Urogenital System

• The earliest role of kidneys was osmoregulation in an aquatic environment. Elimination of nitrogenous wastes is an added excretory role that has become significant in tetrapods. Kidneys replaced gills in this function.
• The basic functional components of kidneys are glomeruli, renal tubules surrounded by peritubular capillaries, and a pair of longitudinal excretory ducts that empty in the cloaca or a derivative thereof.
• Glomeruli are arterial retia that filter water and certain solutes from arterial blood. External glomeruli secrete into coelomic fluid, which is then swept by nephrostomes into the tubules. Internal glomeruli are in the body wall and filter into Bowman's capsules, which are extensions of tubules. Several tubules generally empty into collecting tubules that

  terminate in the longitudinal duct.
• A glomerulus and its associated capsule constitute a renal corpuscle. A renal corpuscle and its tubule constitute a nephron.
• Osmoregulation is effected by the tubules, which add to or subtract from, glomerular filtrate in accordance with the metabolic needs of the organism.
• Kidney tissue arises from a ribbon of intermediate (nephrogenic) mesoderm. Primitive pronephric tubules form at the anterior end of the ribbon, and thereafter a wave of differentiation sweeps along the ribbon, giving rise to tubules of increased complexity. As tubules form farther caudad, many that appear earlier regress.
• An archinephros is a hypothetical archaic kidney with external glomeruli, nephrostomes, and simple tubules arranged metamerically the length of the trunk. The holonephros of larval hagfishes most nearly resembles the archinephros.
• The anteriormost tubules constitute a temporary pronephric kidney in all vertebrate embryos and then regress. The tubules caudal to the pronephros constitute a mesonephros. The duct of the pronephros continues to serve as the mesonephric duct.
• The mesonephros is the functional kidney of adult fishes and amphibians and a temporary functional kidney in embryonic amniotes. In adult anamniotes it is also called an opisthonephros.
• Some of the more anterior mesonephric tubules invade the developing testis and become vasa efferentia that conduct sperm to the mesonephric duct. This duct in generalized fishes and amphibians carries both sperm and urine. It persists in amniotes to serve as the spermatic

  duct (vas deferens).
• The caudal end of the nephrogenic ribbon is displaced laterad and cephalad to form the metanephric kidney of adult amniotes. Thereafter, the mesonephros regresses.
• The metanephros consists of cortex, medulla, and pelvis. Renal corpuscles pack the cortex; long loops of tubules (loops of Henle) and their peritubular capillaries comprise much of the medulla. The tubules empty into the hilumlike pelvis that is drained by a ureter.
• Vertebrates that imbibe excessive quantities of salt (marine vertebrates, shore reptiles and birds whose food comes from the sea, and reptiles that inhabit arid environments) excrete salt extrarenally via gills or rectal, nasal, orbital, oral, or integumentary salt glands.
• Bladders of most fishes (tubal bladders) are derived from the conjoined ends of mesonephric ducts. They are absent in cyclostomes and elasmobranchs. Those of dipnoans arise as dorsal evaginations of the cloaca.
• Tetrapod bladders arise from the floor of the cloaca and, except in placental mammals, empty directly into it. Bladders of placental mammals develop from the base of the allantois, a cloacal derivative, and empty into a urogenital sinus or, in higher female primates, to the exterior. They are drained by a urethra.
• The primary role of urinary bladders in tetrapods other than placental mammals is to serve as a reservoir for urine, the water of which is recycled when needed by body tissues.
• Gonads arise from paired genital ridges adjacent to developing kidneys. Germinal epithelium, the immediate source of gametes, arises on the surface of the embryonic gonad and may remain there or be transposed to a deeper location.
• Mature eggs are released into the coelom before entering the genital tract; sperm are transported in a closed system of ducts that commences within the testes.
• The germinal epithelium of most male vertebrates lines seminiferous tubules that empty into a rete testis. The latter is drained by vasa efferentia that conduct sperm to the spermatic duct.
• The testes remain in the abdominal cavity in a few mammals, descend permanently in some, and are retractile in others.
• Except in a few lower vertebrates, the mesonephric duct is the spermatic duct of adult males. Below placental mammals spermatic ducts open into the cloaca. In placentals they open into the urethra.
• Male intromittent organs of fishes are modifications of pelvic fins (claspers) or anal fins (gonopodia). Snakes and lizards have paired hemipenes. Turtles, crocodilians, some birds, and monotremes have an unpaired penis in the cloacal floor containing a corpus spongiosum. Placental mammals have an external penis with a corpus spongiosum and

  two corpora cavernosa.
• In females of reptilian species in which there is a male penis and in female mammals, a clitoris develops. It is the female homologue of the penis.
• The ovaries of teleosts have a central cavity lined by the germinal epithelium. The cavity is generally continuous with the cavity of the oviduct. Anuran and urodele ovaries are thin-walled sacs, and those of some reptiles and of birds and monotremes have irregular fluid-filled lacunae. Other ovaries are generally solid, with the germinal epithelium

  on the surface. Following ovulation, the ovarian (graafian) follicles of mammals organize a corpus luteum that becomes a source of the progesterone that assists in the maintenance of pregnancy.
• Mammalian external genitalia of both sexes differentiate from a median genital tubercle and paired genital swellings and folds. The tubercle gives rise to the penis or clitoris. Genital swellings give rise to scrotal sacs or labia majora. Genital folds contribute to the penis or become labia minora.
• Ligaments in mammals connect the embryonic testes to the floor of the scrotal sacs and the embryonic ovaries to the labia majora. Shortening of the male ligaments (gubernacula) assists caudal migration of the testes toward or into the scrotum. Shortening of the female ligaments

  (ovarian ligaments and round ligaments of the uterus) causes lesser displacement of ovaries.
• A generalized female genital tract consists of an oviduct that commences in a funnel-shaped ostium and terminates in the cloaca. Segments are specialized for species-specific functions such as secretion of egg envelopes, temporary storage of ova or developing young, and expulsion of

  eggs or offspring.
• Female genital tracts from ostium to cloaca or its derivatives differentiate, except in teleosts, from muellerian ducts present in embryos of both sexes. Those in males do not differentiate. Teleost female tracts arise from peritoneal folds rather than from muellerian ducts.
• The female tracts of placental mammals consist of Fallopian tubes (oviducts); duplex, bipartite, bicornuate, or simplex uteri; a uterine cervix; and a vagina. The latter empties into a urogenital sinus in most female mammals, into the vestibule of the vulva in monkeys, apes, and human

  beings.
• The ovaries of some placental mammals occupy an ovarian bursa, a culde-sac of the coelom that may be partly or completely closed off from the main coelom. It ensures that ovulated eggs will not become implanted in an ectopic location.
• An adult cloaca receives the large intestine, urinary bladder when present, and urinary and genital ducts. A cloaca is shallow or absent in many fishes. A single aperture opens to the exterior.
• In reptiles, birds, and monotremes a urorectal fold divides the anterior portion of the embryonic cloaca into a urodeum that receives the urinary and genital ducts, and a coprodeum that receives the rectum. The caudal portion of the cloaca remains undivided.
• In placental mammals except female monkeys, apes, and humans, the embryonic cloaca becomes completely divided into a urogenital sinus and rectum and two apertures open to the exterior.
• In some female rodents and in female monkeys, apes, and humans, the embryonic cloacal region is divided into a urethra that drains the bladder, a vagina, and a rectum, and three apertures open to the exterior.
•  urinary system, is a group of organs in the body that filters out excess fluid and other substances from the bloodstream. The purpose of the renal / urinary system is to eliminate wastes from the body,  regulate blood volume and pressure, control levels of electrolytes and metabolites, and  regulate blood pH.
• Kidneys: The kidneys are a pair of bean-shaped organs  which filter blood and produce urine.
• • Ureter: Urine passes from the renal tube through tubes called ureters and into the bladder.
• • Bladder: The bladder is flexible and is used as storage until the urine is allowed to pass through the urethra and out of the body.
• • Urethra: Duct that transmits urine from the bladder to the exterior of the body during urination. The female and male urinary system are very similar, differing only in the length of the urethra.
• The kidneys lie on either side of the spine in the retroperitoneal space between the parietal peritoneum and the posterior abdominal wall, well protected by muscle, fat, and ribs.