Human Physiology: The Endocrine System

Overview

• The endocrine system is a control system consisting of ductless glands that secrete hormones.
• Hormones act as messengers, carried by the bloodstream to various cells in the body.
• Essential for homeostasis, growth, metabolism, and reproduction.
• Two types of hormones: steroidal and non-steroidal (protein-based).
• Regulated mainly through negative feedback mechanisms.

Types of Glands

• Exocrine Glands: Release secretions through ducts (e.g., sweat, salivary glands).
• Endocrine Glands: Release hormones directly into the blood; include pituitary, thyroid, adrenal, pancreas, gonads.

Major Hormones and Functions

• Hormones serve as chemical signals affecting target cells/organs.
• Hypothalamus: Produces releasing/inhibiting hormones affecting the anterior pituitary.
• Pituitary Gland: Often called the master gland; divided into anterior and posterior lobes.
  • Anterior Pituitary: Secretes TSH, ACTH, FSH, LH, prolactin, GH, and others.
  • Posterior Pituitary: Releases ADH and oxytocin.
• Thyroid: Produces T3 and T4 (regulate metabolism), and calcitonin (lowers blood calcium).
• Parathyroid: Secretes PTH, raising blood calcium levels.
• Adrenal Glands: Located above kidneys; secrete glucocorticoids (cortisol), mineralocorticoids (aldosterone), and catecholamines (epinephrine, norepinephrine).
• Pancreas: Dual function organ; produces insulin and glucagon (regulate blood sugar).
• Gonads (Testes/Ovaries): Produce sex hormones like testosterone and estrogen.
• Pineal Gland: Secretes melatonin, regulating sleep patterns.

Hormone Classification

• Amino acid-derived: Modified amino acids.
• Polypeptides and Proteins: Chains of amino acids.
• Steroids: Lipids synthesized from cholesterol.
• Eicosanoids: Synthesized from fatty acids.
• Hormones operate through specific receptors on target cells, leading to physiological changes.

Feedback Mechanisms

• Negative Feedback: Most common; hormone levels are adjusted to maintain homeostasis.
• Positive Feedback: Less common; hormone levels increase to intensify a condition (e.g., during childbirth).

Important Glands in Detail

• Pituitary Gland: Controlled by the hypothalamus; influences many body functions through hormone secretion.
• Thyroid Gland: Requires iodine to produce hormones; regulates metabolism and calcium levels.
• Parathyroid Glands: Regulate calcium and phosphorus levels.
• Adrenal Glands: Help manage stress, metabolism, and blood pressure.
• Pancreas: Important in digestion and blood sugar regulation.

Key Concepts

• Antagonistic Hormones: Work together to maintain balance; e.g., insulin and glucagon.
• Thyroid Hormones: Require iodine; imbalances can lead to conditions like goiter or hypo/hyperthyroidism.
• Adrenal Hormones: Vital for stress response; include cortisol and adrenaline.
• Pancreatic Hormones: Regulate blood glucose; insulin lowers and glucagon raises glucose levels.

Study Questions

• Understand the role of major hormones and their impact on the body.
• Explore how feedback mechanisms regulate hormone levels.
• Recognize the symptoms and causes of thyroid and adrenal gland disorders.

References

• Wikibooks Human Physiology
• Endocrine system images and diagrams licensed under Public Domain.