10.5 — The Bladder and Urination

The bladder does something that sounds simple and is not: it fills at a steady rate for hours while keeping its internal pressure almost unchanged, then empties completely on command in about 20 seconds, and does both under conscious control that has to be learned in childhood and can be lost.

Bladder problems are among the most common and least discussed conditions in medicine. Around a third of women and a substantial proportion of older men have significant urinary symptoms, and most never mention them.

The bladder

A hollow muscular organ in the pelvis, behind the pubic bone.

Empty, it is entirely within the pelvis and cannot be felt. As it fills it rises into the abdomen, and a full bladder is palpable and dull to percussion above the pubic bone — which is how urinary retention is diagnosed at the bedside.

Capacity: first sensation of filling at 100 to 200 ml, a definite desire to void at 300 to 400 ml, and comfortable capacity around 400 to 600 ml. It can hold considerably more under pressure, and in acute retention volumes over a litre are common.

The wall is detrusor muscle — smooth muscle arranged in interlacing bundles running in all directions rather than in neat layers.

That arrangement is functional. Muscle fibres running every way means that when they all contract, the bladder shrinks in every dimension at once and empties completely rather than squeezing in one direction and leaving pockets.

The lining is transitional epithelium (Chapter 4.2), which changes shape as the bladder stretches, and it sits on a layer that lets the wall fold and unfold.

The trigone is a smooth triangular area at the base, between the two ureteric openings and the urethral opening. It does not stretch or fold, and it is the most sensitive part — which is why bladder infection and stones cause pain referred to the tip of the urethra.

Compliance — the ability to fill without pressure rising — is the crucial property. A normal bladder fills from empty to 400 ml with a pressure rise of only a few centimetres of water.

Why that matters: the ureters have to drain into it against that pressure. If bladder pressure rises during filling, urine cannot drain from the kidneys, and back-pressure damages them. A poorly compliant bladder is a kidney problem, and this is exactly what happens in some neurological bladders and in long-standing obstruction — the kidney damage is caused by the bladder.

The two sphincters

Internal urethral sphincter — smooth muscle at the bladder neck, involuntary, under sympathetic control.

In men it is well developed and has a second function: it closes during ejaculation to prevent semen entering the bladder. Damage to it — most often after prostate surgery — causes retrograde ejaculation, where semen passes backward into the bladder. It is harmless but causes infertility and is a recognised consequence people should be warned about.

External urethral sphincter — skeletal muscle surrounding the urethra as it passes through the pelvic floor. Voluntary, and this is the one you consciously hold shut.

And there is a difference in urethral length that matters (Chapter 10.1). The female urethra is about 4 cm; the male about 20 cm, passing through the prostate, the pelvic floor and the penis. Short urethra means easier infection; long urethra means more places to obstruct.

The filling and emptying reflex

This is one of the more sophisticated reflexes in the body, because it must be both automatic and overrideable.

During filling:

Stretch receptors in the wall send signals up the spinal cord. At low volumes, the response is to inhibit the detrusor and keep the sphincters closed — sympathetic activity relaxes the bladder body and contracts the bladder neck. The bladder actively accommodates rather than passively stretching.

As volume increases, the signals become more frequent, and a centre in the pons is informed. This is where the decision is made, and it is why the pontine micturition centre is sometimes called the "storage and voiding switch".

Above about 300 to 400 ml, the sensation reaches consciousness as a desire to void.

During voiding:

Voluntary relaxation of the external sphincter and the pelvic floor comes first. The pons then switches: parasympathetic activity contracts the detrusor and relaxes the internal sphincter, and the two are coordinated so the sphincter opens before the bladder squeezes.

That coordination is the whole point. Contracting the bladder against a closed sphincter would generate high pressure and damage the kidneys, and this is exactly what happens in the condition called detrusor–sphincter dyssynergia after spinal cord injury.

Emptying should be complete. Post-void residual volume should be under about 50 ml, and it is measured by ultrasound.

Why urination is hard to start when someone is watching is a genuine physiological question with a genuine answer: voiding requires relaxation of a voluntary muscle, and voluntary relaxation is disrupted by sympathetic arousal, which also contracts the bladder neck. It is not psychological in the dismissive sense — the anxiety produces a real muscular effect.

And this is why continence has to be learned. The reflex in an infant is purely spinal — the bladder fills, the reflex fires, the bladder empties. Toilet training is the development of cortical inhibition over that spinal reflex, and it takes 2 to 4 years.

Which is why any condition that removes cortical control returns the bladder to the infant reflex. Dementia, stroke and spinal cord injury above the sacral level all do this, producing a small, overactive bladder that empties without warning.

The prostate

A walnut-sized gland in men, sitting below the bladder and surrounding the urethra, about 20 grams in a young adult.

Its position is the entire clinical story. The urethra passes through it. Anything that enlarges it compresses the urethra.

Benign prostatic enlargement affects around 50 percent of men by 60 and 80 percent by 80. It is not cancer and it is not a risk factor for cancer.

Symptoms divide into two groups, and the distinction guides treatment.

Obstructive (voiding) symptoms — weak stream, hesitancy, straining, intermittent flow, dribbling at the end, incomplete emptying.

Irritative (storage) symptoms — frequency, urgency, getting up at night, and urge incontinence. These come from the bladder muscle thickening and becoming overactive in response to working against the obstruction.

Treatment maps onto the two mechanisms.

Alpha-blockers (tamsulosin) relax the smooth muscle of the bladder neck and prostate. They work within days. Their main side effects are dizziness on standing, from blocking the same receptors in blood vessels, and retrograde ejaculation.

5-alpha-reductase inhibitors (finasteride) block the conversion of testosterone to its more potent form in the prostate, shrinking the gland over 6 to 12 months. Slow, and they genuinely reduce the need for surgery.

Surgery — most commonly removing the obstructing tissue through the urethra.

Acute urinary retention is a common emergency: sudden complete inability to pass urine, with a painful distended bladder. It is relieved by catheterisation, and the relief is immediate and dramatic.

Common triggers are worth knowing because several are avoidable: anticholinergic drugs, decongestants containing pseudoephedrine, constipation, alcohol, and immobility after surgery.

Chronic retention is more dangerous precisely because it is painless. The bladder distends slowly over months, and the person may present with overflow incontinence, or with kidney failure from back-pressure. A palpable painless bladder with a rising creatinine is a picture worth recognising.

Incontinence

Extremely common, substantially under-reported, and highly treatable — which is the reason to cover it plainly.

Around a third of women over 40 have some degree of urinary incontinence. Most do not raise it, and most are not asked.

Stress incontinence — leaking on coughing, sneezing, laughing or exercise. The pelvic floor cannot resist the sudden rise in abdominal pressure (Chapter 6.4). Commonest in women after childbirth and after menopause.

Treatment: supervised pelvic floor muscle training is first-line and it works. Cure or substantial improvement in the majority, with no side effects. Duloxetine and surgery are options when it does not.

The word "supervised" is doing work there. Studies consistently show that unsupervised exercises, described in a leaflet, are far less effective — largely because a substantial proportion of people contract the wrong muscles when told to.

Urge incontinence (overactive bladder) — a sudden compelling urge followed by leakage, with frequency and getting up at night. The detrusor contracts when it should not.

Treatment: bladder training — deliberately extending the interval between voids to retrain the reflex — plus reducing caffeine and alcohol, both of which are bladder irritants and diuretics. Then antimuscarinic drugs or a beta-3 agonist, and botulinum toxin injected into the bladder wall for resistant cases (Chapter 6.2).

Mixed incontinence — both, and it is common.

Overflow incontinence — a chronically full bladder dribbling. The treatment is to relieve the obstruction, and giving drugs for urgency makes it worse.

Functional incontinence — the bladder works but the person cannot reach the toilet in time. Mobility, dexterity with clothing, and access. This is frequently the actual problem in older people and is frequently treated as if it were a bladder problem, with drugs that cause confusion and falls. A commode by the bed and a walking aid outperform any medication.

And a practical point that matters more than it should have to. Incontinence is a leading reason people are admitted to residential care, and a leading cause of social isolation. It is treatable in most cases. The main barrier is that it is not mentioned, and asking about it directly is one of the higher-value questions a clinician can ask.

Urinary tract infection

Very common, and the anatomy explains the epidemiology entirely (Chapter 10.1).

E. coli causes 70 to 90 percent, arriving from the bowel.

Lower urinary tract infection (cystitis) — burning on passing urine, frequency, urgency, lower abdominal discomfort, cloudy or strong-smelling urine. No fever.

Upper urinary tract infection (pyelonephritis) — fever, rigors, flank pain and vomiting, with or without the lower symptoms. This is a different illness in severity and treatment.

In older people, a urinary infection frequently presents as confusion or a fall rather than urinary symptoms.

And that observation has been substantially overapplied, which is worth stating. Asymptomatic bacteriuria — bacteria in the urine with no symptoms — is very common in older people, particularly in care homes, and should not be treated. Treating it does not improve outcomes, causes side effects and drives resistance. A confused older person with bacteria in their urine may have a urinary infection or may have something else entirely with incidental bacteriuria, and reaching for the urine dipstick first is a well-documented cause of missed diagnoses.

Prevention that has evidence: adequate fluid intake — one good trial showed that increasing water intake by 1.5 litres a day halved recurrent infections in women; not delaying urination; and for recurrent infections after intercourse, voiding afterwards and considering a single post-coital antibiotic dose.

Cranberry products have weak and inconsistent evidence. The mechanism is plausible — compounds that reduce bacterial adhesion — and the trials are unimpressive. It is not harmful and should not replace treatment.

Antibiotic treatment is usually 3 days for uncomplicated cystitis in women, and longer for men, pregnancy, or upper tract infection.

Two situations where a urinary infection is never trivial: in pregnancy, where even asymptomatic bacteriuria is treated because it causes preterm labour and pyelonephritis; and in men, where it usually indicates an underlying abnormality.

Catheters

A urinary catheter is one of the most commonly used devices in hospital and one of the commonest causes of hospital-acquired infection.

The risk of bacteriuria rises by about 3 to 8 percent per day of catheterisation, so within a month essentially everyone is colonised.

Two principles follow, and they are the whole of catheter care. Do not insert one without a clear indication, and remove it as soon as the indication ends. Most preventive measures are far less effective than simply not having one.

Legitimate indications: acute retention, accurate output monitoring in critical illness, some surgery, and comfort in end-of-life care. Incontinence alone is not an indication — it is convenience, and it trades a manageable problem for infections and immobility.

What the next page fixes

The kidney can fail, and when it does everything in this Part fails with it. Chapter 10.6 covers kidney failure — acute and chronic — what dialysis actually replaces and what it does not, and why a transplanted kidney is one of the most successful operations in medicine.