# 1911 Encyclopædia Britannica/Urinary System

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​URINARY SYSTEM. The urinary system in the fully developed human being consists of (r) the kidneys, (2) the ureters, (3) the urinary bladder, and (4) the urethra.
As the greater part of the male urethra is a generative as well as a urinary canal, its description will be found in the article on the Reproductive System.
The kidneys are two bean-shaped granular masses, firm in consistence and reddish brown in colour, about 4% in. long, and placed obliquely behind the other abdominal viscera-one on each side of the last thoracic and three upper lumbar vertebrae. Each is imperfectly covered on its ventral surface by peritoneum and is moulded to some extent by the viscera which press on it. Around them there is usually a considerable amount of fat and areolar tissue, by which, as well as by the peritoneum and b the presence of the surrounding viscera, the kidneys are retainecl, in their place. In rare cases the kidney may slip from its usual place in the loins to a lower position (movable kidney), and may even be movable
2 The lots may have been small pebbles, or small tablets of wood or bone. ​in the abdominal cavity (floating kidney)-a condition often productive of serious consequences. The kidney in the foetus is lobulated, but the intervals between the lobes become smoothed out in later years of childhood. Each gland is invested
Fig. I.—Vertical Section through the Kidney. A, branch of renal artery; U, ureter. I, cortical substance with cortical pyramids, and labyrinth substance of tortuous tubes; 2 and 3, medullary pyramids of straight tubules; 4, fatty masses around blood vessels (5); 6, papilla; 7, pelvis.
From A. F. Dixon, Cunningham's Text-Book of Anatomy.
Fig. 2.—Diagrammatic Representation of the Structures forming a Kidney Lobe.
In the middle part of the figure the course of one of the kidney tubules is indicated, and in the lateral parts the disposition of the larger arteries. A, cortex; B, intermediate zone; C, papillary portion.
The diagram at the right-hand side of the lower part of the figure illustrates the Connexions of the structures composing a Malpighian corpuscle.
by a firm, closely adherent, fibrous capsule, under which is an imperfect lamina of unstriped muscle. The inner and ventral margin of each kidney is concave, and into this hilum or concavity the renal artery from the aorta passes. Here also the renal vein escapes and joins the vena cava inferior. The ureter or metanephric duct, always behind and below the blood vessels, emerges here and passes downward to the bladder. When the kidney is longitudinally divided from hilum to outer edge, the cut surface is seen to consist of two parts—an outer layer, the cortex, and an inner part, the medulla (fig. 1). The latter consists of a series of eight to sixteen pyramids, whose bases and sides are invested with cortical matter, and whose apices or papillae project into the hilum, where they are severally surrounded by membranous tubes (calices), which by their union make up the ureter. The part of the ureter situated in the hilum is dilated, and is named the pelvis of the kidney.
In minute structure the kidney is the most complex gland in the body. Each of the papillae consists of a large number of straight tubes—collecting tubules—which open by pores on its surface. When these are traced into the pyramid, they are seen to divide several times, their line end-branches projecting in little tufts into the cortical matter at the base of each pyramid. Here the branches coming from the tube change in structure and become convoluted in the cortex-the convoluted tubules. Next, each suddenly dips back again as a long straight loop—the loop of Henle—into the pyramid, reaching nearly to the papillary region; then turning sharply on itself, passes back straight to the cortex, where it again becomes convoluted, ultimately ending by dilating into a flask-like bulb called a Malpighian corpuscle. The renal artery, after breaking
Fig. 3.—Vertical Section through Pelvis, showing urinary bladder and rectum in situ. 1, peritoneum; 2, pubic symphysis; 3, muscular coat of bladder; 5, mucous membrane folded and wrinkled; 6, opening of ureter; 8, prostate; 10, vena dorsalis penis; 12, corpus spongiosum; 14, testis in its sac; 15, bulbocavernosus muscle; 16, bulb; 17, sphincters of the anus; 22, anal opening; 30, coccyx; *, vesicula seminalis.
up into branches between the pyramids, ends in minute end-arteries in the cortex. Each of these pierces into one of the flasks just described, and there becomes branched, the branches being collected into a little ball or glomerulus which nearly fills the flask. From this an efferent vessel escapes, which, joining with its neighbouring vessels of the same kind, makes a close network around the convoluted tubes, ultimately ending in the renal vein. It is supposed that the different constituents of the urine are eliminated in different parts of these tubes—some, especially the watery parts, in the flask, and some, especially the more solid constituents, in the convoluted tubular apparatus. A peculiar form of glandular epithelium lines the two convoluted areas of the tubes and the limb of the loop nearer the straight or collecting tubes.
Bladder and urethra.
The ureter or duct of the kidney begins at the hilum and descends on the back wall of the abdominal cavity to open into the bladder. It is usually about 12 in. in length and as thick as a goose quill. At its termination it passes obliquely through the coats of the bladder, so that when the bladder is distended the lumen of its end is closed. The urinary bladder is a membranous bag lying in the pelvic cavity directly behind and above the dorsal surface of the pubes. In the foetus and infant, however, the bladder lies in the abdomen, not in the pelvis. During life it is seldom distended so as to hold more than about 10 oz., but when the abdomen is opened it can be dilated to more than double that size. When distended it rises and is applied closely against the back of the ventral abdominal wall. The bladder has a strong muscular investment of unstriped muscle in several layers, which ​ are innervated by branches from the sacral nerves. It has a peculiar epithelial lining of several strata, the superficial cells of which are cubical when the sac is collapsed, but become flattened and scale like when it is distended. At the lower part of the bladder there is a triangular space known as the trigone, the angles of which are formed by the openings of the two ureters and the urethra. In this space the mucous membrane is smooth and firmly bound to the subjacent muscle; elsewhere it is thrown into numerous folds when the bladder is empty. A muscular band called the torus uretericus Infero-lateral
area
Urethra
Urelel
Posterior surface of prostate
Seminal vesicle
From A. F. Dixon, Cunningham's Tex!-Book of Anatomy. FIG. 4.-The Bladder, Prostate and Seminal Vesicles, viewed from below. Taken from a subject in which the viscera were hardened in situ. The bladder contained but a small amount of fluid. or Mercier's bar joins the orifices of the ureters. The female urethra is only 1% in. in length and is comparable only with that part of the male urethra which extends from the bladder to the openings of the seminal ducts (fig. 3).
Embryology.
The excretory organs of the embryo are developed as a series of small tubes in the intermediate cell mass (see fig. 5). the ventral part of which projects to form the Wolffian ridge. Three sets of these tubes appear in succession and occupy the whole length of the body from the cervical to the lumbar region. The most anteriorpronephros or head kidney-is represented in man by only two or three small tubules on each side which appear as in growths from the neighbouring coelom (fig. 6, Pro. N.). From the study of comparative anatomy it is probable that these are mere vestiges. Although the pronephros is rudimentary, the duct which in lower Neuraltube
Notochord Intermediate cell mass
and
Wolfiian duct
and mesonephros avitY
From A. F. Dixon, Cunningham's Tex!-Book of Anatomy. FIG. 5.-Transverse Section through the Body of a Fowl Embryo. types carries away its excretion is well developed. This is the Woljian duct, which appears in man before the pronephric tubes are formed, and runs longitudinally back in each intermediate cell mass to open into the cloaca (fig. 6, Wv.D.). In certain parts of its course it is at an early date in very close relation with the skin on the dorsal side of the intermediate cell mass, and many embryologists hold that it is originally ectodermal in origin, and has sunk into the mesoderm secondarily. Others think that it is primarily mesodermal but has gained secondary connexions with the ectoderm. From a morphological point of view, as will be explained in the comparative anatomy section, the former view seems the more likely. When the pronephric tubules disappear, which they do at an early stage of the embryo's development, the Wolffian diict persists and acts as the drain for another and much more important series of tubules, which are formed in the intermediate cell mass behind the region of the pronephros, and make up the mesonephros or middle kidney (fig. 6, M.N.). There is some doubt as to whether these tubes are strictly homologous and in series with those of the Dronephros; but they are certainly of later development. By about the sixth week of intra-uterine life these tubules reach their maximum development and form the WoL[7'ian body, which projects into the coelom as the now very definite Woljian ridge and acts as the functional excretory organ of the embryo (see fig. 7). When the permanent kidney is formed this organ degenerates and its ultimate fate is discussed in the article on the REPRO-DUCTIVE SYSTEM.
The metanephros or hind kidney begins as a diverticulum from the dorsal side of the Wolfiian duct close to its opening into the
FIG. 6.-Diagram of the Formation of the Genito-Urinary Apparatus. The first figure is the generalized type, the second the male and the third the female
Suppressed parts are dotted.
specialized arrangements.
Pro. N. Pronephros. N. Nephrostome. M. N. Mesonephros. M. C. Malpighian corpuscle. Mt. N. Metanephros. T. Testis.
B. Bladder. E. Epididymis.
Clo. Cloaca. 0. G. Organ of Giraldés. R. Rectum. V. D. Vas defcrens.
M. D. Mullerian duct. U. M. Uterus masculines. W. D. Wolnian duct. O. Ovary.
Ur. Ureter. Ep. O. Epoiiphoron.
S. H. Sessile hydatid. Par. O. Paroiiphoron. P. H. Pedunculated hydatid. F. 'I'. Fallopian tube. S. G. Sexual gland. U. Uterus.
cloaca (see fig. 6, Mt.N.); this occurs about the fourth week of intra-uterine life, and the diverticulum grows forward (cephalad), dorsal to the hind end of the Wolffian body. In doing this it forms a duct-the metanephric duct or meter-the cephalic end of which enlarges and divides to form the calices of the kidney. From the calices numerous smaller ducts grow into the mesoderm of the hind (caudal) end of the intermediate cell mass and become the collecting Neural
nerve
-ndvdl
Inzesrine
From A. F. Dixon, Cunningl1am's Text-Book of Anatomy. FIG. 7.-Transverse Section through the Body of a Rat Embryo. The position where the germinal epithelium arises is indicated at a. tubes of the kidney. While this is going on another set of tubules, probably in series with the mesonephric tubules, develop independently in the intermediate cell mass and so form all the rest of the tubular system of the kidney. Toward these tubules, at one point, branches from the aorta push their way and invaginate each tube, thus forming the Malpighian corpuscles. By the eighth week the kidney is definitely formed and takes over the excretory work of the mesonephros, which now atrophies; its surface is distinctly lobulated, a condition which persists until after birth. ​At first, as has been stated, the ureters open into the Wolffian ducts, but later on each gains a separate opening into the cloaca, and eventually these shift in a ventral direction until they reach their permanent connexion with the allantoic bladder.
The bladder is developed from that part of the cloaca from which the allantois has grown out, and also from that part of the allantois which is nearest the cloaca. At first it is a tubular structure, but after the second month becomes more pyriform, the stalk of the pear corresponding to the fibrous urachus which reaches the umbilicus. Most of that part of the tubular allantois which lies between the permanent openings of the ureters and the Wolffian ducts becomes the urinary sinus and does not dilate in the same way that the permanent bladder does. This, in the female, forms the whole of the urethra, and in the male the upper part of the pro static urethra. Behind (caudad) the urinary sinus is the urogenital sinus, which is treated of in the article on the Reproductive System.
The Müllerian ducts (fig. 6, M.D.) are formed after the Wolffian ducts are fully developed. A ridge appears in the intermediate cell mass ventral to the Wolffian duct, and into the anterior (cephalic) end of this a tubular process of the coelom forces its way backward (caudad). Before reaching the cloaca the two Mullerian ducts coalesce and open between the orifices of the two Wolffian ducts. These ducts, as is shown in the article. on the Reproductive System, form the oviducts, uterus and at least part of the vagina.
For further details and literature see Quain's Anatomy, vol. i. (Longmans, Green & Co., London, 1908); J. M‘Murrich, The Development of the Human Body (Rebman, London, 1906), and A. Keith, Human Embryology and Morphology (Arnold, London).
Comparative Anatomy.
In the Acrania (Amphioxus) the nephridial tubules are segmentally arranged and are only found in the pharyngeal region; each opens into the coelom by several ciliated funnels called nephrostomes, and also into the atrium, which is practically the exterior of the animal, by an opening called the nephridiopore. There is reason to believe that we have here a pronephros of a very primitive type and arranged on the same plan, in many respects, as the simple nephridia of such lowly forms as the earthworm. There is nothing to indicate that a mesonephros is present, nor are there any Malpighian corpuscles or longitudinal ducts.
Among the Cyclostomata (lampreys and hags) the pronephros persists throughout life in Bdellostoma and probably in the hag (Myxine), but a Wolffian (archinephric) duct has been evolved so that the tubules no longer open on the surface by nephridiopores. It has been surmised that in a transitional type the tubules opened into a groove on each side of the surface of the animal and that the edges of this, coming together, formed a duct. At any rate the superficial openings of the primitive nephridia make it probable that the Wolffian duct was originally of ectodermal origin. A mesonephros has now appeared behind (caudad) the pronephros, though it is not certain whether its tubules (mesonephridia) are in series with those of the pronephros or whether they are structures on a more dorsal plane; but they certainly open into the Wolffian duct, which also drains the pronephros, and so this duct is functionally simply a ureter and has nothing to do with the sexual glands. No Müllerian duct has yet been evolved.
In the Teleostomi (bony and ganoid fish) the pronephros is usually aborted in the adult and the mesonephros is the functional kidney. As the genital glands have special coelomic relations the Wolffian duct is still merely a ureter, and in the Teleostei at least there is no true Müllerian duct.
In the Elasmobranchii (sharks and rays) the pronephros is more completely and more early aborted than in the last subclass, and the mesonephros is divided into an anterior or genital part, which receives the vasa efferentia in the male from the testis and thus is the first appearance phylogenetically of an epididymis and a posterior or renal part. The Wolffian duct therefore acts both as a vas deferens or the sperm and a ureter for the urine, though in the female it is merely a ureter. In the hindmost part of the mesonephros there are separate ducts which are called ureters and open into the lower part of the Wolffian duct in the same way that the metanephric ducts of the Amniota do; it is, however, very doubtful whether they are really homologous with these ducts. The Müllerian duct (see Reproductive System) is present in elasmobranchs and according to modern views arises as a back growth from the coelom as in the Amniota.
The Dipnoi or mudfish are remarkable for having a cloacal caecum which probably functions as an urinary bladder. It is situated on the dorsal wall of the cloaca and is not homologous with the allantoic bladder of higher forms. A good deal of the kidney (mesonephros) as it appears to the naked eye is composed of lymphoid tissue.
In the Amphibia the snake-like forms (Gymnophiona) show a very primitive arrangement of the kidney tubules, each having its nephrostome, Malpighian capsule and short convoluted part leading to the Wolffian duct which acts both as ureter and vas deferens.
In the adult Anura (frogs and toads) the nephrostomes lose their connexion with the nephridia and communicate with the renal veins. In the amphibians a true allantoic bladder first appears as a diverticulum from the ventral wall of the cloaca; in different forms it may be single, bilobed or even double.
In Reptilia the hind kidney or metanephros is developed and takes over all the excretory work; it is usually lobulated, its nephridia are never provided with nephrostomes and its duct (the ureter) opens into the Wolffian duct or vas deferens before reaching the cloaca. The allantoic bladder is present in the Lacertilia (lizards) and Chelonia (turtles), but is absent in others. Birds resemble reptiles very closely in their urinary system except that there is never any bladder and that the ureters and vasa deferentia open independently into the cloaca.
In the Mammalia the bean shape of the kidney is fairly characteristic. In foetal life the organ is always lobulated, and this sometimes persists throughout adult life as in the ox, bear, seal and whale. More often the lobulation disappears on the surface and is only imperfectly represented, on making a section, by the pyramids; even these in some cases fuse so closely that their apices appear as a single papilla. This is the case in many monkeys, carnivores and rodents.
In the Monotremata (Ornithorhynchus and Echidna) there is an allantoic bladder, but the ureters open into the cloaca as they do in birds. In all other mammals they have reached the bladder and open into it by valvular orifices.
On comparing the embryology (ontogeny) of the urinary system with its comparative anatomy (phylogeny) the harmony of the two from a broad point of view is very striking.
For further details see Parker and Haswell, Text-Book of Zoology (Macmillan, London, 1397); Wiedersheim’s Comparative Anat. of Vertebrates, translated by W. N. Parker (London, 1907); Gegenbaur, Vergleich. Anat. der Wirbeltiere (Leipzig, 1901).