The urinary bladder is a hollow, musculomembranous and unpaired organ that is responsible for collecting urine from the kidneys and is channeled inside it by the ureters. It therefore acts as a temporary reservoir, filling up between one urination and another and occasionally emptying itself to eliminate the accumulated urine outside through the urethra.
Macroscopically the bladder is divided into three regions: fundus (or base), body and apex. The urethral outlets are located on the bladder bottom; the area between these and the urethra orifice is called the bladder trigone.
The bladder has a maximum capacity of about 200-400 ml, with considerable individual variability; in particular situations, for example in episodes of urinary stasis, the organ can still accumulate more than a liter of urine. This ability is linked to the peculiar structure of the bladder wall, in which four tunics are recognized, which from the inside to the outside take the name of: mucous tunic, submucosal cassock, muscular cassock and serous tunic.
The mucous membrane is characterized by a transitional coating epithelium, consisting of several cell layers that adapt their shape to the degree of filling of the bladder. When the organ is empty the superficial cells have an umbrella or mushroom head shape, the intermediate ones resemble a club and the lower ones have a rounded shape. In the full bladder, on the other hand, the superficial cells flatten out and the intermediate ones wedge at basal level, making the epithelium much thinner and bistratified.
The transitional epithelium rests on a lamina propria, rich in connective tissue which, with the exception of the so-called trigone, can be raised in folds. These folds are reserve surfaces, as they flatten out in case of strong bladder filling. Also the submucosal cassock consists of a thin layer of connective tissue with the interposition of elastic fibers; its function is comparable to a sliding surface, thanks to which the mucous membrane can modify its characteristics in relation to the degree of fullness of the bladder.
More in depth than the submucosa, the muscular habit is characterized by three layers of smooth muscle fibers that together form the so-called detrusor muscle of the bladder. Although educationally divided into these three layers, in reality these muscular structures are not well differentiable, but they interpenetrate one another. In general, however, in the most superficial layer the muscular fibrocellulas are arranged in longitudinal bundles, which intertwine under the mucosa; in the middle layer, on the other hand, the fibrocellules take on a circular pattern and gather at the base of the bladder around the internal urethral orifice, constituting the so-called smooth sphincter of the bladder. Furthermore, at the outlet of the ureter, a part of this intermediate muscular layer forms a valve that prevents the backflow of urine into it. Like the superficial one, the deeper muscular layer is made up of longitudinal fibers.
The contraction of the detrusor muscle of the bladder and the relaxation of the urethral sphincter are controlled by the parasympathetic nervous system, which therefore favors urination. Conversely, the contraction of the sphincter and the releasing of the detrusor (filling phase), are under the control of the sympathetic system.
The serous tunic is represented by the parietal peritoneum, which covers only the upper region of the bladder and its postero-lateral faces. In the remaining portions the bladder wall is covered with fibroadipose connective tissue.
