outer layer (serous coat) of the bladder wall consists of two types of tissue, parietal peritoneum and fibrous connective tissue. Micturition (Urination) Micturition  is  the  process  by  which  urine  is expelled from the bladder. It involves the contraction of the detrusor muscle, and pressure from surrounding structures. Urination also involves the relaxation of the external  urethral  sphincter.  The  external  urethral sphincter  surrounds  the  urethra  about  3  centimeters from  the  bladder,  and  is  composed  of  voluntary muscular tissue. Urination is usually stimulated by the distention of the bladder as it fills with urine. When the walls of the bladder contract, nerve receptors are stimulated, and the urination reflex is triggered. The urination reflex causes the internal urethral sphincter to open and the external  urethral  sphincter  to  relax.  This  relaxation allows the bladder to empty. The bladder can hold up to 600 ml of urine. The desire to urinate may not occur until the bladder contains 250-300 ml. URETHRA The urethra is the tube that carries urine from the bladder to the outside of the body (fig. 1-59, views A and B). The  urinary meatus  is the external urethral orifice.  In  the  male,  the  urethra  is  common  to  the urinary  and  reproductive  systems;  in  the  female,  it belongs only to the urinary system. Female Urethra The female urethra is about 4 cm long, extending from the bladder to the external orifice, (fig. 1-59, view A). Male Urethra The  male  urethra  is  about  20  cm  long  and  is divided  into  three  parts:  the  prostatic,  membranous, and penile portions. See view B of figure 1-59 for an illustration of the male urethra. PROSTATIC   URETHRA.—The   prostatic urethra is surrounded by the prostate gland; it contains the orifices of the prostatic and ejaculatory ducts. This portion of the male urethra is about 2.5 cm long. MEMBRANOUS   URETHRA.—The   mem- branous   urethra   is   about   2   cm   in   length   and   is surrounded by the external urethral sphincter. PENILE  URETHRA.—The  penile  urethra,  the longest  portion,  is  about  15  cm  long.  It  lies  in  the ventral  portion  of  the  penis.  The  urethra  terminates with the external orifice at the tip of the penis. MALE REPRODUCTIVE SYSTEM LEARNING OBJECTIVE:  Recall  the  parts of   the   male   reproductive   system   and   their function(s). The  organs  of  the  male  and  female  reproductive systems are concerned with the process of reproducing offspring,   and   each   organ   is   adapted   to   perform specialized tasks. The primary male sex organs of the reproductive system are the testes. The other structures of the male reproductive system are termed accessory reproductive  organs.  The  accessory  organs  include both  internal  and  external  reproductive  organs.  See figure 1-60 for an illustration of the male reproductive system. 1-57 HM3F0160 LARGE INTESTINE SEMINAL VESICLE EJACULATORY DUCT PROSTATE GLAND BULBOURETHRAL GLAND ANUS EPIDIDYMIS TESTIS SCROTUM URINARY BLADDER SYMPHYSIS PUBIS VAS DEFERENS URETHRA PENIS GLANS PENIS PREPUCE Figure 1-60.—The male reproductive system.