outer layer (serous coat) of the bladder wall consists of
two types of tissue, parietal peritoneum and fibrous
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
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.
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.
The female urethra is about 4 cm long, extending
from the bladder to the external orifice, (fig. 1-59, view
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
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
Figure 1-60.The male reproductive system.