The Endocrine System: Hormones, Glands, and Mechanisms of Action

The Endocrine System

Introduction to the Endocrine System

The endocrine system is one of the two major control and coordinating systems of the body, alongside the nervous system. It regulates a wide range of physiological processes through the secretion of hormones, which are chemical messengers transported via the bloodstream to target organs and tissues.

• Hormones are chemical substances secreted by endocrine glands.

• They are transported via blood to distant target cells.

• Hormones regulate the metabolic functions of other cells and typically act more slowly than nervous impulses.

• Based on chemical composition, hormones are classified as amino acid-based hormones or steroids.

Hormone Classification

Chemical Structure of Hormones

• Amino acid-based hormones: Include derivatives of amino acids, peptides, and proteins. Most hormones fall into this category.

• Steroid hormones: Synthesized from cholesterol. Includes gonadal (sex hormones) and adrenocortical hormones.

Target Cell Specificity

How Hormones Find Their Targets

Although hormones circulate throughout the body, they only affect specific cells known as target cells. These cells possess specific receptors for the hormone, which may be located on the plasma membrane or within the cell (intracellular).

• Target cells must have specific receptors for a hormone to respond.

• Receptors can be intracellular (for lipid-soluble hormones) or on the plasma membrane (for most amino acid-based hormones).

Major Endocrine Glands and Organs

Overview of Endocrine Glands

• Pituitary gland

• Thyroid gland

• Parathyroid glands

• Adrenal glands

• Pineal gland

• Thymus

• Pancreas and gonads (ovaries and testes) have both endocrine and exocrine functions.

Other hormone-producing tissues include adipose tissue, the walls of the small intestine, stomach, kidneys, and heart.

• Kidney: Produces erythropoietin, which stimulates red blood cell synthesis in the bone marrow.

Locations of Major Endocrine Organs

The major endocrine glands are distributed throughout the body. (See diagram for anatomical locations.)

• Pineal gland: Brain

• Hypothalamus: Brain

• Pituitary gland: Base of brain

• Thyroid gland: Neck

• Parathyroid glands: Dorsal aspect of thyroid gland

• Thymus: Upper chest

• Adrenal glands: On top of kidneys

• Pancreas: Curvature of the small intestine, below the stomach

• Gonads: Ovaries (female), Testes (male)

Mechanisms of Hormone Action

How Hormones Regulate Cellular Processes

Hormones influence target cells by altering their activity. The main effects include:

• Altering plasma membrane permeability

• Stimulating protein synthesis and cell division

• Activating enzymes

• Inducing secretory activities of cells

Hormone Action: Chemical Nature and Receptor Location

Mechanisms Based on Hormone Type

• Amino acid-based hormones (except thyroid hormone):

  • Act on plasma membrane receptors

  • Use G protein and second messenger systems (e.g., cyclic AMP, calcium)

  • Cannot enter the cell

• Lipid-soluble hormones (steroid and thyroid hormones):

  • Act on intracellular receptors that directly activate genes

  • Can enter the cell

Second Messenger Systems

• Most amino acid-based hormones exert their effects through second messenger systems.

• Main second messengers: cyclic AMP (cAMP) and IP3-calcium.

Control of Hormone Release

Regulation of Hormone Levels

Hormone levels in the blood are tightly regulated, primarily by negative feedback mechanisms. Hormones are synthesized and released in response to three types of stimuli:

• Humoral stimuli: Changes in blood levels of ions or nutrients (e.g., blood calcium levels regulate calcitonin and parathyroid hormone secretion).

• Neural stimuli: Nerve fibers stimulate hormone release (e.g., sympathetic nervous system stimulates adrenal medulla to secrete catecholamines during stress).

• Hormonal stimuli: Hormones stimulate other endocrine glands to release their hormones (e.g., hypothalamic hormones stimulate the anterior pituitary, which in turn stimulates other glands).

Pancreas: Structure and Function

Dual Role of the Pancreas

• The pancreas contains both exocrine (acinar cells producing digestive enzymes) and endocrine (islets of Langerhans) components.

• Islets of Langerhans contain:

  • Alpha (α) cells: Produce glucagon

  • Beta (β) cells: Produce insulin

Glucagon

• Raises blood glucose levels (hyperglycemic effect).

• Secreted in response to low blood glucose (hypoglycemia).

• Major target: Liver

  • Stimulates breakdown of glycogen to glucose (glycogenolysis).

  • Stimulates synthesis of glucose from non-carbohydrate sources (gluconeogenesis).

Insulin

• Lowers blood glucose levels (hypoglycemic effect).

• Promotes glucose uptake by cells.

• Stimulates conversion of glucose to glycogen (glycogenesis) for storage in the liver.

• Facilitates breakdown of glucose for ATP production.

Regulation of Blood Glucose Levels

Blood glucose is maintained within a narrow range by the opposing actions of insulin and glucagon.

• Glucagon: Increases blood glucose (hyperglycemic effect).

• Insulin: Decreases blood glucose (hypoglycemic effect).

Diabetes Mellitus (DM)

Overview and Symptoms

• Results from hyposecretion or hypoactivity of insulin.

• Three cardinal signs:

  • Polyuria: Excessive urination

  • Polydipsia: Excessive thirst

  • Polyphagia: Excessive hunger

• Hyperinsulinism: Excessive insulin secretion, resulting in hypoglycemia (low blood sugar). Symptoms include anxiety, sweating, and possible loss of consciousness.

Types of Diabetes Mellitus

• Type I DM (Insulin Dependent Diabetes Mellitus, IDDM): Usually juvenile onset; requires insulin therapy.

• Type II DM (Insulin Independent Diabetes Mellitus, NIDDM): Usually adult onset; often managed with lifestyle changes and oral medications.

Summary Table: Comparison of Hormone Types

Hormone Type	Chemical Nature	Receptor Location	Mechanism of Action
Amino acid-based	Peptides, proteins, derivatives	Plasma membrane	Second messenger systems (e.g., cAMP)
Steroid	Cholesterol-derived	Intracellular	Direct gene activation

Key Equations

• Glycogenolysis: $\text{Glycogen} \xrightarrow{\text{Glucagon}} \text{Glucose}$

• Gluconeogenesis: $\text{Amino acids/fats} \xrightarrow{\text{Glucagon}} \text{Glucose}$

• Glycogenesis: $\text{Glucose} \xrightarrow{\text{Insulin}} \text{Glycogen}$