9.1 - Endocrine System

9.1 The Glands and Hormones of the Endocrine System

The Endocrine System

• Serves as the second messenger system of the body, with the nervous system being the first.

• Effects of the endocrine system are slower but have longer-lasting outcomes.

• Utilizes chemical messages (hormones) released directly into the bloodstream.

Functions of the Endocrine System

• Reproduction

• Growth and development

• Mobilization of body defenses

• Maintenance of homeostasis

• Regulation of metabolism

• Regulation of stress response

• Regulation of blood sugar

• Regulation of water balance

• Other functions as necessary.

Endocrinology

• Focuses on the medical study of endocrine system disorders.

• The endocrine system in vertebrates parallels the nervous system in maintaining homeostasis via hormone release from various glands.

• Hormones are defined as chemical messengers that travel through the bloodstream to effect changes in target cells or organs.

Location of Major Endocrine Organs

• Pineal gland

• Hypothalamus

• Pituitary gland

• Thyroid gland

• Parathyroid glands (posterior aspect of thyroid)

• Thymus gland

• Adrenal glands

• Pancreas

• Ovaries (female)

• Testes (male)

The Endocrine System: Gland Types

• Exocrine glands: Transport substances to target tissues via ducts; examples include tear, salivary, and mammary glands.

• Endocrine glands: Ductless; release hormones directly into the bloodstream. Only endocrine glands can produce hormones, and hormone concentration in blood is typically very low.

Hormone Action on Target Cells

• Target cells possess receptor proteins which specifically bind to hormones.

• Binding of a hormone triggers various reactions within the target cell, activating the cell's function as needed.

Steroid and Water-Soluble Hormones

Steroid Hormones

• Lipid-based and can diffuse easily through the cell membrane to bind with internal receptors.

• Examples include testosterone, estrogen, and cortisol.

Water-Soluble Hormones

• Often amino-acid based and bind to surface receptors on target cells, initiating a cascade of intracellular reactions.

• Examples: Epinephrine (triggers glycogen breakdown), Human Growth Hormone, Thyroxine, Insulin.

Hormone Mechanism Overview

Steroid Hormones

1. Diffuse through the plasma membrane (due to lipid solubility).

2. Bind to intracellular receptor in cytoplasm or nucleus.

3. Activate genes to produce specific mRNA, triggering protein synthesis.

Water-Soluble Hormones

1. Bind to receptor on cell membrane (first messenger).

2. Activates a secondary messenger like cAMP using ATP.

3. Triggers enzymatic and cellular changes, leading to physiological effects (e.g., glucose release).

Hormonal Stimuli of Endocrine Glands

• Endocrine glands can be activated by hormones.

• Tropic Hormones are those that target other endocrine glands, promoting hormonal release.

Humoral Stimuli of Endocrine Glands

• Example: Low calcium levels in blood stimulate the release of parathyroid hormone (PTH) by parathyroid glands.

Neural Stimuli of Endocrine Glands

• Nerve impulses can instigate hormone release, notably under sympathetic nervous system control.

Maintaining Homeostasis - Feedback Loops

1. Hormones released by the hypothalamus (hormone 1) can inhibit their own release through feedback mechanisms.

2. Anterior pituitary produces stimulating hormones (hormone 2) which also utilize feedback inhibition.

3. Target glands release their hormones (hormone 3), contributing to the systemic regulation of bodily functions.