Hormones don't carry energy or nutrients but convey information that controls cell functioning throughout the body.
These molecules influence development from the womb through childhood to adulthood and will impact bodily functions for life.
Hormones are released into the bloodstream in response to nervous system demands or messages from other molecules.
Definition of Hormones
Hormones: signaling molecules produced by glands that regulate physiological processes in the body.
Production and Types of Hormones
Hormones are primarily produced and secreted by specialized organs known as glands.
Some major organs, like the kidneys, also produce hormones.
Hormones can be classified into four chemical classes:
Modified amino acids
Peptides and proteins
Steroids
Prostaglandins
Types of Hormones by Chemical Class
Modified Amino Acids: Few in number, hormones like epinephrine (adrenaline), norepinephrine (noradrenaline), and thyroxine.
Peptide Hormones: Long chains of amino acids, e.g., insulin and antidiuretic hormone (ADH).
Steroid Hormones: Derived from cholesterol, e.g., estrogen and progesterone from ovaries, testosterone from testes, and aldosterone from adrenal cortex.
Prostaglandins: Composed of fatty acids, with biological effects including lowering blood pressure and muscle contraction. They act locally over short distances and are often termed 'local hormones.'
Hormone Action and Target Cells
Hormones act on target cells with specific receptors; cells without these receptors are unaffected.
Hormones can result in different effects in different cell types (e.g., epinephrine triggers glucose production in liver cells and insulin release in pancreatic cells).
Receptor Locations:
Surface receptors or internal receptors (often in the nucleus).
Peptide Hormones: Cannot cross cell membranes, require binding to cell surface protein receptors leading to rapid short-term responses.
Steroid and Thyroid Hormones: Lipid soluble, can cross membranes, initiating long-term responses by altering gene activity.
Secondary Messenger Systems: Involves changing receptor shape, leading to biochemical reactions within the cell.
Cyclic AMP (cAMP): A common second messenger initiated through ATP conversion triggered by the receptor's interaction with the hormone, amplifying the signal.
Calmodulin: Acts as a secondary messenger in smooth muscle contraction by facilitating calcium's role.
Hormonal Effects Example: Depending on the cell type, responses can include opening channels, synthesizing substances, or activating enzymes.
Regulation of Hormonal Release
Hormonal release is mainly regulated through negative feedback mechanisms to maintain homeostasis (stable internal conditions).
Example: Blood glucose levels after a meal lead to insulin release, which helps return glucose levels to normal.
Positive feedback mechanisms can also occur, exemplified by oxytocin during childbirth, enhancing contractions.
Glands and Organ Systems Involved in Endocrinology
Types of Glands
Exocrine Glands: Release secretions outside the body or into the digestive tract via ducts (e.g., sweat glands, salivary glands).
Endocrine Glands: Release hormones directly into the bloodstream (e.g., pancreas, pituitary gland).
Major Endocrine Glands/Organs
Hypothalamus: Controls hormonal release from anterior and posterior pituitary, synthesizing hormones like ADH and oxytocin.
Pituitary Gland: Consists of two lobes – anterior (produces several hormones) and posterior (stores/releases hormones produced by the hypothalamus).
Thyroid and Parathyroid Glands: Produce thyroxine, triodothyronine, and calcitonin, regulating metabolism and calcium levels.
Pancreas: Contains islets of Langerhans, producing insulin and glucagon to regulate blood glucose levels.
Hormones work antagonistically to maintain glucose homeostasis; insulin lowers glucose while glucagon increases it.
Adrenal Glands: Produce stress-response hormones like epinephrine, norepinephrine, glucocorticoids, and aldosterone impacting various physiological processes.
Gonads (Testes/Ovaries): Secrete androgens, estrogens, and progestins, influencing reproductive functions and secondary sexual characteristics.
Other Important Organs:
Kidneys: Produce erythropoietin and renin, impacting blood cell production and pressure regulation.
Heart: Releases atrial natriuretic peptide to manage blood volume and pressure.
Glucagon aids glucose release when levels drop (fasting or excessive exercise).
Diabetes Mellitus: Insufficient insulin leads to uncontrolled blood glucose, frequent urination, and high reliance on fat for energy.
The Gonads
Testes secrete testosterone promoting male characteristics; absence leads to female development.
Ovaries produce estrogens and progesterone crucial for female reproductive system and menstrual cycles.
The Adrenal Gland
Adrenal Medulla: Produces epinephrine and norepinephrine for fight-or-flight response.
Adrenal Cortex: Secretes glucocorticoids for stress response and aldosterone for sodium/water regulation.
The Pineal Gland
Produces melatonin, implicated in regulating circadian rhythms and potentially involved in seasonal affective disorder (SAAD).
Conclusion
The endocrine system is vital for maintaining homeostasis via hormone signaling. Advances in endocrinology continue to unveil discoveries about hormone functions and interactions, emphasizing the complexity of this regulatory system.