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Lecture notes, cheat sheets
Histology. Excretory system
Directory / Lecture notes, cheat sheets Table of contents (expand) Topic 24. EXTRACTIVE SYSTEM The excretory system includes the kidneys, ureters, bladder and urethra. Development of the excretory system The urinary and reproductive systems develop from the intermediate mesoderm. In this case, the pronephros, mesonephros and metanephros are successively formed. The pronephros is rudimentary and does not function, the mesonephros acts in the early stages of intrauterine development, the metanephros forms the permanent kidney. Pronephros. At the end of the 3rd - beginning of the 4th week of development, the intermediate mesoderm of the cervical region separates from the somites and forms segmented cell clusters that have the shape of a stalk with an internal cavity - nephrotomes growing in the lateral direction. Nephrotomes give rise to nephric tubules, the medial ends of which open into the body cavity, and the lateral ends grow in the caudal direction. The nephric tubules of adjacent segments unite and form paired longitudinal ducts growing towards the cloaca (primary renal duct). Small branches separate from the dorsal aorta, one of which penetrates into the wall of the nephritic tubule, and the other into the wall of the coelomic cavity, forming, respectively, the inner and outer glomeruli. The glomeruli consist of a spherical plexus of capillaries and together with tubules form excretory units (nephrons). As subsequent nephrotomes appear, degeneration of the previous ones occurs. By the end of the 4th week of intrauterine development, all signs of nephrotomes are absent. Mesonephros. As the pronephros degenerates, the first tubules of the mesonephros appear more caudally. They lengthen, forming an s-shaped loop, the medial end of which reaches the capillary glomerulus. The glomerulus is embedded in the wall of the tubule, and in this place the tubule forms an epithelial capsule. The capsule and glomerulus form the renal corpuscle. The lateral end of the tubule drains into the primary renal duct, now called the Wolffian (mesonephric duct). In the future, the tubules lengthen, becoming more and more tortuous. They are surrounded by a plexus of capillaries formed by postglomerular vessels. By the middle of the 2nd month, the mesonephros reaches its maximum value. It is a large ovoid organ located on either side of the midline. On its medial side is the rudiment of the gonads. The elevation formed by both organs is known as the urogenital ridge. When the caudal tubules of the mesonephros are still being formed, the cranial tubules and glomeruli are already degenerating; by the end of the 2nd month, most of them disappear. A small portion of the caudal tubules and the mesonephric duct, however, are preserved in the male fetus. A number of structures of the male reproductive system are subsequently formed from the tubules of the mesonephros. With the beginning of the degeneration of the mesonephros, the formation of the metanephros begins. The function of the mesonephros is similar to the function of the tubules of the nephron of the definitive kidney. The blood filtrate from the glomerulus enters the capsule, then into the tubule, then into the mesonephric duct. At the same time, a number of substances are reabsorbed in the tubule. However, urine is poorly concentrated in the mesonephros, which is associated with the absence of medulla structures necessary for water retention. The metanephros (or permanent kidney) develops from a metanephrogenic blastoma, the source of the nephron tubules, and a metanephric diverticulum, the source of the collecting ducts and larger urinary tracts. Metanephros appears during the 5th week of development. Its tubules develop similarly to how it happened in the mesonephros. Metanephric diverticulum and metanephrogenic blastoma. When it flows into the cloaca, the mesonephric duct forms an outgrowth - a metanephric diverticulum. This outgrowth is embedded in the caudal part of the intermediate mesoderm, which thickens around the diverticulum, forming a metanephrogenic blastoma. Further, the diverticulum dichotomously divides, forming a system of collecting ducts, gradually deepening into the tissue of the metanephros. The derivative of the metanephric diverticulum - the collecting duct - is covered at the distal end with a "cap" of the metanephrogenic blastoma. Under the inductive influence of the tubules, small bubbles form from this tissue, giving rise to tubules. In turn, the developing tubules induce further branching of the collecting ducts. The tubules, uniting with the capillary glomerulus, form the nephrons. The proximal end of the nephron forms a capsule into which the glomerulus is deeply embedded. The distal end connects to one of the collecting ducts. Further, the tubule lengthens, resulting in the formation of the proximal convoluted tubule, the loop of Henle and the distal convoluted tubule. First, the kidney is located in the pelvic area. In the future, it moves more cranially. The apparent rise of the kidney is associated with a decrease in the curvature of the body during the development of the fetus and its growth in the lumbar and sacral regions. Functions in the fetus. Fetal urine is hypotonic relative to plasma, slightly acidic (pH 6,0). Maintaining the volume of amniotic fluid is one of the main functions of the fetal urinary system. Beginning at about the 9th week of development, the fetus excretes urine into the amniotic cavity (10 ml/kg/h) and also absorbs up to 0,5 liters of amniotic fluid per day. Nitrogenous residues from the body of the fetus are removed by diffusion through the placenta into the mother's blood. Kidney of a newborn. In a newborn, the kidney has a pronounced lobular appearance. Lobulation subsequently disappears as a result of growth, but not the formation of new nephrons. Nephrogenesis is completed by the 36th week of development, by which time there are about 1 million nephrons in each kidney. Kidneys They are a urinary organ. The rest of the organs make up the urinary tract, through which urine is excreted from the body. Together with urine, over 80% of the end products of metabolism are excreted. The kidneys are paired organs that continuously produce urine. They are located on the inner surface of the posterior abdominal wall and are bean-shaped. Their concave surface is called the gate. The renal arteries enter the gates of the kidneys and the renal veins and lymphatic vessels exit. Here the urinary tract begins - the renal calyces, renal pelvis and ureters. Structure. The kidney is covered with a connective tissue capsule and a serous membrane. The substance of the kidney is divided into cortical and medulla. The cortex is dark red in color, located in a common layer under the capsule. The medulla is lighter in color, divided into 8 - 12 pyramids. The tops of the pyramids, or papillae, protrude freely into the renal calyces. In the process of kidney development, its cortical substance, increasing in mass, penetrates between the bases of the pyramids in the form of renal columns. In turn, the medulla grows into the cortical substance with thin rays, forming brain rays. The kidney is supported by loose connective tissue rich in reticular cells and reticular fibers. The parenchyma of the kidney is represented by epithelial renal tubules, which, with the participation of blood capillaries, form nephrons. There are about 1 million of them in each kidney. Nephron is the structural and functional unit of the kidney. The length of its tubules is from 18 to 50 mm, and of all nephrons, on average, about 100 km. The nephron begins with the renal corpuscle, which includes a capsule enclosing the glomerulus of blood capillaries. At the other end, the nephron passes into the collecting duct. The collecting duct continues into the papillary canal, which opens at the top of the pyramid into the cavity of the renal calyx. There are four main sections in the nephron - the renal corpuscle, the proximal section, the nephron loop with descending and ascending parts, and the distal section. The proximal and distal sections are represented by convoluted tubules of the nephron. The descending and ascending parts of the loop are the direct tubules of the nephron. About 80% of nephrons are located almost entirely in the cortex, and only the knees of their loops are in the medulla. They are called cortical nephrons. The remaining 20% of nephrons are located in the kidney so that their renal corpuscles, proximal and distal parts lie in the cortex on the border with the medulla, while the loops go deep into the medulla. These are the pericerebral (juxtamedullary) nephrons. The collecting ducts into which the nephrons open begin in the cortex, where they form part of the brain rays. Then they pass into the medulla and at the top of the pyramids flow into the papillary canal. Thus, the cortical and medulla of the kidney is formed by different parts of the nephrons. The cortex consists of renal corpuscles, proximal and distal nephrons, which look like convoluted tubules. The medulla consists of straight descending and ascending parts of the nephron loops, as well as the terminal sections of the collecting ducts and papillary canals. Blood is brought to the kidneys through the renal arteries, which, having entered the kidneys, break up into interlobar arteries that run between the cerebral pyramids. At the border between the cortical and medulla, they branch into arcuate arteries, from which the direct arteries branch into the medulla, and the interlobular arteries into the cortex. Afferent arterioles diverge from the interlobular arteries. The upper ones go to the cortical nephrons, the lower ones go to the juxtamedullary nephrons. In this regard, in the kidneys, the cortical circulation, serving the cortical nephrons, and the juxtamedullary circulation, associated with the pericerebral nephrons, are conditionally distinguished. In the cortical circulatory system, the afferent arterioles break up into capillaries that form the vascular glomeruli of the renal corpuscles of the cortical nephrons. There is a collection of glomerular capillaries into efferent arterioles, which are approximately 2 times smaller in diameter than the afferent arterioles. Due to this, in the capillaries of the glomeruli of the cortical nephrons, the blood pressure is unusually high (70 - 90 mm Hg). This is the cause of the first phase of urination, which has the character of the process of filtering substances from the blood plasma into the nephron. The efferent arterioles, having passed a short path, again break up into capillaries, braiding the tubules of the nephron and forming a peritubular capillary network. In these secondary capillaries, the blood pressure, on the contrary, is relatively low (about 10 - 12 mm Hg), which contributes to the second phase of urination, which is in the nature of a process of reabsorption of a number of substances from the nephron into the blood. From the secondary capillaries, blood is collected in the upper sections of the cortex, first into the stellate veins, and then into the interlobular veins, in the middle sections of the cortical substance - directly into the interlobular veins. The interlobular veins flow into the arcuate veins, which pass into the interlobar veins, which form the renal veins that exit the renal hilum. Thus, cortical nephrons, as a result of the characteristics of the cortical circulation (high blood pressure in the capillaries of the vascular glomeruli and the presence of a peritubular network of capillaries with low blood pressure), are actively involved in urination. In the juxtamedullary circulatory system, the afferent and efferent arterioles of the vascular glomeruli of the renal bodies of the paracerebral nephrons are almost the same in size or the efferent arterioles are even somewhat larger, due to which the blood pressure in the capillaries of these glomeruli does not exceed 40 mm Hg. Art., i.e., significantly lower than in the glomeruli of cortical nephrons. The efferent arterioles do not break up into a wide peritubular network of capillaries, which is typical for cortical nephrons, but, by the type of arteriovenular anastomoses, they pass into straight veins that flow into arcuate venous vessels. Therefore, pericerebral nephrons, in contrast to cortical ones, are less active when participating in urination. At the same time, the juxtamedullary circulation plays the role of a shunt, i.e., a short and easy path, which is the place where blood passes through the kidneys under conditions of their strong blood supply, for example, when a person performs hard physical work. The nephron begins with the renal corpuscle, represented by the vascular glomerulus and its capsule. The vascular glomerulus consists of more than 100 blood capillaries. Their endothelial cells have numerous fenestrae (possibly, in addition, pores). Endothelial cells of capillaries are located on the inner surface of a thick, three-layer basement membrane. On the outer side, the epithelium of the inner leaf of the glomerular capsule lies on it. The capsule of the glomerulus in shape resembles a double-walled cup, in which, in addition to the inner leaf, there is an outer leaf, and between them there is a slit-like cavity - the cavity of the capsule, passing into the lumen of the proximal tubule of the nephron. The inner leaf of the capsule penetrates between the capillaries of the vascular glomerulus and covers them from almost all sides. It is formed by large (up to 30 microns) irregularly shaped epithelial cells - podocytes. From the bodies of podocytes, several large wide processes depart - cytotrabeculae, from which, in turn, numerous small processes (cytopodia) begin, which are attached to the three-layer basement membrane. Narrow slits are located between the cytopodia, communicating through the gaps between the bodies of podocytes with the cavity of the capsule. The three-layer basement membrane, which is common to the endothelium of the blood capillaries and podocytes of the inner leaf of the capsule, includes the outer and inner layers (less dense (light)) and the middle layer (more dense (dark)). In the middle layer of the membrane there are microfibrils that form a mesh with a cell diameter of up to 7 nm. All three of these components (the wall of the glomerular capillaries, the inner sheet of the capsule and the three-layer basement membrane common to them) constitute a biological barrier through which the components of the blood plasma that form the primary urine are filtered from the blood into the cavity of the capsule. Thus, in the composition of the renal corpuscles there is a renal filter. He participates in the first phase of urination, which has the character of a filtration process. The renal filter has a selective permeability, retaining everything that is larger than the size of the cells in the middle layer of the basement membrane. Normally, blood cells and some blood plasma proteins with the largest molecules do not pass through it: immune bodies, fibrinogen, etc. If the filter is damaged in cases of kidney disease (for example, with nephritis), they can be found in the urine of patients. In the vascular glomeruli of the renal corpuscles, in those places where the podocytes of the inner leaf of the capsule cannot penetrate between the capillaries, there is another type of cell - mesangial cells. After endotheliocytes and podocytes, they are the third type of cellular elements of the renal bodies, forming their mesangium. Mesangiocytes, like capillary pericytes, have a process shape capable of phagocytosis, and in pathological conditions, in addition, to fiber formation. The outer sheet of the glomerular capsule is represented by a single layer of flat and low cubic epithelial cells located on the basement membrane. The epithelium of the outer leaf of the capsule passes into the epithelium of the proximal nephron. The proximal part has the appearance of a convoluted tubule with a diameter of up to 60 microns with a narrow, irregularly shaped lumen. The wall of the tubule is formed by high cylindrical border epithelium. It carries out obligate reabsorption - reverse absorption into the blood (into the capillaries of the peritubular network) from the primary urine of a number of substances contained in it. The mechanism of this process is associated with the histophysiology of proximal epithelial cells. The surface of these cells is covered with a brush border with a high activity of alkaline phosphatase, which is involved in the complete reabsorption of glucose. In the cytoplasm of cells, pinocytic vesicles are formed and there are lysosomes rich in proteolytic enzymes, with the help of which complete reabsorption of proteins is carried out. The cells have a basal striation formed by the inner folds of the cytolemma and mitochondria located between them. Mitochondria containing succinate dehydrogenase and other enzymes play an important role in the active reabsorption of some electrolytes, and cytolemma folds are of great importance for the passive reabsorption of some of the water. As a result of obligate reabsorption, primary urine undergoes significant qualitative changes: sugar and protein completely disappear from it. In kidney diseases, these substances can be found in the final urine of the patient due to damage to the proximal nephrons. The nephron loop consists of a descending thin portion and an ascending thick portion. The descending part is a straight tubule with a diameter of about 13 - 15 microns. Its wall is formed by flat epithelial cells, the nucleated parts of which swell into the lumen of the tubule. The cytoplasm of the cells is light, poor in organelles. The cytolemma forms deep internal folds. Passive absorption of water into the blood occurs through the wall of this tubule. The ascending part of the loop also looks like a straight epithelial tubule, but with a larger diameter - up to 30 microns. In structure and role in reabsorption, this tubule is close to the distal nephron. The distal nephron is a convoluted tubule. Its wall is formed by a cylindrical epithelium, which is involved in facultative reabsorption: the reabsorption of electrolytes into the blood. The epithelial cells of the tubule lack a brush border, but due to the active transfer of electrolytes, they have a pronounced basal striation - the accumulation of a large number of mitochondria in the basal regions of the cytoplasm. Facultative reabsorption is a key link in the entire process of urination, since the amount and concentration of urine excreted depend on it. The mechanism of this process, called countercurrent-multiplier, seems to be as follows: with the reverse absorption of electrolytes in the distal section, the osmotic pressure in the blood and in the connective tissue surrounding the nephron changes, and the level of passive reverse absorption of water from the tubules of the nephron depends on this. The collecting ducts in the upper cortical part are lined with a single layer of cuboidal epithelium, and in the lower brain part - with a single layer of low cylindrical epithelium. In the epithelium, light and dark cells are distinguished. Light cells are poor in organelles, their cytoplasm forms internal folds. Dark cells in their ultrastructure resemble parietal cells of the gastric glands that secrete hydrochloric acid. In the collecting ducts, with the help of light cells, passive reabsorption of part of the water from the urine into the blood is completed. In addition, acidification of urine occurs, which is probably associated with the secretory activity of dark epithelial cells. Thus, urination is a complex process that takes place in the nephrons. In the renal corpuscles of nephrons, the first phase of this process, or filtration, occurs, resulting in the formation of primary urine (more than 100 liters per day). In the tubules of nephrons, the second phase of urination occurs, i.e., reabsorption (obligatory and facultative), resulting in a qualitative and quantitative change in urine. Sugar and protein completely disappear from it, and its amount also decreases (up to 1,5 - 2 liters per day), which leads to a sharp increase in the concentration of excreted toxins in the final urine: creatine bodies - 75 times, ammonia - 40 times and etc. The final (third) secretory phase of urination is carried out in the collecting ducts, where the urine reaction becomes slightly acidic. All phases of urine formation are biological processes, that is, the result of the vigorous activity of nephron cells. The juxtaglomerular apparatus of the kidneys (JGA), or the periglomerular apparatus, secretes renin into the blood, which is a catalyst for the formation of angiotensins in the body, which have a strong vasoconstrictive effect, and also stimulates the production of the hormone aldosterone in the adrenal glands. In addition, it is possible that JGA plays an important role in the production of erythropoietins. JGA consists of juxtaglomerular cells, macula densa, and Gurmagtig cells. The location of juxtaglomerular cells is the wall of afferent and efferent arterioles under the endothelium. They have an oval or polygonal shape, and in the cytoplasm there are large secretory (renin) granules that are not stained by conventional histological methods, but give a positive PAS reaction. A dense spot is a section of the wall of the distal nephron where it passes next to the renal corpuscle between the afferent and efferent arterioles. In the dense patch, epithelial cells are taller, almost devoid of basal folding, and their basement membrane is extremely thin (according to some sources, it is completely absent). It is assumed that the macula, like a sodium receptor, detects changes in the sodium content in the urine and affects the periglomerular cells that secrete renin. Gurmagtig cells lie in a triangular space between the afferent and efferent arterioles and the macula densa. Their shape may be oval or irregular, they form stretching processes that have a connection with the cells of the mesangium of the glomerulus. Fibrillar structures are revealed in their cytoplasm. Some authors also classify mesangial cells of vascular glomeruli as JGA. It is suggested that Gurmagtig and mesangium cells are involved in renin production when juxtaglomerular cells are depleted. Inpersitial cells (IC) of the kidneys of mesenchymal origin are located in the stroma of the cerebral pyramids in a horizontal direction. Their elongated body has processes, some of which are woven into tubules of the nephron loop, while others are blood capillaries. In the cytoplasm of IC, organelles are well developed and there are lipid (osmiophilic) granules. There are two hypotheses about the role of these cells: 1) participation in the work of the countercurrent-multiplier system; 2) the production of one of the types of prostaglandins, which has an antihypertensive effect, i.e., lowers blood pressure. Thus, JGA and IC are the endocrine complex of the kidneys, which regulates the general and renal circulation, through which it influences urination. Aldosterone (adrenal glands) and vasopressin, or antidiuretic hormone (hypothalamus), directly affect nephron function. Under the influence of the first hormone, sodium reabsorption in the distal nephrons is enhanced, and under the influence of the second, water reabsorption in the nephron tubules and in the collecting ducts is enhanced. The lymphatic system of the kidney is represented by a network of capillaries surrounding the tubules of the cortex and renal corpuscles. There are no lymphatic capillaries in the vascular glomeruli. Lymph from the cortex flows through a sheath-shaped network of lymphatic capillaries surrounding the interlobular arteries and veins to the first order lymphatic vessels that surround the arcuate arteries and veins. Lymphatic capillaries of the medulla surrounding the direct arteries and veins flow into these plexuses of lymphatic vessels. Lymphatic vessels of the XNUMXst order form larger lymphatic collectors of the XNUMXnd, XNUMXrd and XNUMXth order, which flow into the interlobar sinuses of the kidney. From these vessels, lymph enters the regional lymph nodes. The kidney is innervated by efferent sympathetic and parasympathetic nerves and afferent posterior root nerve fibers. The distribution of nerves in the kidney is different. Some of them are related to the vessels of the kidney, others - to the renal tubules. The renal tubules are supplied by the nerves of the sympathetic and parasympathetic systems. Their endings are localized under the epithelium membrane. However, according to some reports, nerves can pass through the basement membrane and terminate on the epithelial cells of the renal tubules. In structure, these nerves resemble secretory nerve endings. Polyvalent endings are also described, when one branch of the nerve ends on the renal tubule, and the other on the capillary. Urinary tract The urinary tract includes the renal calyces and pelvises, the ureters, the bladder and the urethra, which in men simultaneously performs the function of removing seminal fluid from the body and therefore will be described in the chapter on the reproductive system. The structure of the walls of the renal calyces and pelvis, ureters and bladder is similar in general terms. They distinguish between the mucous membrane, consisting of the transitional epithelium and the lamina propria, the submucosa, the muscular and outer membranes. In the wall of the renal calyces and renal pelvis, after the transitional epithelium, there is a lamina propria of the mucous membrane, imperceptibly passing into the connective tissue of the submucosa. The muscular coat consists of two thin layers of smooth muscle cells - inner (longitudinal) and outer (circular). However, only one circular layer of smooth muscle cells remains around the papillae of the renal pyramids. The outer shell without sharp boundaries passes into the connective tissue surrounding the large renal vessels. The ureters have a pronounced ability to stretch due to the presence of deep longitudinal folds of the mucous membrane in them. The submucosa of the lower part of the ureters has small alveolar-tubular glands, similar in structure to the prostate gland. The muscular membrane of the ureters in the upper half consists of two layers - the inner (longitudinal) and the outer (circular). The muscular membrane of the lower part of the ureters has three layers - the inner and outer layers of the longitudinal direction and the middle layer - circular. In the muscular membrane of the ureters, in the places where they pass through the wall of the bladder, the bundles of smooth muscle cells run only in the longitudinal direction. Contracting, they open the opening of the ureter, regardless of the state of the smooth muscles of the bladder. Outside, the ureters are covered with a connective tissue adventitial membrane. The mucous membrane of the bladder consists of a transitional epithelium and its own plate. In it, small blood vessels are especially close to the epithelium. In a collapsed or moderately distended state, the bladder mucosa has many folds. They are absent in the anterior section of the bottom of the bladder, where the ureters flow into it and the urethra exits. This section of the bladder wall, which has the shape of a triangle, is devoid of a submucosa, and its mucous membrane is tightly fused with the muscular membrane. Here, in the own plate of the mucous membrane, glands are laid, similar to the glands of the lower part of the ureters. The muscular membrane of the bladder consists of three limited layers - inner, outer with a longitudinal arrangement of smooth muscle cells and the middle - circular. Smooth muscle cells often resemble split spindles. Layers of connective tissue divide the muscle tissue in this sheath into separate large bundles. In the neck of the bladder, the circular layer forms the muscular sphincter. The outer shell on the upper-posterior and partially on the lateral surfaces of the bladder is characterized by a sheet of peritoneum (serous membrane), in the rest of it it is adventitious. The wall of the bladder is richly supplied with blood and lymphatic vessels. The bladder is innervated by both sympathetic and parasympathetic and spinal (sensory) nerves. In addition, a significant number of nerve ganglia and scattered neurons of the autonomic nervous system were found in the bladder. There are especially many neurons at the place where the ureters enter the bladder. In the serous, muscular and mucous membranes of the bladder there are also a large number of receptor nerve endings. Authors: Selezneva T.D., Mishin A.S., Barsukov V.Yu. << Back: Leather and its derivatives >> Forward: Sexual system
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