Introduction to Endocrinology: Endocrine System Basics

Introduction to Endocrinology

• The endocrine system regulates the body's internal environment via endocrine glands, which produce and release hormones.
• Hormones are released into the bloodstream, transported to distant parts of the body, and bind to target cells to elicit effects.

Characteristics of Hormones

• Hormones are chemical messengers that have an effect on target cells distant from the hormone-secreting cells.
• Signal transduction pathways mediate the sensing and processing of external stimuli.
• Steps in signal transduction: a stimulus arrives at an endocrine gland, inducing a secretory cell to release a signaling molecule (hormone), which binds to a receptor on a target cell's plasma membrane, initiating a signaling cascade inside the cell.

Classification of Hormones

• Classified based on composition and where they interact with target cells.
• 2 main types: steroids (cholesterol-derived, lipid-soluble, easily pass through phospholipid plasma membrane of target cells) and non-steroids (protein-derived, water-soluble, interact with receptor sites on the cell surface).

Non-Steroid Hormones

• Proteins: made of long chains of amino acids (e.g., insulin and glucagon).
• Glycoproteins: attached to a carbohydrate group that attaches to an amino acid (e.g., thyroid-stimulating hormone).
• Peptides: short chains of amino acids (e.g., oxytocin).
• Amino acid derivatives: single amino acids (e.g., epinephrine, norepinephrine, melatonin).

Endocrine Glands

• Produce and excrete hormones, working together to achieve a response.
• Locations: brain, neck, lungs, pancreas, kidney, pelvic region.

Hypothalamus and Pituitary Gland

• Hypothalamus: "control center" linking the CNS and endocrine system.
• Senses impulses from the brain and passes them to the pituitary gland.
• Pituitary gland: "master gland" that controls other endocrine glands by activating their release of hormones.
• Divided into two sections: anterior (communicates with hypothalamus via hormone) and posterior (communicates with hypothalamus via neurons).

Thyroid and Parathyroid Glands

• Thyroid gland: produces thyroid hormones (T3 and T4) that regulate metabolism, development, and catecholamine release.
• Parathyroid glands: produce parathyroid hormone (PTH), which maintains calcium homeostasis and opposes calcitonin's activity.

Adrenal Glands

• Located on top of the kidneys.
• Adrenal cortex (shell): produces steroids, including cortisol, which increases blood glucose and suppresses the immune system, and aldosterone, which increases sodium reabsorption in the blood and maintains hydration.
• Adrenal medulla (core): produces neurotransmitters for the sympathetic nervous system, including norepinephrine and epinephrine.

Ovaries and Testes

• Produce sex hormones (estrogen and progesterone in ovaries, testosterone in testes) that regulate reproductive processes.
• FSH stimulates follicle and egg maturation in ovaries, and sperm production in testes.
• LH stimulates ovulation and progesterone production in ovaries, and testosterone production in testes.