Urinary bladder and urethra
The bladder is a hollow round structure that is part of the urinary system. It has strong muscular walls, and is characterized by its ability to distend. The bladder’s main function is to collect, temporarily store, and expulse urine. It is situated in the pelvic cavity when empty, and expands superiorly into the abdominal cavity when full. Anteriorly, it rests on the pubic bones and pubic symphysis. Posteriorly, it rests on the prostate in males, and the anterior wall of the vagina in females. The empty bladder is shaped like a tipped three sided pyramid. It has a base (fundus), a neck, an apex, as well as a superior and two inferolateral surfaces.
The base of the bladder faces posteroinferiorly. It is triangular in shape, with the tip facing backwards. The left and right ureters enter the bladder at each of the upper corners of the base. The trigone is the smooth walled triangular area present between the openings of the ureters and urethra on the inside of the bladder. The neck is the most inferior portion of the bladder. It is also the most fixed portion, anchored in position by a pair of tough fibromuscular ligaments, which connect it, along with the pelvic part of urethra, to each of the pubic bones. At the intersection point between the two inferolateral surfaces and the base, the neck surrounds the origin of the urethra. The apex of the bladder is directed towards the pubic symphysis. It is located superiorly, and is interconnected with the umbilicus by the median umbilical ligament. The superior surface of the bladder is slightly domed when empty, and balloons up when the bladder starts to fill with urine. It is entirely covered by peritoneum in males. However, in females, the posterior part of the superior surface is devoid of any peritoneum. The two inferolateral surfaces of the bladder are nestled between the levator ani muscles of the pelvic diaphragm, and the nearby obturator internus muscles.
The musculature of the bladder is an important element that plays a major role in the function of the bladder. The bladder is comprised of a specialized smooth muscle called the detrusor muscle. Its fibers travel in three directions, which allows the bladder to retain its structural integrity when stretched. During micturition, the detrusor muscle contracts. The bladder also has two types of muscular sphincters located around the urethral orifices. The internal urethral sphincter is present only in males. Its function is to prevent seminal regurgitation during ejaculation. The external urethral sphincter is present in both sexes. During micturition, it relaxes to allow the passage of urine.
The arterial supply of the bladder is primarily provided by the internal iliac artery. It is delivered by the superior vesical branch of the internal iliac artery. This is supplemented by the inferior vesical artery in males, and the vaginal arteries in females. In both sexes, the obturator and inferior gluteal arteries contribute small branches as well. The venous blood of the bladder flows through the vesical venous plexus, and empties into the internal iliac vein. The innervation of the bladder is complex. It receives input from both the autonomic and somatic nervous systems.