Encyclopedia of Health and Medicine

placenta 319
tissue and surgery to remove the plaque. A key
risk of either procedure is ERECTILE DYSFUNCTION
(inability to obtain erections). Generally these
treatments are most appropriate when the contracture
completely prevents sexual intercourse.
See also CHORDEE; PARAPHIMOSIS; PHIMOSIS; PRI-
APISM.
phimosis A condition in which the foreskin
becomes fused to the glans of an uncircumcised
PENIS and will not retract. Phimosis occasionally
occurs as a congenital condition (present at birth)
though more often develops later in life, typically
as a consequence of poor PERSONAL HYGIENE. Phimosis
is the leading cause of BALANITIS, a fungal
INFECTION of the inner surface of the foreskin, and
can interfere with URINATION and cause PAIN with
ERECTION. Recurrent phimosis increases a man’s
risk for CANCER OF THE PENIS.
The doctor is sometimes able to gently free the
adhered foreskin after anesthetizing the penis.
Frequent retraction of the foreskin and diligent
cleansing are necessary to prevent phimosis from
recurring. When this is not effective or phimosis
becomes chronic, the recommended treatment is
CIRCUMCISION, an OPERATION to surgically remove
the foreskin. Though in some men an unusually
tight foreskin (congenital phimosis) is the primary
cause of phimosis, diligent personal hygiene can
prevent most phimosis. It is important for uncircumcised
boys and men to clean beneath the foreskin
every day by retracting the foreskin, washing
the glans gently but thoroughly to remove any
accumulated secretions, and allowing the foreskin
to return to its natural position.
See also CHORDEE; CONGENITAL DISORDERS; PARAPHI-
MOSIS; PEYRONIE’S DISEASE.
placenta An organ of PREGNANCY that nourishes
and sustains the FETUS. The placenta also secretes a
number of hormones that maintain the biochemical
environment within the woman’s body to support
the pregnancy. The placenta develops within
the first two weeks after the blastocyst implants
into the endometrium of the UTERUS, arising from
the outer layer of the blastocyst’s cells, the trophoblast.
The amniotic sac, which encloses the
developing fetus, and the UMBILICAL CORD also arise
from the trophoblast.
activin
CHORIONIC GONADOTROPIN
CORTICOTROPIN-RELEASING
HORMONE (CRH)
GONADOTROPIN-RELEASING
HORMONE (GNRH)
INHIBIN
PROGESTERONE
RELAXIN
PLACENTAL HORMONES
chorionic adrenocorticotropin
chorionic somatomammotropin
CORTISOL
ESTROGENS
GROWTH HORMONE–RELEASING
HORMONE (GHRH)
placental actinogen
PROLACTIN
THYROTROPIN-RELEASING HORMONE
(TRH)
The placenta uniquely belongs to both the
mother and the fetus. Though the maternal BLOOD
circulation delivers NUTRIENTS and oxygen to the
fetal blood circulation and carries away fetal
wastes, the two circulations do not normally mix
with each other. The side of the placenta that faces
the fetus is the chorion. Fringelike extensions
called the chorionic villi permeate the tissue of the
maternal portion of the placenta. Fetal blood circulates
through the chorionic villi. Arterioles (tiny
arteries) and venules (tiny veins) extend from the
myometrium (muscular wall of the uterus) into
the spaces between the chorionic villi. The arterioles
carry maternal blood into the spaces where it
circulates around the chorionic villi. Nutrients,
oxygen, and wastes pass across the thin membranes
that enclose the chorionic villi.
Problems that can arise with the placenta during
pregnancy include
• placenta abruptio (also called placental abruption),
in which the placenta partially or completely
separates from the uterus; partial
separation reduces nutrition to the fetus and
complete separation is fatal to the fetus
• placenta accreta, in which the tissues that
anchor the placenta to the wall of the uterus
penetrate the myometrium too deeply, making
it difficult for the placenta to separate after
birth
• placenta previa, in which the placenta grows
partially or completely across the CERVIX, necessitating
CESAREAN SECTION to prevent hemorrhage
during labor
After the fetus is born a second round of contractions
separate the placenta from the uterine