Endocrine-Disrupting Pesticides - Pesticides: Evaluation of Environmental Pollution - page 609

20.2 Endocrine Systems of Animals and Men
Glands are organs of an animal body that manufacture some liquid products that are
secreted from the cells (both internal and external secretions).
There are two types of glands:
1. Ducted glands : Ducted glands secrete their product through well-defined ducts, for
example, sweat glands. These are numerous small, tubular glands that are found
nearly everywhere in the skin.
2. Ductless glands : These glands are also called endocrine glands or internally secret-
ing glands. They secrete hormones directly into the bloodstream in response to
instructions from the brain. The blood then carries them around the body where
they control the internal chemical processes. Hence, they are linked with the ner-
vous system and also with the circulatory system.
The various endocrine glands are interrelated. Some even regulate the secretion of
hormones from other glands. An abundance of a particular hormone ensures that
the production of the hormone that stimulates its production is inhibited. Together
with the nervous system, the endocrine system controls the activities of all organs
and systems in the human body.
Secretions from endocrine glands are known as hormones. The ductless glands
are briefly described below (the list is not exhaustive):
• Thymus: The thymus is situated in front of the trachea, behind the top of the
breastbone. It secretes thymic hormones. In early childhood, it plays a role in
building up resistance (immunity) to diseases and physical development.
• Thyroid : This two-lobed gland in all vertebrates is located in front of and on
either side of the trachea. It secretes thyroxine, which contains 65% iodine.
The rate of production is regulated by the pituitary gland. Hormones of the
thyroid regulate metabolism. A deficiency of these hormones causes dwarf-
ness in children called cretinism or myxedema in adults. Other malfunctions
of the thyroid are goiter or Grave's disease, that is, enlargement of the thyroid
gland.
• Parathyroid: The four, small kidney-shaped glands lie in pairs near or within
the lateral lobes of the thyroid gland and secrete parathyroid hormones, also
called parathormone. The function of parathormone is to control the level of
calcium in blood.
• Gonads : The gonads are the reproductive glands, that is, ovaries in females
and testes in males. These glands secrete sex hormones and are related to the
reproductive system.
• Adrenal: These are small dissimilar glands, one above each kidney, consisting
of a cortex and medulla. The cortex secretes corticosteroids and sex hormones;
the medulla secretes adrenaline. Adrenaline causes acceleration of the breath,
contraction of small blood vessels, and dilation of the pupil and increases the
metabolic rate. It also causes heightened emotion and a sudden increase in
physical strength, as during fear or anger.
• Pancreas: The pancreas is a soft, irregular gland lying near the stomach, 15 cm
long. It secretes pancreatic juice into the duodenum and contains the Islets of