# 6.1 Hormones **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. ### Definition and Function of Hormones **Key Functions Regulated by Hormones:** - *Metabolism:* The rate at which the body uses energy. - *Homeostasis:* Maintaining a constant internal balance (e.g., blood pressure, blood sugar, body temperature, water balance). - *Growth and Development:* Regulating the growth of tissues and the body as a whole. - *Sexual Function and Reproduction:* Controlling the development of sexual characteristics and the reproductive cycle. - *Sleep-Wake Cycle:* Influencing circadian rhythms. - *Mood:* Affecting emotional states. --- ### Hormone as a Chemical Messenger 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. **Mechanism:** 1. A hormone (the *key*) circulates in the bloodstream. 2. It binds to a specifically shaped receptor (the *lock*) on or inside a target cell. 3. This binding triggers a specific response or action within the target cell. > **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:** 1. **Endocrine Gland to Endocrine Gland:** One gland releases a hormone that stimulates another gland to release its own hormones. - *Example:* The pituitary gland secretes *Thyroid-Stimulating Hormone (TSH)*, which causes the thyroid gland to release its hormones (e.g., thyroxine). 2. **Endocrine Gland to Target Organ:** A gland releases a hormone that acts directly on a non-glandular organ or tissue. - *Example:* The pancreas secretes *insulin*, which acts on muscle and liver cells to facilitate glucose uptake and processing. --- ### Chemical Nature of Hormones 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 | --- ### Regulation and Feedback 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 . The hypothalamus plays a crucial role in controlling pituitary hormone release. Additionally, hormone secretion is regulated through feedback mechanisms. discusses how hormones like ADH and Aldosterone involve the 'Counter Current Multiplier' mechanism.