Endocrine System Basics

Endocrine System Overview

• The endocrine system works with the nervous system to coordinate and integrate body cell activities.
• It influences metabolic activities via hormones carried in the blood.
• Endocrine responses are slower but longer lasting than nervous system responses.
• Endocrinology is the study of hormones and endocrine organs.

Endocrine vs. Exocrine Glands

• Endocrine glands are ductless, releasing hormones directly into surrounding tissues or the bloodstream.
• Exocrine glands have ducts, releasing non-hormonal substances onto epithelial surfaces.

Hormone Types and Actions

• Hormones are long-distance chemical signals, traveling via blood or lymph.
• Autocrine hormones affect the same cell that secretes them.
• Paracrine hormones affect nearby cells.
• Endocrine hormones affect distant cells.

Hormone Classification

• Amino acid-based hormones are most common. These hormones cannot cross the plasma membrane.
• Steroid hormones (like estrogen and testosterone) are lipid-based and can cross the plasma membrane. These are synthesized from cholesterol.

Hormone Action

• Target cells with specific receptors are affected by hormones.
• Hormones alter target cell activity.
• The effects of hormones can be diverse, including changes in membrane permeability or potential, protein synthesis, activation or deactivation of enzymes, stimulation of secretory activity, or mitosis.

Mechanisms Controlling Hormone Release

• The release of hormones is controlled by negative feedback mechanisms to regulate blood hormone levels in a desirable range.
• Humoral stimuli: Changes in blood ions or nutrients trigger hormone release (example: low blood calcium stimulating parathyroid hormone release).
• Neural stimuli: Nerve impulses trigger hormone release (example: sympathetic nervous stimulation triggering adrenal medulla to release catecholamines).
• Hormonal stimuli: Hormones from one endocrine gland trigger the release of hormones from another gland (example: hypothalamic hormones trigger anterior pituitary hormone release).

Nervous System Modulation of Endocrine System

• The nervous system can modify endocrine gland stimulation.
• The nervous system can override normal endocrine controls under stressful conditions (e.g., increasing blood glucose in response to stress).

Target Cell Specificity

• Target cells must have specific receptors to respond to hormones.
• Blood hormone levels, number of receptors, and receptor-hormone affinity influence target cell activation.
• Up-regulation: Increases in the number of receptors in response to low hormone levels.
• Down-regulation: Reduces the number of receptors in response to high hormone levels.

Half-Life, Onset and Duration of Hormone Activity

• Hormone half-life, onset, and duration depend on whether they are water-soluble or lipid-soluble.
• Water-soluble hormones usually have a short half-life and act through second-messenger systems, usually affecting the cell membrane.
• Lipid-soluble hormones usually have a long half-life and act via direct gene activation, affecting the cell's nucleus.

Interaction of Hormones at Target Cells

• Multiple hormones may act on a target cell at the same time.
• Permissiveness: One hormone may need another hormone to exert its effects.
• Synergism: Multiple hormones work together to produce a greater effect.
• Antagonism: One hormone opposes the effect of another hormone.

Hormones and Regulation

• cAMP (cyclic AMP) is a common second messenger in hormone signaling.
• There are other second messenger systems.

Important Endocrine Organs

• Hypothalamus, Pituitary, Thyroid, Parathyroid, Adrenal, Pineal, Pancreas, Gonads (ovaries and testes).
• Other tissues/organs also produce hormones.

Test your knowledge on the endocrine system and its functions. This quiz covers key concepts such as hormone types, gland characteristics, signaling mechanisms, and regulatory systems. Ideal for students studying biology or health sciences.