LESSON 23 Urinary system.pdf

_____________ LESSON 23 _____________ URINARY SYSTEM The urinary system is made up of two kidneys and the urinary passages (two ureters, the urinary bladder, and the urethra). The urinary system is responsible for the formation and removal of urine, process which regulates the volume and the compos...

_____________ LESSON 23 _____________ URINARY SYSTEM The urinary system is made up of two kidneys and the urinary passages (two ureters, the urinary bladder, and the urethra). The urinary system is responsible for the formation and removal of urine, process which regulates the volume and the composition of body fluids, retain fluids components (salts, glucose, proteins and water) necessary to maintain homeostasis (regulation of blood pressure, hemodynamic changes and acid-base balance of the body) while removing metabolic waste products, water excess and electrolytes from the body. This process is carried out by filtration of the blood, reabsorption and secretion. The urinary bladder acts as a store and intermittent evacuator for urine, while the ureters and urethra conduct urine. It also has an endocrine function since it produces the hormones reninangiotensin, erythropoietin and prostaglandins, among others. I. KIDNEY Anatomically, the kidneys of domestic animals have a great variety of forms, such as the smooth and bean-shaped ones (cat, dog, goat and sheep); the smooth, elongated and flattened (pigs); the smooth and heart-shaped (horse) and the lobed and oval (bovines). The kidney is a pair organ, parenchymal, where we can identify a highly vascularized scarce stroma and abundant parenchyma. Externally, the kidney is surrounded by a thin capsule of dense irregular connective tissue rich in collagen fibers, which can include smooth muscle fibers, such as of the kidney of ruminants and pigs. The capsule does not emit fibrous trabeculae into the organ, so it can be detached easily, which makes evident the surface of the organ for macroscopic examination. In a sagittal section, macroscopically, kidney appears clearly divided peripherally a brown zone - reddish dark called cortex (external and internal) and another innermost pale zone called medulla (external and internal) (Figure 1). Depending on the domestic species, we can identify multi-lobed kidneys with a lobed outer surface such as that of cattle, multi-lobed kidneys with a 1 smooth outer surface such as that of pigs, and unilobed kidneys with a smooth outer surface such as that of dogs, cats, goats and sheeps. The renal parenchyma is mainly made up of uriniferous tubules, the structural and functional units of the kidney, composed of a nephron and a collecting duct system. A kidney can contain between 200,000 (cat) and 400,000 nephrons (dog). A B C Figure 1. Diagrams of the kidney (A), lobe (B) and nephron (C). The nephron is made up of different parts: the renal or Malpighi corpuscle, the proximal and distal tubules (with their convoluted and straight portions) and the Henle´s loop. Each of the parts of the nephron is specifically distributed in the cortex and /or medulla. Thus, in the cortex we can identify two zones: the cortical labyrinth (corpuscles and convoluted tubules) and medullary rays (straight tubules). In the renal medulla we can exclusively observe straight tubules and Henle's loop, in addition to the collecting tubules Nephrons can be classified according to the location of the corpuscle in the cortex into cortical nephrons (subdivided into two groups, superficial and midcortical) and juxtamedullary nephrons; or according to the length of the Henle´s loop in: short loop nephrons (cortical nephrons) and long loop nephrons (juxtamedullary nephrons). Several nephrons are drained by a single collecting tubule, which anastomoses other tubules into the medulla to form the papillary duct, forming a perforated area called the area cribrosa. 1. Nephron (Figure 2) It is made up of different parts, each of which has its own histological and functional characteristics. The nephron is composed of: 1. Renal corpuscle 2. Proximal convoluted and straight tubules 3. Thin tubule 4. Distal straight and convoluted tubules The straight proximal tubule is brought into contact with the straight distal tubule through the thin tubule, these three tubular elements constituting the 2 Henle´s loop. The distal convoluted tubule continues with the collecting tubules which leads to the minor calyxes. Some authors do not consider them as part of the nephron because it has an embryonic origin different from that of the nephron. Figure 2. Kidney and nephron diagram. It can be seen where the different parts of the nephron are located, in the cortex and/or medulla. Image taken from the Histology book. Gartner, 2007. 1.1. Renal corpuscle The renal corpuscle is located exclusively in the renal cortex, and it is the first portion of the nephron where ultrafiltration of the blood takes place. It has a spherical morphology and varies in size with the species (220 µm in horse to 120 µm in cat). The renal corpuscle is composed of: • • Renal glomerulus. Bowman´s capsule, which delimits a space called Bowman´s space or urinary space. The renal corpuscle contains a vascular pole where it enters the afferent arteriole and exits the efferent arteriole and a urinary pole where the glomerular ultrafiltration exits and which is continuous with the proximal convoluted tubule. The glomerulus is composed of a network of fenestrated capillaries that come from the afferent glomerular arteriole and drains into the efferent glomerular 3 arteriole, and of the mesangial cells. However, the term glomerulus is widely used in the literature to refer to the entire renal corpuscle. The endothelial cells of these fenestrated capillaries are thin and have abundant fenestrations (pores of 50-150 nm in diameter) and rest on a basal membrane or lamina with peculiar characteristics. The mesangial cells are the supporting cellular element of the glomerulus since the connective tissue that accompanies the afferent arteriole does not penetrate the capsule. They are specialized cells immersed in a mesangial matrix, rich in glycoproteins. Mesangial cells are pluripotent mesenchymal cells (modified pericytes) with multiple cytoplasmic processes that are arranged occupying the spaces between the capillaries and can be located between the endothelial cells and the basal lamina and even reach the capillary lumen. They have contractile capacity, present receptors for angiotensin II, have phagocytic capacity, have capacity for synthesizing collagen fibers and mesangial matrix, participate in the continuous renewal of the basal lamina of the glomerular capillaries, and they can also synthesize inflammatory mediators. The Bowman´s capsule is composed of two distinct sheets, an outer layer or parietal and an inner or visceral layer, separated by the urinary space which is continuous with the lumen of the proximal convoluted tubule at the level of the corpuscle urinary pole. The parietal layer is made up of a simple squamous epithelium that rests on a thick basement membrane. The visceral layer surrounds the glomerular capillaries and is formed of modified epithelial cells called podocytes. The visceral layer continues with the parietal layer at the vascular pole. The podocytes are arranged on the basal lamina of the fenestrated capillaries without contacting it, being separated by a space of 1 to 3 µm. From the body of the podocyte, which is where the nucleus is located, several primary extensions arise, which tend to surround the fenestrated capillary, and from these other thinner secondaries are differentiated that in turn emit other very small extensions or pedicles, called pedicels. These pedicels contact the basal lamina and interdigitate (cross over) with other pedicels of contiguous podocytes without fusing with them. In this way, narrow intercellular spaces are formed, which constitute the filtration slits, that are not completely open, instead they are covered by a 6 nm thick porous membrane called the filtration slit diaphragm. Along these fissures, the plasma filtration from the capillaries to the urinary space takes place. Podocytes are associated with an underlying basement membrane, which separates them from the endothelial cells of the fenestrated capillaries. This basal lamina is thick and consists of three layers: an outer layer contiguous to the podocytes or “lamina rara externa”, an internal layer adjacent to the capillary endothelium or “lamina rara interna”, both of low electron density and, between them there is a third thick layer and more electron dense called “lamina densa”. The components of the basement membrane include the network of type IV collagen fibers and glycoproteins as laminin in the lamina densa, and heparan sulfate and fibronectin in the laminae rare (PAS +). 4 In the glomerulus there is a very important structure is called filtration barrier, responsible for producing the ultrafiltrate of blood. This filtration barrier is made up of three elements of the renal corpuscle, which are: 1) The filtration slits formed by the pedicels of the podocytes; 2) the basal lamina, and; 3) the fenestrated endothelium of the glomerular capillaries. The glomerular filtration barrier selectively filters molecules according to their size (maximum 70,000 Da), electrical charge (positively charged molecules pass, negative ones do not) and blood pressure. A B C Figure 3. Diagrams of glomerulus (A) and renal tubules (B), filtration barrier (C) and podocyte (D). 1.2. Proximal tubule This segment of the nephron is the longest, it is made up of the proximal convoluted tubule and the proximal straight tubule. The convoluted tubule is in the cortex, in the vicinity of the renal corpuscle, while the straight tubule descends to form part of the medullary rays (inner cortex) and juxtacortical medulla (outer medulla). The proximal convoluted tubule is a tubule that collects ultrafiltrate from the urinary space and begins at the urinary pole of the renal corpuscle. The tubule is lined by a simple cuboidal to low columnar epithelium. The epithelial cells, at their apical border, have numerous microvilli, giving an image called "brush border" and on the lateral and basal borders they contain numerous invaginations and evaginations. In the apical portion, where they contact with contiguous cells, the lateral walls are brought together by means of tight junctions, intermediate junctions and desmosomes. The proximal tubule lumen is small due to the highly developed microvilli. In the cytoplasm of epithelial cells there are large number of mitochondria with elongated morphology, located at a basal complex consisting of compartments formed by the numerous invaginations of the cytoplasmic membrane, typical of cells with active transport. In the apical region there is an endocytic apparatus involved in absorption phenomena, consisting of short canaliculi that connect with invaginations of the cytoplasmic membrane and small endocytic vesicles which fuse with primary lysosomes. They contain many 5 D peroxisomes. On the other hand, they contain a perinuclear Golgi complex and a developed smooth and rough endoplasmic reticulum. This epithelium not only has the function of lining the duct but participates in absorption and excretion phenomena to form the urine: 1. In the apical zone the microvilli increase the surface of contact with the glomerular filtration; 2. the invaginations and evaginations of the lateral and basal surfaces significantly extend intercellular relationship and the surface for ion transport, and; 3. The basal surface shows some fine extensions that are arranged under the neighbouring cells. The modifications of the lateral and basal membranes increase the surface area of the cell to increase the activity of active transport and mitochondria are the energy source. The cells of the straight tubule exhibit a less complex image that constitute the convoluted tubule. They are cells with less height, their microvilli and mitochondria are less numerous and the lateral and basal projections are less developed. Characteristic of cats and dogs is the presence of lipid droplets in the epithelial cells of the proximal convoluted tubule and straight tubule, respectively, which gives the kidney a macroscopically yellowish coloration. 1.3. Thin tubule The thin tubule is the continuation of the straight proximal tubule that abruptly changes in thickness from a simple cuboidal epithelium to a simple squamous epithelium. This segment is seen exclusively in the renal medulla. The nuclei of the epithelial cells make prominence towards the lumen, giving an image very similar to that of the capillaries. The thin tubule together with the straight proximal tubule and the straight distal tubule form the structure called Henle´s loop. 1.4. Distal tubule Similar to the proximal convoluted tubule, the distal tubule is made up of two segments: 1. Straight tubule (ascending), which runs along the outer region of the medulla to reach the cortex contacting the vascular pole of the corresponding renal corpuscle and there forms the macula densa as part of the juxtaglomerular apparatus and, 2. Convoluted tubule located in the cortex and extending from the macula densa to the beginning of the collecting duct system. This part of the distal tubule is shorter than that of the proximal tubule, so it is common to find more proximal than distal tubule sections in histological sections of the kidney. These sections are clearly differentiated since the diameter of the lumen is greater in distal convoluted tubules than in proximal ones. The straight and convoluted distal tubules are lined by a simple cuboidal epithelium like tubule proximal but with less development of microvilli (lacking the brush border) and of the invaginations and evaginations of the lateral surface of 6 the cytoplasmic membrane. However, in the basal area of the straight tubule cells, the basal interdigitations and mitochondria are more numerous than in the proximal and distal convoluted tubules. These cells form zonula occludens with their neighbouring cells. Juxtaglomerular apparatus The juxtaglomerular apparatus is a complex made up of three differentiated structures, which are located in the vascular pole of the renal corpuscle: 1) the macula densa; 2) juxtaglomerular cells, and; 3) extraglomerular mesangial cells The macula densa of the distal tubule is made up of epithelial cells located in the distal convoluted tubule that differentiate into cells specialized in sodium metabolism, thus having the ability to detect the concentration of sodium ion (Na+) transported in the tubule distal, thus intervening in the amount of renin released. These cells are arranged in a palisade with the nuclei close together. To develop the proper function of this structure, they arrange their organoids in the opposite situation to how they are found in the rest of the cells of the distal tubule, that is, towards the direction of the juxtaglomerular cells. Thus, the cells of the macula densa have the nucleus in the apical area while the Golgi complex and the rest of the organoids are in the basal region. Juxtaglomerular cells are in the wall of the afferent arteriole, in the vicinity of the corresponding renal corpuscle. They are smooth muscle fibers from the tunica media of the afferent arteriole that have become epithelioid cells. They are large cells, with a round nucleus and a large cytoplasm, which have organoids characteristic of protein-secreting cells. They also contain myofilaments and secretion granules of renin, which acts as a blood pressure control mechanism, by participating in the formation of angiotensin II and intervening in the secretion of aldosterone. The extraglomerular mesangial cells are arranged between the space delimited by the afferent and efferent arterioles, and the macula densa. They are cells with pale cytoplasm, which show continuity with the intraglomerular mesangial cells. The function of these elements appears to be supportive, although it is still largely unknown. 1.5. Collecting duct system The collecting duct system is considered not to be part of the nephron since they have an embryonic origin different from that of the nephron. Under the denomination of collecting duct system three differentiated tubular elements are included: arched connecting ducts, straight collecting ducts and papillary ducts. The arched collecting ducts begin at the end of the distal convoluted tubule and connect with the straight collecting ducts. The wall is made up of a 7 simple cuboidal epithelium. The cells of this epithelium have a poorly stained cytoplasm and an evident nucleus. The straight collecting ducts are located in the medullary rays. They are lined by a simple cubical epithelium, although the height of their cells is greater than those of the arched connecting ducts. The epithelium present two types of cells: principal or light cells and intercalated or dark cells. The papillary ducts correspond to the final portion of this collecting system, opening it their lumens in the renal papilla or in the minor calyxes, constituting the area cribosa. The epithelium that lines these tubules is variable and can be identified from a simple columnar epithelium to a stratified or transitional epithelium. Renal circulation The kidney presents a segmental circulation in lobes, important in case of injuries such as renal infarction that will be treated in other subjects of the Degree (APS). Renal circulation begins in the abdominal aorta, which emits two branches, the right and left renal arteries, each directed to the corresponding kidney. These arteries enter the corresponding kidney through the hilum and once inside they emit a certain number of lobar or primary arteries located in the renal pelvis, from which the interlobar arteries arise, which cross the renal medulla and that give origin in the corticomedullary union to the arcuate arteries. From these originate the interlobular arteries that reach the capsule ending under it as fine vessels or capsular branches (stellate veins). The interlobular arteries on their way to the capsule generate various vessels, each of which supplies arterial blood to a renal glomerulus. They are the afferent glomerular arterioles that emit the glomerular capillary network and then continue with the efferent glomerular arterioles, which also determine the origin of the peritubular arterial vessels. Likewise, the efferent glomerular arterioles generate a cortical intertubular vascular network and the efferent glomerular arterioles of the juxtamedullary glomeruli originate the arteriolae rectae that project towards the medulla and branch in the form of hairpins accompanying the Henle loops and then going towards the renal cortex. The venous drainage of the kidney is performed by the accompanying vessels of the corresponding arteries and branches indicated. This blood circulation originates under the capsule through the interlobular veins, ending in the right and left renal veins that exit the kidneys through the hilum, reaching the blood transported by them to the caudal vena cava. 8 A B Figure 4. Photomicrograph of the renal cortex with glomeruli and convoluted tubules (A). Juxtaglomerular apparatus scheme (B). II. URINARY PASSAGES Include the calyxes, pelvis, ureters, urinary bladder and urethra. All these structures have a similar structural organization that includes: Ø Mucosa, with a transitional epithelium or urothelium and a lamina propria-submucosa made up of loose connective tissue. Ø Muscularis, made up of an internal longitudinal layer and a middle circular layer and a third external longitudinal layer of smooth muscle Ø Serosa or adventitia, depending on if it is in the abdominal or pelvic cavity. The urinary passages provide urine a conduction and storage site as well as a filling and elimination mechanism. 9