The bladder is a subperitoneal, hollow muscular organ that acts as a reservoir for urine. The bladder is located in the lesser pelvis when empty and extends into the abdominal cavity when full. In children, the bladder is located in the abdomen and does not completely descend into the pelvis until puberty. The bladder is a distensible organ and is typically able to hold up to 500 milliliters of urine. 

The size of the pediatric bladder can be predicted by the following: (Age + 2) x 30 mL.

The bladder is situated just posterior to the pubic symphysis. Posteriorly, the anterior wall of the vagina sits behind the bladder in females. In males, the rectum is located posterior to the bladder. Inferiorly, the muscles of the pelvic diaphragm support the bladder. The bladder is a relatively free organ in the subcutaneous fat of the sub peritoneum except for some fixed ligamentous connections at the bladder neck. The superior and part of the posterior surfaces of the bladder are covered by peritoneum. The inferior portion and inferolateral sides of the bladder are covered by endopelvic fascia.

Anatomically, the bladder is contiguous with the ureters above and the urethra below. It is divided into four anatomical parts: the apex or dome, body, fundus, and neck. The apex is the anterosuperior part of the bladder that points towards the abdominal wall. The fundus, or base, is the posteroinferior part of the bladder. The body is the large area situated between the apex and the fundus. The neck of the bladder is the constricted part of the bladder that leads to the urethra.

The bladder functions as a reservoir and active excretory organ for urine. Urine excreted by the kidneys passes through the ureters and the ureterovesical junction into the bladder. As the bladder fills with urine, sensory nerves traveling back to the central nervous system communicate with efferent somatic and autonomic nerves to control the release of urine through stimulation of the detrusor (bladder) muscle with simultaneous relaxation of the internal and external urethral sphincters.

The bladder is mostly a mesodermal organ but does contain some endoderm as well. Between weeks 4 to 7 of fetal development, the cloaca divides into the urogenital sinus ventrally, and the anal canal dorsally. The urogenital sinus is continuous with the allantois anteriorly. The base of the allantois expands and develops into the bladder while its anterior portion gives rise to the urachus. The urachus later develops into a fibrous cord, the median umbilical ligament.[1]

Through mechanisms not clearly explained, the mesonephric ducts and ureteric buds connect to the posterior bladder wall and form a part of the trigone. The mesonephric ducts also migrate ventrally and come together to give rise to the urethra.[1]

The superior and inferior vesicle arteries provide the blood supply to the bladder. These are indirect branches of the internal iliac arteries. The bladder also receives some of its blood supply from the obturator artery and the inferior gluteal artery. In some cases, the inferior vesicle artery may be a branch of the internal pudendal artery.

Blood is drained from the bladder via the vesical venous plexus that empties into the internal iliac vein. Lymphatics are drained via various lymph nodes associated with the veins in the area, with most of the drainage occurring via the external iliac lymph nodes.[2]

The bladder receives its innervation through a network of parasympathetic, sympathetic and somatic nerve fibers. Parasympathetic fibers arise from sacral spinal nerves (S2-S4) that coalesce to form the pelvic splanchnic nerves. Sympathetic control arises from the lower thoracic and upper lumbar spinal levels  (T10-L2) in the form of the superior and inferior hypogastric plexuses and nerves. The pudendal nerve (S2-S4), a branch of the sacral plexus, provides voluntary somatic control to the striated muscles of the external urethral sphincter.[3] These fibers originate in the ventral horn of the sacral spinal cord which is called Onuf's nucleus.

Mechanoreceptors in the bladder wall respond to the stretch of the muscle with distension. These receptors send sensory information to the central nervous system (CNS) via general visceral afferent (GVA) fibers of the hypogastric and pelvic splanchnic nerves. Sensory information from the superior bladder wall follows the path of the sympathetic innervation, while that from the inferior part of the bladder follows the parasympathetic nerves. The speed of the stimulus sent back to the CNS corresponds to the degree of distension of the bladder, such that a mildly distended bladder will generate slow stimuli, while a fully distended bladder will increase the speed of the stimuli to the CNS.[3]

The micturition center, which regulates the process of bladder filling and voiding is located in the pons of the brainstem (pontine micturition center). In general, the parasympathetic response stimulates bladder emptying, and the sympathetic response promotes bladder filling.[3]

Parasympathetic nerves stimulate the detrusor muscle to contract and the internal urethral sphincter to relax to allow for micturition. Sympathetic stimulation allows for relaxation or filling of the detrusor muscle and constriction of the internal sphincter. Local spinal reflexes primarily regulate the process of bladder filling; that, of voiding, however, requires action from the pontine micturition center in the brainstem.[3][4][3]

As an empty bladder fills with urine, mechanoreceptors detect the stretch in the wall of the detrusor muscle and send slow afferent fibers back to the spinal cord. This stimulates the sympathetic response on the detrusor and internal sphincter via the hypogastric nerves, causing them to relax and constrict, respectively. The parasympathetic response is inhibited at this time. Signals from the pudendal nerve to the external urethral sphincter are also stimulated to keep the external urethra contracted and closed. When the bladder becomes fully distended, sensory fibers travel to the pontine micturition center, stimulating the parasympathetic response and inhibiting the sympathetic tone to allow for contraction of the detrusor muscle and relaxation of the internal sphincter. Throughout this, somatic innervation to the external urethral sphincter remains tonically active to keep the sphincter contracted. When voiding is appropriate, activity from the pudendal nerve is inhibited to allow relaxation of the sphincter and the release of urine. Stimuli sent to the cortex during the filling process also play a role in voiding. The cortex processes emotional, sensory and social stimuli and acts on cells in the PMC and the periaqueductal gray (PAG) to prevent voiding until the appropriate time.[5]