The Endocrine System Overview

The Endocrine System

• The endocrine system is a system of ductless glands that secrete hormones directly into the bloodstream.
• These hormones act as chemical messengers, controlling a wide range of physiological processes.
• Organs like the pancreas also secrete hormones in addition to other functions.

Components of the Endocrine System

• Endocrine glands: Produce and secrete hormones.
• Hormones: Chemical messengers that regulate bodily functions.

Endocrine Glands

• Ductless glands composed of endocrine cells.
• Do not have ducts that lead to the outside of the body.
• Secrete hormones directly into the bloodstream.

Hormones

• Organic chemical messengers produced by endocrine cells.
• Secreted into the bloodstream that circulate throughout the body.
• Act on distant target cells (i.e. remote communication)
• Activate specific responses in targeted cells, and not in all cells.
• Activation is dependent on specific receptor binding.
• There are three primary hormone types: peptide, amino acid derivatives, and steroid.
• Can function as paracrine or autocrine regulators (where the hormones do not travel in the blood).
• Their effects on the target cells depend on the programmed response of the target cells.

Hormone Receptors

• Found on the target cells (surface, cytoplasm, or nucleus).
• Activated only when specific hormones bind to them.
• No physiologic effect if the hormone can't bind to its receptor.

Hormone Classification

• Peptide hormones: Chains of amino acids, synthesized on ribosomes, stored in vesicles, and are water-soluble (hydrophilic).
• Amino acid derivatives: Modified amino acids, not stored in vesicles, and are mostly water-soluble (hydrophilic). Including tyrosine derivatives like catecholamines and thyroid hormones, and tryptophan derivatives like melatonin.
• Steroid hormones: Derived from cholesterol, not stored in vesicles, are lipid-soluble (lipophilic), and bind to intracellular receptors.

Actions of Hormones

• Regulate various physiological processes like fetal development, cell growth, metabolism, cardiovascular function, renal function, skeletal function, reproductive function, immune function, and central nervous system function.

Pheromones

• Hormones outside the body used to signal to others of the same species.
• Primarily found in animals, but some evidence suggests pheromones exist in humans.
• In animals, they mainly relate to alarm, food, and sex signals.

Homeostasis

• Maintains the internal environment of organisms in a steady and balanced state (e.g. body temperature, blood volume).

Fine Regulation Feedback

• Constant monitoring and adjustments are made in the body for maintaining homeostasis.
• Includes neural and endocrine controls that are intertwined.

Negative Feedback

• Turns off a signal in response to a change in the body to maintain homeostasis.

Positive Feedback

• Amplifies signals in a vicious cycle (e.g., child birth, blood clotting).
• Not homeostatic as it doesn't maintain a stable internal environment.

Neurohormones

• Neurotransmitters released into the bloodstream instead of the synaptic cleft (epinephrine, norepinephrine, dopamine).
• Hormones produced by neurons (oxytocin, vasopressin).
• Releasing hormones that stimulate glands instead of target organs (TRH, CRH).

Mechanisms of Hormone Release

• Humoral: In response to changing blood levels of ions and nutrients.
• Neural: Stimulation by nerves.
• Hormonal: Stimulation by other hormones.

Key Points

• The endocrine system comprises primary and secondary endocrine organs.
• Hormones act as chemical signals that enter the blood and act on distant target sites.
• Hypothalamus and pituitary gland work together as major regulators of the endocrine system.
• Normal pathway: Hypothalamus releases hormones that activate the pituitary, which then releases hormones to activate the end organ.
• Hormonal regulation is controlled by positive and negative feedback mechanisms.

The Pituitary Gland

• Pea-sized gland located in a bony hollow in the base of the skull.
• Known as the "master gland" due to its controlling role over other endocrine glands.
• Closely linked to the hypothalamus.

The Hypothalamus

• Main regulator of the endocrine system in close interplay with the pituitary gland.

Anterior Pituitary Gland (Adenohypophysis)

• A true endocrine gland composed of several cell types.
• Controlled by releasing and inhibiting hormones from the hypothalamus.
• Secretes various tropic hormones that regulate the function of other endocrine glands. (TSH, ACTH, GH, FSH, LH, PRL)

Posterior Pituitary Gland (Neurohypophysis)

• Composed of neural tissue and not a true endocrine gland.
• Storage site for and releases hormones synthesized in the hypothalamus. (ADH and oxytocin).

Pituitary Hormones

• List of hormones produced by anterior and posterior pituitary glands and their actions.

The Thyroid Gland

• Butterfly-shaped gland located in the anterior neck, on the trachea, just inferior to the larynx.
• Produces thyroid hormone (T3 and T4), and calcitonin.
• T3 and T4: Essential for metabolism and growth and development.
• Calcitonin: Regulates blood calcium levels.

The Parathyroid Glands

• Four small glands located on the posterior surface of the thyroid gland.
• Secrete parathyroid hormone (PTH), which regulates blood calcium levels.

The Adrenal Glands

• Two glands (cortex and medulla) situated atop the kidneys.

Adrenal Cortex

• Produces steroid hormones necessary for regulating metabolism, electrolyte balance, inflammation, and stress response. (corticosteroids, aldosterone)

Adrenal Medulla

• Produces catecholamine hormones (epinephrine and norepinephrine) and is part of the sympathetic nervous system.

Aldosterone

• Acts specifically on cells of the distal tubules and collecting ducts.
• Regulates salt and water balance by promoting sodium (Na+) reabsorption and potassium (K+) secretion.

Renin-Angiotensin-Aldosterone System (RAAS)

• Complex hormonal system that regulates blood pressure and electrolyte balance involving renin, angiotensin I and II and aldosterone.

Cortisol

• A glucocorticoid produced in the adrenal cortex.
• Plays a vital role in the body's response to stress.
• Affects metabolism, immune system, and blood glucose levels.

Cushing's Syndrome

• Caused by excess cortisol secretion.
• Symptoms include obesity, hypertension, and osteoporosis.

Addison's Disease

• Caused by insufficient cortisol and aldosterone secretion.
• Symptoms include low blood pressure, weakness, and weight loss.

Explore the vital components of the endocrine system, including its glands and hormones. This quiz covers how these chemical messengers regulate various physiological processes in the body. Test your knowledge about the functions and significance of hormone secretion.