Last's Anatomy: Regional and Applied

Part fourteen. Urinary bladder and ureters in the pelvis

Urinary bladder

The empty bladder is situated entirely within the pelvic cavity. As the bladder distends it domes up into the abdominal cavity. The empty bladder is a flattened three-sided pyramid, with the sharp apex pointing forwards to the top of the pubic symphysis and a triangular base facing backwards in front of the rectum or vagina. There are two inferolateral surfaces cradled by the anterior parts of levator ani, a neck where the urethra opens, and a superior surface on which the small intestine and sigmoid colon or uterus lie.

The apex has the remains of the urachus attached to it, the latter forming the median umbilical ligament which runs up the midline of the anterior abdominal wall in the median umbilical fold of peritoneum (see p. 234).

Most of the base, or posterior surface, lies below the level of the rectovesical pouch and only the uppermost portion is covered by peritoneum between the vas deferens on each side (Fig. 5.62). In addition to the latter the seminal vesicles are applied to this surface, and the ureters enter at the upper outer corner. In the female the base has a firm connective tissue union with the anterior vaginal wall and upper part of the uterine cervix with no peritoneum intervening (Fig. 5.64).

Each inferolateral surface slopes downwards and medially to meet its fellow, lying against the front part of the pelvic diaphragm and obturator internus. Where the surfaces meet below the apex there is a (retroperitoneal) space behind the pubic bones and symphysis, the retropubic space (of Retzius), containing loose fatty tissue and the fibromuscular pubovesical ligaments that extend from the bladder neck to the inferior aspect of the pubic bones.

The lowest part of the bladder is its neck, where the base and inferolateral surfaces meet and which is pierced by the urethra at the internal urethral orifice. In the male it lies against the upper surface or base of the prostate. In the female the neck is above the urethra in the connective tissue of the anterior vaginal wall.

The superior surface is covered by peritoneum which sweeps upwards on to the anterior abdominal wall. The distending bladder strips peritoneum from behind rectus abdominis, leaving the transversalis fascia on the back of the muscle; the distended bladder may thus be approached by cannula or scalpel in the midline above the pubic symphysis without entering the peritoneal cavity. At the posterior margin of this surface in the male the peritoneum continues on to the uppermost part of the base and is then continued backwards as the floor of the the rectovesical pouch, but in the female it is reflected from a little in front of the posterior margin of this surface on to the undersurface of the uterus.

The appearance of the interior of the bladder depends upon the state of distension of the organ. When collapsed the mucous membrane is thick and thrown into folds, when distended it is thin and smooth. The trabeculae of the muscle fibres can be seen through the mucous membrane. These remarks do not apply to the trigone, which varies but little with the state of distension of the organ.

The trigone is a triangular area at the base of the bladder lying between the two ureteral orifices (above and laterally) and the internal uretheral orifice (centrally and below) (Fig. 5.60). In the empty bladder these three openings are 2.5cm apart from each other but when distended (as during cystoscopy) the ureteral orifices may be 5cm apart. Being fixed on top of the prostate by the urethra, the trigone is the least mobile part of the bladder. In the female it is stabilized by the connective tissue surrounding the upper urethra at the front of the vagina. The trigone is smooth-walled and the mucous membrane is rather firmly adherent to the underlying muscle. The ureteric orifices are connected by a transverse ridge, the interureteric bar, prominent when viewed through the cystoscope; this is produced by continuity of the longitudinal muscle of the two ureters across the bladder wall. The orifices of the ureters lie at the ends of the bar; they are usually in the shape of an oblique slit, but considerable variations exist. The ureters pierce the muscle and mucosal walls very obliquely, an important factor in preventing reflux of urine when intravesical pressure rises. The ureteric orifices are closed by this pressure, except to open rhythmically in response to ureteric peristalsis each time a jet of urine is injected into the bladder (four or five times a minute normally).

In the male the trigone overlies the median part of the central zone of the prostate which, after middle age, may project above the internal urethral orifice as a rounded elevation, the uvula of the bladder.

Blood supply

The superior and inferior vesical arteries provide most of the arterial blood but there are small contributions to the lower part of the bladder from the obturator, inferior gluteal, uterine and vaginal arteries.

The veins of the bladder do not follow the arteries. They form a plexus that converges on the vesicoprostatic plexus in the groove between bladder and prostate and which drains backwards across the pelvic floor to the internal iliac veins. There is a similar plexus in the female, communicating with veins in the base of the broad ligament.

Lymph drainage

The lymphatics of the bladder drain mainly to external iliac nodes. Some lymph drains to internal iliac nodes including nodes in the obturator fossa.

Nerve supply

Parasympathetic fibres which provide the main motor innervation of the bladder reach it via the pelvic splanchnic nerves (see pp. 20 and 311). Sympathetic fibres come from L1 and 2 segments of the cord via the superior and inferior hypogastric plexuses. For most of the bladder the sympathetic fibres are vasomotor and probably inhibitory to the detrusor muscle, but they are motor to the superficial trigonal muscle and (in the male) the muscle of the bladder neck (see below). The sensation of normal bladder distension travels with parasympathetic fibres and in the spinal cord is conveyed in the gracile tract, but it appears that bladder pain (e.g. from a stone) reaches the spinal cord (lateral spinothalamic tract) by both parasympathetic and sympathetic pathways.

Control of micturition

Normal emptying of the bladder occurs by contraction of the detrusor muscle and reciprocal relaxation of the external sphincter and pelvic floor (levator ani). The accumulation of urine distends the bladder wall with adjustment of tone (accommodation) so that tension does not at first increase. Later increased tension stimulates stretch receptors from which afferent impulses pass along the pelvic splanchnic nerves to sacral segments of the cord. Here the parasympathetic cell bodies are in turn stimulated and efferent impulses travel down the pelvic splanchnic nerves to synapse with the postganglionic cells within the bladder wall and so cause contraction. This autonomic stretch reflex giving bladder control at the spinal level is typical of the infant; with training, control by higher centres becomes superimposed on the spinal activity, and bladder evacuation is assisted by voluntary contraction of abdominal muscles. There is a cortical inhibitory centre in the inferior frontal gyrus (on the medial surface of the cerebral hemisphere, some distance in front of the motor ‘perineal’ area) with fibres passing to a detrusor motor centre in the medial part of the pontine reticular formation. From there reticulospinal fibres run down the cord mixed with those of the lateral corticospinal tract to the sacral segments.

The skeletal muscle of the sphincter urethrae (external urethral sphincter; p. 307 female, p. 317 male) is controlled by the perineal branch of the pudendal nerve (see p. 321), carrying fibres predominantly from anterior horn cells of S2 segment (Onuf's nucleus). A storage centre in the lateral part of the pontine reticular formation exerts central control on this nucleus. During micturition the sphincter relaxes as the detrusor contracts. In the female the pubovaginalis part of levator ani (see p. 291) assists the external sphincter at the end of micturition.

In spinal cord transection above the level of S2 segment, afferent impulses indicating distension cannot reach consciousness, cortical control of the sacral reflex is lost, and relaxation of the sphincter urethrae cannot be prevented. Because the ‘sacral centre’ itself is intact, the bladder automatically empties when distended, as in the infant (or as in the senile where cortical control has been lost through cerebral vascular disease). If the sacral segments themselves are destroyed, the detrusor muscle is paralysed and the bladder becomes abnormally distended until overflow incontinence occurs.

Structure

The smooth muscle of the bladder wall (detrusor muscle) is composed of an interlacing network of fibres running in various directions. Both externally and internally (beneath the mucous membrane) they produce a trabeculated appearance, which is exaggerated when muscular hypertrophy occurs as a result of progressive chronic obstruction to micturition, for example by prostatic enlargement or urethral stricture. They are well supplied by parasympathetic (cholinergic) nerve fibres. However, the trigone possesses a superficial triangular layer of muscle (superficial trigonal muscle) that is histologically and histochemically different from the rest of the bladder musculature (including the deep part of the trigone) and extends into the proximal urethra in both sexes. In further contrast to the detrusor muscle, the superficial trigonal muscle receives predominantly sympathetic (adrenergic) fibres. Contraction of this muscle may help to close the ureteral orifices.

At the bladder neck in the male, circular smooth muscle fibres form a collar around the internal urethral orifice, and extend distally to surround the proximal part of the prostatic urethra (the preprostatic part). This muscle, the preprostatic sphincter, too is profusely supplied with sympathetic (adrenergic) fibres. In the female the muscle in this region is arranged longitudinally and extends into the urethral wall. The muscle of the bladder neck has nothing to do with urinary continence; in the male it acts to prevent seminal regurgitation into the bladder during ejaculation.

The mucous membrane is thick and lax and lined by transitional epithelium. Glands are usually absent; mucus in shed urine has come from urethral glands. There is no muscularis mucosae.

Development

The endoderm of the vesicourethral part of the urogenital sinus (see p. 29) becomes the bladder epithelium, and the surrounding mesenchyme forms the muscle and connective tissue. Continued growth leads to the incorporation of the lower ends of the mesodermal mesonephric ducts into the posterior part of the bladder, so forming the trigone. These developmental differences may account for the structural differences in this region. The mesonephric ducts in the male end up at a lower level as the ejaculatory ducts entering the urethra (see p. 300). The allantois regresses to form a fibrous cord, the urachus (median umbilical ligament).

Ureters in the pelvis

The pelvic part of the ureter forms about half of its 25cm length (see p. 286). It crosses the pelvic brim in the region of the bifurcation of the common iliac artery. On the left it underlies the apex of the sigmoid mesocolon (Figs 5.56 and 5.64). It usually runs over the external iliac artery and vein and then down the side wall of the pelvis in front of the internal iliac artery (and behind the ovary). In order from above downwards, it crosses the obturator nerve, obliterated umbilical (superior vesical) artery, obturator artery and obturator vein (Fig. 5.65). On the right the appendix, if in a pelvic position, may lie adjacent. Reaching the level of the ischial spine, it turns forwards and medially above the pelvic floor to enter the base of the bladder at its upper lateral angle. Here in the male the vas deferens crosses above the ureter and then runs down medial to the ureter. The upper end of the seminal vesicle usually lies just below the point where the ureter enters the bladder wall.

On the pelvic floor in the female, the ureter lies in the base of the broad ligament (see p. 304), where it is crossed above by the uterine artery (Fig. 5.63). Under the broad ligament the ureter penetrates the condensed tissue that forms the lateral cervical ligament (see p. 304), crossing the lateral vaginal fornix 1–2cm from the cervix before entering the bladder in front of the fornix. The ureters are major hazards during hysterectomy, when ligating vessels and transecting ligaments.

In both sexes the ureters run obliquely through the bladder wall for 1–2cm before reaching their orifices at the upper lateral angles of the trigone.