The structure of the renal glomerulus. The structure of the kidney and nephron The structure of the nephron table

In each kidney of an adult, there are at least 1 million nephrons, each of which is capable of producing urine. At the same time, about 1/3 of all nephrons usually function, which is sufficient for the full implementation 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 ability to regenerate. In many people at the age of 80, the number of nephrons decreases by 40% compared to 40-year-olds. However, the loss of such a large number of nephrons is not a threat to life, since the rest of them can fully perform the 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 be the cause of chronic renal failure.

Everyone nephron consists of a renal (Malpighian) corpuscle, in which ultrafiltration of blood plasma and the formation of primary urine, and a system of tubules and tubules, 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 during its movement through the pelvis (cups, cups), ureters, temporary retention in the bladder and through the urinary canal does not change significantly. Thus, healthy person the composition of the final urine excreted during urination is very close to the composition of the urine excreted into the lumen (minor calyces) of the pelvis.

renal corpuscle is located in the cortical layer of the kidneys, is the initial part of the nephron and is formed capillary glomerulus(consisting of 30-50 intertwining capillary loops) and capsule Shumlyansky - Boumeia. On the cut, 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) adhere tightly to the wall of the glomerular capillaries. The outer leaf of the capsule is located at some distance from the inner. 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 cavity of the capsule, the primary urine passes into the lumen of the tubules of the nephron: proximal tubule(curved and straight segments), loop of Henle(descending and ascending divisions) and distal tubule(straight and twisted 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 (dense spot - maculadensa), close to them. The 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 in the cortex of the kidney, and the loop of Henle is in the medulla.

Urine flows from the convoluted distal tubule into the connecting canal, from it to collecting duct and collecting duct cortical substance of the kidneys; 8-10 collecting ducts join 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 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 the hyperosmolarity of the renal medulla, concentrating and diluting urine, and are important structural components of the formation of the final urine.

The structure of the nephron

Each nephron begins with a double-walled capsule, inside which there is a vascular glomerulus. The capsule itself consists of two sheets, between which there is a cavity that passes into the lumen of the proximal tubule. It consists of the proximal convoluted and proximal straight tubules that make up the proximal segment of the nephron. characteristic feature 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, which can descend deeply into the medulla, where it forms a loop and turns 180 ° towards the cortical substance in the form of an ascending thin, turning into a thick part of the nephron loop. The ascending section of the loop rises to the level of its glomerulus, where the distal convoluted tubule begins, which passes into a short connecting tubule connecting the nephron to the collecting ducts. The collecting ducts begin in the renal cortex, merge to form larger excretory ducts that pass through the medulla and drain into the calyx cavity, 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. The 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 — the descending thin knee of a loop of a nephron; 5 — the ascending thin knee of a loop of a nephron; 6 — a distal direct tubule (the thick ascending knee of a loop of a nephron); 7 — a dense spot of a distal tubule; 8 - distal convoluted tubule; 9 - connecting tubule; 10 - collecting duct of the cortical substance of the kidney; 11 - collecting duct of the outer medulla; 12 - collecting duct of the internal medulla

Different types of nephrons differ not only in localization, but also in the size of the glomeruli, the depth of their location, as well as the length of individual sections of the nephron, especially the loop of Henle, and 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, the 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 the renal blood flow is that the blood flow of the kidney remains constant when changing within a fairly wide range of systemic blood pressure. This is ensured by special mechanisms of self-regulation of blood circulation in the kidney. Short renal arteries depart from the aorta, in the kidney they branch into smaller vessels. The afferent (afferent) arteriole enters the renal glomerulus, which breaks up into capillaries in it. When capillaries merge, they form the efferent (efferent) arteriole, through which the outflow of blood from the glomerulus is carried out. After departing from 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 split into a peritubular capillary network, but forms straight vessels that descend into the medulla of the kidney.

Types of Nephrons

Types of nephrons

According to the features of the structure and functions, they are distinguished two main types of nephrons: cortical (70-80%) and juxtamedullary (20-30%).

Cortical nephrons subdivided into superficial, or superficial, cortical nephrons, in which the renal corpuscles are located in the outer part of the cortical substance, and intracortical cortical nephrons, in which the renal corpuscles are located in the middle part of the cortical substance of the kidney. Cortical nephrons have a short loop of Henle penetrating only the outer part of the 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 cortical substance on the border with the medulla. They have a long loop of Henle penetrating deep into the medulla, up to the tops of the pyramids. The main purpose of the juxtamedullary nephrons is to create a high osmotic pressure in the renal medulla, which is necessary for concentrating and reducing the volume of the final urine.

Effective filtration pressure

  • EFD \u003d R cap - R bk - R onk.
  • R cap- hydrostatic pressure in the capillary (50-70 mm Hg);
  • R 6k- hydrostatic pressure in the lumen of Bowman's capsule - Shumlyansky (15-20 mm Hg);
  • R onk- oncotic pressure in the capillary (25-30 mm Hg).

EPD \u003d 70 - 30 - 20 \u003d 20 mm Hg. Art.

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

The kidneys are a complex structure. Them structural unit is the nephron. The structure of the nephron allows it to fully perform its functions - filtration, the process of reabsorption, excretion and secretion of biologically active components take place in it.

Primary, then secondary urine is formed, which is excreted through bladder. During the day, a large amount of plasma is filtered through the excretory organ. Part of it is later returned to the body, the rest is removed.

The structure and functions of nephrons are interrelated. Any damage to the kidneys or their smallest units can lead to intoxication and further disruption of the entire body. The consequence of the irrational use of certain drugs, improper treatment or diagnosis can be kidney failure. The first manifestations of symptoms are the reason for visiting a specialist. Urologists and nephrologists deal with this problem.

The nephron is the structural and functional unit of the kidney. There are active cells that are directly involved in the production of urine (a third of the total), the rest are in reserve.

Reserve cells become active in emergency cases, for example, in trauma, critical conditions, when a large percentage of kidney units is abruptly lost. The physiology of excretion implies partial cell death, therefore, reserve structures are able to be activated in the shortest possible time to maintain the functions of the organ.

Every year, up to 1% of structural units are lost - they die forever and are not restored. With the right lifestyle, the absence of chronic diseases, loss begins only after 40 years. Given that the number of nephrons in a kidney is approximately 1 million, the percentage seems small. By old age, the work of the body can deteriorate significantly, which threatens to disrupt the functionality of the urinary system.

The aging process can be slowed down by making lifestyle changes and drinking enough clean drinking water. Even in the best case, only 60% of active nephrons remain in each kidney over time. This figure is not critical at all, since plasma filtration is disturbed only with the loss of more than 75% of the cells (both active and those in reserve).

Some people live with the loss of one kidney, and then the second one does all the work. The work of the urinary system is significantly disrupted, so it is necessary to carry out the prevention and treatment of diseases in time. In this case, you need a regular visit to the doctor for the appointment of maintenance therapy.

Anatomy of the nephron

The anatomy and structure of the nephron is quite complex - each element plays a specific role. In the event of a malfunction in the work of even the smallest component of the kidney, they cease to function normally.

  • capsule;
  • glomerular structure;
  • tubular structure;
  • loops of Henle;
  • collecting ducts.

The nephron in the kidney consists of segments communicated with each other. Shumlyansky-Bowman's capsule, a tangle of small vessels, are the components of the renal body, where the filtration process takes place. Next come the tubules, where substances are reabsorbed and produced.

From the body of the kidney begins the proximal section; further out the loops that go to the distal section. The nephrons, when unfolded, individually have a length of about 40 mm, and when they are folded, it turns out to be about 100,000 m.

Capsules of nephrons are located in the cortex, are included in the medulla, then again in the cortex, and at the end - in the collecting structures that go into the renal pelvis, where the ureters begin. They remove secondary urine.

Capsule

The nephron originates from the Malpighian body. It consists of a capsule and a tangle of capillaries. The cells around the small capillaries are located in the form of a cap - this is the renal corpuscle, which passes the delayed plasma. Podocytes cover the wall of the capsule from the inside, which, together with the outer one, forms a slit-like cavity with a diameter of 100 nm.

Fenestrated (fenestrated) capillaries (components of the glomerulus) are supplied with blood from the afferent arteries. In another way, they are called the "fairy grid" because they do not play any role in gas exchange. The blood passing through this grid does not change its gas composition. Plasma and solutes under the influence blood pressure enter the capsule.

The nephron capsule accumulates an infiltrate containing harmful products purification of blood plasma - this is how primary urine is formed. The slit-like gap between the layers of the epithelium acts as a pressure filter.

Thanks to the adductor and efferent glomerular arterioles, the pressure changes. The basement membrane plays the role of an additional filter - it retains some blood elements. The diameter of protein molecules is larger than the pores of the membrane, so they do not pass through.

Unfiltered blood enters the efferent arterioles, which pass into a network of capillaries that envelops the tubules. In the future, substances that are reabsorbed in these tubules enter the bloodstream.

The human kidney nephron capsule communicates with the tubule. The next section is called the proximal, where the primary urine goes further.

Collection of tubules

The proximal tubules are either straight or curved. The surface inside is lined with epithelium of a cylindrical and cubic type. The brush border with villi is an absorbing layer of nephron tubules. Selective capture is provided by a large area of ​​the proximal tubules, close dislocation of the peritubular vessels, and a large number of mitochondria.

Fluid circulates between cells. Plasma components in the form of biological substances are filtered. The convoluted tubules of the nephron produce erythropoietin and calcitriol. Harmful inclusions that enter the filtrate using reverse osmosis are excreted with urine.

Nephron segments filter creatinine. The amount of this protein in the blood is an important indicator of the functional activity of the kidneys.

Loops of Henle

The loop of Henle captures part of the proximal and distal section. At first, the diameter of the loop does not change, then it narrows and passes Na ions outward, into the extracellular space. By creating osmosis, H2O is sucked in under pressure.

The descending and ascending ducts are the components of the loop. The descending section with a diameter of 15 µm consists of epithelium, where multiple pinocytic vesicles are located. The ascending portion is lined with cuboidal epithelium.

Loops are distributed between cortical and brain substance. In this area, water moves to the descending part, then returns.

At the beginning, the distal canal touches the capillary network at the site of the incoming and outgoing vessel. It is rather narrow and is lined with smooth epithelium, and on the outside there is a smooth basement membrane. Ammonia and hydrogen are released here.

collecting ducts

The collecting ducts are also known as Bellini's ducts. Their inner lining is light and dark epithelial cells. The former reabsorb water and are directly involved in the production of prostaglandins. Hydrochloric acid is produced in the dark cells of the folded epithelium, has the property of changing the pH of the urine.

Collecting tubules and collecting ducts do not belong to the structure of the nephron, as they are located slightly lower in the renal parenchyma. In these structural elements, passive reabsorption of water occurs. Depending on the functionality of the kidneys, the amount of water and sodium ions in the body is regulated, which, in turn, affects blood pressure.

Structural elements are subdivided depending on the structural features and functions.

  • cortical;
  • juxtamedullary.

Cortical are divided into two types - intracortical and superficial. The number of the latter is approximately 1% of all units.

Features of superficial nephrons:

  • small volume of filtration;
  • the location of the glomeruli on the surface of the cortex;
  • the shortest loop.

The kidneys mainly consist of nephrons of the intracortical type, of which more than 80%. They are located in the cortical layer and play a major role in the filtration of primary urine. Due to the greater width of the output arterioles, blood enters the glomeruli of intracortical nephrons under pressure.

Cortical elements regulate the amount of plasma. With a lack of water, it is taken back from the juxtamedullary nephrons, located in a larger amount in the medulla. They are distinguished by large renal corpuscles with relatively long tubules.

Juxtamedullary make up more than 15% of all nephrons of the organ and form the final amount of urine, determining its concentration. Their structural feature is the long loops of Henle. The efferent and adductor vessels are of the same length. From the efferent loops are formed, penetrating the medulla in parallel with Henle. Then they enter the venous network.

Functions

Depending on the type, the nephrons of the kidneys perform the following functions:

  • filtration;
  • reverse suction;
  • secretion.

The first stage is characterized by the production of primary urea, which is further cleared by reabsorption. At the same stage, useful substances, micro and macro elements, water are absorbed. The last stage of urine formation is represented by tubular secretion - secondary urine is formed. It removes substances that the body does not need.
The structural and functional unit of the kidney is the nephrons, which:

  • maintain water-salt and electrolyte balance;
  • regulate the saturation of urine with biologically active components;
  • maintain acid-base balance (pH);
  • control blood pressure;
  • remove metabolic products and other harmful substances;
  • participate in the process of gluconeogenesis (obtaining glucose from non-carbohydrate type compounds);
  • provoke the secretion of certain hormones (for example, regulating the tone of the walls of blood vessels).

The processes occurring in the human nephron make it possible to assess the state of the organs of the excretory system. This can be done in two ways. The first is the calculation of the content of creatinine (protein breakdown product) in the blood. This indicator characterizes how units of the kidneys cope with the filtering function.

The work of the nephron can also be assessed using the second indicator - the glomerular filtration rate. Blood plasma and primary urine should normally be filtered at a rate of 80-120 ml/min. For people aged, the lower limit may be the norm, since after 40 years the kidney cells die (glomeruli become much smaller, and it is more difficult for the body to fully filter fluids).

Functions of some components of the glomerular filter

The glomerular filter consists of fenestrated capillary endothelium, basement membrane, and podocytes. Between these structures is the mesangial matrix. The first layer performs the function of coarse filtration, the second one sifts out proteins, and the third purifies the plasma from small molecules of unnecessary substances. The membrane has a negative charge, so albumin does not penetrate through it.

Blood plasma is filtered in the glomeruli, and mesangiocytes, the cells of the mesangial matrix, support their work. These structures perform a contractile and regenerative function. Mesangiocytes regenerate the basement membrane and podocytes and, like macrophages, they engulf dead cells.

If each unit does its job, the kidneys function as a well-coordinated mechanism, and the formation of urine passes without returning toxic substances to the body. This prevents the accumulation of toxins, the appearance of puffiness, high blood pressure and other symptoms.

Violations of the functions of the nephron and their prevention

In the event of a malfunction of the functional and structural units of the kidneys, changes occur that affect the work of all organs - the water-salt balance, acidity and metabolism are disturbed. The gastrointestinal tract ceases to function normally, due to intoxication, allergic reactions. The load on the liver also increases, since this organ is directly related to the elimination of toxins.

For diseases associated with transport dysfunction of the tubules, there is a single name - tubulopathies. They are of two types:

  • primary;
  • secondary.

The first type is congenital pathology, the second is acquired dysfunction.

Active death of nephrons begins when drugs are taken, in side effects which are indicated possible diseases kidneys. Some drugs from the following groups have a nephrotoxic effect: non-steroidal anti-inflammatory drugs, antibiotics, immunosuppressants, antitumor drugs, etc.

Tubulopathies are divided into several types (according to location):

  • proximal;
  • distal.

With complete or partial dysfunction of the proximal tubules, phosphaturia, renal acidosis, hyperaminoaciduria and glucosuria can be observed. Impaired phosphate reabsorption leads to the destruction of bone tissue, which is not restored with vitamin D therapy. Hyperaciduria is characterized by a violation of the transport function of amino acids, which leads to various diseases(depending on the type of amino acid).
Such conditions require immediate medical attention, as well as distal tubulopathies:

  • renal water diabetes;
  • tubular acidosis;
  • pseudohypoaldosteronism.

Violations are combined. With the development of complex pathologies, the absorption of amino acids with glucose and the reabsorption of bicarbonates with phosphates can simultaneously decrease. Accordingly, the following symptoms appear: acidosis, osteoporosis and other pathologies of bone tissue.

Prevent kidney dysfunction by proper diet, drinking enough clean water and active image life. It is necessary to contact a specialist in time in case of symptoms of impaired kidney function (to prevent the transition acute form diseases into chronic).

For the existence of the human body, it provides not only a system for delivering substances to it for building the body or extracting energy from them.

There is also a whole complex of various highly efficient biological structures for the removal of its waste products.

One of these structures is the kidneys, the working structural unit of which is the nephron.

general information

This is the name of one of the functional units of the kidney (one of its elements). There are at least 1 million nephrons in the body, and together they form a well-functioning system. Due to their structure, nephrons allow blood to be filtered.

Why - blood, because it is well known that the kidneys produce urine?
They produce urine precisely from the blood, where the organs, having selected everything they need from it, send substances:

  • or at the moment absolutely not required by the body;
  • or their surplus;
  • which can become dangerous for him if they continue to stay in the blood.

To balance the composition and properties of blood, it is necessary to remove unnecessary components from it: excess water and salts, toxins, low molecular weight proteins.

The structure of the nephron

The discovery of the method made it possible to find out: not only the heart has the ability to contract, but all organs: the liver, kidneys and even the brain.

The kidneys contract and relax in a certain rhythm - their size and volume either decrease or increase. In this case, there is a compression, then a stretching of the arteries passing in the bowels of the organ. The level of pressure in them also changes: when the kidney relaxes, it decreases, when it contracts, it increases, making it possible for the nephron to work.

With an increase in pressure in the artery, the system of natural semi-permeable membranes in the structure of the kidney is triggered - and substances that are unnecessary for the body, having squeezed through them, are removed from the bloodstream. They fall into the formations, which are the initial sites urinary tract.

On certain segments of them there are areas where the reabsorption (return) of water and part of the salts into the bloodstream occurs.

The nephron fulfilling its filtering (filtering) function with blood purification and the formation of urine from its components is possible due to the presence in it of several areas of extremely close contact of the semi-permeable structures of the primary urinary tract with a network of capillaries (having an equally thin wall).

In the nephron, there are:

  • primary filtration zone (renal corpuscle, consisting of a renal glomerulus located in the Shumlyansky-Bowman capsule);
  • reabsorption zone (capillary network at the level of the initial sections of the primary urinary tract - renal tubules).

renal glomerulus

This is the name of a network of capillaries that really looks like a loose ball, into which the afferent (another name: supply) arteriole breaks up here.

This structure provides the maximum contact area of ​​the capillary walls with an intimately (very close) selectively permeable three-layer membrane adjacent to them, which forms the inner wall of the Bowman's capsule.

The thickness of the walls of the capillaries is formed by only one layer of endothelial cells with a thin cytoplasmic layer, in which there are fenestrae (hollow structures) that ensure the transport of substances in one direction - from the lumen of the capillary to the cavity of the renal corpuscle capsule.

The spaces between the capillary loops are filled with mesangium, a connective tissue of a special structure containing mesangial cells.

Depending on the localization in relation to the capillary glomerulus (glomerulus), they are:

  • intraglomerular (intraglomerular);
  • extraglomerular (extraglomerular).

After passing through the capillary loops and freeing them from toxins and excess, the blood is collected in the outlet artery. That, in turn, forms another network of capillaries, braiding the renal tubules in their convoluted areas, from which blood is collected in the efferent vein and thus returned to the bloodstream of the kidney.

Bowman-Shumlyansky capsule

The structure of this structure can be described by comparison with a well-known object in everyday life - a spherical syringe. If you press its bottom, a bowl is formed from it with an internal concave hemispherical surface, which is at the same time independent. geometric shape, and serves as a continuation of the outer hemisphere.

Between the two walls of the formed form, a slit-like space-cavity remains, continuing into the spout of the syringe. Another example for comparison is a thermos flask with a narrow cavity between its two walls.

In the Bowman-Shumlyansky capsule there is also a slit-like internal cavity between its two walls:

  • outer, called the parietal plate and
  • internal (or visceral plate).

Their structure is significantly different. If the outer one is formed by one row of squamous epithelial cells (which also continues into the single-row cubic epithelium of the efferent tubule), then the inner one is composed of elements of podocytes - cells of the renal epithelium of a special structure (literal translation of the term podocyte: cell with legs).

Most of all, the podocyte resembles a stump with several thick main roots, from which thinner roots evenly extend on both sides, and the entire system of roots spread over the surface both extends far from the center and fills almost the entire space inside the circle formed by it. Main types:

  1. Podocytes- these are giant-sized cells with bodies located in the cavity of the capsule and at the same time - elevated above the level of the capillary wall due to support on their root-like processes-cytotrabeculae.
  2. Cytotrabecula- this is the level of primary branching of the "leg"-process (in the example with the stump - the main roots). But there is also secondary branching - the level of cytopodia.
  3. cytopodia(or pedicles) are secondary processes with a rhythmically maintained distance from the cytotrabecula (“main root”). Due to the similarity of these distances, a uniform distribution of cytopodia is achieved in the areas of the capillary surface on both sides of the cytotrabecula.

Cytopodial outgrowths of one cytotrabecula, entering the gaps between similar formations of a neighboring cell, form a figure, in relief and pattern very reminiscent of a zipper, between the individual “teeth” of which only narrow parallel linear slits remain, called filtration slits (slit diaphragms) .

Due to this structure of podocytes, all outside surface capillaries, facing the cavity of the capsule, is completely covered with weaves of cytopodia, whose zippers do not allow the wall of the capillary to be pushed into the cavity of the capsule, counteracting the force of blood pressure inside the capillary.

renal tubules

Starting with a flask-shaped thickening (Shumlyansky-Bowman capsule in the structure of the nephron), the primary urinary tract then has the character of tubes of diameter that varies along their length, moreover, in some areas they acquire a characteristically convoluted shape.

Their length is such that some of their segments are in the cortical, others are in the medulla.
On the way of fluid from blood to primary and secondary urine, it passes through the renal tubules, consisting of:

  • proximal convoluted tubule;
  • the loop of Henle, which has a descending and ascending knee;
  • distal convoluted tubule.

The proximal section of the renal tubule is distinguished by its maximum length and diameter; it is made of highly cylindrical epithelium with a “brush border” of microvilli, which provides a high resorption function due to an increase in the area of ​​the suction surface.

The same purpose is served by the presence of interdigitations - finger-like indentations of the membranes of neighboring cells into each other. Active resorption of substances into the lumen of the tubule is a very energy-intensive process; therefore, the cytoplasm of the tubule cells contains many mitochondria.

In the capillaries braiding the surface of the proximal convoluted tubule,
reabsorption:

  • ions of sodium, potassium, chlorine, magnesium, calcium, hydrogen, carbonate ions;
  • glucose;
  • amino acids;
  • some proteins;
  • urea;
  • water.

So, from the primary filtrate - the primary urine formed in the Bowman's capsule, a liquid of intermediate composition is formed, following to the loop of Henle (with a characteristic bend of the hairpin shape in the renal medulla), in which a descending knee of small diameter and an ascending knee - large diameter are isolated.

The diameter of the renal tubule in these sections depends on the height of the epithelium, which performs different functions in different parts of the loop: in the thin section it is flat, ensuring the efficiency of passive water transport, in the thick section it is higher cubic, ensuring the activity of reabsorption of electrolytes (mainly sodium) into the hemocapillaries and passively water following them.

In the distal convoluted tubule, urine of the final (secondary) composition is formed, which is created during facultative reabsorption (reabsorption) of water and electrolytes from the blood composition of the capillaries that braid this section of the renal tubule, which completes its history by falling into the collecting duct.

Types of nephrons

Since the renal corpuscles of most nephrons are located in the cortical layer of the kidney parenchyma (in the outer cortex), and their loops of Henle of short length pass through the outer medulla of the kidney along with most of the blood vessels of the kidney, they are called cortical, or intracortical.

The rest of them (about 15%), with a longer loop of Henle, deeply immersed in the medulla (up to reaching the tops of the renal pyramids), is located in the juxtamedullary cortex - the border zone between the medulla and the cortical layer, which allows us to call them juxtamedullary.

Less than 1% of nephrons located shallow in the subcapsular layer of the kidney are called subcapsular, or superficial.

Urine ultrafiltration

The ability of the “legs” of podocytes to contract with simultaneous thickening makes it possible to narrow the filtration gaps even more, which makes the process of cleaning the blood flowing through the capillary as part of the glomerulus even more selective in terms of the diameter of the filtered molecules.

Thus, the presence of "legs" in podocytes increases the area of ​​their contact with the capillary wall, while the degree of their contraction regulates the width of the filtration slits.

In addition to the role of a purely mechanical obstacle, slit diaphragms contain proteins on their surfaces that have a negative electrical charge, which limits the transmission of also negatively charged molecules of proteins and other chemical compounds.

Such an effect on the composition and properties of blood, carried out by a combination of physical and electrochemical processes, makes it possible to ultrafilter blood plasma, leading to the formation of urine at first of the primary, and in the course of subsequent reabsorption, of the secondary composition.

The structure of nephrons (regardless of their localization in the kidney parenchyma), designed to perform the function of maintaining the stability of the internal environment of the body, allows them to perform their task, regardless of the time of day, the change of seasons and other external conditions, throughout a person's life.

A lot depends on the work of the kidneys in the body: both how successfully the water and electrolyte-salt balance will be maintained, and how the waste products of metabolism will be excreted. About how the urinary organs function, and what is the name of the main structural unit of the kidney, read in our review.

How is the nephron arranged?

The main anatomical and physiological unit of the kidney is the nephron. During the day, up to 170 liters of primary urine is formed in these structures, its further thickening with reabsorption (reverse absorption) of useful substances and, finally, the release of 1-1.5 liters of the end product of metabolism - secondary urine.

How many nephrons are there in the body? According to scientists, this number is about 2 million. The total area of ​​the excretory surface of all structural elements of the right and left kidneys is 8 square meters, which is three times the area of ​​the skin. At the same time, no more than a third of the nephrons work at the same time: this creates a high reserve for the urinary system and allows the body to function actively even with one kidney.

So, what does the main functional element in the human urinary system consist of? The nephron of the kidney includes:

  • renal corpuscle - blood is filtered in it and diluted, or primary urine is formed;
  • tubular system - the part responsible for the reabsorption of the body and the secretion of waste substances.

renal corpuscle


The structure of the nephron is complex and is represented by several anatomical and physiological units. It begins with the renal corpuscle, which also consists of two formations:

  • renal glomeruli;
  • Bowman-Shumlyansky capsules.

The glomeruli contain several dozen capillaries that receive blood from the ascending arteriole. These vessels do not participate in gas exchange (after passing through them, the saturation of blood with oxygen practically does not change), however, along the pressure gradient, the liquid and all components dissolved in it are filtered into the capsule.

The physiological rate of blood passing through the glomeruli of the kidneys (GFR) is 180-200 l/day. In other words, in 24 hours the entire volume of blood in the human body passes through the glomeruli of nephrons 15-20 times.

The nephron capsule, consisting of outer and inner sheets, receives the liquid that has passed through the filter. Water, chloride and sodium ions, amino acids and proteins weighing up to 30 kDa, urea, glucose freely penetrate through the glomerular membranes. Thus, essentially the liquid part of the blood, devoid of large protein molecules, enters the space of the capsule.

renal tubules

During microscopic examination, one can notice the presence in the kidney of many tubular structures, consisting of elements with different histological structure and the functions performed.

In the system of tubules of the nephron, the kidneys secrete:

  • proximal tubule;
  • loop of Henle;
  • distal convoluted tubule.

The proximal tubule is the longest and longest part of the nephrons. Its main function is to transport filtered plasma into the loop of Henle. In addition, it reabsorbs water and electrolyte ions, as well as the secretion of ammonia (NH3, NH4) and organic acids.

The loop of Henle is a segment of the part of the path connecting two types of tubules (central and marginal). It reabsorbs water and electrolytes in exchange for urea and processed substances. It is in this section that the osmolarity of urine increases sharply and reaches 1400 mOsm / kg.

In the distal section, transport processes continue, and concentrated secondary urine is formed at the outlet.

Collecting tubes

The collecting ducts are located in the periglomerular zone. They are distinguished by the presence of the juxtaglomerular apparatus (JGA). It, in turn, consists of:

  • dense spot;
  • juxtaglomerular cells;
  • juxtavascular cells.

In the SGA, renin is synthesized - the most important participant in the renin-angiotensin system, which controls blood pressure. In addition, the collecting ducts are the final part of the nephron: they receive secondary urine from many distal tubules.

Classification of nephrons


Depending on what structural and functional feature nephrons possess, they are divided into:

  • cortical;
  • juxtaglomerular.

In the cortical layer of the kidneys there are two types of nephrons - superficial and intracortical. The former are few in number (their number is less than 1%), are located superficially and have a small volume of filtration. Intracortical nephrons make up the majority (80-83%) of the basic structural unit of the kidney. They are located in the central part of the cortical layer and carry out almost the entire volume of the ongoing filtration.

The total number of juxtaglomerular nephrons does not exceed 20%. Their capsules are located on the border of two renal layers - cortical and cerebral, and the loop of Henle descends to the pelvis. This type of nephron is considered key to the ability of the kidneys to concentrate urine.

Physiological features of the kidneys

Such a complex structure of the nephron allows for a high functional activity of the kidneys. Getting through the afferent arterioles into the glomerulus, the blood undergoes a filtration process, in which proteins and large molecules remain in the vascular bed, and the liquid with ions and other small particles dissolved in it enters the Bowman-Shumlyansky capsule.

Then the filtered primary urine enters the tubular system, where the fluid and ions necessary for the body are reabsorbed into the blood, as well as the secretion of processed substances and metabolic products. Ultimately, the formed secondary urine through the collecting ducts enters the small renal calyces. This completes the process of urination.

The role of nephrons in the development of PN


It has been proven that after the age of 40 in a healthy person, about 1% of all functioning nephrons die each year. Given the huge "reserve" of the structural elements of the kidney, this fact does not affect the health and well-being even after 80-90 years.

In addition to age, the causes of death of the glomeruli and the tubular system include inflammation of the renal tissue, infectious and allergic processes, acute and chronic intoxications. If the volume of dead nephrons exceeds 65-67% of the total volume, a person develops renal failure (RF).

PN is a pathology in which the kidneys are unable to filter and form urine. Depending on the main causative factor, there are:

  • acute, acute renal failure - sudden, but often reversible;
  • chronic, chronic renal failure - slowly progressive and irreversible.

Thus, the nephron is an integral structural unit of the kidney. This is where the process of urination takes place. It contains several functional elements, without the clear and coordinated work of which the work of the urinary system would be impossible. Each of the renal nephrons not only provides constant blood filtration and promotes urination, but also allows timely cleansing of the body and maintaining homeostasis.

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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. structural organization 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 histological and physiological characteristics each component 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. Filaments lamina densa with a thickness of 1.2-2.5 nm 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.

To explain the mechanism of proteinuria in damage to the basement membrane, methods using markers, which take into account the electric charge of molecules, were of great importance.

Changes in the BM of the glomerulus are characterized by its thickening, homogenization, loosening and fibrillation. BM thickening occurs in many diseases with proteinuria. In this case, 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 mesangiocyte processes to the periphery of capillary cells, lead to thickening of the BM glomeruli. loops that exfoliate the endothelium from the BM.

In many diseases with proteinuria, in addition to membrane thickening, various deposits (deposits) in the membrane or in its immediate vicinity are detected by electron microscopy. 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 able to react to certain damages of the glomerular filter by production of the 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 processes of cells 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 case 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 mesangiocyte processes with the 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

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