C.A.B.M. | 2024
1
The Endocrine System
I. Overview of the Endocrine System
The endocrine system is the second major control
system of the body, using chemical messengers called
hormones to regulate various processes.
• Functions:
1. Reproduction.
2. Growth and development.
3. Mobilization of body defenses.
4. Maintenance of homeostasis.
5. Regulation of metabolism.
• Key Characteristics:
o Hormones are secreted by specialized
cells into extracellular fluids.
o Blood carries hormones to target
cells/organs.
o Endocrinology: Study of hormones and
endocrine organs.
II. Chemistry of Hormones
Hormones are classified based on their chemical
structure:
1. Amino Acid-Based Hormones:
o Includes proteins, peptides, and amines.
2. Steroid Hormones:
o Derived from cholesterol (e.g., sex
hormones, adrenal cortex hormones).
3. Prostaglandins:
o Made from lipids; act as local hormones.
III. Mechanism of Hormone Action
Hormones target specific cells with compatible
receptors and alter their activity.
1. General Effects of Hormones:
• Alter membrane permeability.
• Activate/inactivate enzymes.
• Stimulate/inhibit cell division.
• Promote/inhibit product secretion.
• Influence gene transcription.
2. Two Mechanisms of Hormone Action:
a) Direct Gene Activation (Steroid and Thyroid
Hormones):
1. Hormone diffuses through the plasma
membrane.
2. Enters the nucleus and binds to a specific
receptor.
3. Hormone-receptor complex binds DNA.
4. Activates specific genes for mRNA
transcription.
5. mRNA is translated to produce new proteins.
b) Second Messenger System (Protein and Peptide
Hormones):
1. Hormone (first messenger) binds to a membrane
receptor.
2. Activates an enzyme inside the cell.
3. Enzyme generates a second messenger (e.g.,
cyclic AMP).
4. Second messenger triggers cellular changes
(e.g., glycogen breakdown).
IV. Hormone Release Control
1. Negative Feedback Mechanism:
• Hormone release is regulated to maintain a
balance.
• When hormone levels are adequate, further
secretion stops.
