The  average  amount  of  urine  an  adult  excretes varies from 1,000 to 1,500 ml per day. However, the amount   of   urine   excreted   varies   greatly   with temperature,   water   intake,   and   state   of   health.   No matter  how  much  water  one  drinks,  the  blood  will always  remain  at  a  constant  concentration,  and  the excess water will be excreted by the kidneys. A large water  intake  does  not  put  a  strain  on  the  kidneys. Instead it eases the load of concentration placed on the kidneys. URETERS The  ureters'  only  function  is  to  carry  urine  from each kidney to the urinary bladder. The ureters are two membranous tubes 1 mm to 1 cm in diameter and about 25   cm   in   length.   Urine   is   transported   through   the ureters by peristaltic waves (produced by the ureter's muscular walls). URINARY BLADDER The   urinary   bladder   functions   as   a   temporary reservoir for urine. The bladder possesses features that enable urine to enter, be stored, and later be released for evacuation from the body. Structure The  bladder  is  a  hollow,  expandable,  muscular organ located in the pelvic girdle (fig. 1-59). Although the shape of the bladder is spherical, its shape is altered by  the  pressures  of  surrounding  organs.  When  it  is empty, the inner walls of the bladder form folds. But as the   bladder   fills   with   urine,   the   walls   become smoother. The   internal   floor   of   the   bladder   includes   a triangular   area   called   the   trigone   (fig.   1-59).   The trigone has three openings at each of its angles. The ureters are attached to the two posterior openings. The anterior opening, at the apex of the trigone, contains a funnel-like continuation called the neck of the bladder. The neck leads to the urethra. The wall of the bladder consists of four bundles of smooth muscle fibers. These muscle fibers, interlaced, form   the   detrusor   muscle   (which   surrounds   the bladder neck) and comprise what is called the internal urethral  sphincter.  The  internal  urethral  sphincter prevents  urine  from  escaping  the  bladder  until  the pressure  inside  the  bladder  reaches  a  certain  level. Parasympathetic  nerve  fibers  in  the  detrusor  muscle function  in  the  micturition  (urination)  process.  The 1-56 HM3F0159 B URETER URETER URINARY BLADDER URINARY BLADDER DETRUSOR MUSCLE (INTERNAL URETHRAL SPHINCTER) PROSTATE GLAND BULBOURETHRAL GLAND PENIS EXTERNAL URETHRAL ORIFICE EXTERNAL URETHRAL ORIFICE PENILE URETHRA PROSTATIC URTHRA TRIGONE TRIGONE A MEMBRANOUS URETHRA URETHRA Figure 1-59.—Urinary bladder and urethra: A. Frontal section of the female urinary bladder and urethra; B. Frontal section of the male urinary bladder and urethra.