into cells. When an adequate level of insulin is not present in the bloodstream,
excessive quantities of sugar remain in the blood, and are detected in urine. Diabetes
mellitus is the name of the disease that results from failure of the pancreas to secrete
adequate quantities of insulin. The alpha cells found within the islets of Langerhans
secrete glucagon. Glucagon effects a rapid breakdown of glycogen to glucose in liver
cells. Blood glucose levels rise after glucagon is secreted by alpha cells. Insulin and
glucagon are, therefore, antagonists. Insulin effects an active cellular absorption of
glucose from extracellular fluids while glucagon increases blood glucose levels.
a. Ovaries (Female). The ovaries, located in the pelvic cavity, secrete
estrogens and progesterone. Estrogens are also secreted by the adrenal cortex and
during pregnancy in very large amounts by the placenta.
(1) The main function of estrogens is to promote cellular proliferation and
growth of the sexual organs and other reproductive tissues. Estrogens cause the
endometrium (lining of the uterus) to thicken, and they play an essential role in the
regulation of the menstrual cycle. Estrogens produce the secondary sex characteristics
of females: enlargement of the uterus and vagina, growth of pubic hair, development of
mammary glands, development of the pelvic girdle, and deposition of fat in the mons
pubis and labia majora.
(2) Progesterone is secreted by the corpus luteum and during pregnancy by
the placenta. (The corpus luteum is a glandular mass formed in an ovary when a follicle
discharges its ovum. If the ovum is impregnated, the corpus luteum increases in size
and lasts for several months. Without pregnancy, the corpus luteum actively secretes
for only about 14 days.) The function of progesterone is primarily to provide adequate
nutrients for an embryo if it should begin to develop. During pregnancy, it maintains the
placenta, prevents further ovulation, enlarges the breasts, and stimulates the
development of the milksecreting cells in the mammary glands.
b. Testes (Male). The testes, located in the scrotum, secrete testosterone. The
testes of a fetus secrete testosterone to stimulate the development of the male
anatomy. After birth the testes become dormant and remain so until puberty, when
testosterone is secreted once again and stimulates the development of secondary male
sex characteristics, including enlargement of the male sex organs, growth of facial,
pubic, and chest hair, growth of the larynx to deepen the voice, and deposition of
protein to increase muscularity and general body size.
248. THE PITUITARY GLAND (HYPOPHYSIS)
The pituitary gland lies at the base of the brain and is attached to it by a short
stem. The pituitary gland is made up of an anterior and a posterior lobe.