Introduction

Urination or micturition removes metabolic products and toxic wastes filtered from the kidneys and is a vital function of the human body. The micturition reflex requires a complex network of signals between the nervous system and the urinary tract system. Urine storage and emptying are highly dependent on these pathways. For these reasons, neurological function and anatomic abnormalities can involve many disease states such as incontinence, obstruction, or infections.

Cellular Level

The upper urinary-collecting system consists of the kidneys of which the renal papillae are the first gross structure of the upper urinary tract. These papillae are individually cupped by the minor calyx, which then goes to the infundibulum and combines to form the major calyces. The major calyces then come together to form the renal pelvis. Urine then flows from the renal pelvis into the bilateral ureters, which consist of fibromuscular tubes and terminate in the bladder. The ureters are narrowest at the ureteropelvic junction, ureterovesical junction, and when they cross the common iliac vessels, which will be clinically important. The lower urinary tract system consists of the bladder and urethra. The urethra connects with the bladder neck and is the final destination for urine to flow through before exiting the body.[1][2] 

The histological anatomy of the urinary tract is composed of a specialized epithelium called urothelium (transitional epithelium), which is characterized by proteins known as uroplakins. However, recent research suggests that different parts of the urinary tract have distinctive epithelium. Some areas, such as the ureters, contain fewer amounts of uroplakins and cytoplasmic fusiform vesicles than bladder urothelium. The bladder, in particular, contains a large number of ‘umbrella cells,’ which are large, multinucleated luminal cells that can form large urothelial plaques. Urothelial stem cells are believed to reside within the basal cell layers. The bladder is also mostly composed of smooth muscle and collagen.[3][4]

Development

The development of the urinary tract system starts at week four with the pronephros as a set of rudimentary and nonfunctional kidneys. These disappear and pave the way for the mesonephros, which functions during week five through week ten as the permanent kidneys are forming. The mesonephric structures make a pathway for urine production. The metanephros appears in week five, become functional at week ten, and continues to develop to become the adult kidney. The metanephroi consist of the metanephric mesenchyme (where nephrogenesis will occur) and the ureteric bud (which gives rise to the collecting duct system-the collecting tubules, major and minor calyces, renal pelvis, and ureter).[5]

Function

Urination or micturition primarily functions in the excretion of metabolic products and toxic wastes. The urinary tract also serves as a storage vessel of the waste filtered from the kidneys. Urine stored in the bladder is released from the bladder through the urethra upon a complex network of neurological function.

Mechanism

The brain, spinal cord, and peripheral ganglia all affect the micturition reflex. Afferent pathways from the bladder to the brain include the dorsal system and the spinothalamic tract. Efferent pathways from the brain to the bladder exist for micturition and storage, requiring the bladder and an outlet (bladder neck, urethra, and urethral sphincter). Sympathetic fibers originating from the T11-L2 spinal segments travel in the hypogastric nerve and link to the base of the bladder and urethra through the B3 adrenergic and a1 adrenergic receptor, respectively.

Parasympathetic preganglionic fibers from the S2-S4 spinal segments travel in the pelvic nerves and link to the bladder wall through the M3 muscarinic receptor. Somatic motor nerves from the S2-S4 motor neurons travel in the pudendal nerves and link to the striated muscles of the external urethral sphincter through the nicotinic cholinergic receptor.

When sympathetic postganglionic neurons release noradrenaline (NA), the B3 receptors are stimulated to relax the bladder smooth muscle while the A1 receptors become stimulated to contract the urethral smooth muscle to store urine. When the parasympathetic postganglionic axons release acetylcholine (ACh), the M3 receptors are stimulated to contract the bladder smooth muscle (detrusor). When the somatic axons release ACh, the nicotinic receptors become stimulated to contract the external urethral sphincter. These pathways conjunctively lead to urination.[6][7][8]

Clinical Significance

Due to the intrinsic and extrinsic nature of the urinary tract system, pathologies can range from neurological, obstruction, or infection. 

Incontinence is a common issue that can occur in the elderly, patients with diabetes, or post-menopausal females, for example. Urge incontinence characteristically presents as the sudden feeling to pass urine with the cause most likely idiopathic. Stress incontinence is voiding after an increase in intrabdominal pressure, such as from coughing or sneezing, most likely from dysfunctional urethral closure. Overflow incontinence is an increase in post-void residual urine due to incomplete emptying of the bladder. Mixed urinary incontinence is a condition combining of both stress and urge incontinence.[9]

The narrowing of the ureter at certain junctions, as stated above, may cause the formation of ureteral calculi due to stasis and, thus, further obstruction. The most common places are at the ureteropelvic junction, ureterovesical junction, and when they cross the common iliac vessels.[1] Common presenting signs and symptoms of ureterolithiasis or nephrolithiasis include intermittent colicky pain radiating to the groin and hematuria. The complex nature of embryology can lead to congenital abnormalities such as unilateral renal agenesis due to the abnormal interaction of the ureteric bud and metanephric mesenchyme. A common feature that can be present in Turner syndrome is horseshoe kidney. Horseshoe kidneys can occur when the inferior poles of both kidneys fuse most commonly during ascent as the kidneys pass through an arterial fork.[10]

Because of its physiological function, the urinary tract system is prone to urinary tract infections (UTIs). Urethritis is more common in females because of the urethra's shorter length and proximity to the anus than in males. Cystitis is inflammation of the bladder and can present with suprapubic pain and urinary urgency and frequency. Prostatitis is inflammation of the prostate. Pyelonephritis is inflammation of the renal parenchyma and pelvis. It can present with upper UTI symptoms such as fevers, flank pain, and costovertebral angle tenderness along with lower UTI symptoms such as dysuria, urinary urgency, and frequency.[11]