Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

BIOLOGICAL BASIS FOR R ADIATION SAFETY 303
thyroid hormones and may lead to high radiation doses (depending on the quantity
of radioiodine in the thyroid) to the thyroid gland. This, in turn, may lead to cancer
of the thyroid. A large increase in the thyroid cancer rate, predominantly among
youngsters, was observed following the release of massive quantities of radioiodine
during the nuclear reactor accident in Chernobyl in 1986.
Parathyroids. The parathyroids consist of two pea-sized glands located on the posterior
surface of each of the two lobes of the thyroid. They secrete the parathyroid
hormone, which controls plasma levels of calcium and phosphorus.
Adrenals. The adrenal glands sit on top of the kidneys. Each adrenal is made up of
two different endocrine tissues. An outer tissue, called the adrenal cortex, surrounds
an inner tissue, called the adrenal medulla. The medulla secretes a group of hormones
called catecholamines, the most prominent one being adrenaline (epinephrine).
The adrenal cortex secretes the steroidal hormones. One group of steroidal hormones,
called the glucocorticoids, contains mainly hydrocortisone and is involved in
metabolism of carbohydrates, proteins, fats, and electrolytes. Another major group
of adrenocortical hormones is the mineralocorticoids, which consist mainly of aldosterone.
Aldosterone regulates the retention of sodium and chloride ions and elimination
of potassium and hydrogen ions; thus, it is involved in control of blood
pressure and blood volume.
Islets of Langerhans. The islets of Langerhans are widely distributed within the
pancreas. They contain two hormone-producing cells: alpha cells, which produce
glucagon, and beta cells, which produce insulin. The function of a third islet cell,
called the delta cell, is presently not known. Insulin is essential to carbohydrate
metabolism. In the disease known as insulin-dependent diabetes mellitus, or Type 1
diabetes, the beta cells are destroyed through an autoimmune mechanism and insulin
must be injected if the patient is to remain alive. In Type 2, or non-insulindependent
diabetes mellitus, an insufficient amount of insulin is produced. This
disease is treated either with a medicine that stimulates insulin production or by
decreasing the quantity of insulin needed through dietary restriction. Glucagon is
secreted from the alpha cells and is then carried by the blood to the liver, where it
stimulates the liver to produce glucose. The action of glucagon is opposite to that
of insulin. As a result, the coordinated release of insulin and glucagon provides a
sensitive means for the control of blood glucose.
Gonads. The gonads—the ovaries in the female and the testes in the male—produce
the sex hormones. The female sex hormones include the estrogens and progesterone.
The androgenic or male sex hormones consist mainly of testosterone. The
sex hormones determine the characteristics that distinguish males from females and
regulate sexual activity and reproduction.
The Reproductive System
In the male, the reproductive system includes the penis, testicles, ducts through
which semen passes into the urethra and from whence it is ejaculated during orgasm,
the prostate gland, and Cowper’s gland, which produce the seminal fluids.
The testicles produce the male sex hormone, testosterone, as well as the sperms that