The urinary bladder is an organ that serves to collect urine to be voided through urination after the urine is filtered through the kidneys (where the necessary ions are reabsorbed if physiologically needed through feedback mechanisms found throughout the body and in the nephrons of the kidneys, such as the macula densa). Histologically, the urinary bladder is lined with transitional epithelium and does not produce mucus.
Generally, the bladder is a hollow, muscular, and pear-shaped distensible elastic organ that sits on the pelvic floor. It receives urine via the ureters, which are thick tubes running from each kidney down to the superior part of the bladder. Urine is collected in the body of the bladder, and finally it is voided through the urethra. The fundus is the base of the bladder, which is formed by the posterior wall and contains the trigone of the bladder, and is lymphatically drained by the external iliac lymph nodes. The trigone is the structure that contains the outlet (urethra) of the bladder.
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. One mnemonic often heard in clinical settings related to the bladder is: “water (ureters) under the bridge (uterine arteries or vas deferens).” This phrase describes an anatomical relationship, where in females, the ureters pass through the mesometrium and under the uterine arteries on the way to the urinary bladder. Or to give another mental image, the ureters are posterior to the ovarian/testicular artery.
The muscles in the bladder that allow for conscious control of when you are or are not in a suitable situation to urinate are especially meaningful in civilized societies. There are 2 important pathways involving your bladder: 1) the sensation that lets you know your urinary bladder is full and needs to be voided, and 2) the motor control of your bladder to allow you to urinate at will.
First, as the bladder walls are stretched when it is full or getting closer to maximum capacity, there are signals that are transmitted through the parasympathetic nervous system to contract 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. This signal will encourage the bladder to expel urine through the urethra. Sensations from the bladder are transmitted to the central nervous system (CNS) via general visceral afferent fibers (GVA). Whereas GVA fibers on the superior surface of the bladder follow the course of the sympathetic effect nerves back to the CNS, GVA fibers on the inferior portion follow along with the parasympathetic efferent fibers. Surprisingly (or maybe not), 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.
To control the act of urination voluntarily, motor control is achieved through innervation by both sympathetic fibers, most of which arise from the hypogastric plexuses and nerves, and parasympathetic fibers, which come from the pelvic splanchnic nerves and the inferior hypogastric plexus. Finally, there are two important sphincters the urine must pass through in order to leave the body: both the autonomically controlled internal sphincter and the voluntarily controlled external sphincter must be opened.