Basic structure, function and control of the lower urinary tract

CHAPTER 2 Basic structure, function and control of the lower urinary tract



INTRODUCTION


The urinary tract consists of two distinct and mutually dependent components:


• upper tract – comprising the kidneys and ureters

• lower tract – comprising the urinary bladder and urethra.

These provide a highly sophisticated system of conduits and a reservoir that converts the continuous involuntary production of urine by the kidneys into the intermittent, consciously controlled voiding of urine (micturition at a convenient time and place).


A thorough understanding of the structure, function and control of the lower urinary tract is vital for the accurate interpretation of urodynamic investigations.



THE KIDNEYS AND URETERS


Both kidneys continuously produce greater than 0.5 ml of urine per kg of body weight per hour (i.e. >35 ml per hour in a 70 kg man) when functioning properly and adequately hydrated. This urine empties into the kidney’s collecting systems which drain via the ureters.


The ureters function as low-pressure distensible conduits with intrinsic peristalsis, which transport urine from the kidneys to the bladder. The urine drains into the bladder at the vesico-ureteric junction (VUJ) at the termination of each ureter. Each junction if correctly functioning only allows the one way flow of urine and contains a mechanism to prevent retrograde transmission of urine back into the ureters from the bladder. This serves to protect the upper tract from the high pressures encountered within the bladder during voiding and to prevent infection entering the upper tracts.



THE URINARY BLADDER


The bladder is a hollow, muscular organ. It has two main functions:


• low pressure storage of urine

• expulsion of urine at an appropriate time and place.

Histologically the bladder is composed of three distinct layers:


1. Serosa – an outer adventitial connective tissue layer.

2. Detrusor muscle – a middle smooth muscle layer, comprising a functional syncytium of interlacing muscle bundles with fibres running in all directions.

3. Urothelium – an innermost lining composed of transitional cell epithelium providing an elastic barrier that is impervious to urine and which has a high metabolic rate and an important role in the control of bladder function (Figure 2.1).

4. Suburothelial layer – this lies immediately beneath the urothelim, is also highly active metabolically and acts in concert with the urothelium to subserve a key afferent role.

image

Figure 2.1 Structure of the bladder wall. The bladder wall is composed of four distinct layers and includes highly metabolically active urothelial and suburothelial layers.


The base of the bladder extends circumferentially from the ureteric orifices. This region contains the trigone which is a small triangular muscular area between the two ureteric orifices and the bladder neck. The trigone contains a complex plexus of nerves. Above the ureteric orifices is the main body of the bladder.



THE URETHRA AND SPHINCTERIC MECHANISMS


The urethra has two main functions:


1. To provide an effective continence mechanism, for the majority of the time (storage phase).

2. To allow adequate emptying from the bladder with the minimum of resistance during micturition (voiding phase).

A further role for the urethra which seems likely but remains hypothetical is to provide afferent feedback which may have important implications in influencing bladder function. The innermost mucosal layer in both sexes is organized in longitudinal folds and during the storage phase when the urethra is ‘closed’ this appears in a stellate configuration on cross section. Such a configuration allows significant distensibility, which is necessary during urethral ‘opening’.


The submucosal layer contains a vascular plexus which may be involved in improving the seal of a ‘closed urethra’ by transmitting the tension of the urethral muscle to the mucosal folds.


Apart from the obvious anatomical differences there are important differences in the configuration of the sphincter mechanisms between males and females (Figure 2.2).


image

Figure 2.2 Anatomy of the lower urinary tract in both sexes.


Table 2.1 contrasts the longer urethra, a prostate and two powerful sphincter mechanisms in the male compared to the single weaker intrinsic sphincter mechanism with a weaker bladder neck and also a shorter urethra in the female.


Table 2.1 Comparison of male and female sphincteric mechanisms – showing why females are much more likely to develop an incompetent urethral mechanism and be prone to urinary incontinence from intrinsic sphincter deficiency (ISD).

























Comparison of male and female sphincteric mechanisms
  Male Female
Proximal bladder neck mechanism Powerful Weak
Distal urethral mechanism/urethal sphincter mechanism (in the female) Powerful Prone to the effect of exogenous influences such as pelvic floor weakness and damage or denervation consequent upon childbirth
Prostate Further increases bladder outlet resistance Not present
Urethra Long Short (∼3.5 cm)


Male sphincteric mechanisms


In the male there are two important sphincteric mechanisms:


1. A proximal ‘bladder neck mechanism’.

2. A distal urethral mechanism at the apex of the prostate.

The proximal sphincter in the male bladder neck provides a powerful mechanism in both maintaining urinary continence and also preventing retrograde ejaculation of semen during sexual activity. In patients with a damaged distal urethral sphincter (e.g. a pelvic fracture-associated urethral disruption) continence can be maintained solely by the proximal bladder neck mechanism. Ultrastructurally it consists of a powerful inner layer of muscle bundles arranged in a circular orientation.


The distal sphincteric mechanism is also extremely important, as evidenced by its ability to maintain continence even when the proximal bladder neck mechanism has been rendered totally incompetent by surgical bladder neck incision or a prostatectomy. It is confined to the 3–5 mm thickness of the wall of the membranous urethra from the level of the verumontanum down to the distal aspect of the membranous urethra. It is composed mainly of extrinsic striated muscle which is capable of the sustained contraction necessary for continence and to a lesser degree by intrinsic smooth muscle.



Prostate gland


The prostate is made up of smooth muscle and glandular tissue, with the proportion of smooth muscle being increased in benign prostatic hyperplasia (BPH). The prostatic smooth muscle is controlled by the sympathetic nervous system, which acts by releasing noradrenaline onto α1a-adrenoceptors located on the smooth muscle cells; the resulting contraction increases the bladder outlet resistance and further aids continence in the male.

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Related posts:

Paediatric urodynamics Clinical evaluation of the lower urinary tract Urodynamic procedures Pressure/flow cystometry Voiding disorders and bladder outlet obstruction Storage disorders and incontinence

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Jul 20, 2016 | Posted by in UROLOGY | Comments Off on Basic structure, function and control of the lower urinary tract

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