Endocrine System Overview

The Endocrine System: An Overview

• Regulates physiological processes via hormone secretion.
• Composed of endocrine glands and specialized cells throughout the body.
• Hormones travel via bloodstream to target tissues.
• Chemical communication relies on principles governing hormone release and action.

Principles of Chemical Communication

• Autocrine: Hormones act on the same cells that secrete them.
• Paracrine: Hormones act on nearby cells.
• Neurotransmitter: Hormones released by neurons at synapses.
• Endocrine: Hormones released into the bloodstream for wide distribution.
• Hormone secretion: Specialized glands produce and release hormones into the bloodstream.
• Target cells: Possess specific receptors for hormone binding; location can be on the cell surface, in the cytoplasm, or nucleus.
• Specificity: Each hormone interacts with cells having appropriate receptors.
• Feedback loops: Often negative feedback; hormone levels trigger signals to halt production once a set point is reached.
• Hormone concentration: Different concentrations can have varied effects.
• Half-life: Duration of a hormone's action varies; short half-life hormones have rapid, transient effects, while longer half-life hormones produce sustained effects.
• Synergy/Antagonism: Hormones can work together (synergy) or oppose each other (antagonism) for balanced regulation.
• Nervous system regulation: Nervous system influences hormone secretion, especially in response to stress.
• Hormone clearance: Bloodstream removal via liver metabolism, kidney excretion, or target cell uptake.
• Hormone transport: Some require binding proteins for effective transport; others travel freely in the blood.

Hormone Types

• Lipid-soluble hormones: Degrade slowly, insoluble in water, small molecules.
• Water-soluble hormones: Dissolve in blood, often circulate freely, diffuse slower into tissues.

Control of Hormone Secretion

• Stimulation:
  • Humoral stimuli: Blood chemical changes trigger hormone release.
  • Neural stimuli: Neuron action potentials stimulate hormone release.
  • Hormonal stimuli: One hormone triggers the release of another.
• Inhibition:
  • Humoral stimuli: Hormone release is sensitive to a humoral stimulus; a companion hormone’s release is inhibited by the same stimulus.
  • Neural stimuli: Inhibitory neurotransmitters prevent hormone secretion.
  • Hormonal stimuli: Hormone release inhibits other hormones' release.

Regulation of Hormone Blood Levels

• Negative feedback: A hormone inhibits production of stimulatory hormones or its own production.

Endocrine Glands

• Major glands: Pituitary, thyroid, parathyroid, adrenal, pancreas, ovaries (female), testes (male).
• Function: Each produces specific hormones regulating various bodily functions.
• Homeostasis: Endocrine system maintains homeostasis (internal balance).

Hypothalamus and Pituitary Gland

• Hypothalamus: Controls pituitary gland ("master gland"). Produces releasing and inhibiting hormones.
• Pituitary gland: Releases hormones stimulating other glands; located at the base of the brain in the sella turcica; has anterior (adenohypophysis) and posterior (neurohypophysis) lobes.
• Infundibulum: Connects hypothalamus and pituitary gland.

Specific Endocrine Glands and Hormones

• Thyroid gland: Regulates metabolism, energy production, body temperature, growth and development; located in the neck below the larynx.
• Parathyroid glands: Four glands on the thyroid's posterior surface; produce parathyroid hormone (PTH), regulating calcium and phosphate levels.
• Adrenal glands: On top of kidneys; secrete adrenaline and cortisol; involved in stress response, blood pressure, and inflammation.
• Pancreas: Behind stomach; islets of Langerhans produce insulin (lowers blood sugar) and glucagon (raises blood sugar).
• Ovaries (female): Produce estrogen and progesterone; regulate menstrual cycle, pregnancy, and secondary sexual characteristics.
• Testes (male): Produce testosterone; regulate male sexual characteristics and reproductive functions. Specific hormones include testosterone and inhibin.
• Pineal gland: Secretes melatonin, regulating circadian rhythms and sleep-wake cycles; located deep in the brain attached to the roof of the 3rd ventricle.
• Thymus: Secretes thymosin, crucial for T cell development and immune function.

Effects of Aging on the Endocrine System

• Hormone production: Decreased production of growth hormones, sex hormones, and thyroid hormones.
• Thyroid function: Hypothyroidism (underactive thyroid) is common, with symptoms including fatigue, weight gain, and cold intolerance.
• Andropause (men): Gradual decline in sperm production and testosterone levels.
• Menopause (women): Significant decline in estrogen and progesterone, usually in the late 40s to early 50s, leading to lower chances of pregnancy.
• Insulin sensitivity: Decreased insulin sensitivity increases the risk of type 2 diabetes, influencing body composition and lifestyle changes.
• Cortisol regulation: Dysregulation can lead to stress-related conditions and metabolic changes.
• Bone density: Decreased bone density and increased risk of osteoporosis due to changes in parathyroid hormone and calcitonin regulation of Ca2+ levels.
• Cardiovascular changes: Hormonal changes (e.g., decreased estrogen) impact cardiovascular health, increasing the risk of heart disease.
• Cognitive function: Hormone changes can influence cognitive function and mood, potentially leading to cognitive decline and mood disorders.
• Body composition: Aging leads to decreased muscle mass and increased body fat.
• Pituitary function: Changes in pituitary function can affect overall hormone balance and regulation.

Explore the fascinating world of the endocrine system and its role in regulating physiological processes through hormone secretion. This quiz covers the principles of chemical communication, including various hormone actions and feedback mechanisms. Test your knowledge on how endocrine glands function and the specificity of hormone-target interactions.