The kidney, ren, is a paired organ in which urine is constantly produced by filtering fluid from the capillaries into the Shumlyansky-Bowman capsule.
The kidneys perform various functions: - Regulate the exchange of water and electrolytes; - Maintain the acid-base state of the body; - Carry out excretion of metabolic end products (urea, uric acid, creatinine and others) and foreign substances from the blood and their excretion in the urine; - Synthesize glucose from non-carbohydrate components (gluconeogenesis); - Produce hormones (renin, erythropoietin and others).
The kidney of an adult is bean-shaped with a bright brown color. Its weight ranges from 120 to 200 g, length - 10-12 cm, width - 5-6 cm, thickness - 3-4 cm. There are two surfaces of the kidney: anterior and posterior, two edges: lateral and medial, directed to the side spinal column; as well as two ends (poles): rounded top. The medial edge of the kidney in the middle part has depressions, the renal sinus. The entrance to the sinus is limited by the anterior and posterior lips and is called the renal hilum, in which the renal pedicle is located, consisting of renal artery, renal vein, renal pelvis, renal nerve plexus and lymphatic vessels.
The kidneys are located in upper section retroperitoneal space on both sides of the spine. In relation to the posterior abdominal wall, the kidneys lie in the lumbar region. In relation to the peritoneum they lie extraperitoneally. The kidneys are projected onto the anterior abdominal wall in the subcostal regions, partially in the epigastric region; the right kidney with its lower end can reach the right lateral region. The right kidney, as a rule, is located below the left, most often by 1.5-2 cm.
Every minute, about 1.2 liters of blood passes through the kidneys, which is up to 25% of the blood entering the aorta. The renal artery arises directly from abdominal aorta. At the hilum of the kidney it branches into smaller arteries to arterioles. Their terminal branches are called afferent arterioles. Each of these arterioles enters the Shumlyansky-Bowman capsule, where it breaks up into capillaries and forms a vascular glomerulus - the primary capillary network of the kidney. Numerous capillaries of the primary network, in turn, gather in efferent arteriole, the diameter of which is two times smaller than the diameter of the bringing one. Thus, blood from an arterial vessel enters the capillaries and then into another arterial vessel. In almost all organs, after the capillary network, blood collects in venules. Therefore, this fragment of the intraorgan vascular bed was called the “miraculous network of the kidney.” The efferent arteriole again breaks up into a network of capillaries intertwining the tubules of all parts of the nephron. This forms a secondary capillary network of the kidney. Consequently, the kidney has two capillary systems, which is associated with the function of urine formation. The capillaries intertwining the tubules finally merge and form venules. The latter, gradually merging and passing into intraorgan veins, form the renal vein.
The kidneys are innervated by the renal nerve plexus. The sources of its formation are nn. splanchnicimajoretminor, branches lumbar region trunc.us sympaticus, branches of the abdominal, superior mesenteric plexus and renal aortic ganglia. Afferent innervation is carried out due to the sensory nodes of the vagus nerve and spinal nodes, in which sensory neurons are located. Efferent nerve fibers of the autonomic nervous system(sympathetic and parasympathetic) reach the smooth muscle cells of the walls blood vessels kidneys, calyces and pelvis. At the renal hilum, the renal plexus divides into the perivascular plexus, the accompanying renal vessels and, together with them, penetrate the renal parenchyma. In the medulla and cortex, nerve fibers entwine the pyramids and lobules of the kidney, accompany the afferent glomerular arterioles and reach the glomerular capsules. (Unmyelinated) nerve fibers approach the walls of the urinary tubules and the renal calyces.
The nephron is the main structural and functional unit of the kidneys. It is responsible for the production of urine. There are approximately 1.2 million nephrons in the human body.
Nephrons function periodically: first some nephrons work, while others do not participate in the work at this time, then vice versa. The nephron consists of sections located in the medulla and cortex of the kidneys.
Urine formation occurs in three stages:
1) tubular secretion;
3) tubular reabsorption.
The kidneys are located retroperitoneally (retroperitoneally) on both sides of the spine, with the right kidney slightly lower than the left. The lower pole of the left kidney lies at the level of the upper edge of the body of the third lumbar vertebra, and the lower pole right kidney corresponds to its middle. The XII rib crosses the posterior surface of the left kidney almost in the middle of its length, and the right one - closer to its upper edge.
The buds are bean-shaped. The length of each bud is 10–12 cm, width - 5–6 cm, thickness - 3–4 cm. The mass of the bud is 150–160 g. The surface of the buds is smooth. In the middle section of the kidney there is a depression - the renal gate (hilus renalis), into which the renal artery and nerves flow. The renal vein and lymphatic ducts emerge from the renal hilum. The renal pelvis is also located here, which passes into the ureter.
On a section of the kidney, 2 layers are clearly visible: the cortex and medulla of the kidney. The tissue of the cortex contains renal (Malpighian) corpuscles. In many places, the cortex penetrates deeply into the thickness of the medulla in the form of radially located renal columns, which divide the medulla into renal pyramids, consisting of straight tubules forming a nephron loop, and collecting ducts passing through the medulla. The apices of each renal pyramid form renal papillae with openings that open into the renal calyces. The latter merge and form the renal pelvis, which then passes into the ureter. The renal calyces, pelvis and ureter make up urinary tract kidneys The top of the kidney is covered with a dense connective tissue capsule.
The bladder is located in the pelvic cavity and lies behind pubic symphysis. When the bladder fills with urine, its tip protrudes above the pubis and comes into contact with the anterior abdominal wall. In women back surface The bladder is in contact with the anterior wall of the cervix and vagina, and in men it is adjacent to the rectum.
The female urethra is short - 2.5–3.5 cm long. The length of the male urethra about 16 cm; its initial (prostatic) part passes through the prostate gland.
The main feature of the blood supply to the renal (cortical) nephron is that the interlobular arteries split twice into arterial capillaries. This is the so-called “miraculous network” of the kidney. The afferent arteriole, after entering the glomerular capsule, breaks up into glomerular capillaries, which then unite again and form the efferent glomerular arteriole. The latter, after leaving the Shumlyansky-Bowman capsule, again disintegrates into capillaries, densely entwining the proximal and distal sections tubules, as well as the loop of Henle, providing them with blood.
The second important feature of blood circulation in the kidney is the existence of two circles of blood circulation in the kidneys: large (cortical) and small (juxtamedullary), corresponding to two types of nephrons of the same name.
The glomeruli of juxtamedullary nephrons are also located in the renal cortex, but somewhat closer to the medulla. The loops of Henle of these nephrons descend deeply into the renal medulla, reaching the apexes of the pyramids. The efferent arteriole of the juxtamedullary nephrons does not break up into a second capillary network, but forms several straight arterial vessels, which go to the tops of the pyramids, and then, forming a turn in the form of a loop, return back to the cortex in the form of venous vessels. The direct vessels of the juxtamedullary nephrons, located next to the ascending and descending parts of the loop of Henle and being essential elements of the countercurrent-turning system of the kidneys, play an important role in the processes of osmotic concentration and dilution of urine.
Kidney structureThe kidneys are the main excretory organ. They perform many functions in the body. Some of them are directly or indirectly related to excretion processes, others do not have such a connection.
A person has a pair of kidneys located at back wall abdominal cavity on both sides of the spine at the level of the lumbar vertebrae. The weight of one kidney is about 0.5% of the total body weight, left kidney slightly advanced in comparison with the right kidney.
Blood enters the kidneys through the renal arteries and flows out of them through the renal veins, which flow into the inferior vena cava. Urine produced in the kidneys flows through two ureters into bladder, where it accumulates until it is excreted through the urethra.
A cross-section of the kidney shows two clearly distinguishable zones: the renal cortex, which lies closer to the surface, and the inner medulla. The renal cortex is covered with a fibrous capsule and contains renal glomeruli, barely visible to the naked eye. The medulla consists of renal tubules, renal collecting ducts and blood vessels, collected together to form the renal pyramids. The apices of the pyramids, called the renal papillae, open into the renal pelvis, which forms the dilated orifice of the ureter. Many vessels pass through the kidneys, forming a dense capillary network.
The main structural and functional unit of the kidney is the nephron with its blood vessels (Fig. 1.1).
Nephron is the structural and functional unit of the kidney. In humans, each kidney contains about a million nephrons, each about 3 cm long.
Each nephron includes six sections that differ greatly in structure and physiological functions: the renal corpuscle (Malpighian corpuscle), consisting of Bowman's capsule and the renal glomerulus; proximal convoluted renal tubule; descending limb of the loop of Henle; ascending limb of the loop of Henle; distal convoluted renal tubule; renal collecting tube.
There are two types of nephrons - cortical nephrons and juxtamedullary nephrons. Cortical nephrons are located in the renal cortex and have relatively short loops of Henle that extend only a short distance into the renal medulla. Cortical nephrons control the volume of blood plasma during normal quantity water in the body, and with a lack of water, increased reabsorption occurs in the juxtamedullary nephrons. In the juxtamedullary nephrons, the renal corpuscles are located near the border of the renal cortex and the renal medulla. They have long descending and ascending limbs of the loop of Henle, penetrating deeply into the medulla. Juxtamedullary nephrons intensively reabsorb water when there is a lack of it in the body.
Blood enters the kidney through the renal artery, which branches first into the interlobar arteries, then into the arcuate arteries and interlobular arteries, from the latter the afferent arterioles depart, supplying the glomeruli with blood. From the glomeruli, blood, the volume of which has decreased, flows through the efferent arterioles. It then flows through a network of peritubular capillaries located in the renal cortex and surrounding the proximal and distal convoluted tubules of all nephrons and the loop of Henle of cortical nephrons. From these capillaries arise the renal vasa recta, which run in the renal medulla parallel to the loops of Henle and collecting ducts. Function of both vascular systems- return of blood containing nutrients valuable to the body to the general circulatory system. Significantly less blood flows through the vasa recta than through the peritubular capillaries, due to which the high osmotic pressure necessary for the formation of concentrated urine is maintained in the interstitial space of the renal medulla.
The vessels are straight. The narrow descending and wider ascending renal capillaries of the vasa recta run parallel to each other throughout their entire length and form branching loops at different levels. These capillaries pass very close to the tubules of the loop of Henle, but there is no direct transfer of substances from the filtrate of the loop to the vasa recta. Instead, solutes exit first into the interstitial spaces of the renal medulla, where urea and sodium chloride are delayed due to the low speed of blood flow in the vasa recta, and the osmotic gradient of tissue fluid is maintained. The cells of the walls of the vasa recta freely allow water, urea and salts to pass through, and since these vessels are adjacent, they function as a countercurrent exchange system. When the descending capillary enters the medulla, water leaves the blood plasma through osmosis due to a progressive increase in the osmotic pressure of the tissue fluid, and sodium chloride and urea enter back through diffusion. In the ascending capillary the reverse process occurs. Thanks to this mechanism, the osmotic concentration of plasma leaving the kidneys remains stable regardless of the concentration of plasma entering them.
Since all movement of solutes and water occurs passively, countercurrent exchange in straight vessels occurs without the expenditure of energy.
Convoluted proximal tubule. The proximal convoluted tubule is the longest (14 mm) and widest (60 µm) part of the nephron, through which the filtrate enters the loop of Henle from Bowman's capsule. The walls of this tubule consist of a single layer of epithelial cells with numerous long (1 μm) microvilli forming a brush border on the inner surface of the tubule. The outer membrane of the epithelial cell is adjacent to the basement membrane, and its invaginations form the basal labyrinth. The membranes of neighboring epithelial cells are separated by intercellular spaces, and fluid circulates through them and the labyrinth. This fluid bathes the cells of the proximal convoluted tubules and the surrounding network of peritubular capillaries, forming a link between them. In the cells of the proximal convoluted tubule, numerous mitochondria are concentrated near the basement membrane, generating ATP, necessary for the active transport of substances.
The large surface area of the proximal convoluted tubules, their numerous mitochondria, and the proximity of the peritubular capillaries are all adaptations for the selective reabsorption of substances from the glomerular filtrate. Here, more than 80% of substances are reabsorbed, including all glucose, all amino acids, vitamins and hormones, and about 85% of sodium chloride and water. About 50% of urea is also reabsorbed from the filtrate by diffusion, which enters the peritubular capillaries and thus returns to the general circulatory system, the rest of the urea is excreted in the urine.
Proteins with a molecular weight of less than 68,000, which enter the lumen of the renal tubule during ultrafiltration, are extracted from the filtrate by pinocytosis occurring at the base of the microvilli. They find themselves inside pinocytotic vesicles, to which primary lysosomes are attached, in which hydrolytic enzymes break down proteins into amino acids, which are used by tubule cells or pass by diffusion into peritubular capillaries.
In the proximal convoluted tubules, the secretion of creatinine and the secretion of foreign substances also occur, which are transported from the intercellular fluid washing the tubules into the tubular filtrate and excreted in the urine.
Convoluted distal tubule. The distal convoluted tubule approaches the Malpighian corpuscle and lies entirely in the renal cortex. The cells of the distal tubules have a brush border and contain many mitochondria. It is this section of the nephron that is responsible for fine regulation water-salt balance and regulation of blood pH. The permeability of distal convoluted tubule cells is regulated by antidiuretic hormone.
Collecting tube. The collecting duct begins in the renal cortex from the renal distal convoluted tubule and passes down through the renal medulla, where it joins several other collecting ducts to form larger ducts (the ducts of Bellini). The permeability of the walls of the collecting ducts for water and urea is regulated by antidiuretic hormone, and thanks to this regulation, the collecting duct participates, together with the distal convoluted tubule, in the formation of hypertonic urine, depending on the body's need for water.
Loop of Henle. The loop of Henle, together with the capillaries of the renal vasa recta and the renal collecting duct, creates and maintains a longitudinal gradient of osmotic pressure in the renal medulla from the renal cortex to the renal papilla by increasing the concentration of sodium chloride and urea. Thanks to this gradient, it is possible to remove more and more water by osmosis from the lumen of the tubule into the interstitial space of the renal medulla, from where it passes into the direct renal vessels. Ultimately, hypertonic urine is produced in the renal connecting tube. The movement of ions, urea and water between the loop of Henle, the vasa recta and the collecting duct can be described as follows:
The short and relatively wide (30 µm) upper segment of the descending limb of the loop of Henle is impermeable to salts, urea and water. Along this section, the filtrate passes from the proximal convoluted renal tubule into a longer, thin (12 µm) segment of the descending limb of the loop of Henle, which freely allows water to pass through.
Due to the high concentration of sodium chloride and urea in the tissue fluid of the renal medulla, high osmotic pressure is created, water is sucked out of the filtrate and enters the renal vasa recta.
As a result of the release of water from the filtrate, its volume decreases by 5% and it becomes hypertonic. At the apex of the medulla (in the renal papilla), the descending limb of the loop of Henle bends and passes into the ascending limb, which is permeable to water along its entire length.
The lower portion of the ascending limb - the thin segment - is permeable to sodium chloride and urea, and sodium chloride diffuses out of it and urea diffuses in.
In the next, thick segment of the ascending limb, the epithelium consists of flattened cuboidal cells with a rudimentary brush border and numerous mitochondria. In these cells, active transfer of sodium and chlorine ions from the filtrate occurs.
Due to the release of sodium and chlorine ions from the filtrate, the osmolarity of the renal medulla increases, and a hypotonic filtrate enters the distal convoluted tubules. Epithelial cells that perform a barrier function (mainly) epithelial cells of the genitourinary tract that perform a barrier function.
The glomerulus is renal. The renal glomerulus consists of approximately 50 capillaries collected in a bundle, into which the only afferent arteriole approaching the glomerulus branches and which then merge into the efferent arteriole.
As a result of ultrafiltration, which occurs in the glomeruli, all substances with a molecular weight of less than 68,000 are removed from the blood, and a liquid called the glomerular filtrate is formed.
Malpighian corpuscle. Malpighian body - primary department nephron, it consists of the renal glomerulus and Bowman's capsule. This capsule is formed as a result of invagination of the blind end of the epithelial tubule and encloses the renal glomerulus in the form of a two-layer sac. The structure of the Malpighian corpuscle is entirely related to its function - blood filtration. The walls of the capillaries consist of a single layer of endothelial cells, between which there are pores with a diameter of 50 - 100 nm. These cells lie on a basement membrane that completely surrounds each capillary and forms a continuous layer that completely separates the blood in the capillary from the lumen of Bowman's capsule. The inner layer of Bowman's capsule consists of cells with processes called podocytes. The processes support the basement membrane and the capillary surrounded by it. The cells of the outer layer of Bowman's capsule are flat, unspecialized epithelial cells.
As a result of ultrafiltration, which occurs in the glomeruli, all substances with a molecular weight of less than 68,000 are removed from the blood and a liquid called the glomerular filtrate is formed.
In total, 1,200 ml of blood passes through both kidneys in 1 minute (i.e., in 4 - 5 minutes, all the blood in the blood passes through circulatory system). This volume of blood contains 700 ml of plasma, of which 125 ml is filtered in the Malpighian corpuscles. Substances filtered from the blood in the glomerular capillaries pass through their pores and basement membrane under the influence of pressure in the capillaries, which can vary with changes in the diameter of the afferent and efferent arterioles, which are under nervous and hormonal control. The narrowing of the efferent arteriole leads to a decrease in the outflow of blood from the glomerulus and an increase in hydrostatic pressure in it. In this condition, substances with a molecular weight of more than 68,000 can pass into the glomerular filtrate.
By chemical composition glomerular filtrate is similar to blood plasma. It contains glucose, amino acids, vitamins, some hormones, urea, uric acid, creatinine, electrolytes and water. Leukocytes, red blood cells, platelets and plasma proteins such as albumins and globulins cannot leave the capillaries - they are retained by the basement membrane, which acts as a filter. The blood flowing from the glomeruli has an increased oncotic pressure, since the concentration of proteins in the plasma is increased, but its hydrostatic pressure is reduced.
Renal circulation. Average speed renal blood flow at rest is about 4.0 ml/g per minute, i.e. in general, for kidneys weighing about 300 g, approximately 1200 ml per minute. This represents approximately 20% of total cardiac output. The peculiarity of the renal circulation is the presence of two consecutive capillary networks. The afferent arterioles divide into the glomerular capillaries of the kidneys, separated from the peritubular capillary bed of the kidneys by the efferent arterioles. The efferent arterioles are characterized by high hydrodynamic resistance. The pressure in the glomerular capillaries of the kidneys is quite high (about 60 mm Hg), and the pressure in the peritubular capillaries of the kidneys is relatively low (about 13 mm Hg).
Humanity knew about arteries and veins more than two thousand years ago. People learned about capillaries only at the end of the 17th century, after the discovery of a microscope by the Dutch biologist Leeuwenhoek.
Almost 250 years ago, the Italian physiologist Malpighi, for the first time seeing blood circulation in the capillaries under a microscope, was amazed by the magnificence of the spectacle unfolding before his eyes and exclaimed: “With more right than Homer once did, I can say: I see truly great things with my own eyes.”
Centuries have passed.
Many amazing discoveries have been made by scientists in various fields of science. And, despite this, every person, examining the blood circulation under a specially designed capillaroscope or a modern microscope, can hardly tear himself away from the eyepiece, fascinated by the delightful picture of circulating blood.
The capillaries were called hair vessels. This emphasized that they were as thin as hair. In fact, capillaries are much thinner than a hair: their cross-section area is no more than 0.00008 mm 2, and their radius is 0.005 mm, and the radius of the hair is 0.15 mm. Only one thing can pass through the lumen of the capillary blood cell. Red blood cells, passing through them, are even somewhat flattened. The length of the capillary does not exceed 0.5 mm. It is here, in these short and thin vessels, that life flows. important processes. They consist in the fact that through the walls of the capillaries, the blood gives oxygen to the tissues and receives carbon dioxide from them. In addition, nutrients pass from the blood into the tissues through them, and decay products, or waste substances, enter the blood from the tissues.
The structure of capillaries corresponds to the fulfillment of this function. Their walls are devoid of muscle and consist of only one layer of cells. Therefore, oxygen and carbon dioxide, as well as various substances, easily pass from the blood into the tissues and from the tissues into the blood.
There are a lot of capillaries - several billion. The superior mesenteric artery alone splits into 72 million capillaries. Such an abundance of them sharply increases the surface of contact, and this in turn contributes to better exchange between blood and tissues.
Let's give a small calculation. The circumference of one capillary is 22 microns (1 micron-0.001 mm); if we take into account that the superior mesenteric artery breaks up into 72 million capillaries, then the sum of their circumferences will be 1584 m; Meanwhile, the circumference of the superior mesenteric artery is 9.4 mm. Thus, the sum of the circumferences of all capillaries that are formed by the upper mesenteric artery, 170,000 times the circumference of the artery itself. This means that the blood comes into contact with a surface that is almost 170,000 times larger than the surface of the arteries.
Total length of capillaries human body- 100,000 km. By stretching them in one line, you can wrap the globe around the equator two and a half times.
The abundant and dense capillary network has another very important feature. Comparative observations of a muscle at rest and in a state of work found that the number of capillaries through which blood flows depends on the condition of the muscle.
In a resting muscle, only a small part of the capillaries is open (approximately 2 to 10%) and only blood flows through them.
The remaining capillaries are tightly closed.
When the muscle begins to work, almost the entire dense capillary network opens. Here are some examples.
The almost complete opening of the entire capillary network in the working muscle has a large physiological significance. The opened network of capillaries promotes increased supply of oxygen to the muscle and nutrients and removal of decomposition products. This is very important, since during work, due to increased energy expenditure, the muscle’s need for oxygen and nutrients increases sharply. At the same time, the amount of decomposition products increases and there is a need to quickly remove them.
Wide open during physical work the capillary network, abundantly washing the tissues with blood and supplying them with oxygen and nutrients, provides best conditions for the life of the body.
This is why moderate physical labor, sports, morning exercises, etc. cause vigor and wellness. Important condition long-term preservation of working capacity throughout life, late onset of old age - a combination of mental and physical labor from a very early age.
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