Urinary bladder

Anatomy

The urinary bladder is a hollow, muscular, and distensible organ that sits on the pelvic floor and below the peritoneum. Generally, when empty it is pear-shaped with an apex, a base (fundus), neck, superior surface and two inferolateral surfaces. However, the size, shape, position, and relationships to adjacent structures of the bladder can change considerably depending on the amount of urine it holds.

The apex of the urinary bladder is oriented towards the upper part of the pubic symphysis. Extending from the apex to the umbilicus is the median umbilical ligament, a vestige of the embryological urachus. The ligament ascends behind the anterior abdominal wall forming the median umbilical fold.

The base (fundus) of the urinary bladder is situated opposite the apex, on the posteroinferior surface of the urinary bladder. It has an inverted triangle shape and receives the two ureters at the upper corners, which transport urine from each kidney. The urethra descends from the lower corner of the base. Internally, the base of the urinary bladder bears a smooth, triangular region called the trigone. The openings of the ureters (ureteric orifices) and the internal opening of the urethra (internal urethral orifice) form the angles of the trigone.

The main section of the bladder situated between the apex and the fundus is known as the body. Urine is collected in the body of the bladder and ultimately voided through the urethra. The base of the bladder connects with the inferolateral surfaces at the lower part to form the neck of the bladder, which is continous with the urethra inferiorly.

The walls of the urinary bladder are primarily composed of the detrusor muscle. The detrusor muscle is a layer of the bladder wall made of smooth muscle fibers that are arranged in spiral, longitudinal, and circular bundles. Near the neck of the bladder in males, these muscle fibers create the involuntary internal urethral sphincter.

While the general volume of the human bladder will vary from person to person, the range of urine that can be held in the bladder is roughly 400 mL (~13.5 oz) to 1000 mL (~34 oz), with the average capacity being 400 to 600 mL.

For more information about the urinary bladder, take a look at the study units below:

Blood supply

The urinary bladder receives arterial supply from branches of the anterior trunk of the internal iliac arteries. Its anterosuperior aspects are supplied by the superior vesical arteries. In males, the fundus and neck of the bladder are supplied by the inferior vesical arteries. In females, however, this role is taken over mainly by the vaginal arteries, which provide small branches to the posterior and lower parts of the bladder. Small branches from the obturator and inferior gluteal arteries also contribute to the blood supply of the urinary bladder.

The veins that drain blood from the urinary bladder form a plexus around its inferolateral surfaces which ultimately drains into the internal iliac veins. In males, the vesical venous plexus merges with the prostatic venous plexus forming a combined network that surrounds the base of the bladder and prostate, along with the seminal glands, the ductus deferentes, and the lower portions of the ureters. In females, the vesical venous plexus surrounds the pelvic segment of the urethra and the neck of the bladder. This plexus typically empties into the internal iliac veins via the inferior vesical veins but could potentially empty into the vertebral venous plexuses through the sacral veins. 

Innervation

The urinary bladder receives autonomic innervation from the vesical plexuses composed of sympathetic and parasympathetic nerve fibres.

• Sympathetic fibers reach the vesical plexus via hypogastric nerves from the inferior mesenteric ganglion, which is supplied by the lumbar splanchnic nerves from the sympathetic lumbar outflow T10-T11. These fibers prevent urination by promoting detrusor relaxation and internal sphincter contraction.
• Parasympathetic nerve fibers, reach the vesical plexus via pelvic splanchnic nerves which are from the parasympathetic sacral outflow. In contrast to the sympathetic, the parasympathetic innervation facilitates urination by causing contraction of the detrusor muscle and relaxation of the internal sphincter.

Somatic innervation of the urinary bladder comes via the pudendal nerves (S2-S4), which also innervate the striated muscle of the pelvic floor and the external urethral sphincter.

The hypogastric, pelvic splanchnic and pudendal nerves all have afferent components mainly involved in the sensation of pain and the perception of stretching.

Functions

The muscles in the bladder allow for conscious control of urination. There are two important pathways involved: the sensation that lets you know your urinary bladder is full and needs to be emptied, and the motor control of the bladder to allow for urination.

First, as the bladder fills and its walls stretch closer to maximum capacity, sensory signals are transmitted via the parasympathetic nervous system, prompting the contraction of the detrusor muscle. This signal encourages the bladder to expel urine through the urethra. Secondly, to control the act of urination voluntarily, motor control is achieved through innervation by both sympathetic fibers and parasympathetic fibers.

Urine must pass through two important sphincters to exit the body: the autonomically controlled internal sphincter and the voluntarily controlled external sphincter, both of which need to be opened for urination to occur.

Contraction of the detrusor muscle during urination, not only expels urine but also helps close the ureteric orifices, thus, preventing backflow of urine into the ureters and kidneys.

Interestingly, a meta-analysis conducted on the effects of different voiding positions in male urodynamics reports that sitting down allows for improved contraction of the detrusor muscle.

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It is important to note as well that in males, the internal urethral sphincter contracts during ejaculation to inhibit the backward flow of semen into the bladder, preventing retrograde ejaculation.