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.