The ureters

The ureters are tubes that transport urine from the kidneys to the urinary bladder. In adults, the ureters are generally 25 to 35 centimeters (10 to 14 inches) long and around 3 mm in diameter. Each ureter is extended from the hilus of the kidney to the lower, posterior side of the urinary bladder.

Like the kidneys, the ureters are retroperitoneal, i.e., behind the peritoneum of the dorsal abdominal cavity, in front of the psoas muscle into the pelvic cavity. The ureter is inserted into the bladder at the (ureterovesical junction), which passes obliquely through the posterior wall of the bladder.

As a result of this arrangement, when the urine accumulates and the pressure in the bladder increases, it expands and compresses the lower ends of the ureters to avoid the backflow of urine into the ureters (towards the kidneys) when the bladder fills and as the pressure increases when the muscular bladder wall contracts.

The ureters consist of three layers of tissue:

1. The outer lining of the fibrous tissue, continuous with the kidney fibrous capsule.
2. A middle muscular layer composed of interlacing smooth muscle fibers that form a functional unit around the ureter and an additional outer longitudinal layer in the lower third. The walls of the ureters contain smooth muscles organized in a spiral, longitudinal, and circular bundles, but distinct muscle layers are not evident.
3. The inner layer of mucosa, consisting of a transitional epithelium.

The peristaltic waves, of the smooth muscle in the wall of the ureter contracts to propel urine to the urinary bladder. This is an innate feature of the smooth muscle and is not under autonomic nerve control. Peristalsis originates in the pacemaker of the minor calyces. Regular peristaltic contractions, occurring one to five times per minute, transfer the urine from the renal pelvis to the bladder, where it enters into synchronous spurts with each peristaltic wave (sending small spurts of urine to the bladder).

Urinary bladder

The urinary bladder is a triangular hollow organ situated in the pelvic cavity which varies in size and location depending on the amount of urine it contains. If the bladder is distended, it rises into the abdominal cavity.

In adults, the hollow bladder resides completely inside the pelvis. In a young child, the bladder is an abdominal organ, even if it is empty; It starts to enter the pelvis at 6 years of age but does not become a pelvic organ until after puberty. It is a muscular sac below the peritoneum and behind the pubic bones. In men, the bladder is superior to the prostate gland; in women, the bladder is inferior to the uterus. It is a reservoir for the accumulation of urine and contracts to eliminate urine.

The Urinary bladder is a chamber of smooth muscle consisting of two main parts (1) the body, which is the major part of the bladder in which the urine is collected; the main body of the empty bladder is pyramidal, with an apex, a base, and a superior and two inferolateral surfaces; and (2) the neck, which is a funnel-shaped extension of the body, passing inferiorly and anteriorly into the urogenital triangle and connecting with the urethra, it is the most fixed part of the bladder. The lower part of the bladder neck is sometimes called the internal urethral orifice (posterior urethra) Due to its relation to the urethra.

The mucosa of the bladder has a transitional epithelium, which allows for expansion without tearing the lining. When the bladder is empty, the mucosa appears wrinkled; these folds are called (rugae) which gradually disappear as it fills and also allows for expansion. The bladder is distensible, but awareness of the need to pass urine is felt when it contains between 300 and 400 ml. The total capacity is rarely more than 800 ml.

There is a small triangular area called the (trigone) on the posterior wall of the bladder, lying immediately above the bladder neck. At the lower apex of the trigone, the bladder neck opens into the internal urethral orifice (posterior urethra) and the two ureters join the bladder at the upper corners of the trigone.

The trigone can be identified by the fact that its mucosa, the inner lining of the bladder, is smooth, as opposed to the remaining mucosa of the bladder, which is folded to form a rugae like that of the ureters, the smooth muscle of the bladder is organized in spiral, circular, and longitudinal bundles.

Circular muscle contraction, called the (detrusor) muscle, is primarily responsible for emptying the bladder during urination (micturition). Detrusor muscle fibers when contracted can increase the pressure in the bladder to 40 to 60 mm Hg. And extend in all directions. Thus, the contraction of the detrusor muscle is a significant step in emptying the bladder. When Smooth muscle cells of the detrusor muscle fuse with each other, a low resistance electrical pathway exist from one muscle cell to another, as a result, the action potential will spread through the detrusor muscle, from one muscle cell to the next, triggering contraction of the entire bladder at once.

Each ureter, as it reaches the bladder, travels obliquely through the detrusor muscle and then passes another 1 to 2 centimeters under the bladder mucosa before being drained into the bladder.

Urine transport from the Kidney via the Ureters and into the Bladder

Urine that is released from the bladder has the same composition as the fluid that flows from the collecting ducts; as the urine flows through the renal calyces and ureters to the bladder there are no major differences in the composition of the urine.

Urine flowing from the collecting ducts into the renal calyces stretch the calyces and increase their inherent pacemaker activity, which successively initiates peristaltic contractions that spread to the renal pelvis then downward along the length of the ureter, thereby forcing urine from the renal pelvis toward the bladder.

The walls of the ureters contain smooth muscles and are innervated by both sympathetic and parasympathetic nerves, furthermore as by the intramural plexus of neurons and nerve fibers that extends along the entire length of the ureters. Like other visceral smooth muscles, peristaltic contractions within the ureter are stimulated by parasympathetic stimulation and inhibited by sympathetic stimulation.

In the trigone region of the bladder, the ureters enter the bladder through the detrusor muscle. Normally, the ureters pass obliquely through the bladder wall for a few centimeters. The normal tone of the detrusor muscle within the bladder wall tends to compress the ureter when the pressure builds up in the bladder during urination or bladder compression, preventing the backflow (reflux) of urine from the bladder. Each peristaltic wave along the ureter raises the pressure within the ureter so that the region passing through the bladder wall expands and allows the urine to flow into the bladder.

In some people, the distance the ureter travels through the bladder wall is less than the normal average, so the contraction of the bladder during micturition does not always lead to total occlusion of the ureter. As a result, some of the urine in the bladder is propelled back into the ureter, a condition called (vesicoureteral reflux). Such reflux can lead to ureter enlargement of the ureter and, if extreme, can increase the pressure in the renal calyces and structures of the renal medulla, causing damage to these regions.

Summary

The urinary bladder is a muscular sac below the peritoneum and behind the pubic bones; in men above the prostate gland, in women below the uterus. It is a temporary reservoir for urine. In an empty bladder, the greater part of the mucosa has irregular folds called (rugae) due to lose attachments to the muscular coat. There is a triangular region on the bladder floor at the posterior surface of the bladder wall called the (trigone). Two ureters join the bladder at the upper corners of this trigone.

The lower part of bladder is narrow and forms the neck. It opens into the internal urethral orifice located at the apex (lower angle) of the trigone. Here, the mucous membrane is always smooth (doesn't have rugae) and very sensitive to expansion.

The muscular layer of the urinary bladder is formed by smooth muscle fibers that form the detrusor muscle. The detrusor muscle is structured in interlacing longitudinal, circular, and spiral bundles. The detrusor muscle contracts to extract the urine. Urine is extracted from the bladder and passes through the urethra before it exits the body.