essay 9

General Principles of Humoral Control of Physiological Functions

• Hormones are chemical signaling molecules that regulate:

  • Metabolism, growth, fluid balance, reproduction, stress response, etc.

• Hormones help maintain homeostasis by coordinating the function of distant organs.

• They are secreted into the bloodstream and act on target cells with specific receptors.

• The CNS influences endocrine function via:

  • Neurohormones from the hypothalamus

  • Autonomic nervous system effects on glands

Telecrine and Paracrine Functions

Telecrine function:

• Hormones secreted into bloodstream and act on distant target organs

• Most classical endocrine hormones (e.g., insulin, cortisol)

📍 Paracrine function:

• Hormones/chemicals act on neighboring or adjacent cells

• Example: Insulin secreted by β-cells inhibits glucagon release from nearby α-cells in the pancreas

Classification of Hormones

A. Amino Acid Derivatives (Water-soluble)

• Smallest hormones (up to 3 amino acids)

• Examples: Adrenaline, noradrenaline, thyroid hormones

B. Peptide and Protein Hormones (Water-soluble)

• Chains longer than 3 amino acids

• Examples: Insulin, glucagon, oxytocin, ADH

C. Lipid-Derived Hormones

i. Steroid hormones (from cholesterol)

• Examples: Cortisol, testosterone, oestrogen, progesterone

ii. Eicosanoids (from fatty acids)

• Examples: Prostaglandins, leukotrienes

4. Mechanism of Action of Hormones

A. Water-Soluble Hormones (cannot enter cells directly)

1. Bind to receptors on cell membrane

2. Activate G-proteins, which activate secondary messengers:

   • Adenylyl cyclase → cAMP

   • Guanylyl cyclase → cGMP

   • Phospholipase C → DAG

   • Calcium → calmodulin

3. These messengers activate enzyme pathways → physiological effects

B. Lipid-Soluble Hormones (can enter cells)

1. Diffuse through the cell membrane

2. Bind to carrier proteins in cytoplasm to form a hormone-protein complex

3. This complex enters the nucleus and binds to nuclear receptors

4. Activates DNA transcription → mRNA

5. Ribosomes produce new proteins → specific cell effects

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5. Control of Hormonal Secretion

🧠 A. Nervous system control:

• Hypothalamus produces neurohormones that regulate pituitary gland

• ANS can stimulate or inhibit secretory cells (e.g., adrenal medulla)

🔁 B. Negative feedback (most common):

1. Hormone is secreted → acts on target

2. Target's response inhibits further hormone release

   • Example: High cortisol levels inhibit ACTH release

➕ C. Positive feedback (rare but important):

• Hormone action stimulates more release

• Example: Oxytocin increases uterine contractions → more oxytocin released

⚖ 6. Physiological Effects of Hormones

• Protein metabolism: Catabolic (e.g., cortisol) and anabolic (e.g., GH, insulin)

• Lipid metabolism: Control of lipid breakdown and storage

• Blood glucose regulation (normal: 2.8–6.1 mmol/L):

  • ↓ by: Insulin (only hormone that lowers glucose)

  • ↑ by: Glucagon, GH, T3/T4, catecholamines, cortisol

• Calcium-phosphate metabolism:

  • Calcium: 2.12–2.62 mmol/L

  • Phosphate: 0.7–1.7 mmol/L

  • Regulated by: Parathyroid hormone, calcitonin, vitamin D3

• Water-electrolyte and acid-base balance: Via ADH, aldosterone, etc.