bladders

Bladders are hollow, expandable organs found in many animals, serving as reservoirs for fluids. The most familiar is the urinary bladder in terrestrial vertebrates, which stores urine before it is excreted. Some fish possess a gas-filled swim bladder, used primarily to regulate buoyancy rather than to store urine.

In humans, the urinary bladder sits in the pelvis, behind the pubic bone. In males it lies anterior to the rectum; in females it sits anterior to the uterus and vagina. The bladder wall consists of an innermost mucosa with transitional epithelium, a submucosa, a thick muscular layer called the detrusor, and an outer adventitia or serosa. The trigone region between the ureteral openings remains relatively smooth and is a key anatomical feature. The bladder’s capacity in adults typically ranges from about 400 to 600 milliliters, with sensations signaling the need to void before it becomes overly full.

Functionally, the bladder serves as a storage organ that releases urine through the urethra during micturition. Filling stretches the detrusor minimally at first, while the internal urethral sphincter relaxes and the external sphincter remains under voluntary control until a suitable time to void. The process is coordinated by autonomic and somatic nerves, including parasympathetic signals that stimulate contraction and urination, and voluntary control of the external sphincter.

Common bladder-related conditions include urinary tract infections, cystitis, overactive bladder, neurogenic bladder, and bladder cancer. Diagnostic approaches include urinalysis, imaging, and cystoscopy, while treatments range from fluids and medications to catheterization or surgery, depending on the condition.

Swim bladders in fish are a separate type of bladder. They are gas-filled and used to adjust buoyancy, not for urine storage, and their structure and regulatory mechanisms differ from the mammalian urinary bladder.