Human Physiology, 14th edition (2016)

Reproduction 719

Erection, Emission, and Ejaculation

Erection, accompanied by increases in the length and width of

the penis, is achieved as a result of blood flow into the “erectile

tissues” of the penis. These erectile tissues include two paired

Figure 20.19 The organs of the male reproductive system. The male organs are seen here in a sagittal view.

Ductus (vas)

deferens

Ductus (vas)

deferens

Ampulla of ductus

deferens

Urinary bladder

Symphysis pubis

Urethra

Penis

Glans penis

Prepuce

Seminal vesicle

Ejaculatory duct

Bulbourethral

gland

Testis

Anus

Epididymis

Scrotum

Prostate

CLINICAL APPLICATION

Benign prostatic hyperplasia ( BPH ) is a common disorder,

with incidence that increases from 40% to 50% for men in

their fifties to more than 80% for men in their eighties. The

growth of the prostate can constrict the urethra, increasing

the resistance to flow and slowing urination. Drug treatments

for BPH usually begin with a 1 -adrenergic receptor blockers,

which cause smooth muscle relaxation in the prostate and the

neck of the bladder, promoting urination. Because dihydrotes-

tosterone (DHT) supports the growth of the prostate, a reduc-

tion in DHT helps to reduce the size of the prostate. This is

achieved by 5 a -reductase inhibitors, which inhibit the enzyme

that converts testosterone to DHT. (Because DHT acts on

hair follicles to promote male-pattern baldness, 5 a -reductase

inhibitors are also used to treat androgenic alopecia, or male

hair loss). There are also surgical options available to treat

BPH if required, including transurethral resection of the pros-

tate ( TURP ), transurethral needle ablation ( TUNA ), and others.

the sperm are fully motile and become capable of fertilizing
an ovum once they spend some time in the female reproduc-
tive tract. Sperm obtained from the seminiferous tubules, by
contrast, cannot fertilize an ovum. The epididymis serves as a
site for sperm maturation and for the storage of sperm between
ejaculations.
The ductus deferens carries sperm from the epididymis
out of the scrotum into the pelvic cavity. The seminal vesicles
then add secretions that pass through their ducts; at this point,
the ductus deferens becomes an ejaculatory duct. The ejacu-
latory duct is short (about 2 cm), however, because it enters
the prostate and soon merges with the prostatic urethra. The
prostate adds its secretions through numerous pores in the
walls of the prostatic urethra, forming a fluid known as semen
( fig. 20.19 ).
The seminal vesicles and prostate are androgen-dependent
accessory sex organs—they will atrophy if androgen is with-
drawn by castration. The seminal vesicles secrete fluid containing
fructose, which serves as an energy source for the spermatozoa.
This fluid secretion accounts for about 60% of the volume of the
semen. The fluid contributed by the prostate contains citric acid,
calcium, and coagulation proteins. Clotting proteins cause the
semen to coagulate after ejaculation, but the hydrolytic action of
fibrinolysin later causes the coagulated semen to again assume a
more liquid form, thereby freeing the sperm.