Page 50 - 86 human physiology part-2

The Endocrine System

1. Amino acid-derived: Hormones that are modified amino acids.
2. Polypeptide and proteins: Hormones that are chains of amino acids of less than or more
than about 100 amino acids, respectively. Some protein hormones are actually glycoproteins,
containing glucose or other carbohydrate groups.
3. Steroids: Hormones that are lipids that are synthesized from cholesterol. Steroids are
characterized by four interlocking carbohydrate rings.
4. Eicosanoids: Are lipids that are synthesized from the fatty acid chains of phospholipids
found in plasma membrane.

Hormones circulating in the blood diffuse into the interstitial fluids surrounding the cell. Cells with
specific receptors for a hormone respond with an action that is appropriate for the cell. Because of the
specificity of hormone and target cell, the effects produced by a single hormone may vary among
different kinds of target cells.

Hormones activate target cells by one of two methods, depending upon the chemical nature of the
hormone.

• Lipid-soluble hormones (steroid hormones and hormones of the thyroid gland) diffuse
through the cell membranes of target cells. The lipid-soluble hormone then binds to a receptor
protein that, in turn, activates a DNA segment that turns on specific genes. The proteins
produced as result of the transcription of the genes and subsequent translation of mRNA act as
enzymes that regulate specific physiological cell activity.

• Water-soluble hormones (polypeptide, protein, and most amino acid hormones) bind to a
receptor protein on the plasma membrane of the cell. The receptor protein, in turn,stimulates the
production of one of the following second messengers:

Cyclic AMP (cAMP) is produced when the receptor protein activates another membrane-bound
protein called a G protein. The G protein activates adenylate cyclase, the enzyme that catalyzes the
production of cAMP from ATP. Cyclic AMP then triggers an enzyme that generates specific cellular
changes.

Inositol triphosphate (IP3) is produced from membrane phospholipids. IP3, in turn, triggers the
release of CA2+ from the endoplasmic reticulum, which then activates enzymes that generate cellular
changes.

Endocrine glands release hormones in response to one or more of the following stimuli:

1. Hormones from other endocrine glands.
2. Chemical characteristics of the blood (other than hormones).
3. Neural stimulation.

Most hormone production is managed by a negative feedback system. The nervous system and
certain endocrine tissues monitor various internal conditions of the body. If action is required to
maintain homeostasis, hormones are released, either directly by an endocrine gland or indirectly
through the action of the hypothalamus of the brain, which stimulates other endocrine glands to release
hormones. The hormones activate target cells, which initiate physiological changes that adjust the body
conditions. When normal conditions have been recovered, the corrective action - the production of
hormones - is discontinued. Thus, in negative feedback, when the original (abnormal) condition has

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