Urinary bladder by pengtt

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									Urinary bladder

The urinary bladder is a reservoir and varies in size, shape, position and relations,
according to its content and the state of neighbouring viscera. When empty, it is
entirely in the lesser pelvis but as it distends it expands anterosuperiorly into the
abdominal cavity. When empty, has a fundus, neck, apex, a superior and two
inferolateral surfaces. The fundus or base is triangular and postero-inferior. In females
it is closely related to the anterior vaginal wall; in males it is related to the rectum
although separated from it above by the rectovesical pouch and below that by the
seminal vesicles and deferent ducts.
In a triangular area between the deferent ducts, the bladder and rectum are separated
only by rectovesical fascia; the inferior part of this area may be obliterated by
approximation of the deferent ducts above the prostate. Although the vesical fundus
should be, by definition, the lowest region, the neck is in fact lowest and also the
most fixed; it is 3–4 cm behind the lower part of the symphysis pubis. It is pierced by
the internal urethral orifice and alters little in position with varying conditions of the
bladder and rectum. There is no special constriction of the bladder at its neck.

In males the neck rests on, and is in direct continuity with, the base of the prostate; in
females it is related to the pelvic fascia which surrounds the upper urethra.
The vesical apex in both sexes faces towards the upper part of the symphysis pubis;
from it the median umbilical ligament (urachus) ascends behind the anterior
abdominal wall to the umbilicus, the peritoneum over it being the median umbilical
fold.
The triangular superior surface is bounded by lateral borders from the apex to the
ureteric entrances and by a posterior border joining them. In males the superior
surface is completely covered by peritoneum, extending slightly on to the base and
continued posteriorly into the rectovesical pouch, laterally into the paravesical fossae
and anteriorly into the median umbilical fold. It is in contact with the sigmoid colon
and the terminal coils of the ileum.
In females the superior surface is also largely covered by peritoneum but posteriorly
this is reflected to the uterus at the level of the internal os (the junction of the uterine
body and cervix), forming the vesico-uterine pouch. The posterior part of the superior
surface, devoid of peritoneum, is separated from the supravaginal cervix by fibro-
areolar tissue. Each inferolateral surface in males is separated anteriorly from the
pubis and puboprostatic ligaments by an adipose retropubic pad and posteriorly by
fascia from the levator ani and obturator internus. In females the relations are similar,
but the puboprostatic are replaced by pubovesical ligaments. The inferolateral
surfaces are not covered by peritoneum.
As the bladder fills it becomes ovoid. In front it displaces the parietal peritoneum
from the suprapubic region of the abdominal wall, so that the inferolateral surfaces
become anterior and rest against the abdominal wall without intervening peritoneum
for a distance above the symphysis pubis, varying with the degree of distension.
At birth, the bladder is relatively higher than in the adult, the internal urethral orifice
being level with the upper symphyseal border; the bladder is abdominal rather than
pelvic, extending about two-thirds of the distance towards the umbilicus. It
progressively descends, reaching the adult position shortly after puberty.
Ligaments of the Bladder
In both sexes stout bands of fibromuscular tissue extend from the bladder neck to the
inferior aspect of the pubic bones. These structures are the pubovesical ligaments and
constitute the superior extensions of the pubourethral ligaments in the female or the
puboprostatic ligaments in the male. The two pubovesical ligaments lie one on each
side of the median plane, leaving a midline hiatus through which pass numerous small
veins. In addition to the pubovesical ligaments a number of other so-called ligaments
have been described in relation to the base of the urinary bladder. These are formed
by condensation of connective tissue around neurovascular structures and as such do
not merit the distinction of having specific names assigned to them.
The apex of the bladder is joined to the umbilicus by the remains of the urachus
(below), which forms the median umbilical ligament.. From the superior surface of
the bladder the peritoneum is carried off in a series of folds which are termed the
'false' ligaments of the bladder. Anteriorly there are three folds: the median umbilical
fold over the median umbilical ligament and two medial umbilical folds over the
obliterated umbilical arteries. The reflexions of the peritoneum from the bladder to the
side walls of the pelvis form the lateral false ligaments while the sacrogenital folds
constitute the posterior false ligaments.

Interior of the Bladder
Vesical Mucosa
Attached only loosely to subjacent muscle for the most part, it folds when the bladder
empties, the folds being effaced as it fills. Over the trigone (13.35), immediately
above and behind the internal urethral orifice, it is adherent to the subjacent muscle
layer and always smooth. The trigone's anteroinferior angle is formed by the internal
urethral orifice, its posterolateral angles by the ureteric orifices. The superior trigonal
boundary is a slightly curved interureteric crest, connecting the two ureteric orifices
and produced by the continuation into the vesical wall of the ureteric internal
longitudinal muscle. (For details of trigonal musculature, consult Uhlenhuth et al
1952; Woodburne 1965, 1968.) Laterally this ridge extends beyond the ureteric
openings as ureteric folds, produced by the terminal parts of the ureters running
obliquely through the bladder wall. At cystoscopy the interureteric crest appears as a
pale band and is a guide to the ureteric orifices in catheterization.
Ureteric Orifices
Placed at the posterolateral trigonal angles (13.35), they are usually slit-like. In empty
bladders they are about 2.5 cm apart and about the same from the internal urethral
orifice; in distension these measurements may be doubled.
Internal Urethral Orifice
Sited at the trigonal apex, the lowest part of the bladder, this is usually somewhat
crescentic in section; in adult males, particularly past middle age, immediately behind
it is a slight elevation caused by the median prostatic lobe, the uvula of the bladder.
Bladder Capacity
Mean vesical capacity in male adults varies from 120–320 ml (Thompson 1919);
micturition commonly occurs at about 280 ml. Filling to about 500 ml may be
tolerated but beyond this pain is caused by tension in the wall, leading to reflex
contractions and the urgent desire to micturate. Pain is referred to the cutaneous areas
supplied by spinal segments supplying the bladder (T11–L2, S2–4), including the
lower anterior abdominal wall, perineum and penis.
Bladder Microstructure
Histologically the wall of the urinary bladder consists of three layers (13.36): an outer
adventitial layer of soft connective tissue which in some regions possesses a serosal
covering of peritoneum, a nonstriated muscle coat (the detrusor muscle) and an inner
layer of mucous membrane which lines the interior of the bladder.
Serous Layer
The serous layer is restricted to the superior and, in males, part of the posterior
surfaces of the bladder, the rest being devoid of peritoneum. It consists of
mesothelium and underlying connective tissue as elsewhere in the peritoneum.
Muscular Layer
The muscular layer, the detrusor muscle, is composed of relatively large (in diameter)
interlacing bundles of nonstriated muscle cells arranged as a complex meshwork.
Three ill-defined layers are present and arranged in such a way that longitudinally
orientated muscle bundles predominate on the inner and outer aspects of a substantial
middle circular layer. Posteriorly some of the outer longitudinal bundles pass over the
bladder base and fuse with the capsule of the prostate or with the anterior vaginal
wall. Other bundles are carried on to the anterior aspect of the rectum and named the
rectovesical muscle. Anteriorly some of the outer longitudinal bundles continue into
the pubovesical ligaments and contribute to the muscular component of these
structures. As in the muscle coat of the ureter, exchange of fibres between adjacent
muscle bundles within the bladder wall frequently occurs so that from a functional
viewpoint, the detrusor comprises a single unit of interlacing smooth muscle. An
electron-dense basal lamina surrounds each nonstriated myocyte except at certain
junctional regions. The most frequently observed type of junction between muscle
cells is the region of close approach at which an intercellular separation of 10–20 nm
occurs over distances occasionally in excess of 1 mm in length. Junctions of the 'peg
and socket' and 'intermediate' types are observed occasionally but gap junctions
(nexuses) have not been reported in the detrusor. Since electrotonic spread excitation
occurs in the nonstriated muscle of the bladder wall, the regions of close approach
may represent the morphological feature which enables this physiological event to
take place, although it is more likely that gap junctions exist, albeit few of them.
Within muscle bundles, the smooth muscle cells are closely packed together such that
the basal lamina of one cell very often becomes confluent with that of its neighbours.
Trigone
The smooth muscle of this region consists of two distinct layers, sometimes termed
the superficial and deep trigonal muscles. The latter is composed of muscle cells
which are indistinguishable from the muscle cells of the detrusor. Hence this deep
trigonal muscle is simply the postero-inferior portion of the detrusor muscle proper
and confusion might be avoided if the term deep trigonal muscle was abandoned in
favour of the more accurate definition as trigonal detrusor muscle. The superficial
trigonal muscle represents a morphologically distinct component of the trigone which,
unlike the detrusor, is composed of relatively small diameter muscle bundles
continuous proximally with those of the intramural ureters. The muscle layer
comprising the superficial trigone is relatively thin but is generally described as
becoming thickened along its superior border to form the interureteric crest. Similar
thickenings occur along the lateral edges of the superficial trigone (Bell 1812). In both
sexes the superficial trigone muscle becomes continuous with the smooth muscle of
the proximal urethra, extending in the male along the urethral crest as far as the
openings of the ejaculatory ducts. However, the arrangement of nonstriated muscle in
the human trigone has been re-examined recently by Dorschner et al (1994a) who
describe the presence of two muscular structures, a musculus interuretericus
originating from the muscular systems of both ureters and forming the muscular
component of the interureteric ridge, and a musculus sphincter trigonalis or musculus
sphincter vesicae, the latter surrounding the internal urethral orifice but not extending
into the urethra or its surroundings. This description is at variance with that of Bell
(1812). Dorschner et al (1994a) reported that in men the lower part of the sphincter
trigonalis is pervaded with prostatic tissue, and suggested that the muscle could have a
dual function, assisting in urinary continence and also preventing retrograde
ejaculation whilst aiding the release of prostatic secretions.
Ureterovesical Junction
The distal 1–2 cm of each ureter is surrounded by an incomplete collar of the detrusor
nonstriated muscle which forms a sheath (of Waldeyer) separated from the ureteric
muscle coat by a connective tissue sleeve. The ureters pierce the posterior aspect of
the bladder and run obliquely through its wall for a distance of 1.5–2.0 cm before
terminating at the ureteric orifices. This arrangement is believed to assist in the
prevention of reflux of urine into the ureter, since the intramural ureters are thought to
be occluded during increases in bladder pressure. There is no evidence of a classic
ureteral sphincter mechanism in man (Noordzij & Dabhoiwala 1993). The
longitudinally oriented muscle bundles of the terminal ureter continue into the bladder
wall and at the ureteric orifices become continuous with the superficial trigonal
muscle (Tanagho et al 1970). More recently this muscular continuity has been
disputed, no evidence of it having been found in the porcine ureterovesical junction
(Thomson et al 1994).
Bladder Neck
The nonstriated muscle of this region is histologically, histochemically and
pharmacologically distinct from that which comprises the detrusor muscle proper
(Klück 1980). Hence the bladder neck should be considered as a separate functional
unit. The arrangement of nonstriated muscle in this region is quite different in males
and females and consequently each sex will be described separately.
Male
In the male bladder neck, the nonstriated muscle cells form a complete circular collar
which extends distally to surround the preprostatic portion of the urethra. Because of
the location and orientation of its constituent fibres, the terms internal, proximal or
preprostatic urethral sphincter are suitable alternatives for this particular component
of urinary tract smooth muscle. Distally, the bladder neck muscle merges with and
becomes indistinguishable from the musculature in the stroma and capsule of the
prostate gland.
Female
The female bladder neck also consists of morphologically distinct nonstriated muscle,
since the large diameter fasciculi characteristic of the detrusor are replaced in the
region of the bladder neck by those of small diameter. However, unlike the circularly
orientated preprostatic nonstriated muscle, the muscle fasciculi in the female extend
obliquely or longitudinally into the urethral wall. The female does not, therefore,
possess a smooth muscle sphincter at the bladder neck and it is unlikely that active
contraction of this region plays a significant part in the maintenance of female urinary
continence.
Mucosa
The mucosa has a structure similar to that of the ureters and consists of an epithelium
(urothelium, transitional epithelium, p. 72) supported by a layer of loose connective
tissue, the lamina propria. The latter consists of loose fibro-elastic connective tissue
and forms a relatively thick layer, varying in depth from 500 mm in the fundus and
inferolateral walls to about 100 mm in the trigone. Small-diameter bundles of
nonstriated muscle cells also occur in the subepithelial connective tissue forming an
incomplete and rudimentary muscularis mucosae. The soft connective tissue elements
immediately beneath the urothelium, particularly in the region of the trigone, are
densely packed. At deeper levels they are more loosely arranged, thus allowing the
bladder mucosa to form numerous thick folds when the volume of fluid contained
within the lumen is small. An extensive network of blood vessels is present
throughout the lamina propria and supplies a plexus of thin-walled fenestrated
capillaries lying in grooves at the base of the urothelium.
Nontrigonal urothelium is often up to six cells in thickness. These cells can be
classified according to position and consist of highly differentiated superficial or
luminal cells, one or more layers of smaller intermediate cells and a layer of
undifferentiated basal cells. The large superficial cells frequently bulge into the
bladder lumen and are often binucleate. In contrast the intermediate and basal cells are
smaller and each contains a single darkly-staining nucleus. The flattened urothelium
of the trigone usually consists of only two or three layers of cells and a similar
appearance prevails throughout the bladder when in the distended state.
In addition to the basal, intermediate and superficial cells described above, a fourth
type of cell occurs in the urothelium of the human bladder neck and trigone. These
flask-shaped cells extend throughout the depth of the urothelium and are characterized
by the presence of numerous large membrane-bound vesicles each containing a
central dense granule. Vesicles of this type are believed to be involved in the storage
of amines and it seems likely that these cells belong to the so-called APUD (amine-
precursor-uptake and decarboxylation, see p. 1898) series which have a wide
distribution throughout the body.
Several morphological variations have been described in the mucosa of the bladder
which, because of their occurrence in otherwise normal healthy adults, are not
considered to represent pathological conditions. One of the commonest epithelial
variants found in bladder biopsy samples or at postmortem is the occurrence of so-
called Brunn's nests. These consist of proliferations of morphologically normal basal
urothelial cells which project into the underlying connective tissue of the lamina
propria and are particularly frequent in the trigone. Mucus-secreting glands with
single or branched ducts are another frequently observed feature of the bladder
mucosa. When present these structures are particularly numerous near the ureteric and
internal urethral orifices. Non-keratinizing squamous metaplasia of the vaginal type
also frequently occurs in the urinary bladder mucosa, especially over the trigone. This
histological appearance, whilst occasionally observed in males and in children, is
more common in adult females.
Vascular and Lymphatic Supply of the Bladder
Arteries
The principal arteries of supply to the bladder are the superior and inferior vesical,
derived from the anterior trunk of the internal iliac artery (p. 1559). The obturator and
inferior gluteal arteries also send small branches to it and in the female additional
branches are derived from the uterine and vaginal arteries.
Veins
These form a complicated plexus on the inferolateral surfaces and pass backwards in
the posterior ligaments of the bladder to end in the internal iliac veins (p. 1598).
Lymph Vessels
These are described on page 1623.
Nerve Supply of the Bladder
The nerves supplying the bladder form the vesical plexus (see p. 1309) and consist of
both sympathetic and parasympathetic components, each of which contains both
efferent and afferent fibres. The innervation of the bladder was reviewed in some
detail by Burnstock (1990c) and by de Groat in 1993.
Efferent Fibres
Parasympathetic fibres arise from the second to the fourth sacral segments of the
spinal cord (nervi erigentes); the sympathetic fibres are derived from the lower two
thoracic and upper two lumbar segments of the spinal cord. In addition to the branches
from the vesical plexus, small groups of autonomic neurons occur throughout all
regions of the bladder wall. These multipolar intramural neurons are rich in acetyl
cholinesterase (AChE) and occur in ganglia consisting of five to 20 nerve cell bodies.
Numerous preganglionic autonomic fibres form both axosomatic and axodendritic
synapses with the ganglion cells. The majority of these preganglionic nerve terminals
correspond morphologically to presumptive cholinergic fibres. Noradrenergic
terminals also relay on cell bodies in the pelvic plexus although it is unknown whether
similar nerves synapse on intramural bladder ganglia.
The urinary bladder (including the trigonal detrusor muscle) is profusely supplied
with nerves which form a dense plexus among the detrusor smooth muscle cells. The
majority of these nerves contain AChE and occur in abundance throughout the muscle
coat of the bladder. Axonal varicosities adjacent to detrusor nonstriated muscle cells
possess features which are considered to typify cholinergic nerve terminals and
contain clusters of small (50 nm diameter) agranular vesicles together with occasional
large (80–160 nm diameter) granulated vesicles and small mitochondria. Terminal
regions approach to within 20 nm of the muscle cells' surface and are either partially
surrounded by or more often totally denuded of Schwann cell cytoplasm. The human
detrusor muscle possesses a sparse supply of sympathetic noradrenergic nerves
(Sundin et al 1977). Nerves of this type generally accompany the vascular supply and
only rarely extend among the nonstriated myocytes of the urinary bladder. A further
component plays a part in the autonomic innervation of the urinary bladder (Ambache
& Zar 1970), which has been classified as having a nonadrenergic, noncholinergic
nerve mediated effect. A number of other neurotransmitters or neuromodulators have
been detected in intramural ganglia, including the peptide somatostatin (see p. 937).
The superficial trigonal muscle is associated with few cholinergic (parasympathetic)
nerves while those of the noradrenergic (sympathetic) variety occur relatively
frequently. These differences support the view that the superficial trigonal muscle
should be regarded as 'ureteric' rather than 'vesical' in origin. It should be emphasized
that the superficial trigonal muscle forms a very minor part of the total muscle mass of
the bladder neck and proximal urethra in either sex and is probably of little
significance in the physiological mechanisms which control these regions.
In bladder neck of the male, nonstriated muscle is sparsely supplied with cholinergic
(parasympathetic) nerves but possesses a rich noradrenergic (sympathetic) innervation
(Gosling et al 1977). A similar distribution of autonomic nerves also occurs in the
nonstriated muscle of the prostate gland, seminal vesicles and ducti deferentes. From
a functional standpoint, sympathetic nerves on stimulation cause contraction of
nonstriated muscle in the wall of the genital tract resulting in seminal emission.
Concomitant sympathetic stimulation of the proximal urethral muscle causes
sphincteric closure of the bladder neck, thereby preventing reflux of ejaculate into the
bladder. Although this genital function of the bladder neck of the male is well
established it is not known whether the nonstriated muscle of this region plays an
active role in maintaining urinary continence.
In contrast with this rich sympathetic innervation in the male, the nonstriated muscle
of the bladder neck of the female receives relatively few noradrenergic nerves but is
richly supplied with presumptive cholinergic fibres. The sparse supply of sympathetic
nerves presumably relates to the absence of a functioning 'genital' portion
incorporated within the wall of the female urethra.
The lamina propria of the fundus and inferolateral walls of the bladder is virtually
devoid of autonomic nerve fibres, apart from some noradrenergic and occasional
presumptive cholinergic perivascular nerves. However, as the urethral orifice is
approached the density of nerves unrelated to blood vessels increases. At the bladder
neck and trigone a nerve plexus extends throughout the lamina propria. The
constituent nerves are cholinesterase positive and run through the connective tissue
unassociated with blood vessels. Some of the larger diameter axons are myelinated
and others lie adjacent to the basal urothelial cells. As in the ureter, the subepithelial
nerve plexus of the bladder is assumed to subserve a sensory function in the absence
of any obvious effector target sites (Gosling & Dixon 1974).
Afferent Fibres
Vesical nerves are also concerned with pain and awareness of distension. Pain fibres
are stimulated by distension or spasm due to a stone, inflammation or malignant
disease; they are found in sympathetic and parasympathetic nerves, predominantly the
latter. Hence, simple division of the sympathetic paths (e.g. 'presacral neurectomy'), or
of the superior hypogastric plexus (p. 1308), does not materially relieve vesical pain.
The spinal path for pain is in the anterolateral white columns and considerable relief
follows bilateral anterolateral cordotomy. Since nerve fibres mediating awareness of
distension are in the posterior columns (fasciculus gracilis), after anterolateral
cordotomy the patient still retains awareness of the need to micturate. The nerve
endings detecting noxious stimuli are probably of more than one type; a subepithelial
plexus of fibres containing dense vesicles, probably afferent endings, has been
described.
Clinical Anatomy
A distended bladder may be ruptured in lower abdominal or pelvic injuries, either
extraperitoneally or, if the superior surface is involved, with tearing of the peritoneum
and escape of vesical contents into the peritoneal cavity. In progressive chronic
obstruction to micturition, for example by prostatic enlargement (p. 1861) or urethral
stricture, vesical musculature hypertrophies, its fasciculi increasing in size and
interlacing in all directions to produce an enlarged 'trabeculated bladder'. Mucosa
between the fascicles forms 'diverticula', which may contain phosphatic concretions.
When outflow is thus obstructed, emptying is not complete; some urine remains and
may become infected; infection may extend to the ureters and kidneys. Back pressure
from a distended bladder may gradually dilatate the ureters, renal pelves and even the
renal collecting tubules. Lesions of the fasciculus gracilis (e.g. tabes dorsalis) cause
loss of desire to micturate; the distended bladder may empty merely by overflow.
Severe spinal cord lesions above its sacral segments, interrupting efferent and afferent
tracts involved in normal micturition, may result in 'automatic' emptying.
The vesical interior can be examined with a cystoscope, introduced via the urethra
after distending the bladder with fluid. A special cystoscope is used to catheterize the
ureters, to obtain a direct specimen of urine from either kidney or to inject radio-
opaque fluid for retrograde pyelography (13.6). The vesical outline can be similarly
demonstrated.
The distended bladder may be punctured just above the symphysis pubis without
traversing the peritoneum (suprapubic cystostomy). When the bladder contains about
300 ml its anteroinferior surface contacts the anterior abdominal wall directly for
about 7.5 cm above the pubis. Surgical access to the bladder is usually by this route.
In females, owing to the shorter and more dilatable urethra, small calculi, foreign
bodies and growths may be removed through it.
Congenital abnormalities of the bladder are described on p. 213.
Male Urethra
The male urethra (13.32, 38) is from 18–20 cm long, and extends from an internal
orifice in the urinary bladder to an external opening, or meatus, at the end of the penis.
It may be considered in four regional parts: preprostatic, prostatic, membranous and
spongiose, and presents a double curve while the penis is in its ordinary flaccid state
(13.32). Except during the passage of fluid along it, the urethral canal is a mere slit; in
the prostatic part the slit is transversely arched in transverse section, in the
preprostatic and membranous portions it is stellate, in the spongiose portion
transverse, while at the external orifice it is sagittal in orientation.
Preprostatic Part
The preprostatic urethra possesses a stellate lumen and is approximately 1–1.5 cm in
length, extending almost vertically from the bladder neck to the superior aspect of the
prostate gland. The non-striated muscle bundles surrounding the bladder neck and
preprostatic urethra are arranged as a distinct circular collar which becomes
continuous distally with the capsule of the prostate gland. The bundles which form
this preprostatic or internal sphincter (sphincter vesicae) are separated by connective
tissue containing many elastic fibres (Gilpin & Gosling 1983). Unlike the detrusor
muscle, the nonstriated muscle surrounding the proximal urethra is almost totally
devoid of parasympathetic cholinergic nerves but is richly supplied with sympathetic
noradrenergic nerves. Similar nerves also supply the nonstriated muscle of the
prostate, ducti deferentes and seminal vesicles and are involved in causing muscle
contraction at the time of ejaculation (Learmonth 1931). Contraction of the
preprostatic sphincter serves to prevent the retrograde flow of ejaculate through the
proximal urethra into the bladder.
Prostatic Part
The prostatic urethra is approximately 3–4 cm in length and tunnels through the
substance of the prostate closer to the anterior than the posterior surface of the gland.
It is continuous above with the preprostatic part and emerges from the prostate
slightly anterior to its apex (its most inferior point). Throughout most of its length the
posterior wall possesses a midline ridge, the urethral crest, which projects into the
lumen causing it to appear crescentic in transverse section. On each side of the crest
there is a shallow depression, termed the prostatic sinus, the floor of which is
perforated by the orifices of the prostatic ducts. About the middle of the length of the
urethral crest the colliculus seminalis (verumontanum) forms an elevation on which
the slit-like orifice of the prostatic utricle is situated; on both sides of or just within
this orifice there are the two small openings of the ejaculatory ducts. The prostatic
utricle is a cul-de-sac about 6 mm long, which runs upwards and backwards in the
substance of the prostate behind its median lobe. Its walls are composed of fibrous
tissue, muscular fibres and mucous membrane; the last presents the openings of
numerous small glands. Developed from the paramesonephric ducts or urogenital
sinus, it is thought to be homologous with the vagina of the female (p. 205). The
prostatic utricle is, therefore, called by some the 'vagina masculina', but the more
usual view is that it is a uterine homologue and hence the term 'utricle'. The
ejaculatory ducts are described on page 1856. Distally the prostatic urethra possesses
an outer layer of circularly disposed skeletal muscle cells which are continuous with a
prominent collar of striated skeletal muscle (the external urethral sphincter) within the
wall of the membranous urethra.
Membranous Part
The membranous part is the shortest, least dilatable and, with the exception of the
external orifice, the narrowest section of the urethra. It descends with a slight ventral
concavity from the prostate to the bulb of the penis (13.32), passing through the
perineal membrane about 2.5 cm postero-inferior to the pubic symphysis. The hind
part of the bulb of the penis is closely apposed to the inferior aspect of the urogenital
diaphragm (perineal membrane) but anteriorly it is slightly separated from the latter,
so that the wall of the urethra is related anteriorly neither to the perineal membrane
nor the penile bulb. If this part of the anterior wall of the urethra is regarded as the
'membranous' anteriorly the membranous urethra is about 2 cm long, whilst
posteriorly it is only 1.2 cm. The wall of the membranous urethra consists of a muscle
coat which is separated from the epithelial lining by a narrow layer of fibro-elastic
connective tissue. This muscle coat consists of a relatively thin layer of nonstriated
muscle bundles continuous proximally with those of the prostatic urethra and a
prominent outer layer of circularly orientated skeletal muscle fibres forming the
external urethral sphincter. The skeletal muscle fibres which comprise this external
sphincter are unusually small in cross-section, with diameters of only 15–20 mm. The
fibres are physiologically of the slow twitch type (Gosling et al 1981), unlike the
pelvic floor musculature which is a heterogeneous mixture of slow and fast twitch
fibres of larger diameter. Moreover, the external sphincter is devoid of muscle
spindles and is supplied by the pelvic splanchnic nerves, further distinguishing it
from the periurethral levator ani muscle (Donker et al 1976). The slow twitch fibres of
the external sphincter are capable of sustained contraction over relatively long periods
of time and actively contribute to the tone which closes the urethra and maintains
urinary continence.
Spongiose Part
The spongiose part is contained in the corpus spongiosum penis (p. 1857). It is about
15 cm long and extends from the end of the membranous urethra to the external
urethral orifice on the glans penis. Commencing below the perineal membrane, it
continues the ventrally concave curve of the membranous urethra to a point anterior to
the lowest level of the symphysis pubis. From here, when the penis is flaccid, the
urethra curves downwards in the 'free' part of the penis. It is a narrow, transverse, slit
when empty, with a diameter of about 6 mm when passing urine; it is dilated at its
commencement as the intrabulbar fossa and again within the glans penis, where it
becomes the navicular fossa. The enlargement of the intrabulbar fossa affects the floor
and side walls but not the roof of the urethra. The bulbo-urethral glands open into the
spongiose section of the urethra about 2.5 cm below the perineal membrane (p. 1861).
The external urethral orifice is the narrowest part of the urethra: it is a sagittal slit,
about 6 mm long, bounded on each side by a small labium.
The epithelium of the urethra, except in its most anterior part, presents the orifices of
numerous small mucous glands and follicles situated in the submucous tissue and
named the urethral glands. Besides these there is a number of small pit-like recesses,
or lacunae, of varying sizes; the orifices of these are directed forwards and may
intercept the point of a catheter in its passage along the canal. One, larger than the
rest, the lacuna magna, is situated on the roof of the navicular fossa.
Mucous Membrane of the Male Urethra
The epithelium lining the preprostatic urethra and the proximal part of the prostatic
urethra is of the typical urothelial type and is in continuity with that lining the bladder;
it is also continuous with the ducts of the prostate and bulbo-urethral glands and with
the linings of the seminal vesicles, deferent ducts and ejaculatory ducts, a relationship
which is important in the spread of urinary tract infections. However, below the
openings of the ejaculatory ducts this epithelium changes to a patchily
pseudostratified or stratified columnar variety which lines the membranous urethra
and the major part of the penile urethra (2.71). Mucus-secreting cells are common
throughout this epithelium and frequently occur in small clusters in the penile urethra.
The mucuous membrane of the penile urethra shows many recesses which continue
into deeper branching tubular mucous glands (of Littré) which are especially
numerous on the dorsal aspect. In older men many of the recesses of the urethral
mucosa contain concretions similar to those found within the substances of the
prostate. Towards the distal end of the penile urethra the epithelium changes once
again, becoming stratified squamous in character with well-defined connective tissue
papillae. This type of epithelium lines the navicular fossa and becomes keratinized at
the external meatus.
The epithelial cells lining the navicular fossa are unusual in being glycogen-rich and it
has been suggested that they may act as a substrate for an endogenous flora of
lactobacteria (Holstein et al 1991). They also lack acid phosphatase activity and
lysozyme-like immunoreactivity, both of which are demonstrable in the epithelium
lining other parts of the distal male urethra. The epithelium lining the distal urethra in
men is thus heterogeneous, a condition which may provide a measure of defence
against invasion by pathogenic organisms.
Urethral Sphincters
Of the two urethral sphincters, the internal sphincter vesicae (p. 1842) controls the
vesical neck and the prostatic urethra above the ejaculatory ducts. It is composed of
nonstriated muscle and supplied by sympathetic and parasympathetic fibres from the
vesical plexus (see above and p. 1309). The external sphincter urethrae (p. 834)
surrounds the membranous urethra; it consists of striated muscle and is supplied by
the perineal branches of the pudendal nerve (S2, 3 and 4); it is voluntary after early
infancy.
The existence of an internal sphincter is, however, controversial. Many consider that
its nonstriated muscle fibres are multi-directional as in the vesical wall, no true
circumferential fibres being identifiable (Woodburne 1961; Angell 1969). However,
all agree that a substantial muscular aggregation, with an admixture of elastic and
collagenous fibres, exists at the vesical outlet (Vincent 1966). Its significance in
micturition is noted below.
Female Urethra
The female urethra is about 4 cm long and 6 mm in diameter. It begins at the internal
urethral orifice of the bladder, approximately opposite the middle of the symphysis
pubis, and runs anteroinferiorly behind the symphysis pubis, embedded in the anterior
wall of the vagina. It traverses the perineal membrane and ends at the external urethral
orifice, an anteroposterior slit with rather prominent margins, which is situated
directly anterior to the opening of the vagina and about 2.5 cm behind the glans
clitoridis. Except during the passage of urine the anterior and posterior walls of the
urethra are in apposition and the epithelium is thrown into longitudinal folds, one of
which, on the posterior wall of the canal, is termed the urethral crest. Many small
mucous urethral glands and minute pit-like recesses or lacunae open into the urethra.
On each side, near the lower end of the urethra, a number of these glands are grouped
together and open into a duct, named the para-urethral duct; each duct runs down in
the submucous tissue and ends in a small aperture on the lateral margin of the external
urethral orifice.
Microscopic Structure of the Female Urethra
The wall of the female urethra comprises an outer muscle coat and an inner mucous
membrane which lines the lumen and is continuous with that of the bladder. The
muscle coat consists of an outer sleeve of striated muscle (external urethral sphincter
p. 835) together with an inner coat of smooth muscle fibres. The female external
urethral sphincter is anatomically separate from the adjacent periurethral striated
muscle of the anterior pelvic floor. The constituent fibres of this sphincter are
circularly disposed and form a sleeve which is thickest in the middle one-third of the
urethra. In this region striated muscle completely surrounds the urethra although the
posterior portion lying between the urethra and vagina is relatively thin. The striated
muscle extends into the anterior wall of both the proximal and distal thirds of the
urethra but is deficient posteriorly in these regions. The myocytes forming the
external urethral sphincter are all of the slow twitch variety. As in the male, muscle
fibres of the external urethral sphincter are unusually small and have diameters of 15–
20 mm on average. Although the thickness of the external urethral sphincter in the
female is less than that of the male, its constituent fibres are able to exert tone upon
the urethral lumen over prolonged periods, especially in relation to the middle third of
its length. Periurethral striated muscle (pubococcygeus) aids urethral closure during
events which require rapid, albeit short-lived, elevation of urethral resistance. The
smooth muscle coat extends throughout the length of the urethra and consists of
slender muscle bundles, the majority of which are orientated obliquely or
longitudinally. A few circularly arranged muscle fibres occur in the outer aspect of the
non-striated muscle layer and intermingle with the skeletal muscle fibres forming the
inner part of the external urethral sphincter. Proximally the urethral smooth muscle
extends as far as the bladder neck where it is replaced by fascicles of detrusor
nonstriated muscle. This region in the female is devoid of a well-defined circular non-
striated muscle component comparable with the preprostatic sphincter of the male.
When traced distally, urethral smooth muscle bundles terminate in the subcutaneous
adipose tissue surrounding the external urethral meatus. The smooth muscle of the
female urethra is associated with relatively few noradrenergic nerves but receives an
extensive presumptive cholinergic parasympathetic nerve supply identical in
appearance to that which supplies the detrusor muscle (Ek et al 1977a, b). From a
functional viewpoint it seems unlikely that competence of the female bladder neck
and proximal urethra is solely the result of nonstriated muscle activity, in the absence
of an anatomical sphincter. The innervation and longitudinal orientation of most of
the muscle fibres suggest that urethral smooth muscle in the female is active during
micturition, serving to shorten and widen the urethral lumen. For further details, see
Gilpin and Gosling (1983). The mucous membrane lining the female urethra consists
of a stratified epithelium and a supporting layer of loose fibro-elastic connective
tissue (the lamina propria). The lamina propria contains an abundance of elastic fibres
orientated both longitudinally and circularly around the urethra. Numerous thin-
walled veins are another characteristic feature and in the past have been falsely
likened to erectile tissue. A fine plexus of acetyl cholinesterase (AChE) positive
nerves is present throughout the lamina propria and these fibres are believed to be
sensory branches of the pudendal nerves. The proximal part of the urethra is lined by
urothelium, identical in appearance to that of the bladder neck. Distally the epithelium
changes into a non-keratinizing stratified squamous type which lines the major portion
of the female urethra. This epithelium is keratinized at the external urethral meatus
and becomes continuous with the skin of the vestibule.
Functional Anatomy of the Lower Urinary Tract
The urinary bladder performs a dual function, acting at times as a reservoir for fluid
accumulating within its lumen and at others as a contractile organ actively expelling
its contents into the urethra. In the following account the tissue components and,
where appropriate, their neurological control will be considered under the headings
continence of urine and micturition.
Continence of Urine
To achieve urinary continence, the bladder acts as a passive reservoir retaining fluid
because the forces acting on the urethra produce an intra-urethral pressure greater than
bladder pressure. Several tissue components play a part in generating this urethral
resistance and make either an active or passive contribution. Since the nonstriated
muscle of the bladder is replaced in the bladder neck region by a different type of
nonstriated muscle, the detrusor muscle does not play a part in closing the proximal
urethra.
In the male, a distinct collar of circularly orientated nonstriated muscle occurs in the
bladder neck and preprostatic urethra, continuous distally with the muscular
components of the genital tract. This smooth muscle sphincter is supplied by a rich
plexus of sympathetic nerve fibres which, on stimulation, cause the sphincter to
contract, thereby preventing retrograde flow of semen into the urinary bladder at
ejaculation. During seminal emission, the sympathetic nervous system also prevents
coincidental contraction of detrusor smooth muscle. This inhibitory effect on bladder
contractility is mediated by noradrenergic nerves which synapse in the vesical plexus
upon parasympathetic motor neurons. Despite this well-defined genital role, it is not
known whether the nonstriated muscle of the bladder neck region and preprostatic
urethra plays an active part in the maintenance of continence. Intramural collagen and
elastic fibres within the wall of the bladder neck, proximal urethra and prostate
generate passive forces which help to close the urethral lumen. However,
postoperative incontinence of urine does not usually follow radical surgical excision
of the bladder neck, preprostatic urethra and prostate, suggesting that these regions
make only a minor contribution to urinary continence.
In the female, a nonstriated muscle sphincter cannot be anatomically recognized in the
wall of the bladder neck and proximal urethra. Consequently it is even less likely that
active smooth muscle contraction can be considered as an important factor in the
continence of urine. However, the bladder neck and proximal urethra possess within
their walls innumerable elastic fibres which are of particular importance in producing
passive occlusion of the urethral lumen (Lapides 1958). Indeed, it has been suggested
that the passive elastic resistance offered by the urethral wall is the most important
single factor responsible for the closure of the bladder neck and proximal urethra in
the continent woman. The anatomy and physiology of urinary continence in women
was reviewed in 1990 by DeLancey.
In both sexes the urethra contains within its walls the external urethral sphincter, the
location of which corresponds anatomically to the zone where maximal urethral
closure pressures are normally recorded. This striated muscle sphincter is
morphologically adapted to maintain tone over relatively long periods without fatigue
and plays an important active role in producing urethral occlusion at rest. It remains to
be determined, however, whether the force exerted by the sphincter is maximal at all
times between two consecutive acts of micturition or whether additional motor units
are recruited during coughing, sneezing, etc. to enhance the occlusive force on the
urethra during these events. The external sphincter is innervated by nerve fibres which
travel via several routes, not exclusively via the pudendal nerves. The clinical
relevance of this arrangement is that pudendal blockade or neurectomy performed in
order to reduce urethral resistance will not achieve the desired effect since much of
the motor innervation of the striated sphincter remains intact after these procedures.
Concerning the role of periurethral muscle in the maintenance of continence, the
medial parts of the levator ani muscles in both sexes are related to (but structurally
separate from) the urethral wall. These periurethral fibres are innervated by the
pudendal nerve and consist of an admixture of large diameter fast and slow twitch
fibres. Therefore, unlike the external sphincter, periurethral muscle possesses
morphological features which are similar to other 'typical' voluntary muscles. This
pelvic floor musculature plays an important part (especially in the female) by
providing an additional occlusive force on the urethral wall, particularly during events
which are associated with an increase in intra-abdominal pressure. In addition, the
muscles provide support for the pelvic viscera.
Micturition
To enable fluid to flow along the urethra it is necessary for the pressure in the urinary
bladder to exceed that within the urethral lumen. Under normal circumstances, in
order to initiate micturition, a fall in urethral resistance immediately precedes a rise in
pressure within the lumen of the bladder. The fall in urethral resistance may be due, at
least in part, to the action of what has been termed the 'musculus dilator urethrae'
(Dorschner et al 1994b), a ventral longitudinal muscle system located in the neck of
the bladder and in the wall of the urethra. The fasciculi of the upper part of this
muscle cross in stages the ventral circumference of the trigonal sphincter muscle,
whereas the lower part is closely related to the mucous membrane lining the urethra.
Histomorphological evidence suggests that the muscle should be able to oppose the
action of the musculus sphincter trigonalis (p. 1840) and the musculus sphincter
urethrae (Dorschner & Stolzenburg 1994), causing urethral dilatation. The pressure
rise is usually produced by active contraction of detrusor smooth muscle at the onset
of micturition. The detrusor muscle coat consists of numerous interlacing bundles
forming a complex meshwork of smooth muscle which, on contraction, reduces all
dimensions of the bladder. The muscle coat is collectively involved and it is
unnecessary to attach special significance to the precise orientation of individual
bundles within the wall of the viscus.
The preganglionic nerve supply travels in the pelvic splanchnic nerves before
synapsing on neurons located within the vesical part of the pelvic plexuses and within
the wall of the bladder. These peripheral neurons supply nerve fibres which ramify
throughout the thickness of the detrusor smooth muscle coat. The profuse distribution
of these motor nerves emphasizes the importance of the autonomic nervous system in
initiating and sustaining bladder contracting during micturition. For micturition to
occur the pressure differential between the bladder and urethra must overcome the
elastic resistance of the bladder neck. Immediately prior to the onset of micturition,
the tonus of the external sphincter is reduced by central inhibition of its motor neurons
located in the second, third and fourth sacral spinal segments. This inhibition is
mediated by descending spinal pathways originating in higher centres of the central
nervous system. Concomitantly, other descending pathways activate (either directly or
via sacral interneurons) the preganglionic parasympathetic motor outflow to the
urinary bladder. This central integration of the nervous control of the bladder and
urethra is essential for normal micturition.
Clinical Anatomy
After urethral rupture, the extravasation of urine may complicate micturition; urine
usually extends between the perineal membrane and the membranous layer of the
superficial fascia. As both of these are attached firmly to the ischiopubic rami,
extravasated fluid cannot pass posteriorly because the two layers are continuous
around the superficial transverse perineal muscles. Laterally, the spread of urine is
blocked by the pubic and ischial rami; it cannot enter the lesser pelvis through the
perineal membrane and, if this remains intact, fluid can make its way only anteriorly
into the scrotal and penile loose connective tissue and thence to the anterior
abdominal wall. When the lesser pelvis is crushed the urethra may be ruptured
between the prostatic and membranous parts; extravasation of urine then occurs into
the pelvic extraperitoneal tissue.
The lower urinary tract is subject to many congenital anomalies, some amenable to
surgical correction.

								
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