The urinary bladder is a hollow, muscular, and distensible organ that serves as a temporary reservoir for urine. This article explores the bladder through an anatomical lens, detailing its visible form, microscopic composition, and the structural basis for its actions.
Macroscopic Features
The urinary bladder, a hollow, muscular, and distensible organ, resides in the pelvic cavity, serving as a temporary reservoir for urine produced by the kidneys. When empty, it is largely contained within the pelvic cavity, positioned behind the pubic bones and pubic symphysis, making it the most anterior pelvic organ. As it fills with urine, the bladder expands, extending upwards into the abdominal cavity.
Its shape and size vary with urine volume, appearing pear-shaped when empty and more ovoid or spherical when full. Anatomists divide the bladder into distinct regions: the apex, body, fundus (or base), and neck. The apex, the anterosuperior part, points towards the pubic symphysis and is connected to the umbilicus by the median umbilical ligament. The body represents the main, largest section, located between the apex and the fundus.
The fundus, or base, is the posteroinferior aspect, where the two ureters enter the bladder. The neck, situated inferiorly, is the constricted part continuous with the urethra, forming the lowest point of the bladder. Internally, the bladder features two ureteric orifices where urine enters and one internal urethral orifice where it exits. These three openings delineate a smooth, triangular area on the internal base of the bladder called the trigone, a significant anatomical landmark.
The bladder’s relations to surrounding structures differ between sexes. In males, it lies anterior to the rectum and superior to the prostate gland. In females, the bladder is positioned anterior to the uterus and vagina. Ligaments connect the bladder to other organs and the hip bone, helping to keep it in place within the pelvis.
Microscopic Structure
The wall of the urinary bladder is comprised of several distinct layers, each contributing to its ability to store and expel urine. From the innermost lining to the outermost covering, these layers are the mucosa, submucosa, muscularis propria, and adventitia or serosa.
The innermost layer, the mucosa, is lined by a specialized type of stratified epithelium known as transitional epithelium, or urothelium. This unique epithelium stretches and flattens significantly as the bladder fills, allowing for considerable volume changes without damage. When empty, the urothelium appears thicker, with multiple cell layers and dome-shaped superficial cells; upon distension, these cells flatten, and the layers thin, often to two or three cell layers. The urothelium also forms an impermeable barrier, preventing the reabsorption of urine components back into the body. Beneath the urothelium lies the lamina propria, a layer of connective tissue that contains blood vessels, nerves, and lymphatic vessels, supporting the overlying epithelium.
The submucosa, beneath the mucosa, consists of loose connective tissue with elastic fibers. This layer provides support to the mucous membrane and houses additional blood vessels and nerves.
The muscularis propria, often referred to as the detrusor muscle, forms the substantial middle layer of the bladder wall. It is composed of interlacing bundles of smooth muscle fibers, arranged in a complex, multi-directional pattern. The detrusor muscle is primarily responsible for the contraction that expels urine from the bladder.
The outermost layer of the bladder wall varies depending on its location. The superior surface of the bladder is covered by a serosa, which is a reflection of the peritoneum. The remaining surfaces are covered by an adventitia, a fibrous connective tissue layer that blends with surrounding structures, helping to anchor the bladder in place.
Functional Architecture
The bladder’s anatomical structures are organized to enable its primary functions: efficient storage and controlled expulsion of urine. The bladder’s remarkable distensibility is a direct consequence of its unique microscopic and macroscopic features.
The transitional epithelium lining the bladder’s interior, along with the numerous folds (rugae) present when the bladder is empty, permits significant expansion. As urine accumulates, these folds flatten, and the urothelial cells stretch and thin, allowing the bladder to hold a substantial volume of urine, typically 300 to 500 mL in adults, without a considerable increase in internal pressure. Concurrently, the smooth muscle fibers of the detrusor muscle remain relaxed during the filling phase, contributing to the bladder’s ability to accommodate increasing volumes.
The maintenance of continence during storage is facilitated by the internal urethral sphincter, which is a thickening of the detrusor muscle around the bladder neck, particularly prominent in males. This involuntary sphincter contracts to prevent urine leakage.
When the bladder needs to empty, the detrusor muscle contracts forcefully, effectively reducing the bladder’s volume and increasing internal pressure. Simultaneously, the internal urethral sphincter relaxes, allowing urine to flow into the urethra. The trigone, a smooth triangular region at the bladder base, directs urine into the urethra and prevents backflow into the ureters during contraction. The coordinated action of detrusor contraction and sphincter relaxation allows for the controlled expulsion of urine.
Vascular and Neural Supply
The urinary bladder receives a rich network of blood vessels and nerves that support its metabolic needs and enable its complex storage and emptying functions.
Arterial blood supply to the bladder primarily originates from branches of the internal iliac arteries. Specific branches supply different parts of the bladder, with some variations between sexes.
Venous drainage corresponds to the arterial supply, with a vesical venous plexus forming around the bladder, ultimately draining into the internal iliac veins. Lymphatic drainage from the bladder generally follows the major blood vessels, leading to the internal and external iliac lymph nodes.
The bladder’s function is precisely controlled by an extensive neural network, involving both the autonomic and somatic nervous systems. Autonomic innervation primarily comes from the vesical plexuses, which receive sympathetic and parasympathetic fibers.
Sympathetic fibers generally promote urine storage by inhibiting detrusor contraction and stimulating the contraction of the internal urethral sphincter. Parasympathetic innervation facilitates urination by stimulating the contraction of the detrusor muscle and promote relaxation of the internal urethral sphincter.
Sensory nerves within the bladder wall detect stretch as the bladder fills, transmitting this information to the spinal cord and brain, signaling the urge to urinate. The somatic nervous system provides voluntary control over the external urethral sphincter, which is composed of skeletal muscle. This dual autonomic and somatic innervation allows for both involuntary and voluntary regulation of bladder function.