Page 48 - 86 human physiology part-2

The Endocrine System

the trachea. They are slightly inferior to the larynx.

The parathyroids are four masses of tissue, two embedded posteriorly in each lateral mass of the
thyroid gland.

One adrenal gland is located on top of each kidney. The cortex is the outer layer of the adrenal
gland. The medulla is the inner core.

The pancreas is along the lower curvature of the stomach, close to where it meets the first region
of the small intestine, the duodenum.

The gonads are found in the pelvic cavity.

Hormones and Types

A hormone is a type of chemical signal. They are a means of communication between cells.

The endocrine system produces hormones that are instrumental in maintaining homeostasis and
regulating reproduction and development. A hormone is a chemical messenger produced by a cell that
effects specific change in the cellular activity of other cells (target cells). Unlike exocrine glands
(which produce substances such as saliva, milk, stomach acid, and digestive enzymes), endocrine
glands do not secrete substances into ducts (tubes). Instead, endocrine glands secrete their hormones
directly into the surrounding extracellular space. The hormones then diffuse into nearby capillaries and
are transported throughout the body in the blood.

The endocrine and nervous systems often work toward the same goal. Both influence other cells
with chemicals (hormones and neurotransmitters). However, they attain their goals differently.
Neurotransmitters act immediately (within milliseconds) on adjacent muscle, gland, or other nervous
cells, and their effect is short-lived. In contrast, hormones take longer to produce their intended effect
(seconds to days), may affect any cell, nearby or distant, and produce effects that last as long as they
remain in the blood, which could be up to several hours.

In the following table there are the major hormones, their target and their function once in the
target cell.

Endocrine Hormone Released Chemical Target Major Function of
Gland Class Tissue/Organ Hormone
Hypothalamic releasing Anterior Regulate anterior pituitary
Hypothalamus Peptide
and inhibiting hormones pituitary hormone
Posterior Antidiuretic (ADH) Peptide Kidneys Stimulates water
Pituitary reabsorption by kidneys
Uterus, Stimulates uterine muscle
Posterior Oxytocin Peptide mammary contractions and release of
Pituitary
glands milk by mammary glands
Anterior Thyroid stimulating Glycoprotein Thyroid Stimulates thyroid
Pituitary (TSH)
Adrenocorticotropic Peptide Adrenal cortex Stimulates adrenal cortex

Wikibooks | 263