Hormones and the Endocrine System: Regulation and Homeostasis

Chapter 26: Hormones and the Endocrine System

The Nature of Chemical Regulation

Chemical signals, including hormones, coordinate body functions by transmitting information between cells and organs.

• Hormones are chemical signals carried by the blood that cause specific changes in target cells.

• Hormones can be secreted by endocrine glands or neurosecretory cells.

• Local regulators, such as neurotransmitters and prostaglandins, produce changes in nearby cells.

• Pheromones are chemical signals released into the environment to affect other individuals of the same species.

Endocrine Glands and Hormone Delivery

The endocrine system consists of glands and cells that secrete hormones directly into the bloodstream.

• Endocrine glands include the pituitary, thyroid, adrenal, pancreas, and gonads.

• Neurosecretory cells are specialized nerve cells that release hormones into the blood.

• All hormone-secreting cells together form the endocrine system, which works with the nervous system to regulate body activities.

Mechanisms of Hormone Action

Hormones affect target cells by binding to specific receptors, triggering signal transduction pathways and cellular responses.

• Most hormones derived from amino acids bind to receptor proteins on the target cell's plasma membrane.

• These hormones initiate signal transduction pathways that result in changes inside the target cell.

• Steroid hormones (e.g., estrogen, progesterone, vitamin D) bind to intracellular receptors, often acting as transcription factors to regulate gene expression.

General Steps in Hormone Action:

1. Hormone binds to receptor (membrane or intracellular).

2. Signal transduction pathway is activated.

3. Cellular response occurs (e.g., gene expression, enzyme activation).

Overview of the Vertebrate Endocrine System

The vertebrate endocrine system consists of more than a dozen glands, secreting over 50 hormones.

Gland	Main Hormones	Functions
Pituitary	Growth hormone, ACTH, TSH, etc.	Regulates other glands, growth, metabolism
Thyroid	T3, T4, calcitonin	Metabolism, calcium regulation
Adrenal	Epinephrine, cortisol, aldosterone	Stress response, metabolism, salt balance
Pancreas	Insulin, glucagon	Blood glucose regulation
Gonads	Estrogen, progesterone, testosterone	Reproduction, secondary sex characteristics

The Hypothalamus and Pituitary Gland

The hypothalamus is the master control center of the endocrine system, regulating the pituitary gland.

• The anterior pituitary secretes hormones that regulate other endocrine glands.

• The posterior pituitary releases hormones synthesized by the hypothalamus (e.g., ADH, oxytocin).

• Releasing and inhibiting hormones from the hypothalamus control the anterior pituitary.

• Homeostasis is maintained by negative feedback mechanisms.

The Thyroid Gland and Homeostasis

The thyroid gland regulates development and metabolism by producing thyroid hormones.

• Produces thyroxine (T4) and triiodothyronine (T3).

• Negative feedback maintains hormone levels in the blood.

• Deficiency can cause cretinism, metabolic disorders, and goiter.

Abnormalities of Thyroid Hormones

Condition	Symptoms
Hyperthyroidism	Weight loss, sweating, high heart rate, intolerance to heat
Hypothyroidism	Weight gain, lethargy, intolerance to cold

Calcium Regulation: Thyroid and Parathyroid Hormones

Blood calcium levels are regulated by hormones from the thyroid (calcitonin) and parathyroid glands (parathyroid hormone, PTH).

• Calcitonin lowers blood calcium.

• PTH increases blood calcium.

Pancreatic Hormones and Glucose Regulation

The pancreas regulates blood glucose through the secretion of insulin and glucagon.

• Insulin signals cells to store glucose as glycogen, lowering blood glucose.

• Glucagon signals cells to release stored glucose, raising blood glucose.

Diabetes Mellitus

• A disorder where body cells cannot absorb glucose from the blood.

• Two main types:

  • Type 1 (insulin-dependent): Autoimmune destruction of beta cells; requires insulin supplementation.

  • Type 2 (non-insulin-dependent): Cells fail to respond to insulin; often associated with obesity and develops after age 40.

The Adrenal Glands and Stress Response

The adrenal glands help the body respond to stress by secreting hormones from the adrenal medulla and cortex.

• Adrenal medulla: Secretes epinephrine and norepinephrine (fight-or-flight response).

• Adrenal cortex: Secretes corticosteroids (e.g., glucocorticoids, mineralocorticoids) that regulate metabolism and stress adaptation.

Gonadal Hormones

The gonads (testes and ovaries) secrete sex hormones that regulate reproduction and secondary sexual characteristics.

• Estrogens and progesterone: Maintain female reproductive system and characteristics.

• Androgens (e.g., testosterone): Trigger development of male characteristics and influence behavior.

Testosterone and Aggression

• High levels of testosterone are associated with increased aggression in some species, but social and environmental factors also play a role.

Other Hormonal Functions and Environmental Hormones

• Prolactin (PRL): Stimulates milk production in mammals; has different roles in other vertebrates.

• Environmental hormones: Chemicals in the environment that can mimic or disrupt normal hormone function, affecting health and reproduction.

Key Equations and Concepts

• Negative Feedback Regulation:

$\text{Stimulus} \rightarrow \text{Endocrine gland} \rightarrow \text{Hormone release} \rightarrow \text{Target cell response} \rightarrow \text{Reduced stimulus}$

• Signal Transduction Pathway:

$\text{Hormone} + \text{Receptor} \rightarrow \text{Second messenger} \rightarrow \text{Cellular response}$

Additional info: This summary expands on the provided slides by including definitions, examples, and tables for clarity and completeness, following a mini-textbook format suitable for college-level General Biology students.