URINARY SYSTEM Dr Mujahid Khan Urinary System It
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URINARY SYSTEM Dr. Mujahid Khan
Urinary System It consists of: Ø The kidneys, which excrete urine Ø The ureters, which convey urine from the kidneys to the bladder Ø The urinary bladder, which stores urine temporarily Ø The urethra, which carries urine from the bladder to the exterior
Development of Kidneys and Ureters Ø Three sets of kidneys develop in human embryos Ø The first set is pronephroi, are rudimentary Ø The second set is mesonephroi, well developed but function briefly Ø The third set is metanephroi, become the permanent kidneys
Pronephroi Ø These transitory, non functional structures appear early in the fourth week in the neck region Ø The pronephric ducts run caudally and open into the cloaca Ø They soon degenerate but most of the pronephric ducts persist and are utilized by the next set of kidneys
Mesonephroi Ø Large, elongated, excretory organ appear late in the fourth week, caudal to the rudimentary pronephroi Ø Are well developed and function as interim kidneys for about four weeks, until permanent kidneys develop Ø They consist of glomeruli and mesonephric tubules
Mesonephroi Ø The tubules open into the mesonephric ducts Ø The mesonephric ducts open into the cloaca Ø They degenerate toward the end of the first trimester Ø Their tubules become the efferent ductules of the testes Ø Mesonephric ducts have several adult derivatives in male
Metanephroi Ø These are primordia of permanent kidneys Ø Begin to develop early in the fifth week Ø Start to function about four weeks later Ø Urine formation continues throughout fetal life Ø Urine is excreted into the amniotic cavity and mixes with amniotic fluid
Metanephroi Ø A mature fetus swallows several hundred ml of amniotic fluid each day, which is absorbed by the intestine Ø The waste products are transferred through the placental membrane into the maternal blood for elimination by kidneys Ø They develop from metanephric diverticulum and metanephric mass of intermediate mesoderm
Metanephroi Ø It is an outgrowth from the mesonephric duct near its entrance into the cloaca Ø The metanephric mass of intermediate mesoderm is derived from the caudal part of the nephrogenic cord Ø Both primordia of the metanephros are of mesodermal origin
Metanephric Diverticulum Ø It is the primordium of the ureter, renal pelvis, calices, and collecting tubules Ø As it elongates, the metanephric diverticulum penetrates the metanephric mass of intermediate mesoderm Ø The stalk of the metanephric diverticulum becomes the ureter and its expanded cranial end forms the renal pelvis
Metanephric Diverticulum Ø The straight collecting tubules undergo repeated branching, forming successive generations of collecting tubules Ø First four generations of tubules enlarge to form the major calices Ø Second four generations coalesce to form the minor calices Ø The remaining generations of tubules form the collecting tubules
Metanephric Diverticulum Ø The end of each arched collecting tubule induces clusters of mesenchymal cells in the metanephric mass of mesoderm to form small metanephric vesicles Ø These vesicles elongate and become metanephric tubules Ø As these renal tubules develop, their proximal ends are invaginated by glomeruli
Metanephric Diverticulum Ø The renal corpuscle and its proximal convoluted tubule, loop of Henle, and distal convoluted tubule constitute a nephron Ø Between the tenth and eighteenth weeks of gestation, the number of glomeruli increases gradually Ø They increase rapidly until the thirty second week
Uriniferous Tubule Ø It consists of two embryologically different parts: Ø A nephron derived from the metanephric mass of intermediate mesoderm Ø A collecting tubule derived from the metanephric diverticulum
Fetal Kidneys Ø They are subdivided into lobes Ø This lobulation diminishes toward the end of the fetal period Ø The lobulation usually disappear during infancy as the nephrons increase and grow Ø The lobulated character of kidneys is normally obscured in adults
Fetal Kidneys Ø At term each kidneys contains 800, 000 to 1, 000 nephrons Ø The increase in kidney size after birth results mainly from the elongation of the proximal convoluted tubules, as well as an increase in interstitial tissue
Fetal Kidneys Ø Nephron formation is complete at birth, except in premature infants Ø Functional maturation of kidneys occurs after birth Ø Glomerular filtration begins around the ninth week and the rate of filtration increases after birth
Positional Changes of Kidneys Ø Initially the metanephric kidneys lie close to each other in the pelvis Ø As the abdomen and pelvis grow, the kidneys gradually come to lie in the abdomen and move apart Ø They attain there adult position by the ninth week
Positional Changes of Kidneys Ø Initially the hilum of the kidneys lie ventrally Ø As the kidney ascends, it rotates medially almost 90 degrees Ø By the ninth week the hilum is directed anteromedially
Changes in the Blood Supply of Kidneys Ø As the kidneys ascend from the pelvis, they receive their blood supply from vessels that are close to them Ø Initially the renal arteries are branches of the common iliac arteries Ø As they ascend further, the kidneys receive their blood supply from the distal end of the aorta Ø As they reach higher level, they receive new branches from the aorta
Changes in the Blood Supply of Kidneys Ø Normally the caudal branches disappear Ø Their ascent stops when they come in contact with suprarenal glands in the ninth week Ø The kidneys receive their most cranial branches from abdominal aorta, which becomes the permanent renal arteries Ø The right renal artery is longer and often more superior
Urinary Bladder The urogenital sinus is divided into three parts: Ø A cranial visceral part that forms most of the bladder, continuous with the allantois Ø Middle pelvic part becomes the urethra Ø A caudal phallic part that grows toward the genital tubercle
Urinary Bladder Ø The bladder develops mainly from the vesical part of the urogenital sinus Ø Its trigone region is derived from the caudal ends of the mesonephric ducts Ø Its epithelium is derived from the endoderm of vesical part of urogenital sinus Ø Other layers of its wall develop from adjacent splanchnic mesenchyme
Urinary Bladder Ø Initially it is continuous with the allantois Ø The allantois soon constricts and becomes a thick fibrous cord, called urachus Ø Urachus extends from the apex of the bladder to the umbilicus Ø In the adult the urachus is represented by the median umbilical ligament
Urinary Bladder Ø As the bladder enlarges, distal parts of the mesonephric ducts are incorporated into its dorsal wall Ø Epithelium of the entire bladder is derived from the endoderm of the urogenital sinus Ø As the mesonephric ducts are absorbed, the ureters come to open separately into the urinary bladder
Urinary Bladder Ø The orifices of the mesonephric ducts move close together and enter the prostatic part of the urethra Ø Caudal ends of these ducts become the ejaculatory ducts Ø The distal ends of the mesonephric ducts in females degenerate
Urinary Bladder Ø In infants and children, the urinary bladder is in the abdomen Ø It begins to enter the greater pelvis at about six years of age Ø Does not enter the lesser pelvis and become a pelvic organ until after puberty
Urethra Ø The epithelium of most of the male urethra and the entire female urethra is derived from endoderm of the urogenital sinus Ø The epithelium of the terminal part of the urethra is derived from the surface ectoderm Ø The connective tissue and smooth muscle of the urethra in both sexes are derived from splanchnic mesenchyme
Suprarenal Glands Ø The cortex develops from the mesoderm Ø The medulla differentiates from neural crest cells Ø Cortex appears during the sixth week from the mesothelium lining the posterior abdomnal wall Ø The cells that form the medulla are derived from an adjacent sympathetic ganglion, derived from neural crest cells
Suprarenal Glands Ø Differentiation of the characteristic suprarenal cortical zones begins during the late fetal period Ø The zona glomerulosa and zona fasciculata are present at birth Ø The zona reticularis is not recognizable until the end of the third year
Suprarenal Glands Ø The suprarenal glands of human fetus are 10 to 20 times larger than adult glands relative to body weight Ø This is due to the extensive size of the fetal cortex Ø The glands lose about one-third of their weight during the first 2 or 3 weeks after birth and do not regain their original weight until the end of the second year
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