Human Anatomy and Physiology - I. V. Gayvoronsky 2011

Anatomy and Physiology of the Excretory System
Urinary Tract

Structure. The Ureter is a paired organ in the form of a muscular tube with a variable lumen, measuring 30–35 cm in length. Its primary function is to continuously transport urine from the renal pelvis to the Urinary Bladder. The ureter emerges from the renal hilum, descends inferiorly, and enters the Base of the urinary bladder. It is worth noting that the organ typically follows a slightly tortuous path featuring several gentle, arch-like curves. Anatomically, the ureter is divided into abdominal, pelvic, and intramural parts. The first two parts are approximately equal in length (15–17 cm), while the latter passes through the wall of the urinary bladder over a distance of about 1.5–2.0 cm. Along its course, the ureter exhibits three narrowings: at its origin, at the transition from the abdominal to the pelvic part, and within its intramural segment.

The wall of the ureter consists of three layers: the mucous membrane (mucosa), the muscular coat (muscularis), and the outer layer (adventitia). The mucous membrane forms longitudinal folds. The muscular coat, through its peristaltic contractions, facilitates the flow of urine into the bladder. Throughout its entire length, the organ lies extraperitoneally; that is, only its anterior surface is covered by the Peritoneum, while the remaining surfaces are surrounded by the adventitial layer.

The urinary bladder, vesica urinaria (Greek cystis), is an unpaired organ designed to store urine continuously delivered from the Ureters and to perform an excretory function—Micturition (urination). Its shape and dimensions are variable, depending on the degree of filling. Its capacity is individual, typically ranging from 250 to 700 mL. The urinary bladder is situated within the lesser pelvis, posterior to the Pubic Symphysis. Its anatomical relations with adjacent Organs differ between males and females. In males, the rectum, Seminal Vesicles, and ampullae of the Ductus deferens lie posteriorly; loops of the Small Intestine lie superiorly; and the base of the bladder is in contact with the Prostate Gland. In females, the cervix uteri and Vagina lie posteriorly; the body and base of the Uterus lie superiorly; and the base of the bladder rests on the Muscles of the pelvic floor (Perineum). The reflection of the peritoneum from the bladder onto adjacent organs forms specific recesses: the rectovesical pouch in males, and the vesicouterine pouch in females. The organ is anchored in place by ligaments attached to the pubic bones and the anterior abdominal wall. When fully distended, it can be palpated just above the pubic symphysis as an elastic mass.

The urinary bladder is anatomically divided into a superior part (the apex), an inferior part (the base or fundus), and a middle part (the body) (Fig. 9.5). The region where it funnels into the Urethra is termed the neck. The internal urethral orifice is located within the neck region.

The wall of the bladder is composed of three layers: a mucous membrane, a muscular coat, and an outer layer (serous and adventitial). Due to the presence of a loose submucosa, the mucous membrane is mobile and easily forms numerous folds that flatten out as the bladder distends. The epithelium of the mucosa has a specialized structure. When the bladder is empty, the epithelial Cells overlap, creating the microscopic appearance of a Stratified Epithelium. As the organ fills, these cells stretch, reducing the thickness of the epithelial layer and producing a pseudostratified appearance. Consequently, this specialized type of epithelium is known as transitional epithelium (urothelium).

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Fig. 9.5. Urinary bladder and prostate gland:

1 — apex of urinary bladder; 2 — muscular coat; 3 — serous coat; 4 — mucous membrane; 5 — base of urinary bladder; 6 — ureteric orifice; 7 — trigone of bladder; 8 — neck of urinary bladder; 9 — urethral crest; 10 — membranous part of urethra; 11 — seminal colliculus; 12 — prostatic part of urethra; 13 — prostate gland; 14 — body of urinary bladder

In the region of the bladder base, There is a distinct triangular area devoid of folds, known as the trigone of the bladder (Lieutaud's trigone). Here, the mucosa lacks a submucosal layer and is firmly fused with the underlying muscular coat. The apices of the trigone are marked by the two ureteric orifices (located on the posterior wall near the base) and the internal urethral orifice (at the neck). Small mucosal folds near the ureteric openings, along with their narrowed lumina and oblique course through the bladder wall, serve as a valve-like mechanism that prevents the retrograde flow of urine from the bladder back into the ureters.

The muscular coat of the bladder is quite thick and consists of interwoven smooth Muscle bundles. Traditionally, this coat is divided into three layers: outer and inner longitudinal layers, and a middle circular layer. The muscular wall of the bladder is collectively referred to as the detrusor muscle (m. detrusor vesicae). In the area of the internal urethral orifice, the circular muscle layer thickens to form the internal urethral sphincter, which plays a crucial role in The Mechanism of urinary continence. The internal sphincter is composed of Cytology/cytology/32.html">Smooth Muscle tissue and operates involuntarily (autonomously). Below the prostate gland, within the perineum, lies the external urethral sphincter, which is formed by skeletal (striated) muscle fibers and is under voluntary control.

When empty, the urinary bladder is covered by the peritoneum on only one side—superiorly—and thus lies extraperitoneally. The remaining walls of the bladder are covered by an adventitial coat. When fully distended, the organ occupies a mesoperitoneal position.

The urethra exhibits significant differences in Structure and function between males and females. In males, it has a more complex structure, is considerably longer, and serves as a common passageway for both urine and semen. The female urethra is notably shorter and wider. This topic will be discussed in greater detail in Chapter 11.

Mechanism of Micturition. The urinary bladder fills with urine up to a certain threshold without any significant increase in intravesical pressure. With further accumulation, however, the pressure begins to rise, and once it reaches 15–16 cm $ ext{H}_2 ext{O}$, the stretch receptors in the mucosa and muscular coat are stimulated. The subsequent pathway of micturition depends on the individual's age and neurological maturity. In infants, these processes are regulated exclusively by the Spinal Cord. Upon stimulation of bladder receptors, nerve impulses travel via afferent (sensory) fibers to the spinal cord, where the spinal micturition center is located at the level of the II–IV sacral segments. This center automatically triggers bladder emptying: the

m. detrusor vesicae contracts while the internal sphincter relaxes. Because the cortical center for urinary control has not yet developed in an infant's Brain, urine is evacuated from the body reflexively and without voluntary retention.

At approximately two years of age, a specialized micturition center develops in the cortex of the frontal lobes, enabling conscious, voluntary control to temporarily postpone urination or, conversely, to initiate it even when the bladder is not full. Efferent impulses originating in the frontal lobes travel down through the spinal cord to the external voluntary sphincter, which consists of striated muscle fibers. Contraction of the external sphincter can inhibit bladder emptying or interrupt the flow of urine once micturition has begun.

Despite the presence of the spinal micturition center, it is impossible to suppress urination indefinitely. When the bladder reaches a critical state of overfilling, a protective reflex is triggered, resulting in the relaxation of all sphincters and subsequent bladder evacuation. This protective reflex safeguards the bladder walls from excessive overstretching, urinary stasis, and the reflux of urine into the ureters and renal pelves.

Involuntary urination in older children and adults, as well as nocturnal enuresis (bedwetting), indicates dysfunction of The Nervous system and warrants specialized medical evaluation and Treatment.

Pathologies of the Urinary System may lead to The formation of calculi (stones) within the renal calices, pelvis, and ureters, as well as The Development of Inflammatory Diseases and subsequent renal failure.