Impaired phosphate reabsorption leads to the development of rickets-like bone structure, a condition resistant to treatment with vitamin D. The pathology is associated with the absence of a phosphate transport protein and a lack of calcitriol-binding receptors.

Renal glycosuria is associated with a decreased ability to absorb glucose. Hyperaminoaciduria is a phenomenon in which the transport function of amino acids in the tubules is disrupted. Depending on the type of amino acid, pathology leads to various systemic diseases.

So, if the reabsorption of cystine is impaired, the disease cystinuria develops - an autosomal recessive disease. The disease manifests itself as developmental delay and renal colic. In the urine of cystinuria, cystine stones may appear, which easily dissolve in an alkaline environment.

Proximal tubular acidosis is caused by an inability to absorb bicarbonate, due to which it is excreted in the urine, and its concentration in the blood decreases, and Cl ions, on the contrary, increase. This leads to metabolic acidosis, with increased excretion of K ions.

Distal tubulopathy

Pathologies of the distal sections are manifested by renal water diabetes, pseudohypoaldosteronism, and tubular acidosis. Kidney diabetes is a hereditary damage. The congenital disorder is caused by a failure of distal tubular cells to respond to antidiuretic hormone. Lack of response leads to impaired ability to concentrate urine. The patient develops polyuria; up to 30 liters of urine can be excreted per day.

With combined disorders, complex pathologies develop, one of which is called de Toni-Debreu-Fanconi syndrome. In this case, the reabsorption of phosphates and bicarbonates is impaired, amino acids and glucose are not absorbed. The syndrome is manifested by developmental delay, osteoporosis, pathology of bone structure, and acidosis.

Each adult kidney contains at least 1 million nephrons, each of which is capable of producing urine. At the same time, usually about 1/3 of all nephrons function, which is enough to fully perform the excretory and other functions of the kidneys. This indicates the presence of significant functional reserves of the kidneys. With aging, there is a gradual decrease in the number of nephrons(by 1% per year after 40 years) due to their lack of regeneration ability. For many people in their 80s, the number of nephrons is reduced by 40% compared to those in their 40s. However, the loss of such a large number of nephrons is not a threat to life, since the remaining part can fully perform excretory and other functions of the kidneys. At the same time, damage to more than 70% of the total number of nephrons in kidney diseases can cause the development of chronic renal failure.

Every nephron consists of a renal (Malpighian) corpuscle, in which ultrafiltration of blood plasma and the formation of primary urine occurs, and a system of tubules and tubes in which primary urine is converted into secondary and final (released into the pelvis and into the environment) urine.

Rice. 1. Structural and functional organization of the nephron

The composition of urine does not change significantly when it moves through the pelvis (calyces, cups), ureters, temporary retention in the bladder and through the urinary canal. Thus, in a healthy person, the composition of the final urine released during urination is very close to the composition of urine released into the lumen (small calyxes of the large calyces) of the pelvis.

Renal corpuscle located in the renal cortex, is the initial part of the nephron and is formed capillary glomerulus(consisting of 30-50 interwoven capillary loops) and capsule Shumlyansky - Boumeia. In cross-section, the Shumlyansky-Boumeia capsule looks like a bowl, inside of which there is a glomerulus of blood capillaries. The epithelial cells of the inner layer of the capsule (podocytes) are tightly adjacent to the wall of the glomerular capillaries. The outer leaf of the capsule is located at some distance from the inner one. As a result, a slit-like space is formed between them - the cavity of the Shumlyansky-Bowman capsule, into which blood plasma is filtered, and its filtrate forms primary urine. From the capsule cavity, primary urine passes into the lumen of the nephron tubules: proximal tubule(convoluted and straight segments), loop of Henle(descending and ascending sections) and distal tubule(straight and convoluted segments). An important structural and functional element of the nephron is juxtaglomerular apparatus (complex) of the kidney. It is located in a triangular space formed by the walls of the afferent and efferent arterioles and the distal tubule (solar macula - maculadensa), tightly adjacent to them. Cells of the macula densa have chemo- and mechanosensitivity, regulating the activity of juxtaglomerular cells of arterioles, which synthesize a number of biologically active substances (renin, erythropoietin, etc.). The convoluted segments of the proximal and distal tubules are located in the renal cortex, and the loop of Henle is in the medulla.

Urine flows from the distal convoluted tubule into the connecting tubule, from it to collecting duct And collecting duct renal cortex; 8-10 collecting ducts unite into one large duct ( collecting duct of the cortex), which, descending into the medulla, becomes collecting duct of the renal medulla. Gradually merging, these ducts form large diameter duct, which opens at the top of the papilla of the pyramid into the small calyx of the large calyx of the pelvis.

Each kidney has at least 250 large-diameter collecting ducts, each of which collects urine from approximately 4,000 nephrons. The collecting ducts and collecting ducts have special mechanisms for maintaining hyperosmolarity of the renal medulla, concentrating and diluting urine, and are important structural components of the formation of final urine.

Nephron structure

Each nephron begins with a double-walled capsule, inside of which there is a vascular glomerulus. The capsule itself consists of two leaves, between which there is a cavity that passes into the lumen of the proximal tubule. It consists of the proximal convoluted tubule and the proximal straight tubule, constituting the proximal segment of the nephron. A characteristic feature of the cells of this segment is the presence of a brush border, consisting of microvilli, which are outgrowths of the cytoplasm surrounded by a membrane. The next section is the loop of Henle, consisting of a thin descending part that can descend deeply into the medulla, where it forms a loop and turns 180° towards the cortex in the form of an ascending thin part of the nephron loop, turning into a thick part. The ascending limb of the loop rises to the level of its glomerulus, where the distal convoluted tubule begins, which becomes a short communicating tubule connecting the nephron with the collecting ducts. The collecting ducts begin in the renal cortex, merging to form larger excretory ducts that pass through the medulla and empty into the cavity of the renal calyx, which in turn drain into the renal pelvis. According to localization, several types of nephrons are distinguished: superficial (superficial), intracortical (inside the cortical layer), juxtamedullary (their glomeruli are located on the border of the cortical and medulla layers).

Rice. 2. Structure of the nephron:

A - juxtamedullary nephron; B - intracortical nephron; 1 - renal corpuscle, including the capsule of the glomerulus of capillaries; 2 - proximal convoluted tubule; 3 - proximal straight tubule; 4 - descending thin limb of the nephron loop; 5 - ascending thin limb of the nephron loop; 6 - distal straight tubule (thick ascending limb of the nephron loop); 7 - dense spot of the distal tubule; 8 - distal convoluted tubule; 9 - connecting tubule; 10 - collecting duct of the renal cortex; 11 - collecting duct of the outer medulla; 12 - collecting duct of the internal medulla

Different types of nephrons differ not only in location, but also in the size of the glomeruli, the depth of their location, as well as in the length of individual sections of the nephron, especially the loop of Henle, and in their participation in the osmotic concentration of urine. Under normal conditions, about 1/4 of the volume of blood ejected by the heart passes through the kidneys. In the cortex, blood flow reaches 4-5 ml/min per 1 g of tissue, therefore, this is the highest level of organ blood flow. A feature of renal blood flow is that the blood flow of the kidney remains constant when systemic blood pressure changes within a fairly wide range. This is ensured by special mechanisms of self-regulation of blood circulation in the kidney. Short renal arteries arise from the aorta; in the kidney they branch into smaller vessels. The renal glomerulus includes the afferent (afferent) arteriole, which breaks up into capillaries. When capillaries merge, they form an efferent arteriole, through which blood flows out from the glomerulus. After leaving the glomerulus, the efferent arteriole again breaks up into capillaries, forming a network around the proximal and distal convoluted tubules. A feature of the juxtamedullary nephron is that the efferent arteriole does not break up into a peritubular capillary network, but forms straight vessels that descend into the renal medulla.

Types of Nephrons

Types of nephrons

Based on the characteristics of their structure and functions, they are distinguished two main types of nephrons: cortical (70-80%) and juxtamedullary (20-30%).

Cortical nephrons are divided into superficial, or superficial, cortical nephrons, in which the renal corpuscles are located in the outer part of the renal cortex, and intracortical cortical nephrons, in which the renal corpuscles are located in the middle part of the renal cortex. Cortical nephrons have a short loop of Henle that extends only into the outer medulla. The main function of these nephrons is the formation of primary urine.

Renal corpuscles juxtamedullary nephrons are located in the deep layers of the cortex at the border with the medulla. They have a long loop of Henle that penetrates deep into the medulla, right up to the apexes of the pyramids. The main purpose of juxtamedullary nephrons is to create high osmotic pressure in the renal medulla, which is necessary to concentrate and reduce the volume of final urine.

Effective filtration pressure

EFD = Rcap - Rbk - Ronk. Rcap- hydrostatic pressure in the capillary (50-70 mm Hg); R6k- hydrostatic pressure in the lumen of the Bowman-Shumlyaneki capsule (15-20 mm Hg); Ronk- oncotic pressure in the capillary (25-30 mm Hg).

EPD = 70 - 30 - 20 = 20 mmHg. Art.

The formation of final urine is the result of three main processes occurring in the nephron: filtration, reabsorption and secretion.

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The tubular part of the nephron is usually divided into four sections:

1) main (proximal);

2) thin segment of the loop of Henle;

3) distal;

4) collecting ducts.

Main (proximal) section consists of a sinuous and a straight part. Cells of the convoluted part have a more complex structure than the cells of other parts of the nephron. These are tall (up to 8 µm) cells with a brush border, intracellular membranes, a large number of correctly oriented mitochondria, well-developed lamellar complex and endoplasmic reticulum, lysosomes and other ultrastructures (Fig. 1). Their cytoplasm contains many amino acids, basic and acidic proteins, polysaccharides and active SH groups, highly active dehydrogenases, diaphorases, hydrolases [Serov V.V., Ufimtseva A.G., 1977; Jakobsen N., Jorgensen F. 1975].

Rice. 1. Scheme of the ultrastructure of tubular cells of various parts of the nephron. 1 - cell of the convoluted part of the main section; 2 - cell of the straight part of the main section; 3 - cell of the thin segment of the loop of Henle; 4 - cell of the direct (ascending) part of the distal section; 5 - cell of the convoluted part of the distal section; 6 - “dark” cell of the connecting section and collecting duct; 7 - “light” cell of the connecting section and collecting duct.

Cells of the direct (descending) part of the main section basically have the same structure as the cells of the convoluted part, but the finger-like outgrowths of the brush border are coarser and shorter, there are fewer intracellular membranes and mitochondria, they are not so strictly oriented, and there are significantly fewer cytoplasmic granules.

The brush border consists of numerous finger-like projections of cytoplasm covered with a cell membrane and glycocalyx. Their number on the cell surface reaches 6500, which increases the working area of each cell by 40 times. This information gives an idea of the surface on which exchange occurs in the proximal tubule. The activity of alkaline phosphatase, ATPase, 5-nucleotidase, aminopeptidase and a number of other enzymes has been proven in the brush border. The brush border membrane contains a sodium-dependent transport system. It is believed that the glycocalyx covering the microvilli of the brush border is permeable to small molecules. Large molecules enter the tubule by pinocytosis, which occurs due to crater-shaped depressions in the brush border.

Intracellular membranes are formed not only by the bends of the BM cell, but also by the lateral membranes of neighboring cells, which seem to overlap each other. Intracellular membranes are essentially also intercellular, which serves the active transport of fluid. In this case, the main importance in transport is attached to the basal labyrinth, formed by protrusions of the BM into the cell; it is considered as a “single diffusion space”.

Numerous mitochondria are located in the basal part between the intracellular membranes, which gives the impression of their correct orientation. Each mitochondria is thus enclosed in a chamber formed by folds of intra- and intercellular membranes. This allows the products of enzymatic processes developing in mitochondria to easily leave the cell. The energy produced in mitochondria serves both the transport of matter and secretion, carried out using the granular endoplasmic reticulum and the lamellar complex, which undergoes cyclic changes in different phases of diuresis.

The ultrastructure and enzyme chemistry of the tubule cells of the main section explain its complex and differentiated function. The brush border, like the labyrinth of intracellular membranes, is a kind of device for the colossal reabsorption function performed by these cells. The enzymatic transport system of the brush border, dependent on sodium, ensures the reabsorption of glucose, amino acids, and phosphates [Natochin Yu. V., 1974; Kinne R., 1976]. The intracellular membranes, especially the basal labyrinth, are associated with the reabsorption of water, glucose, amino acids, phosphates and a number of other substances, which is performed by the sodium-independent transport system of the labyrinth membranes.

Of particular interest is the question of tubular reabsorption of protein. It is considered proven that all the protein filtered in the glomeruli is reabsorbed in the proximal tubule, which explains its absence in the urine of a healthy person. This position is based on many studies performed, in particular, using an electron microscope. Thus, protein transport in the cell of the proximal tubule was studied in experiments with microinjection of ¹³¹I-labeled albumin directly into the rat tubule, followed by electron microscopic radiography of this tubule.

Albumin is found primarily in the invaginates of the brush border membrane, then in pinocytotic vesicles, which merge into vacuoles. The protein from the vacuoles then appears in lysosomes and the lamellar complex (Fig. 2) and is cleaved by hydrolytic enzymes. Most likely, the “main efforts” of high dehydrogenase, diaphorase and hydrolase activity in the proximal tubule are aimed at protein reabsorption.

Rice. 2. Scheme of protein reabsorption by the cell of the main segment of the tubules.

I - micropinocytosis at the base of the brush border; Mvb - vacuoles containing the protein ferritin;

II - vacuoles filled with ferritin (a) move to the basal part of the cell; b - lysosome; c - fusion of a lysosome with a vacuole; d - lysosomes with incorporated protein; AG - lamellar complex with tanks containing CF (painted black);

III - release through the BM of low molecular weight fragments of reabsorbed protein formed after “digestion” in lysosomes (shown by double arrows).

In connection with these data, the mechanisms of “damage” to the tubules of the main section become clear. In case of NS of any origin, proteinuric conditions, changes in the epithelium of the proximal tubules in the form of protein dystrophy (hyaline-droplet, vacuolar) reflect the resorption insufficiency of the tubules in conditions of increased porosity of the glomerular filter for protein [Davydovsky I.V., 1958; Serov V.V., 1968]. There is no need to see primary dystrophic processes in the changes in the tubules in NS.

Equally, proteinuria cannot be considered as a result of only increased porosity of the glomerular filter. Proteinuria in nephrosis reflects both primary damage to the kidney filter and secondary depletion (blockade) of the tubular enzyme systems that reabsorb protein.

In a number of infections and intoxications, blockade of the enzyme systems of the tubule cells of the main section can occur acutely, since these tubules are the first to be exposed to toxins and poisons when they are eliminated by the kidneys. Activation of hydrolases of the cell's lysosomal apparatus in some cases completes the dystrophic process with the development of cell necrosis (acute nephrosis). In the light of the above data, the pathology of hereditary “loss” of renal tubular enzymes (the so-called hereditary tubular enzymopathies) becomes clear. A certain role in tubular damage (tubulolysis) is assigned to antibodies that react with the antigen of the tubular basement membrane and brush border.

Cells of the thin segment of the loop of Henle characterized by the peculiarity that intracellular membranes and plates cross the cell body to its entire height, forming gaps up to 7 nm wide in the cytoplasm. It seems that the cytoplasm consists of separate segments, and some of the segments of one cell seem to be wedged between the segments of an adjacent cell. The enzyme chemistry of the thin segment reflects the functional feature of this part of the nephron, which, as an additional device, reduces the filtration charge of water to a minimum and ensures its “passive” resorption [Ufimtseva A. G., 1963].

The subordinate work of the thin segment of the loop of Henle, the canaliculi of the distal part of the rectum, the collecting ducts and the straight vessels of the pyramids ensures the osmotic concentration of urine based on a countercurrent multiplier. New ideas about the spatial organization of the countercurrent multiplying system (Fig. 3) convince us that the concentrating activity of the kidney is ensured not only by the structural and functional specialization of various parts of the nephron, but also by the highly specialized mutual arrangement of tubular structures and vessels of the kidney [Perov Yu. L., 1975 ; Kriz W., Lever A., 1969].

Rice. 3. Diagram of the location of the structures of the countercurrent multiplying system in the renal medulla. 1 - arterial vessel recta; 2 - venous straight vessel; 3 - thin segment of the loop of Henle; 4 - straight part of the distal section; CT - collecting ducts; K - capillaries.

Distal section The tubules consist of straight (ascending) and convoluted parts. The cells of the distal section ultrastructurally resemble the cells of the proximal section. They are rich in cigar-shaped mitochondria filling the spaces between intracellular membranes, as well as cytoplasmic vacuoles and granules around the apically located nucleus, but lack a brush border. The distal epithelium is rich in amino acids, basic and acidic proteins, RNA, polysaccharides and reactive SH groups; it is characterized by high activity of hydrolytic, glycolytic enzymes and Krebs cycle enzymes.

The complexity of the structure of the cells of the distal tubules, the abundance of mitochondria, intracellular membranes and plastic material, high enzymatic activity indicate the complexity of their function - facultative reabsorption, aimed at maintaining the constancy of the physicochemical conditions of the internal environment. Facultative reabsorption is regulated mainly by hormones of the posterior lobe of the pituitary gland, adrenal glands and JGA of the kidney.

The place of application of the action of the pituitary antidiuretic hormone (ADH) in the kidney, the “histochemical springboard” of this regulation is the hyaluronic acid - hyaluronidase system, located in the pyramids, mainly in their papillae. Aldosterone, according to some data, and cortisone influence the level of distal reabsorption by direct inclusion in the cell enzyme system, which ensures the transfer of sodium ions from the lumen of the tubule to the interstitium of the kidney. Of particular importance in this process is the epithelium of the rectal part of the distal part, and the distal effect of aldosterone is mediated by the secretion of renin attached to the cells of the JGA. Angiotensin, formed under the influence of renin, not only stimulates the secretion of aldosterone, but also participates in the distal reabsorption of sodium.

In the convoluted part of the distal tubule, where it approaches the pole of the vascular glomerulus, macula densa is distinguished. Epithelial cells in this part become cylindrical, their nuclei become hyperchromic; they are arranged polysadically, and there is no continuous basement membrane. The cells of the macula densa have close contacts with granular epithelioid cells and lacis cells of the JGA, which ensures the influence of the chemical composition of the urine of the distal tubule on the glomerular blood flow and, conversely, the hormonal effects of the JGA on the macula densa.

The structural and functional characteristics of the distal tubules and their increased sensitivity to oxygen deprivation are to some extent associated with their selective damage during acute hemodynamic kidney damage, in the pathogenesis of which deep disturbances of the renal circulation with the development of anoxia of the tubular apparatus play a major role. Under conditions of acute anoxia, the cells of the distal tubules are exposed to acidic urine containing toxic products, which leads to their damage up to necrosis. In chronic anoxia, the cells of the distal tubule undergo atrophy more often than the proximal tubule.

Collecting ducts, lined with cubic and, in the distal sections, columnar epithelium (light and dark cells) with a well-developed basal labyrinth, highly permeable to water. Secretion of hydrogen ions is associated with dark cells; high activity of carbonic anhydrase was found in them [Zufarov K. A. et al., 1974]. Passive transport of water in the collecting tubes is ensured by the features and functions of the countercurrent multiplying system.

Concluding the description of the histophysiology of the nephron, we should dwell on its structural and functional differences in different parts of the kidney. On this basis, cortical and juxtamedullary nephrons are distinguished, differing in the structure of the glomeruli and tubules, as well as the uniqueness of their function; The blood supply to these nephrons is also different.

Clinical Nephrology

edited by E.M. Tareeva

The kidneys are located retroperitoneally on both sides of the spinal column at the Th 12 –L 2 level. The mass of each kidney of an adult man is 125–170 g, of an adult woman – 115–155 g, i.e. in total less than 0.5% of total body weight.

The kidney parenchyma is divided into those located outward (at the convex surface of the organ) cortical and what is underneath medulla. Loose connective tissue forms the stroma of the organ (interstitium).

Cork substance located under the kidney capsule. The granular appearance of the cortex is given by the renal corpuscles and convoluted tubules of the nephrons present here.

Brain substance has a radially striated appearance, since it contains parallel descending and ascending parts of the nephron loop, collecting ducts and collecting ducts, straight blood vessels ( vasa recta). The medulla is divided into an outer part, located directly under the cortex, and an inner part, consisting of the apices of the pyramids

Interstitium represented by an intercellular matrix containing fibroblast-like cells and thin reticulin fibers, closely associated with the walls of capillaries and renal tubules

Nephron as a morpho-functional unit of the kidney.

In humans, each kidney consists of approximately one million structural units called nephrons. The nephron is the structural and functional unit of the kidney because it carries out the entire set of processes that result in the formation of urine.

Fig.1. Urinary system. Left: kidneys, ureters, bladder, urethra (urethra) On the right6 the structure of the nephron

Nephron structure:

The Shumlyansky-Bowman capsule, inside which there is a glomerulus of capillaries - the renal (Malpighian) corpuscle. Capsule diameter – 0.2 mm

Proximal convoluted tubule. Feature of its epithelial cells: brush border - microvilli facing the lumen of the tubule

Loop of Henle

Distal convoluted tubule. Its initial section necessarily touches the glomerulus between the afferent and efferent arterioles

Connecting tubule

Collecting tube

Functionally distinguish 4 segment:

1.Glomerula;

2.Proximal – convoluted and straight parts of the proximal tubule;

3.Thin loop section – descending and thin part of the ascending part of the loop;

4.Distal – thick part of the ascending limb of the loop, distal convoluted tubule, connecting part.

During embryogenesis, the collecting ducts develop independently, but function together with the distal segment.

Beginning in the renal cortex, the collecting ducts merge to form excretory ducts, which pass through the medulla and open into the cavity of the renal pelvis. The total length of the tubules of one nephron is 35-50 mm.

Types of nephrons

There are significant differences in different segments of the nephron tubules depending on their localization in a particular zone of the kidney, the size of the glomeruli (juxtamedullary ones are larger than the superficial ones), the depth of the location of the glomeruli and proximal tubules, the length of individual sections of the nephron, especially the loops. The zone of the kidney in which the tubule is located is of great functional importance, regardless of whether it is located in the cortex or medulla.

The cortex contains the renal glomeruli, proximal and distal tubules, and connecting sections. In the outer strip of the outer medulla there are thin descending and thick ascending sections of the nephron loops and collecting ducts. The inner layer of the medulla contains thin sections of nephron loops and collecting ducts.

This arrangement of nephron parts in the kidney is not accidental. This is important in the osmotic concentration of urine. There are several different types of nephrons functioning in the kidney:

1. With superofficial ( superficial,

short loop );

2. And intracortical ( inside the cortex );

3. Juxtamedullary ( at the border of the cortex and medulla ).

One of the important differences between the three types of nephrons is the length of the loop of Henle. All superficial - cortical nephrons have a short loop, as a result of which the limb of the loop is located above the border, between the outer and inner parts of the medulla. In all juxtamedullary nephrons, long loops penetrate into the inner medulla, often reaching the apex of the papilla. Intracortical nephrons can have both a short and a long loop.

FEATURES OF THE KIDNEY BLOOD SUPPLY

Renal blood flow is independent of systemic blood pressure over a wide range of changes. This is due to myogenic regulation , caused by the ability of smooth muscle cells to contract in response to their stretching by blood (with an increase in blood pressure). As a result, the amount of blood flowing remains constant.

In one minute, about 1200 ml of blood passes through the vessels of both kidneys in a person, i.e. about 20-25% of the blood ejected by the heart into the aorta. The mass of the kidneys is 0.43% of the body weight of a healthy person, and they receive ¼ of the volume of blood ejected by the heart. 91-93% of the blood entering the kidney flows through the vessels of the renal cortex, the rest is supplied by the renal medulla. Blood flow in the renal cortex is normally 4-5 ml/min per 1 g of tissue. This is the highest level of organ blood flow. The peculiarity of renal blood flow is that when blood pressure changes (from 90 to 190 mm Hg), the blood flow of the kidney remains constant. This is due to the high level of self-regulation of blood circulation in the kidney.

Short renal arteries - depart from the abdominal aorta and are a large vessel with a relatively large diameter. After entering the portal of the kidneys, they are divided into several interlobar arteries, which pass in the medulla of the kidney between the pyramids to the border zone of the kidneys. Here the arcuate arteries depart from the interlobular arteries. From the arcuate arteries in the direction of the cortex there are interlobular arteries, which give rise to numerous afferent glomerular arterioles.

The afferent (afferent) arteriole enters the renal glomerulus, where it breaks up into capillaries, forming the Malpeguian glomerulus. When they merge, they form an efferent arteriole, through which blood flows away from the glomerulus. The efferent arteriole then splits back into capillaries, forming a dense network around the proximal and distal convoluted tubules.

Two networks of capillaries – high and low pressure.

Filtration occurs in high-pressure capillaries (70 mm Hg) - in the renal glomerulus. The high pressure is due to the fact that: 1) the renal arteries arise directly from the abdominal aorta; 2) their length is small; 3) the diameter of the afferent arteriole is 2 times larger than the efferent one.

Thus, most of the blood in the kidney passes through the capillaries twice - first in the glomerulus, then around the tubules, this is the so-called "miraculous network". Interlobular arteries form numerous anastomoses, which play a compensatory role. In the formation of the peritubular capillary network, the Ludwig arteriole, which arises from the interlobular artery or from the afferent glomerular arteriole, is essential. Thanks to the Ludwig arteriole, extraglomerular blood supply to the tubules is possible in the event of death of the renal corpuscles.

Arterial capillaries, creating the peritubular network, become venous. The latter form stellate venules located under the fibrous capsule - interlobular veins flowing into the arcuate veins, which merge and form the renal vein, which flows into the inferior pudendal vein.

In the kidneys there are 2 circles of blood circulation: the large cortical - 85-90% of the blood, the small juxtamedullary - 10-15% of the blood. Under physiological conditions, 85-90% of the blood circulates through the systemic (cortical) circle of the renal circulation; under pathology, the blood moves along a small or shortened path.

The difference in the blood supply of the juxtamedullary nephron is that the diameter of the afferent arteriole is approximately equal to the diameter of the efferent arteriole, the efferent arteriole does not break up into a peritubular capillary network, but forms straight vessels that descend into the medulla. The vasa recta form loops at different levels of the medulla, turning back. The descending and ascending parts of these loops form a countercurrent system of vessels called the vascular bundle. The juxtamedullary circulation is a kind of “shunt” (Truet shunt), in which most of the blood flows not into the cortex, but into the medulla of the kidneys. This is the so-called kidney drainage system.

The nephron is not only the main structural but also the functional unit of the kidney. This is where the most important stages take place. Therefore, information about what the structure of the nephron looks like and what exact functions it performs will be very interesting. In addition, the peculiarities of nephron functioning can clarify the nuances of the renal system.

Structure of the nephron: renal corpuscle

Interestingly, the mature kidney of a healthy person contains between 1 and 1.3 billion nephrons. A nephron is a functional and structural unit of the kidney, which consists of the renal corpuscle and the so-called loop of Henle.

The renal corpuscle itself consists of the Malpighian glomerulus and the Bowman-Shumlyansky capsule. To begin with, it is worth noting that the glomerulus is actually a collection of small capillaries. The blood enters here through the afferent artery - this is where the plasma is filtered. The remainder of the blood is removed by the efferent arteriole.

The Bowman-Shumlyansky capsule consists of two layers - internal and external. And if the outer sheet is an ordinary fabric, then the structure of the inner sheet deserves more attention. The inside of the capsule is covered with podocytes - these are cells that act as an additional filter. They allow glucose, amino acids and other substances to pass through, but prevent the movement of large protein molecules. Thus, primary urine is formed in the renal corpuscle, which differs from it only in the absence of large molecules.

Nephron: structure of the proximal tubule and loop of Henle

The proximal tubule is a formation that connects the renal corpuscle and the loop of Henle. Inside the tubule has villi, which increase the total area of the internal lumen, thereby increasing reabsorption rates.

The proximal tubule smoothly passes into the descending part of the loop of Henle, which is characterized by a small diameter. The loop descends into the medulla, where it bends around its own axis by 180 degrees and rises upward - here begins the ascending part of the loop of Henle, which has a much larger size and, accordingly, diameter. The ascending loop rises to approximately the level of the glomerulus.

Structure of the nephron: distal tubules

The ascending part of the loop of Henle in the cortex passes into the so-called distal convoluted tubule. It is in contact with the glomerulus and is in contact with the afferent and efferent arterioles. This is where the final absorption of nutrients occurs. The distal tubule passes into the terminal part of the nephron, which in turn flows into the collecting duct, which carries fluid into the nephron.

Nephron classification

Depending on their location, it is customary to distinguish three main types of nephrons:

Cortical nephrons make up approximately 85% of the number of all structural units in the kidney. As a rule, they are located in the outer cortex of the kidney, as their name suggests. The structure of this type of nephron is slightly different - the loop of Henle is small;
juxtamedullary nephrons - such structures are located just between the medulla and cortex, have long loops of Henle that penetrate deeply into the medulla, sometimes even reaching the pyramids;
subcapsular nephrons are structures that are located directly under the capsule.

It can be noted that the structure of the nephron is fully consistent with its functions.

Each adult kidney contains at least 1 million nephrons, each of which is capable of producing urine. At the same time, usually about 1/3 of all nephrons function, which is enough for the full performance of excretory and other functions. This indicates the presence of significant functional reserves of the kidneys. With aging, there is a gradual decrease in the number of nephrons(by 1% per year after 40 years) due to their lack of regeneration ability. For many people in their 80s, the number of nephrons is reduced by 40% compared to those in their 40s. However, the loss of such a large number of nephrons is not a threat to life, since the remaining part can fully perform excretory and other functions of the kidneys. At the same time, damage to more than 70% of the total number of nephrons in kidney diseases can cause the development of chronic renal failure.

Rice. 1. Structural and functional organization of the nephron

Renal corpuscle located in the renal cortex, is the initial part of the nephron and is formed capillary glomerulus(consisting of 30-50 interwoven capillary loops) and Shumlyansky-Boumeia capsule. In cross-section, the Shumlyansky-Boumeia capsule has the appearance of a cup, inside of which there is a glomerulus of blood capillaries. The epithelial cells of the inner layer of the capsule (podocytes) are tightly adjacent to the wall of the glomerular capillaries. The outer leaf of the capsule is located at some distance from the inner one. As a result, a slit-like space is formed between them - the cavity of the Shumlyansky-Bowman capsule, into which blood plasma is filtered, and its filtrate forms primary urine. From the capsule cavity, primary urine passes into the lumen of the nephron tubules: proximal tubule(convoluted and straight segments), loop of Henle(descending and ascending sections) and distal tubule(straight and convoluted segments). An important structural and functional element of the nephron is juxtaglomerular apparatus (complex) of the kidney. It is located in a triangular space formed by the walls of the afferent and efferent arterioles and the distal tubule (solar macula - maculadensa), tightly adjacent to them. Cells of the macula densa have chemo- and mechanosensitivity, regulating the activity of juxtaglomerular cells of arterioles, which synthesize a number of biologically active substances (renin, erythropoietin, etc.). The convoluted segments of the proximal and distal tubules are located in the renal cortex, and the loop of Henle is in the medulla.

Nephron structure

Each nephron begins with a double-walled capsule, inside of which there is a vascular glomerulus. The capsule itself consists of two leaves, between which there is a cavity that passes into the lumen of the proximal tubule. It consists of the proximal convoluted tubule and the proximal straight tubule, constituting the proximal segment of the nephron. A characteristic feature of the cells of this segment is the presence of a brush border, consisting of microvilli, which are outgrowths of the cytoplasm surrounded by a membrane. The next section is the loop of Henle, consisting of a thin descending part that can descend deeply into the medulla, where it forms a loop and turns 180 ° towards the cortex in the form of an ascending thin, turning into a thick part of the nephron loop. The ascending limb of the loop rises to the level of its glomerulus, where the distal convoluted tubule begins, which becomes a short communicating tubule connecting the nephron with the collecting ducts. The collecting ducts begin in the renal cortex, merging to form larger excretory ducts that pass through the medulla and empty into the cavity of the renal calyx, which in turn drain into the renal pelvis. According to localization, several types of nephrons are distinguished: superficial (superficial), intracortical (inside the cortical layer), juxtamedullary (their glomeruli are located on the border of the cortical and medulla layers).

Rice. 2. Structure of the nephron:

Types of Nephrons

Types of nephrons

Based on the characteristics of their structure and functions, they are distinguished two main types of nephrons: cortical (70-80%) and juxtamedullary (20-30%).

Effective filtration pressure

EFD = P cap - P bk - P onk.
R cap— hydrostatic pressure in the capillary (50-70 mm Hg);
R 6k- hydrostatic pressure in the lumen of the Bowman-Shumlyaneki capsule (15-20 mm Hg);
R onk— oncotic pressure in the capillary (25-30 mm Hg).

EPD = 70 - 30 - 20 = 20 mm Hg. Art.

The formation of final urine is the result of three main processes occurring in the nephron: and secretion.