Elimination of Urine (Micturition)

Comprehensive anatomical and physiological overview of the micturition process, neural control mechanisms, and clinical pathophysiology of the lower urinary tract.

1. Definition

Elimination of urine, also called micturition or voiding, is the process by which urine is expelled from the urinary bladder through the urethra to the exterior of the body. It is the final step in urine handling, following filtration by the kidneys and transport through the ureters.

2. Anatomy of the Lower Urinary Tract

The Urinary Bladder

The bladder is a hollow, muscular, distensible organ located in the anterior pelvis. In adults, it typically holds 400–600 mL of urine. The physiological desire to void typically begins when the volume reaches 200–300 mL.

Figure: Detailed anatomy of the bladder showing the detrusor muscle, trigone, and rugae

Layers of the bladder wall (inner to outer):

Mucosa: Transitional epithelium (urothelium) that stretches without damage; forms folds called rugae when the bladder is empty.
Submucosa: Connective tissue layer containing blood vessels and nerves.
Muscularis (Detrusor Muscle): Smooth muscle arranged in three interlacing layers; its contraction is responsible for expelling urine.
Adventitia/Serosa: The outermost protective covering.

Key Anatomical Regions:

Apex: The anterior tip, connected to the umbilicus by the median umbilical ligament.
Body: The main central portion of the bladder.
Fundus (Base): The posterior surface containing the trigone—a smooth, triangular area bounded by the two ureteric orifices and the internal urethral orifice.
Neck: The inferior narrowing where the bladder joins the urethra.

Key Concept

The trigone is highly sensitive to stretch. Because it is embryologically distinct from the rest of the bladder (mesodermal origin), it lacks rugae and remains smooth regardless of the bladder's distension state.

Figure 1: Anatomy of the Urinary Bladder and Urethra (Sagittal Section — Female) showing trigone, sphincters, and urethral orifice

The Urethra

Female urethra: Approximately 3–4 cm long. Its short length increases susceptibility to urinary tract infections (UTIs) as bacteria can ascend more easily.
Male urethra: Approximately 18–20 cm long, divided into three main portions: prostatic, membranous, and spongy (penile).

The Urethral Sphincters

Sphincter	Location	Muscle Type	Control	Function
Internal urethral sphincter	Bladder neck	Smooth muscle	Involuntary (autonomic)	Closes during filling; relaxes during voiding.
External urethral sphincter	Deep perineal pouch / pelvic floor	Skeletal muscle	Voluntary (somatic)	Consciously prevents or permits urination.

3. Physiology of Micturition

Micturition involves two alternating phases controlled by a combination of reflex (involuntary) and voluntary mechanisms.

Figure 3: The Two Phases of Micturition — Side-by-side comparison of the Storage Phase (relaxed detrusor, closed sphincters) vs. the Voiding Phase (contracted detrusor, open sphincters)

Phase 1: Storage (Filling)

Urine flows continuously from the kidneys via the ureters into the bladder.
The detrusor muscle remains relaxed to allow for high compliance.
Both sphincters remain contracted to maintain continence.
Sympathetic (Hypogastric) and somatic (Pudendal) nerves are active.
Parasympathetic activity is inhibited.
Stretch receptors in the bladder wall send signals to the spinal cord. At low volumes, these are suppressed by higher brain centres.

Phase 2: Voiding (Emptying)

Triggered consciously when the setting is appropriate through the following sequence:

Voluntary relaxation of the external urethral sphincter (cortical inhibition of the pudendal nerve).
Sympathetic inhibition → internal sphincter relaxes.
Parasympathetic activation (pelvic splanchnic nerves, S2–S4) → detrusor muscle contracts strongly.
Intravesical pressure rises; the bladder neck opens.
Urine flows out through the urethra.
Sensory feedback from the urethra reinforces detrusor contraction until the bladder is completely empty.

4. Neural Control

Afferent (Sensory) Pathways

Stretch receptors in the bladder wall and trigone detect distension. Signals travel via the pelvic splanchnic nerves to the sacral spinal cord (S2–S4), then ascend to the pontine micturition centre (PMC) and the cerebral cortex.

Figure 2: Neural Control of Micturition — Schematic illustrating afferent stretch signals to the sacral cord and PMC, and efferent motor responses

Efferent (Motor) Pathways

Pathway	Nerve	Spinal Origin	Neurotransmitter	Effect
Parasympathetic	Pelvic splanchnic nerves	S2–S4	Acetylcholine (M3 receptors)	Detrusor contraction; internal sphincter relaxation.
Sympathetic	Hypogastric nerve	T11–L2	Noradrenaline (α1 and β3 receptors)	Detrusor relaxation (β3); internal sphincter contraction (α1).
Somatic	Pudendal nerve	S2–S4	Acetylcholine (nicotinic)	External sphincter contraction (voluntary control).

Higher Control Centres

Sacral Micturition Centre (S2–S4): Contains the basic reflex arc. In infants, this produces involuntary voiding.
Pontine Micturition Centre (PMC): Coordinates the switch between storage and voiding; ensures detrusor contraction and sphincter relaxation occur together.
Cerebral Cortex: Provides conscious, voluntary control. Allows for the delay of voiding or initiation at will.

5. Pathophysiology

Urinary Retention

Inability to empty the bladder. Causes include:

Obstructive: Benign prostatic hyperplasia (BPH), urethral stricture, bladder stones.
Neurogenic: Spinal cord injury, cauda equina syndrome, diabetic neuropathy.
Pharmacological: Anticholinergics, opioids, tricyclic antidepressants.
Postoperative: Occurs frequently after pelvic or spinal surgery.

Urinary Incontinence

The involuntary leakage of urine, classified by mechanism:

Type	Mechanism	Key Features
Stress	Weak pelvic floor / sphincter	Leakage with cough, sneeze, or exertion.
Urge	Detrusor overactivity	Sudden urgency, large-volume loss.
Overflow	Chronic retention with passive overflow	Dribbling, palpable bladder, weak stream.
Functional	Physical/cognitive barriers	Inability to reach the toilet in time despite normal tract function.

Special Pathological Conditions

Neurogenic Bladder (Spastic): Lesion above sacral cord → detrusor overactivity, small capacity, and reflux risk.
Neurogenic Bladder (Flaccid): Lesion at/below sacral cord → detrusor areflexia, large capacity, and high residual urine.
Vesicoureteral Reflux: Backflow of urine to ureters due to failure of the ureterovesical valvular mechanism. Predisposes to pyelonephritis.

6. Clinical Relevance

Common Local Scenarios

Acute retention in elderly males: BPH is the leading cause; requires urgent catheterisation.
UTI: Extremely common, especially in females; diagnosed by urinalysis and culture.
Obstetric fistula: Resulting from prolonged obstructed labour; causes continuous incontinence and social stigmatization.
Spinal cord injury: Requires intermittent catheterisation to prevent secondary renal damage.

Patient Assessment

History: Voiding pattern, hematuria, and neurological symptoms.
Examination: Abdominal palpation for bladder distension, digital rectal examination (males).
Investigations: Urine culture, bladder scan, renal ultrasound, and urodynamics.

7. Summary: Storage Phase vs. Voiding Phase

Feature	Storage Phase	Voiding Phase
Detrusor muscle	Relaxed	Contracted
Internal sphincter	Closed	Open
External sphincter	Closed (voluntary)	Open (voluntary relaxation)
Parasympathetic	Inhibited	Active
Sympathetic	Active	Inhibited
Intravesical pressure	Low and stable	Elevated

9. Key Points

Micturition is a reflex act modulated by voluntary cortical control.
Parasympathetic nerves (S2–S4, pelvic nerve) promote voiding.
Sympathetic nerves (T11–L2, hypogastric nerve) promote storage.
The pontine micturition centre coordinates the synergy between detrusor contraction and sphincter relaxation.
In Uganda, BPH, UTIs, and Obstetric fistula are primary drivers of lower urinary tract clinical visits.

Doctors Revision