Anatomy of the lower urinary tract

To be able to understand continence fully it is important to know about the organs involved in storage and evacuation – the bladder and urethra, and how they relate to the lower bowel and the male and female reproductive organs.

The diagram below shows the position of the bladder in the abdominal cavity in a male, as the prostate gland can be seen below the bladder.

^{Figure 2.1: Anatomy of the lower urinary tract.}

Urine is produced by the kidneys and is propelled downwards to the bladder in the ureters.

The bladder

The bladder is situated just behind the pubic bone. As it fills the bladder rises up into the abdomen and becomes ovoid.

Bladder has two functions:

• To store urine
• To expel urine

The muscle in the bladder wall is called the detrusor muscle (from the Latin word ‘detrudo‘ to thrust or push out). It is a smooth muscle under involuntary control and it plays a vital role in pushing urine out of the bladder.

As the bladder fills the detrusor muscle stretches with no significant rise in bladder pressure – this is known as bladder compliance.

This is important and we will come back to it again when looking at causes of urinary incontinence.

The bladder wall is mainly made up of a special type of smooth muscle called the detrusor which is elastic and allows the bladder to expand and contract. At the bottom of the bladder the detrusor muscle forms the internal urethral sphincter.

The lining of the bladder is called the urothelium, a layer of specialised cells and other proteins that are very elastic, yet resistant to the chemical properties of urine and forms the first line of defence against infection.

The trigone is a triangular area of the bladder between the ureters and urethra. The trigone is a very sensitive area of the bladder: it is sensitive to pressure, changes in concentration and composition of urine and oestrogen levels. When stretched as the bladder fills it signals the spine and brain that the bladder needs to be emptied.

Key bladder facts

Facts you need to know and understand about the bladder to be able to interpret a bladder diary/frequency volume chart:

• Urine is produced by the kidneys at a rate of 1-2 millilitres (ml) per minute. Therefore in 1 hour 60-120ml is produced
• An adult bladder holds around 500ml (1 pint)
• Usually the bladder is emptied when it is about 2/3 full – voiding 300-400ml
• Normal voiding frequency is 4-7 times daily
• When passing urine, most people will empty their bladder completely if they use the toilet correctly (sitting down for women) relax and take time.
• For adults, a post void residual urine volume of less than 100ml is considered normal
• Nocturia – waking from sleep by the need to pass urine at night – once nightly, if at all, is normal
• Arginine vasopressin (AVP), previously known as antidiuretic hormone (ADH), reduces urine production and the level of this hormone will normally rise at night to decrease overnight urine volume. This allows us to sleep at night and is the reason why urine is more concentrated in the morning

Clinical practice points.

What could irritate the bladder? Concentrated urine, low oestrogen levels, ‘foreign bodies’ such as a catheter balloon, a bladder stone or bacterial infection affecting the urothelium (bladder lining).

What symptoms might occur? Frequency, urgency, pain (dysuria) and spasm.