Endocrine System Physiology Notes

1. Introduction

• Communication and Coordination Systems in the Body: The body relies on two major systems for communication and coordination:

  1. Nervous System: Uses a complex network of neurons, neurotransmitters, and synapses to integrate functions across various body parts. This rapid-response system is responsible for immediate changes, such as reflex actions and sensory responses.

  2. Endocrine System: Utilizes hormones secreted from specialized endocrine glands into the bloodstream, enabling long-term regulation and coordination of organ functions. Hormones can affect distant target cells and organs, providing a comprehensive regulatory framework for various physiological processes.

Functions of the Endocrine System

• Glands produce and secrete hormones involved in numerous critical biological functions, including but not limited to:

  • Metabolism: Regulating energy utilization and storage.

  • Growth: Influencing the growth patterns and processes in various body systems.

  • Sexual Development: Governing the development and function of sexual organs and characteristics.

  • Reproduction: Controlling reproductive processes and cycles.

  • Sensory Perception: Modulating functions related to the senses.

  • Blood Circulation: Affecting heart rate, blood pressure, and overall cardiovascular health.

  • Respiration: Influencing breathing rates and patterns.

  • Movement: Facilitating motor functions and muscular coordination.

2. Chemical Structure and Synthesis of Hormones

Types of Hormones

• Endocrine Hormones: Released by glands directly into the bloodstream (e.g., thyroxine T4, cortisol, testosterone). These hormones typically influence distant target cells and have widespread effects throughout the body.

• Neuroendocrine Hormones: Secreted by specialized neurons into the bloodstream (e.g., oxytocin, vasopressin); they also target distant cells but originate from neural tissues.

• Paracrines: These hormones affect neighboring target cells without entering the bloodstream (e.g., cortisol in the thymus, oxytocin in the testes/ovaries).

• Autocrines: Affect the same cells that produced them (e.g., insulin acting on pancreatic beta cells).

• Cytokines: A diverse group of signaling molecules that can function as autocrines, paracrines, or endocrine hormones, playing vital roles in various physiological responses.

Basic Structure

1. Proteins and Polypeptides: Include hormones from anterior and posterior pituitary glands, pancreas, and parathyroid glands. They are usually water-soluble and act on receptors located on cell membranes.

2. Steroids: Derived from cholesterol, these hormones are synthesized by the adrenal cortex, ovaries, and testes. They are lipid-soluble and typically act on intracellular receptors, influencing gene expression.

3. Amine Hormones: Derived from the amino acid tyrosine, including thyroxine from the thyroid gland and catecholamines (epinephrine and norepinephrine) from the adrenal medulla. They can act on both membrane-bound and intracellular receptors, depending on their structure.

3. Hormone Secretion, Transport, and Clearance from the Blood

Secretion Patterns

• Linear, Diurnal, and Episodic Secretion:

  • Set Point Regulation: Certain hormones like thyroxine T4 maintain stable levels within the blood over 24 hours, while others can vary significantly over weeks with fine-tuned adjustments.

  • Episodic Secretion: Some hormones fluctuate periodically, as seen with parathyroid hormone (PTH), which regulates calcium homeostasis.

  • Diurnal Variation: The secretion of some hormones, like growth hormone and cortisol, follows a circadian rhythm influenced by sleep and activity levels, ensuring optimal physiological functioning.

Key Hormones and Their Functions

Anterior Pituitary Hormones:

• ACTH (Adrenocorticotropic Hormone): Stimulates the secretion of adrenocortical hormones, crucial for stress response and metabolism.

• FSH (Follicle-Stimulating Hormone): Essential for sperm maturation in the testes and promotes follicle growth in the ovaries.

• GH (Growth Hormone): Plays a pivotal role in stimulating tissue growth and development, impacting overall body composition and metabolism.

• PRL (Prolactin): Promotes breast development and stimulates milk secretion in lactating women, also influencing reproductive health.

• TSH (Thyroid-Stimulating Hormone): Stimulates the secretion of thyroid hormones, which regulate metabolism, growth, and development.

Clearance of Hormones

• The concentrations of hormones in the bloodstream are influenced by various factors, including:

  • Rate of secretion from the endocrine glands.

  • Rate of clearance through metabolic processes such as tissue destruction and excretion

  • Liver excretion: Metabolism and breakdown of hormones for removal from circulation.

  • Kidney filtering: Eliminating water-soluble hormones through urine excretion.