Hormones are organic chemical messengers secreted by cells (often in endocrine glands) that are transported by the blood to target cells, where they coordinate various bodily functions. They are effective even in very low concentrations. Scientists have identified over 50 different hormones in the human body.
Key Functions Regulated by Hormones:
A hormone acts as a signal, transporting a message from one cell or gland to another.
Target Cells and Receptors:
A hormone can only affect cells that have specific receptors for it. These cells are called target cells. The interaction between a hormone and its receptor is highly specific, often compared to a lock and key model, similar to enzyme-substrate interactions. Receptors Are Transducers→
Mechanism:
SCIENCE TITBITS In 1902, Bayliss and Starling discovered 'secretin', an extract from the duodenum that stimulated pancreatic secretions. They coined the term 'hormone', from the Greek for 'to excite' or 'to set in motion', to describe this chemical messenger.
Types of Hormonal Communication:
Hormones are organic substances that can be classified into several chemical categories.
| Chemical Category | Description | Examples |
|---|---|---|
| Steroid Hormones | Composed of steroids (lipid-based molecules derived from cholesterol). | Cortisone, Aldosterone, Testosterone, Estrogen, Progesterone |
| Proteinaceous Hormones | Also called polypeptide hormones; made of long chains of amino acids. | Insulin, Somatotrophic hormone (growth hormone), Thyrotrophic hormone |
| Catecholamines | A class of amine hormones derived from the amino acid tyrosine. | Adrenaline (Epinephrine), Noradrenaline (Norepinephrine) |
| Amino Acid Derivatives | Modified from single amino acids. | Thyroxine (derived from tyrosine) |
| Peptide Hormones | Made of short chains of amino acids. | Oxytocin, Vasopressin, Calcitonin, Parathormone, MSH |
Hormone secretion is strictly regulated to maintain homeostasis. Most hormonal systems are controlled by negative feedback loops.
For more details on the endocrine glands that produce these hormones, refer to Endocrine System of Man→. The hypothalamus plays a crucial role in controlling pituitary hormone release. Additionally, hormone secretion is regulated through feedback mechanisms. Functions of Kidney→ discusses how hormones like ADH and Aldosterone involve the 'Counter Current Multiplier' mechanism.