The urinary bladder is a sack-like dilatation of the urinary passages which serves as a collecting reservoir, the size and shape of which is dependent upon the degree of distention. Three chief portions may be recognized in it: the middle and larger portion of the bladder is the body; the upper portion, which is distinctly pointed, especially in the newborn, is the vertex; and the lowermost portion, directed toward the perineum, is called the fundus.

The shape of the bladder and the thickness of its wall vary with the amount of contained fluid, and its shape is also subject to certain sexual and individual variations. The full bladder is ellipsoidal or ovoid, and in the female particularly is frequently markedly flattened from before backward. The empty bladder is irregularly spherical and flattened from above downward, and in the female is frequently indented by the overlying and anteverted body of the uterus. In the child the full bladder is often pear-shaped rather than ovoid. 

The middle umbilical ligament is attached to the vertex of the bladder and in the adult is a fibrous cord which gradually disappears upon the slightly pointed vertex. 

It represents an obliterated embryonic canal, known as the urachus, the vesical extremity of which is usually still patulous in the newborn. It passes upward to the umbilicus upon the anterior abdominal wall and is situated in front of the peritoneum. Two lateral fibrous cords also run to the umbilicus from the lateral walls of the bladder; they are known as the lateral umbilical ligaments and are the remains of the obliterated hypogastric arteries. In the adult they gradually disappear at the umbilicus, where they join the middle umbilical ligament at an acute angle, forming with it the foveae supravesicales. 

The bladder is situated in the anterior portion of the pelvic cavity behind the pubic symphysis. Its axis is not vertical but approximately parallel to that of the pubic symphysis, passing obliquely from before backward and from above downward. When empty the vertex does not project above the symphysis, although when strongly distended it may extend far above this point. The anterior vesical wall consequently borders upon the anterior pelvic wall and upon the posterior surface of the anterior abdominal wall when the bladder is full, and the lateral surfaces of the empty or slightly filled bladder are in relation with the lateral walls of the pelvis. In the female the posterior surface is in contact with the uterus and the upper portion of the vagina, while in the male it is in relation with the seminal vesicles and the vasa deferentia (particularly the ampullae) and with the rectum (when the bladder is distended) or the intestinal coils lying in the rectovesical pouch. In the female, however, the posterior wall of the bladder even when empty is always in relation with the uterus since the vesico-uterine pouch is always empty when the position of the uterus is normal. 

When the bladder is full the fundus is almost horizontal, but when the bladder is empty it slants obliquely forward and from above downward. Together with the lower portion of the posterior surface it is also partly in relation with the anterior wall of the rectum, being in immediate contact with it below, but separated from it above by the lowermost portion of the rectovesical fold. 

In the male the fundus is intimately adherent to the prostate, by means of which it is attached to the pelvic floor; in the female it lies upon the middle portion of the anterior vaginal wall. The deepest point of the fundus is the site of the internal orifice of the urethra, which, corresponding to the oblique position of the vesical axis, is situated several centimeters behind the symphysis and is attached to the pelvic floor by the urogenital trigone and in the male also by the prostate. 

The urinary bladder possesses three openings, the two orifices of the ureters and the internal orifice of the urethra. All three openings are within 1 or 2 cm. of each other in the region of the fundus, the urethral orifice being at the lowest point of the bladder and the orifices of the ureters in the lower portion of the posterior vesical wall. The ureters pass through the wall quite obliquely, and in so doing form a fold of mucous membrane, known as the ureteric fold, which, since the ureters are directed toward each other during their course through the vesical wall, are correspondingly convergent. Continuations of these folds pass from the orifices of the ureters toward the internal orifice of the urethra, where they gradually disappear, and the orifices of the ureters are connected by a low, transverse, slightly curved fold, which is also a continuation of the ureteric fold, so that an approximately equilateral triangle is formed, the three vesical openings being situated at its angles, its base being directed posteriorly (and above), and the apex anteriorly and below (the internal urethral orifice). This triangle is characterized by the smoothness of its mucous membrane even when the bladder is empty, and is termed the vesical trigone (triangle of Lieutaud). The portion of the bladder corresponding to the vesical trigone together with the urethral orifice and the vesical wall attached to the prostate is also termed the neck. The orifices of the ureters are obliquely placed, slit-like openings upon the ureteric folds of the vesical mucous membrane. 

Near the apex of the vesical trigone there is usually an independent longitudinal elevation which extends to the internal urethral orifice and is known as the uvula of the bladder; it projects into the urethral orifice from behind, so that this opening is semilunar in shape. 

The vesical wall consists of a mucous coat, of a muscular coat, and of a serous coat. The last named coat is not present in all parts of the viscus, but invests only the upper surface of the bladder as far as the vertex and the upper part of the lateral surfaces; at the posterior surface the peritoneum is reflected to the rectum in the male and to the vesical surface of the uterus in the female. It attains the lowest level in the male between the ampullae of the vasa deferentia. The peritoneum is but loosely attached to the lateral portions of the bladder, but at the middle segment of the viscus it is rather firmly adherent. The anterior wall, the lower portions of the lateral walls, and the fundus of the bladder have no serous covering whatever, and between the posterior surface of the symphysis and the anterior surface of the bladder the prevesical space y which is filled chiefly by loose fatty tissue, is situated. 

The entire surface of the bladder is covered by a layer of fascia, which rests upon the external muscular layer and forms the outer coat of the bladder in those situations which are uncovered by peritoneum. It is a portion of the visceral layer of the pelvic fascia. 

The muscular coat is quite thick (about 1 cm.) when the viscus is empty, but when it is distended the muscular fasciculi become markedly separated. It consists of three layers. The outer layer consists essentially of longitudinal fibers and is reinforced by two muscular fasciculi which connect the bladder with surrounding structures. Of these the m. pubavesicalis passes from the posterior surface of the pubis near the symphysis and from the anterior extremity of the tendinous arch of the pelvic fascia (the " white line") to the fundus of the bladder; it is a constituent of the puboprostatic ligament in the male and of the pubovesical ligament in the female. The tn. rectovesicalis is present only in the male; it connects the longitudinal musculature of the rectum with the outer layer of the muscular coat of the bladder, and runs in the peritoneal fold of the same name. 

The middle layer is the strongest of the three muscular coats, and consists of a fairly continuous stratum of circular fibers which are oblique in the upper and exactly transverse in the lower portion of the bladder. A thickening of this layer at the internal orifice of the urethra forms what is termed the urethral annulus and serves as a m. sphincter vesica. 

The internal layer forms a wide-meshed muscular reticulum, the majority of the fibers pursuing a longitudinal direction. It is situated immediately beneath the mucous membrane. 

The musculature of the ureter remains independent even within the wall of the bladder (the intramural portion of the ureter), and is characterized by strong longitudinal fasciculi which penetrate the muscular layers of the bladder obliquely and become lost in the thick muscular wall of the vesical trigone. The ureter penetrates the vesical wall at such an acute angle that it is surrounded by it for 1 to 2 cm. according to the degree of vesical distention, and it retains its fibrous coat almost until it reaches the orifice. As a consequence of this oblique penetration the resulting lip of mucous membrane above the orifice acts as a valve, which is closed by the pressure of the urine contained within the bladder and can be opened only by the contraction of the musculature of the ureter. 

The mucous membrane of the bladder is of a bright red color in the living subject (cystoscopic picture), and when the viscus is empty the mucous membrane is thrown into marked folds, except in the area included within the vesical trigone, which is nearly always smooth. The folds are completely obliterated when the bladder is distended. The vesical mucous membrane contains lymphatic nodules, but no true glands. 

When markedly distended the bladder contains over a liter, but under normal conditions it rarely contains more than 500 c.c, and usually less than this quantity. Even when markedly contracted the viscus is not entirely empty, but still contains about 50 c.c. of urine. 

The urinary bladder is supplied with blood by the superior and inferior vesical arteries, both of which are branches of the internal iliac. The larger inferior vessels are direct branches of the internal iliac, while the superior vesicals are really given off by the hypogastric artery. 

The veins form several plexuses within the vesical wall and empty into the pudendal and vesicoprostatic plexuses. 

The lymphatic vessels pass to the small vesical lymphatic glands and thence partly into the iliac glands. 

The nerves are largely furnished by the sympathetic, but are also derived from the third and fourth sacral nerves; they form the vesical plexus. 

The bladder is developed from the so-called allantois.