Which glandular accessory organ produces the largest
proportion of semen?
What is semen?
For answers, see Appendix H.
Physiology of the Male
Male Sexual Response
Describe the phases of the male sexual response.
Although there is more to it, the chief phases of the male sexual
response are (1)
of the penis, which allows it to pene-
trate the female vagina, and (2)
, which expels semen
into the vagina.
, enlargement and stiﬀening of the penis, results from
engorgement of the erectile bodies with blood. When a man
is not sexually aroused, arterioles supplying the erectile tissue
are constricted and the penis is ﬂaccid. However, sexual excite-
ment triggers a parasympathetic reﬂex that promotes release
of nitric oxide (NO) locally. NO relaxes smooth muscle in the
penile blood vessel walls, dilating these arterioles, and the erec-
tile bodies ﬁll with blood. Expansion of the corpora cavernosa
of the penis compresses their drainage veins, retarding blood
outﬂow and maintaining engorgement. Te corpus spongiosum
expands but not nearly as much as the cavernosa. Its main job is
to keep the urethra open during ejaculation. It is important that
the erect penis not kink or buckle during intercourse. Te longi-
tudinal and circular arrangement of collagen ﬁbers surrounding
the penis prevents this problem.
Erection of the penis is one of the rare examples of parasym-
pathetic control of arterioles. Another parasympathetic eﬀect is
stimulation of the bulbo-urethral glands, the secretion of which
lubricates the glans penis.
A variety of sexual stimuli can initiate erection—touch, me-
chanical stimulation of the penis, and erotic sights, sounds,
and smells. Te CNS responds by activating parasympathetic
neurons that innervate the internal pudendal arteries serving
the penis. Sometimes erection is induced solely by emotional
or higher mental activity (the thought of a sexual encounter).
Emotions and thoughts can also inhibit erection, causing vaso-
constriction and a return to the ﬂaccid penile state.
to shoot forth) is the propulsion of semen
from the male duct system. Although erection is under para-
sympathetic control, ejaculation is under sympathetic control.
When impulses provoking erection reach a critical level, a spi-
nal reﬂex is initiated, and a massive discharge of nerve impulses
occurs over the sympathetic nerves serving the genital organs
(largely at the level of L
). As a result,
Te bladder sphincter muscle constricts, preventing ex-
pulsion of urine or reﬂux of semen into the bladder.
such cases, regular tests to monitor the tumor for any changes
may be all that is necessary.
The Bulbo-Urethral Glands
thral) are pea-sized
glands located inferior to the prostate in the urogenital dia-
phragm (Figures 27.1 and 27.5). Tey produce a thick, clear
mucus, some of which drains into the spongy urethra and lu-
bricates the glans penis when a man becomes sexually excited.
Additionally, the mucus neutralizes traces of acidic urine and
lubricates the urethra just prior to ejaculation.
men) is a milky white, somewhat sticky mixture of
sperm, testicular ﬂuid, and accessory gland secretions. Te liq-
uid provides a transport medium and nutrients and contains
chemicals that protect and activate the sperm and facilitate their
movement. Mature sperm cells are streamlined cellular “mis-
siles” containing little cytoplasm or stored nutrients. Catabo-
lism of the fructose in seminal gland secretions provides nearly
all the fuel needed for sperm A±P synthesis.
Semen contains several substances that play many roles:
Prostaglandins decrease the viscosity of mucus guarding the
entry (cervix) of the uterus and stimulate reverse peristalsis
in the uterus, facilitating sperm movement through the fe-
male reproductive tract.
Te hormone relaxin and certain enzymes in semen enhance
Contained A±P provides energy.
Certain ingredients suppress the immune response in the fe-
male’s reproductive tract.
Antibiotic chemicals destroy some bacteria.
Clotting factors in semen coagulate it just a²er it is ejacu-
lated. Coagulation causes the sperm to stick to the walls of
the vagina and prevents the initially immobile sperm from
draining out of the vagina. Soon a²er semen coagulates, ﬁ-
brinolysin liqueﬁes the sticky mass and the sperm swim out
and begin their journey through the female duct system.
Semen, as a whole, is alkaline (pH 7.2–8.0), which helps
neutralize the acid environment of the male’s urethra and the
female’s vagina, protecting the delicate sperm and enhancing
their motility. Sperm are sluggish under acidic conditions (be-
low pH 6).
Te amount of semen propelled out of the male duct system
during ejaculation is relatively small, only 2–5 ml and only 10%
sperm, but there are between 20 and 150 million sperm per
Check Your Understanding
Adolph, a 68-year-old gentleman, has trouble urinating and
is scheduled for a rectal exam. What is his most probable
condition and what is the purpose of the rectal exam?