The structure of the renal tubule. Nephron - structural and functional unit of the kidney

Normal blood filtration is guaranteed by the correct structure of the nephron. It carries out the processes of reuptake of chemicals from plasma and the production of a number of biologically active compounds. The kidney contains from 800 thousand to 1.3 million nephrons. Aging, an unhealthy lifestyle and an increase in the number of diseases lead to the fact that with age the number of glomeruli gradually decreases. To understand the principles of the nephron, it is worth understanding its structure.

Description of the nephron

The main structural and functional unit of the kidney is the nephron. The anatomy and physiology of the structure is responsible for the formation of urine, the reverse transport of substances and the production of a spectrum of biological substances. The structure of the nephron is an epithelial tube. Further, networks of capillaries of various diameters are formed, which flow into the collecting vessel. The cavities between the structures are filled with connective tissue in the form of interstitial cells and matrix.

The development of the nephron is laid down in the embryonic period. Different types of nephrons are responsible for different functions. The total length of the tubules of both kidneys is up to 100 km. Under normal conditions, not all of the glomeruli are involved, only 35% work. The nephron consists of a body, as well as a system of channels. It has the following structure:

capillary glomerulus;
capsule of the renal glomerulus;
near tubule;
descending and ascending fragments;
distant straight and convoluted tubules;
connecting path;
collecting ducts.

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Functions of the nephron in humans

Up to 170 liters of primary urine are formed per day in 2 million glomeruli.

The concept of nephron was introduced by the Italian physician and biologist Marcello Malpighi. Since the nephron is considered an integral structural unit of the kidney, it is responsible for the following functions in the body:

blood purification;
formation of primary urine;
return capillary transport of water, glucose, amino acids, bioactive substances, ions;
the formation of secondary urine;
ensuring salt, water and acid-base balance;
regulation of blood pressure;
secretion of hormones.

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Diagram of the structure of the renal glomerulus and Bowman's capsule.

The nephron begins as a capillary glomerulus. This is the body. The morphofunctional unit is a network of capillary loops, up to 20 in total, which are surrounded by a nephron capsule. The body receives its blood supply from the afferent arteriole. The vessel wall is a layer of endothelial cells, between which there are microscopic gaps up to 100 nm in diameter.

In capsules, internal and external epithelial balls are isolated. Between the two layers there is a slit-like gap - the urinary space, where the primary urine is contained. It envelops each vessel and forms a solid ball, thus separating the blood located in the capillaries from the spaces of the capsule. The basement membrane serves as a support base.

The nephron is arranged as a filter, the pressure in which is not constant, it changes depending on the difference in the width of the gaps of the afferent and efferent vessels. The filtration of blood in the kidneys takes place in the glomerulus. Blood cells, proteins, usually cannot pass through the pores of the capillaries, since their diameter is much larger and they are retained by the basement membrane.

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Capsule podocytes

The nephron consists of podocytes, which form the inner layer in the nephron capsule. These are large stellate epithelial cells that surround the renal glomerulus. They have an oval nucleus, which includes scattered chromatin and plasmosome, transparent cytoplasm, elongated mitochondria, a developed Golgi apparatus, shortened cisterns, few lysosomes, microfilaments, and several ribosomes.

Three types of podocyte branches form pedicles (cytotrabeculae). The outgrowths closely grow into each other and lie on the outer layer of the basement membrane. Structures of cytotrabeculae in nephrons form a cribriform diaphragm. This part of the filter has a negative charge. They also require proteins to function properly. In the complex, blood is filtered into the lumen of the nephron capsule.

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basement membrane

The structure of the basement membrane of the kidney nephron has 3 balls about 400 nm thick, consists of a collagen-like protein, glyco- and lipoproteins. Between them are layers of dense connective tissue - mesangium and a ball of mesangiocytitis.

There are also gaps up to 2 nm in size - membrane pores, they are important in the processes of plasma purification. On both sides, the sections of connective tissue structures are covered with glycocalyx systems of podocytes and endotheliocytes. Plasma filtration involves some of the matter. The basement membrane of the glomeruli of the kidneys functions as a barrier through which large molecules must not penetrate. Also, the negative charge of the membrane prevents the passage of albumins.

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Mesangial matrix

In addition, the nephron consists of mesangium. It is represented by systems of connective tissue elements that are located between the capillaries of the Malpighian glomerulus. It is also a section between the vessels, where there are no podocytes. Its main composition includes loose connective tissue containing mesangiocytes and juxtavascular elements, which are located between two arterioles. The main work of the mesangium is supportive, contractile, as well as ensuring the regeneration of the components of the basement membrane and podocytes, as well as the absorption of old constituent components.

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proximal tubule

The proximal capillary renal tubules of the nephrons of the kidney are divided into curved and straight. The lumen is small in size, it is formed by a cylindrical or cubic type of epithelium. At the top is placed a brush border, which is represented by long villi. They form an absorbent layer. The extensive surface area of the proximal tubules, the large number of mitochondria, and the close location of the peritubular vessels are designed for selective uptake of substances.

The filtered fluid flows from the capsule to other departments. The membranes of closely spaced cellular elements are separated by gaps through which fluid circulates. In the capillaries of the convoluted glomeruli, 80% of the plasma components are reabsorbed, among them: glucose, vitamins and hormones, amino acids, and in addition, urea. The functions of the nephron tubules include the production of calcitriol and erythropoietin. The segment produces creatinine. Foreign substances that enter the filtrate from the interstitial fluid are excreted in the urine.

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The structural and functional unit of the kidney consists of thin sections, also called the loop of Henle. It consists of 2 segments: descending thin and ascending thick. The wall of the descending section with a diameter of 15 μm is formed by a squamous epithelium with multiple pinocytic vesicles, and the ascending section is formed by a cubic one. The functional significance of the nephron tubules of the loop of Henle covers the retrograde movement of water in the descending part of the knee and its passive return in the thin ascending segment, the reuptake of Na, Cl and K ions in the thick segment of the ascending fold. In the capillaries of the glomeruli of this segment, the molarity of urine increases.

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Distal tubule

The distal parts of the nephron are located near the Malpighian body, as the capillary glomerulus makes a bend. They reach a diameter of up to 30 microns. They have a structure similar to the distal convoluted tubules. The epithelium is prismatic, located on the basement membrane. Mitochondria are located here, providing the structures with the necessary energy.

Cellular elements of the distal convoluted tubule form basement membrane invaginations. At the point of contact of the capillary tract and the vascular pole of the malipighian body, the renal tubule changes, the cells become columnar, the nuclei approach one another. In the renal tubules, an exchange of potassium and sodium ions occurs, affecting the concentration of water and salts.

Inflammation, disorganization or degenerative changes in the epithelium are fraught with a decrease in the ability of the apparatus to properly concentrate or, conversely, dilute urine. Violation of the function of the renal tubules provokes changes in the balance of the internal environment of the human body and is manifested by the appearance of changes in the urine. This condition is called tubular insufficiency.

To maintain the acid-base balance of the blood, hydrogen and ammonium ions are secreted in the distal tubules.

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Collecting tubes

The collecting duct, also known as the Bellinian ducts, is not part of the nephron, although it emerges from it. The epithelium consists of light and dark cells. Light epithelial cells are responsible for water reabsorption and are involved in the formation of prostaglandins. At the apical end, the light cell contains a single cilium, and in the folded dark cells, hydrochloric acid is formed, which changes the pH of the urine. The collecting ducts are located in the parenchyma of the kidney. These elements are involved in the passive reabsorption of water. The function of the tubules of the kidneys is the regulation of the amount of fluid and sodium in the body, which affect the value of blood pressure.

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Classification

Based on the layer in which the nephron capsules are located, the following types are distinguished:

Cortical - capsules of nephrons are located in the cortical ball, the composition includes glomeruli of small or medium caliber with the corresponding length of bends. Their afferent arteriole is short and wide, while the efferent arteriole is narrower.
Juxtamedullary nephrons are located in the renal medulla. Their structure is presented in the form of large renal bodies, which have relatively longer tubules. The diameters of the afferent and efferent arterioles are the same. The main role is the concentration of urine.
Subcapsular. Structures located directly under the capsule.

In general, in 1 minute both kidneys purify up to 1.2 thousand ml of blood, and in 5 minutes the entire volume of the human body is filtered. It is believed that nephrons, as functional units, are not capable of recovery. The kidneys are a delicate and vulnerable organ, therefore, factors that negatively affect their work lead to a decrease in the number of active nephrons and provoke the development of renal failure. Thanks to knowledge, the doctor is able to understand and identify the causes of changes in the urine, as well as to make a correction.

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renal glomeruli

The renal glomerulus consists of many capillary loops that form a filter through which fluid passes from the blood into Bowman's space - the initial section of the renal tubule. The renal glomerulus consists of approximately 50 capillaries assembled into a bundle, into which the only afferent arteriole that approaches the glomerulus branches and which then merge into the efferent arteriole.

Through 1.5 million glomeruli, which are contained in the kidneys of an adult, 120-180 liters of fluid are filtered per day. GFR depends on glomerular blood flow, filtration pressure, and filtration surface area. These parameters are strictly regulated by the tone of the afferent and efferent arterioles (blood flow and pressure) and mesangial cells (filtration surface). As a result of ultrafiltration occurring in the glomeruli, all substances with a molecular weight less than 68,000 are removed from the blood and a liquid is formed, called glomerular filtrate (Fig. 27-5A, 27-5B, 27-5C).

The tone of arterioles and mesangial cells is regulated by neurohumoral mechanisms, local vasomotor reflexes and vasoactive substances that are produced in the capillary endothelium (nitric oxide, prostacyclin, endothelins). Freely passing plasma, the endothelium does not allow platelets and leukocytes to come into contact with the basement membrane, thereby preventing thrombosis and inflammation.

Most of the plasma proteins do not penetrate into the Bowman space due to the structure and charge of the glomerular filter, which consists of three layers - the endothelium, permeated with pores, the basement membrane and the filtration gaps between the legs of the podocytes. The parietal epithelium separates Bowman's space from the surrounding tissue. This is briefly the purpose of the main parts of the glomerulus. It is clear that any damage to it can have two main consequences:

- decrease in GFR;

- the appearance of protein and blood cells in the urine.

The main mechanisms of damage to the renal glomeruli are presented in Table. 273.2.

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The kidney is a paired parenchymal organ located in the retroperitoneal space. 25% of the arterial blood ejected by the heart into the aorta passes through the kidneys. A significant part of the liquid and most of the substances dissolved in the blood (including medicinal substances) are filtered through the renal glomeruli and enter the renal tubular system in the form of primary urine, through which, after certain processing (reabsorption and secretion), the substances remaining in the lumen are excreted from the body. . The main structural and functional unit of the kidney is the nephron.

There are about 2 million nephrons in the human kidney. Groups of nephrons give rise to collecting ducts that continue into the papillary ducts, which end in the papillary foramen at the top of the renal pyramid. The renal papilla opens into the renal calyx. The fusion of 2-3 large renal calyces forms a funnel-shaped renal pelvis, the continuation of which is the ureter. The structure of the nephron. The nephron consists of a vascular glomerulus, a glomerular capsule (Shumlyansky-Bowman capsule) and a tubular apparatus: the proximal tubule, the nephron loop (the loop of Henle), the distal and thin tubules, and the collecting duct.

Vascular glomerulus.

A network of capillary loops, in which the initial stage of urination is carried out - ultrafiltration of blood plasma, forms a vascular glomerulus. Blood enters the glomerulus through the afferent (afferent) arteriole. It breaks up into 20-40 capillary loops, between which there are anastomoses. In the process of ultrafiltration, protein-free fluid moves from the lumen of the capillary into the glomerular capsule, forming primary urine, which flows through the tubules. Unfiltered fluid flows out of the glomerulus through the efferent (efferent) arteriole. The wall of the glomerular capillaries is a filtering membrane (kidney filter) - the main barrier to ultrafiltration of blood plasma. This filter consists of three layers: capillary endothelium, podocytes and basement membrane. The lumen between the capillary loops of the glomeruli is filled with mesangium.

The capillary endothelium has openings (fenestra) with a diameter of 40-100 nm, through which the main flow of the filtering fluid passes, but blood cells do not penetrate. Podocytes are large epithelial cells that make up the inner layer of the glomerular capsule.

Large processes extend from the cell body, which are divided into small processes (cytopodia, or "legs"), located almost perpendicular to the large processes. Between the small processes of podocytes there are fibrillar connections that form the so-called slit diaphragm. The slit diaphragm forms a system of filtration pores with a diameter of 5-12 nm.

Basement membrane of glomerular capillaries (GBM)
is located between the layer of endothelial cells lining its surface from the inside of the capillary, and the layer of podocytes covering its surface from the side of the glomerular capsule. Consequently, the process of hemofiltration passes through three barriers: the fenestrated endothelium of the glomerular capillaries, the basement membrane proper, and the slit diaphragm of the podocytes. Normally, BMC has a three-layer structure 250–400 nm thick, consisting of collagen-like protein filaments, glycoproteins, and lipoproteins. The traditional theory of the BMC structure implies the presence of filtration pores in it with a diameter of no more than 3 nm, which ensures the filtration of only a small amount of low molecular weight proteins: albumin, (32-microglobulin, etc.

And prevents the passage of large molecular components of the plasma. This selective permeability of BMC for proteins is called the size selectivity of BMC. Normally, due to the limited pore size of BMC, large molecular proteins do not enter the urine.

The glomerular filter has, in addition to the mechanical (pore size), also an electrical barrier for filtration. Normally, the surface of the BMC has a negative charge. This charge is provided by glycosaminoglycans, which are part of the outer and inner dense layers of the BMC. It has been established that it is heparan sulfate that is the very glycosaminoglycan that carries an anionic sites that provide a negative charge of BMC. Albumin molecules circulating in the blood are also negatively charged; therefore, approaching the BMC, they repel the similarly charged membrane without penetrating through its pores. This variant of the selective permeability of the basement membrane is called charge selectivity. The negative charge of BMA prevents albumins from passing through the filtration barrier, despite their low molecular weight, which allows them to penetrate through the pores of BMA. With preserved charge selectivity of BMC, urinary albumin excretion does not exceed 30 mg/day. The loss of the negative charge of BMC, as a rule, due to impaired heparan sulfate synthesis, leads to a loss of charge selectivity and an increase in urinary albumin excretion.

Factors determining BMC permeability:
Mesangium is a connective tissue that fills the gap between the capillaries of the glomerulus; with its help, the capillary loops are, as it were, suspended from the pole of the glomerulus. The composition of the mesangium includes mesangial cells - mesangiocytes and the main substance - the mesangial matrix. Mesangiocytes are involved in both the synthesis and catabolism of the substances that make up the BMC, have phagocytic activity, "cleansing" the glomerulus from foreign substances, and contractility.

Glomerulus capsule (Shumlyansky-Bowman capsule). The capillary loops of the glomerulus are surrounded by a capsule that forms a reservoir that passes into the basement membrane of the tubular apparatus of the nephron. The tubular apparatus of the kidney. The tubular apparatus of the kidney includes the urinary tubules, which are divided into proximal tubules, distal tubules and collecting ducts. The proximal tubule consists of convoluted, straight and thin parts. The epithelial cells of the convoluted part have the most complex structure. These are tall cells with numerous finger-like outgrowths directed into the lumen of the tubule - the so-called brush border. The brush border is a kind of adaptation of the cells of the proximal tubule to perform a huge load on the reabsorption of fluid, electrolytes, low molecular weight proteins, and glucose. The same function of the proximal tubule also determines the high saturation of these segments of the nephron with various enzymes involved both in the process of reabsorption and in the intracellular digestion of reabsorbed substances. The brush border of the proximal tubule contains alkaline phosphatase, y-glutamyl transferase, alanine aminopeptidase; cytoplasmic lactate dehydrogenase, malate dehydrogenase; lysosomes - P-glucuronidase, p-galactosidase, N-acetyl-B-D-glucosaminidase; mitochondria - alanine amino transferase, aspartate amino transferase, etc.

The distal tubule consists of the straight and convoluted tubules. At the point of contact of the distal tubule with the pole of the glomerulus, a “dense spot” (macula densa) is distinguished - here the continuity of the basement membrane of the tubule is disturbed, which ensures that the chemical composition of the urine of the distal tubule affects the glomerular blood flow. This site is the site of renin synthesis (see below - "The hormone-producing function of the kidneys"). The proximal thin and distal straight tubules form the descending and ascending limbs of the loop of Henle. Osmotic concentration of urine occurs in the loop of Henle. In the distal tubules, reabsorption of sodium and chlorine, secretion of potassium, ammonia and hydrogen ions are carried out.

The collecting ducts are the final segment of the nephron that transport fluid from the distal tubule to the urinary tract. The walls of the collecting ducts are highly permeable to water, which plays an important role in the processes of osmotic dilution and concentration of urine.

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Nephron as a morpho-functional unit of the kidney.

In humans, each kidney is made up 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)

Shumlyansky-Bowman's capsule, inside which is located a glomerulus of capillaries - the renal (Malpighian) body. Capsule diameter - 0.2 mm

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

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

Connecting tubule

Collecting duct

functional distinguish 4 segment:

1.Glomerulus;

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

3.Slim loop section - descending and thin part of the ascending part of the loop;

4.Distal - thick part of the ascending loop, distal convoluted tubule, connecting section.

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

Beginning in the renal cortex, the collecting ducts merge to form excretory ducts that 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

In various segments of the nephron tubules, there are significant differences depending on their localization in one or another 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 loops. Of great functional importance is the zone of the kidney in which the tubule is located, regardless of whether it is located in the cortex or medulla.

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

This arrangement of parts of the nephron in the kidney is not accidental. This is important in the osmotic concentration of urine. Several different types of nephrons function in the kidney:

1. With superficial ( superficial,

short loop );

2. And intracortical ( inside the cortex );

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

One of the important differences listed 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 knee of the loop is located above the border, between the outer and inner parts of the medulla. In all juxtamedullary nephrons, long loops penetrate 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 does not depend on systemic arterial pressure in a wide range of its changes. It's connected with myogenic regulation , due to the ability of vasafferens smooth muscle cells to contract in response to stretching them with 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. Through the vessels of the renal cortex flows 91-93% of the blood entering the kidney, the rest of it supplies the medulla of the kidney. The 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 the renal blood flow is that when the 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 gates of the kidneys, they are divided into several interlobar arteries that 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, interlobular arteries go, which give rise to numerous afferent glomerular arterioles.

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

Two networks of capillaries – high and low pressure.

In high pressure capillaries (70 mm Hg) - in the renal glomerulus - filtration occurs. A lot of pressure is due to the fact that: 1) the renal arteries depart 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 anostomoses that play a compensatory role. In the formation of the peritubular capillary network, Ludwig's arteriole, which departs from the interlobular artery, or from the afferent glomerular arteriole, is essential. Thanks to Ludwig's arteriole, extraglomerular blood supply to the tubules is possible in case of death of the renal corpuscles.

The arterial capillaries, which form the peritubular network, pass into the venous ones. The latter form stellate venules located under the fibrous capsule - interlobular veins that flow into the arcuate veins, which merge and form the renal vein, which flows into the inferior pudendal vein.

In the kidneys, 2 circles of blood circulation are distinguished: a large cortical - 85-90% of the blood, a small juxtamedullary - 10-15% of the blood. Under physiological conditions, 85-90% of the blood circulates through the large (cortical) circle of the renal circulation; in 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. Direct vessels 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 pathway of blood circulation is a kind of "shunt" (Truet's shunt), in which most of the blood enters not into the cortex, but into the medulla of the kidneys. This is the so-called drainage system of the kidneys.

The nephron, the structure of which directly depends on human health, is responsible for the functioning of the kidneys. The kidneys consist of several thousand of these nephrons, thanks to them, urination is correctly carried out in the body, the removal of toxins and the purification of blood from harmful substances after processing the products obtained.

What is a nephron?

The nephron, the structure and significance of which is very important for the human body, is a structural and functional unit inside the kidney. Inside this structural element, the formation of urine is carried out, which subsequently leaves the body using the appropriate pathways.

Biologists say that there are up to two million of these nephrons inside each kidney, and each of them must be absolutely healthy so that the genitourinary system can fully perform its function. If the kidney is damaged, the nephrons cannot be restored; they will be excreted along with the newly formed urine.

Nephron: its structure, functional significance

The nephron is a shell for a small tangle, which consists of two walls and closes a small tangle of capillaries. The inner part of this shell is covered with epithelium, the special cells of which help to achieve additional protection. The space that is formed between the two layers can be transformed into a small hole and a channel.

This channel has a brush edge of small villi, immediately after it begins a very narrow section of the sheath loop, which descends. The wall of the site consists of flat and small epithelial cells. In some cases, the compartment of the loop reaches the depth of the medulla, and then turns to the crust of the renal formations, which gradually develop into another segment of the nephron loop.

How is the nephron arranged?

The structure of the renal nephron is very complex, so far biologists around the world are struggling with attempts to recreate it in the form of an artificial formation suitable for transplantation. The loop appears predominantly from the rising part, but may also include a delicate one. As soon as the loop is in the place where the ball is placed, it enters a curved small channel.

In the cells of the resulting formation, there is no fleecy edge, however, a large number of mitochondria can be found here. The total area of the membrane can be increased due to the numerous folds that form as a result of the formation of a loop within a single nephron taken.

The scheme of the structure of the human nephron is quite complex, since it requires not only careful drawing, but also a thorough knowledge of the subject. It will be quite difficult for a person far from biology to portray it. The last section of the nephron is a shortened connecting channel that goes into the accumulation tube.

The channel is formed in the cortical part of the kidney, with the help of storage tubes it passes through the "brain" of the cell. On average, the diameter of each shell is about 0.2 millimeters, but the maximum length of the nephron channel, recorded by scientists, is about 5 centimeters.

Sections of the kidney and nephrons

The nephron, the structure of which became known to scientists for certain only after a number of experiments, is located in each of the structural elements of the most important organs for the body - the kidneys. The specificity of kidney functions is such that it requires the existence of several sections of structural elements at once: a thin segment of the loop, distal and proximal.

All channels of the nephron are in contact with the stacked storage tubes. As the embryo develops, they arbitrarily improve, however, in an already formed organ, their functions resemble the distal portion of the nephron. Scientists have repeatedly reproduced the detailed process of nephron development in their laboratories over the course of several years, however, genuine data were obtained only at the end of the 20th century.

Varieties of nephrons in human kidneys

The structure of the human nephron varies depending on the type. There are juxtamedullary, intracortical and superficial. The main difference between them is their location within the kidney, the depth of the tubules and the localization of the glomeruli, as well as the size of the tangles themselves. In addition, scientists attach importance to the features of the loops and the duration of the various segments of the nephron.

The superficial type is a connection created from short loops, and the juxtamedullary type is made from long loops. Such diversity, according to scientists, appears as a result of the need for nephrons to reach all parts of the kidney, including the one that is located below the cortical substance.

Parts of the nephron

The nephron, the structure and significance of which for the body are well studied, directly depends on the tubule present in it. It is the latter that is responsible for the constant functional work. All substances that are inside the nephrons are responsible for the safety of certain types of renal tangles.

Inside the cortical substance, one can find a large number of connecting elements, specific divisions of channels, renal glomeruli. The work of the entire internal organ will depend on whether they are correctly placed inside the nephron and the kidney as a whole. First of all, this will affect the uniform distribution of urine, and only then on its correct removal from the body.

Nephrons as filters

The structure of the nephron at first glance looks like one big filter, but it has a number of features. In the middle of the 19th century, scientists assumed that the filtration of fluids in the body precedes the stage of urine formation, a hundred years later this was scientifically proven. With the help of a special manipulator, scientists were able to obtain the internal fluid from the glomerular membrane, and then conduct a thorough analysis of it.

It turned out that the shell is a kind of filter, with the help of which water and all the molecules that form blood plasma are purified. The membrane with which all fluids are filtered is based on three elements: podocytes, endothelial cells, and a basement membrane is also used. With their help, the fluid that needs to be removed from the body enters the nephron tangle.

The insides of the nephron: cells and membrane

The structure of the human nephron must be considered in terms of what is contained in the nephron glomerulus. Firstly, we are talking about endothelial cells, with the help of which a layer is formed that prevents particles of protein and blood from entering inside. Plasma and water pass further, freely enter the basement membrane.

The membrane is a thin layer that separates the endothelium (epithelium) from connective tissue. The average membrane thickness in the human body is 325 nm, although thicker and thinner variants may occur. The membrane consists of a nodal and two peripheral layers that block the path of large molecules.

Podocytes in the nephron

The processes of podocytes are separated from each other by shield membranes, on which the nephron itself, the structure of the structural element of the kidney and its performance depend. Thanks to them, the sizes of substances that need to be filtered are determined. Epithelial cells have small processes, due to which they are connected to the basement membrane.

The structure and functions of the nephron are such that, taken together, all its elements do not allow molecules with a diameter of more than 6 nm to pass through and filter out smaller molecules that must be removed from the body. The protein cannot pass through the existing filter due to special membrane elements and negatively charged molecules.

Features of the kidney filter

The nephron, whose structure requires careful study by scientists seeking to recreate the kidney using modern technologies, carries a certain negative charge, which forms a limit on protein filtration. The size of the charge depends on the dimensions of the filter, and in fact the component of the glomerular substance itself depends on the quality of the basement membrane and the epithelial coating.

The features of the barrier used as a filter can be implemented in a variety of variations, each nephron has individual parameters. If there are no disturbances in the work of nephrons, then in the primary urine there will be only traces of proteins that are inherent in blood plasma. Particularly large molecules can also penetrate through the pores, but in this case everything will depend on their parameters, as well as on the localization of the molecule and its contact with the forms that the pores take on.

Nephrons are not able to regenerate, therefore, if the kidneys are damaged or any diseases appear, their number gradually begins to decrease. The same thing happens for natural reasons when the body begins to age. The restoration of nephrons is one of the most important tasks that biologists around the world are working on.

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 excreted. During the day, two million kidney nephrons, as many as there are in the body, produce 170 liters of primary urine, thicken to a daily amount of up to one and a half liters. The total area of the excretory surface of nephrons is almost 8 m 2, which is 3 times the area of the skin.

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

The arterial blood passing through the afferent arteriole is purified in the kidneys. Purified blood exits through the outgoing arteriole. The diameter of the afferent arteriole is larger than that of the arteriole, thereby creating a pressure drop.

Structure

The divisions of the kidney nephron are:

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

Diagram of a nephron

Capsule

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

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 its entire length.

Outside, there is a basement membrane, collected in numerous folds, which straighten out when the tubules are filled. The tubule at the same time acquires a rounded shape in diameter, and the epithelium is flattened. In the absence of fluid, the diameter of the tubule becomes narrow, the cells acquire a prismatic appearance.

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 wrap around the tubule in a dense network. At this site, bile acid is absorbed into the cavity of the tubule, oxalic, paraaminohyppuric, uric acids are absorbed, adrenaline, acetylcholine, thiamine, histamine are absorbed, drugs are transported - penicillin, furosemide, atropine, etc.

Loop of Henle

After entering the brain ray, the proximal tubule passes into the initial section of the loop of Henle. The tubule passes into the descending segment of the loop, which descends into the medulla. Then the ascending part rises into the cortex, approaching the Bowman's capsule.

The internal structure of the loop at first does not differ from the structure of the proximal tubule. Then the loop lumen narrows, Na filtration passes through it 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, passes into the distal tubule.

Gentle loop

Distal tubule

This area already, in short, consists 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 is reabsorbed, water reabsorption continues, secretion of hydrogen ions and ammonia into the lumen of the tubule continues.

In the video, a diagram of the structure of the kidney and nephron:

Types of nephrons

According to the structural features, functional purpose, there are such types of nephrons that function in the kidney:

cortical - superficial, intracortical;
juxtamedullary.

Cortical

There are two types of nephrons in the cortex. Superficials make up about 1% of the total number of nephrons. They differ in the superficial location of the glomeruli in the cortex, the shortest loop of Henle, and a small amount of filtration.

The number of intracortical - more than 80% of kidney nephrons, located in the middle of the cortical layer, play a major role in urine filtration. The blood in the glomerulus of the intracortical nephron passes under pressure, since the afferent arteriole is much wider than the outflow 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 cortical and medulla, the rest of it is located in the medulla, the loop of Henle descends almost to the renal pelvis itself.

This type of nephron is of decisive importance in the ability to concentrate urine. A feature of the juxtamedullary nephron is that the outgoing 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, flow into the venous network.

Functions

The functions of the kidney nephron include:

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

Urine is formed in several stages:

in the glomeruli, the 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, the reabsorption of useful compounds, proteins, amino acids, glucose, hormones, minerals. Cortical nephrons are involved in the processes of filtration, reabsorption due to the peculiarities of blood supply, and reabsorbed compounds immediately penetrate into the blood through a closely located 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 the movement of blood in the outgoing arteriole. The arteriole does not pass into the capillary network, but into the venules that flow into the 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.

Violation of the functions of the nephron and how to restore

Violation 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 a violation of the transport functions of nephrons are called tubulopathies, among which there are:

primary tubulopathies - congenital dysfunctions;
secondary - acquired violations of the 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 localization 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 parts of the nephron leads to the formation of:

phosphaturia;
hyperaminoaciduria;
renal acidosis;
glycosuria.

Violation of phosphate reabsorption leads to the development of rickets-like bone structure - a condition resistant to vitamin D treatment. Pathology is associated with the absence of a phosphate carrier protein, a lack of calcitriol-binding receptors.

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

So, if the reabsorption of cystine is impaired, the disease of cystinuria develops - an autosomal recessive disease. The disease is manifested by developmental delay, renal colic. In the urine with cystinuria, cystine stones may appear, which are easily dissolved 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, while 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, tubular acidosis. Renal diabetes is a hereditary disorder. A congenital disorder is caused by a lack of response of cells in the distal tubules to antidiuretic hormone. Lack of response leads to a violation of the 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. At the same time, the reabsorption of phosphates, bicarbonates is impaired, amino acids and glucose are not absorbed. The syndrome is manifested by developmental delay, osteoporosis, pathology of the bone structure, acidosis.

The renal glomerulus consists of many capillary loops that form a filter through which fluid passes from the blood into Bowman's space - the initial section of the renal tubule. The renal glomerulus consists of approximately 50 capillaries collected in a bundle, into which the only afferent arteriole suitable for the glomerulus branches and which then merge into the efferent arteriole.

Through 1.5 million glomeruli, which are contained in the kidneys of an adult, 120-180 liters of fluid are filtered per day. GFR depends on glomerular blood flow, filtration pressure, and filtration surface area. These parameters are strictly regulated by the tone of the afferent and efferent arterioles (blood flow and pressure) and mesangial cells (filtration surface). As a result of ultrafiltration occurring in the glomeruli, all substances with a molecular weight of less than 68,000 are removed from the blood and a liquid is formed, called glomerular filtrate (Fig. 27-5A, 27-5B, 27-5C).

Most of the plasma proteins do not penetrate into Bowman's space due to the structure and charge of the glomerular filter, consisting of three layers - endothelium, permeated with pores, basement membrane and filtration gaps between the legs of podocytes. The parietal epithelium separates Bowman's space from the surrounding tissue. This is briefly the purpose of the main parts of the glomerulus. It is clear that any damage to it can have two main consequences:

Decreased GFR;

The appearance of protein and blood cells in the urine.

The main mechanisms of damage to the renal glomeruli are presented in

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The peculiarities and specificity of the functions of the kidneys are explained by the peculiarity of the specialization of their structure. The functional morphology of the kidneys is studied at different structural levels - from macromolecular and ultrastructural to organ and systemic. Thus, the homeostatic functions of the kidneys and their disorders have a morphological substrate at all levels of the structural organization of this organ. Below we consider the originality of the fine structure of the nephron, the structure of the vascular, nervous and hormonal systems of the kidneys, which makes it possible to understand the features of the functions of the kidneys and their disturbances in the most important kidney diseases.

The nephron, which consists of the vascular glomerulus, its capsule, and renal tubules (Fig. 1), has a high structural and functional specialization. This specialization is determined by the histological and physiological characteristics of each constituent element of the glomerular and tubular parts of the nephron.

Rice. 1. The structure of the nephron. 1 - vascular glomerulus; 2 - the main (proximal) department of the tubules; 3 - thin segment of the loop of Henle; 4 - distal tubules; 5 - collecting tubes.

Each kidney contains approximately 1.2-1.3 million glomeruli. The vascular glomerulus has about 50 capillary loops between which anastomoses are found, allowing the glomerulus to function as a "dialysis system". The capillary wall is glomerular filter, consisting of epithelium, endothelium and a basement membrane (BM) located between them (Fig. 2).

Rice. 2. Glomerular filter. Scheme of the structure of the capillary wall of the renal glomerulus. 1 - capillary lumen; endothelium; 3 - BM; 4 - podocyte; 5 - small processes of the podocyte (pedicles).

Glomerular epithelium, or podocyte, consists of a large cell body with a nucleus at its base, mitochondria, a lamellar complex, an endoplasmic reticulum, fibrillar structures and other inclusions. The structure of podocytes and their relationship with capillaries have been well studied recently with the help of a scanning electronic microphone. It is shown that large processes of the podocyte depart from the perinuclear zone; they resemble "pillows" covering a significant surface of the capillary. Small processes, or pedicles, depart from large processes almost perpendicularly, intertwine with each other and cover all the capillary space free from large processes (Fig. 3, 4). Pedicles are closely adjacent to each other, the interpedicular space is 25-30 nm.

Rice. 3. Filter electron diffraction pattern

Rice. 4. The surface of the capillary loop of the glomerulus is covered with the body of the podocyte and its processes (pedicles), between which interpedicular fissures are visible. Scanning electron microscope. X6609.

Podocytes are interconnected by beam structures - peculiar junction ", formed from the ininmolemma. Fibrillar structures are especially distinctly disguised between the small processes of podocytes, where they form the so-called slit diaphragm - slit diaphragma

Podocytes are interconnected by beam structures - "peculiar junction", formed from the plasmalemma. Fibrillar structures are especially distinctly sharpened between the small processes of podocytes, where they form the so-called slit diaphragm - slit diaphragma (see Fig. 3), which plays a large role in glomerular filtration. The slit diaphragm, having a filamentary structure (thickness 6 nm, length 11 nm), forms a kind of lattice, or a system of filtration pores, the diameter of which in humans is 5-12 nm. From the outside, the slit diaphragm is covered with glycocalyx, i.e., the sialoprotein layer of the podocyte cytolemma; inside, it borders on the lamina rara externa BM of the capillary (Fig. 5).

Rice. 5. Scheme of relationships between the elements of the glomerular filter. Podocytes (P) containing myofilaments (MF) are surrounded by a plasma membrane (PM). The filaments of the basement membrane (VM) form a slit diaphragm (SM) between the small processes of podocytes, covered on the outside by the glycocalyx (GK) of the plasma membrane; the same VM filaments are associated with endothelial cells (En), leaving only its pores (F) free.

The filtration function is performed not only by the slit diaphragm, but also by the myofilaments of the podocyte cytoplasm, with the help of which they contract. Thus, “submicroscopic pumps” pump the plasma ultrafiltrate into the cavity of the glomerular capsule. The system of microtubules of podocytes also serves the same function of primary urine transport. Podocytes are associated not only with the filtration function, but also with the production of BM substance. In the cisterns of the granular endoplasmic reticulum of these cells, material similar to that of the basement membrane is found, which is confirmed by an autoradiographic label.

Changes in podocytes are most often secondary and are usually observed in proteinuria, nephrotic syndrome (NS). They are expressed in hyperplasia of the fibrillar structures of the cell, the disappearance of pedicles, vacuolization of the cytoplasm and disorders of the slit diaphragm. These changes are associated with both primary damage to the basement membrane and proteinuria itself [Serov VV, Kupriyanova LA, 1972]. Initial and typical changes in podocytes in the form of the disappearance of their processes are characteristic only for lipoid nephrosis, which is well reproduced in the experiment using an aminonucleoside.

endothelial cells glomerular capillaries have pores up to 100-150 nm in size (see Fig. 2) and are equipped with a special diaphragm. The pores occupy about 30% of the endothelial lining covered with glycocalyx. The pores are considered as the main ultrafiltration pathway, but a transendothelial pathway that bypasses the pores is also allowed; This assumption is supported by the high pinocytotic activity of the glomerular endothelium. In addition to ultrafiltration, the endothelium of glomerular capillaries is involved in the formation of BM substance.

Changes in the endothelium of the glomerular capillaries are diverse: swelling, vacuolization, necrobiosis, proliferation and desquamation, however, destructive-proliferative changes that are so characteristic of glomerulonephritis (GN) predominate.

basement membrane glomerular capillaries, in the formation of which not only podocytes and endothelium participate, but also mesangial cells, has a thickness of 250-400 nm and looks three-layer in an electron microscope; the central dense layer (lamina densa) is surrounded by thinner layers on the outer (lamina rara externa) and inner (lamina rara interna) sides (see Fig. 3). The BM itself serves as the lamina densa, which is made up of protein filaments like collagen, glycoproteins, and lipoproteins; the outer and inner layers containing mucosubstances are essentially the glycocalyx of podocytes and endothelium. The lamina densa filaments, 1.2–2.5 nm thick, enter into “mobile” compounds with the molecules of their surrounding substances and form a thixotropic gel. It is not surprising that the substance of the membrane is spent on the implementation of the filtration function; BM completely renews its structure during the year.

The presence of collagen-like filaments in the lamina densa is associated with the hypothesis of filtration pores in the basement membrane. It was shown that the average pore radius of the membrane is 2.9±1 nm and is determined by the distance between normally located and unaltered collagen-like protein filaments. With a drop in hydrostatic pressure in the glomerular capillaries, the initial “packing” of collagen-like filaments in the BM changes, which leads to an increase in the filtration pore size.

It is assumed that under normal blood flow, the pores of the basement membrane of the glomerular filter are large enough and can pass albumin, IgG, and catalase molecules, but the penetration of these substances is limited by a high filtration rate. Filtration is also limited by an additional barrier of glycoproteins (glycocalix) between the membrane and the endothelium, and this barrier is damaged under conditions of disturbed glomerular hemodynamics.

Methods with the use of markers, which take into account the electric charge of molecules, were of great importance to explain the mechanism of proteinuria in damage to the basement membrane.

Changes in the BM of the glomerulus are characterized by its thickening, homogenization, loosening and fibrillation. BM thickening occurs in many diseases with proteinuria. At the same time, an increase in the gaps between the membrane filaments and depolymerization of the cementing substance are observed, which is associated with an increased porosity of the membrane for blood plasma proteins. In addition, membranous transformation (according to J. Churg), which is based on excessive production of the BM substance by podocytes, and mesangial interposition (according to M. Arakawa, P. Kimmelstiel), represented by the "eviction" of processes of mesangiocytes to the periphery of capillary cells, lead to thickening of BM glomeruli. loops that exfoliate the endothelium from the BM.

In many diseases with proteinuria, in addition to thickening of the membrane, electron microscopy reveals various deposits (deposits) in the membrane or in its immediate vicinity. At the same time, each deposit of a particular chemical nature (immune complexes, amyloid, hyaline) has its own ultrastructure. Most often, deposits of immune complexes are detected in BM, which leads not only to profound changes in the membrane itself, but also to destruction of podocytes, hyperplasia of endothelial and mesangial cells.

The capillary loops are connected to each other and suspended like a mesentery to the glomerular pole by the connective tissue of the glomerulus, or mesangium, the structure of which is mainly subordinated to the filtering function. With the help of an electron microscope and histochemistry methods, a lot of new things have been introduced into the previous ideas about fibrous structures and mesangial cells. The histochemical features of the main substance of the mesangium are shown, bringing it closer to the fibromucin of fibrils capable of receiving silver, and mesangium cells, which differ in ultrastructural organization from the endothelium, fibroblast and smooth muscle fiber.

In mesangial cells, or mesangiocytes, a lamellar complex, a granular endoplasmic reticulum are well drawn out, they contain many small mitochondria, ribosomes. The cytoplasm of cells is rich in basic and acidic proteins, tyrosine, tryptophan and histidine, polysaccharides, RNA, glycogen. The peculiarity of the ultrastructure and the richness of the plastic material explain the high secretory and hyperplastic potencies of the mesangial cells.

Mesangiocytes are capable of reacting to certain damages of the glomerular filter by producing BM substance, which manifests a reparative reaction in relation to the main component of the glomerular filter. Hypertrophy and hyperplasia of mesangial cells lead to the expansion of the mesangium, to its interposition, when the cell processes surrounded by a membrane-like substance, or the cells themselves, move to the periphery of the glomerulus, which causes thickening and sclerosis of the capillary wall, and in the event of a breakthrough of the endothelial lining, obliteration of its lumen. The development of glomerulosclerosis is associated with interposition of mesangium in many glomerulopathies (GN, diabetic and hepatic glomerulosclerosis, etc.).

Mesangial cells as one of the components of the juxtaglomerular apparatus (JGA) [Ushkalov A. F., Vikhert A. M., 1972; Zufarov K. A., 1975; Rouiller S., Orci L., 1971] are capable of incretion of renin under certain conditions. This function is apparently served by the relationship of the processes of mesangiocytes with elements of the glomerular filter: a certain number of processes perforate the endothelium of the glomerular capillaries, penetrate into their lumen and have direct contact with the blood.

In addition to the secretory (synthesis of a collagen-like substance of the basement membrane) and endocrine (synthesis of renin) functions, mesangiocytes also perform a phagocytic function - "cleansing" the glomerulus and its connective tissue. It is believed that mesangiocytes are capable of contraction, which is subject to the filtration function. This assumption is based on the fact that fibrils with actin and myosin activity were found in the cytoplasm of mesangial cells.

glomerulus capsule represented by BM and epithelium. Membrane, continuing into the main department of the tubules, consists of reticular fibers. Thin collagen fibers anchor the glomerulus in the interstitium. epithelial cells are fixed to the basement membrane with filaments containing actomyosin. On this basis, the epithelium of the capsule is considered as a kind of myoepithelium that changes the volume of the capsule, which serves as a filtering function. The epithelium is cuboidal but functionally similar to that of the main tubule; in the region of the glomerular pole, the epithelium of the capsule passes into podocytes.

Clinical Nephrology

ed. EAT. Tareeva