Basic clinical background to the lower urinary tract

See also Urodynamic theory of the lower urinary tract.

The Bladder and Urethra

The three main conduits for urine once it leaves the kidneys are the ureters which lead from kidney to bladder, the bladder itself and the urethra which leads from bladder to the outside world.

The bladder can be thought of as a muscular sphere. It would not be appropriate to give a capacity as this derives more from the sensitivity of the individual than morphological constraints. The wall of the bladder consists mainly of layers of smooth muscle called the detrusor. In urodynamics we frequently talk of detrusor pressure which is the contribution to intravesical pressure (pressure inside the bladder) exerted by this muscle. A triangular region between the 2 ureteric orifices and the bladder neck is called the trigone, a different type of muscle but the detrusor is usually assumed to determine urodynamic characteristics.

Although the terminology of urodynamics has been well developed by the International continence Society, there are still variations in anatomical terminology. For example the main urethral sphincter may be called the external sphincter or the distal sphincter. I shall use external sphincter. The term internal sphincter describes the bladder neck.

The male and female urethras are significantly different. The main differences being that in the female the urethra is shorter and has a low external sphincter pressure. In males the bladder neck has a greater sphincteric function, the prostate gland also has some contribution and there is a higher external sphincter pressure. In the male there is a longer urethra.

fig 1. Anatomy of the female bladder & urethra

fig 2 Anatomy of the male bladder & urethra

In both sexes there are the striated muscles of the levator ani or pelvic floor. These are under voluntary control and can be used to aid continence or interrupt urinary flow.

The urethra is a highly distensible muscular tube. During bladder filling, it is closed and sealed so that it is watertight. The musculature of the urethra is complex - being a combination of both smooth and striated muscle. There are longitudinally oriented fibres and circularly oriented. Clearly, for a sphincteric action circularly oriented fibres are required. Although striated muscle typically fatigues easily, it seems that in the urethra there is circularly oriented striated muscle of a type which fatigues less and maintains urinary continence by means of sustained contraction. This is the external urethral sphincter. When filling these muscles are under a constant stimulation to contract. This can be proven by performing electromyelogram (emg) or by inference from the development of urethral pressure in spinal injury.

The urethra is all that stands between the bladder and outside world and therefore continence depends on several active and passive properties of detrusor and urethra. The detrusor is passive when it is filling and active when it is emptying or undergoing unstable contraction. The urethra is active when it is filling - under constant innervation from and passive when it is emptying.

In the male the bladder neck is much more strongly developed with circularly oriented smooth muscle. Furthermore, the urethra passes through the prostate gland which from the age of 40 onwards may begin to swell and can ultimately completely obstruct flow. Thirdly the closure pressure exerted by the male external sphincter is considerably in excess of that of the female. Thus the male is relatively unlikely to suffer from stress incontinence.

Differences between male and
female urethra under filling

Urethra under stress