Module 16.1 Overview of the Endocrine System

Comparison of Endocrine and Nervous System Control

The endocrine system and nervous system are the two main regulatory systems in the human body. Both coordinate body functions, but they differ in mechanisms, speed, and duration of action.

• Endocrine system: Uses hormones released into the bloodstream to regulate distant target organs. Effects are generally slower to begin but longer-lasting.

• Nervous system: Uses electrical impulses and neurotransmitters for rapid, short-term responses. Effects are localized and brief.

• Integration: Many physiological processes involve both systems working together for homeostasis.

Major Structures and Functions of the Endocrine System

• Primary endocrine organs: Pituitary gland, thyroid gland, parathyroid glands, adrenal glands, pancreas, pineal gland, thymus, gonads (ovaries and testes).

• Functions: Regulation of metabolism, growth, reproduction, stress response, and homeostasis of blood composition.

Chemical Signaling Types

• Endocrine signaling: Hormones travel through the bloodstream to distant targets.

• Paracrine signaling: Chemical messengers affect nearby cells.

• Autocrine signaling: Cells respond to substances they themselves secrete.

• Neuroendocrine signaling: Neurons release hormones into the blood (e.g., hypothalamic hormones).

Chemical Classes of Hormones and Receptors

• Amino acid-based hormones: Includes peptides, proteins, and amines. Usually bind to cell surface receptors and use second messengers.

• Steroid hormones: Derived from cholesterol. Lipid-soluble; bind to intracellular receptors and directly affect gene transcription.

• Receptor types: Cell membrane receptors (for water-soluble hormones) vs. intracellular receptors (for lipid-soluble hormones).

Control of Hormone Secretion

• Stimuli: Hormone release can be triggered by humoral (blood levels of ions/nutrients), neural (nerve fibers), or hormonal (other hormones) stimuli.

• Feedback mechanisms:

  • Negative feedback: Most common; hormone secretion decreases as its effects are achieved.

  • Positive feedback: Less common; hormone secretion increases in response to its effects (e.g., oxytocin during childbirth).

Module 16.2 The Hypothalamus and the Pituitary Gland

Anatomical and Functional Relationships

• Hypothalamus: Located in the diencephalon; links the nervous and endocrine systems.

• Pituitary gland: Sits below the hypothalamus; consists of anterior (adenohypophysis) and posterior (neurohypophysis) lobes.

• Connection: The hypothalamus controls the pituitary via releasing/inhibiting hormones (anterior) and direct neural connections (posterior).

Tropic Hormones

• Tropic hormones: Hormones that stimulate other endocrine glands to secrete hormones (e.g., TSH, ACTH).

Posterior Pituitary Hormones

• Hormones: Antidiuretic hormone (ADH) and oxytocin.

• Stimulus for release: Neural signals from the hypothalamus.

• Target tissues: Kidneys (ADH), uterus and mammary glands (oxytocin).

• Effects: Water retention (ADH), uterine contraction and milk ejection (oxytocin).

Anterior Pituitary Hormones

• Hormones: Growth hormone (GH), prolactin (PRL), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), luteinizing hormone (LH).

• Stimulus for release: Hypothalamic releasing/inhibiting hormones.

• Target tissues and effects: Vary by hormone (e.g., GH targets most tissues for growth; TSH stimulates thyroid hormone release).

Negative Feedback Regulation

• Anterior pituitary hormone secretion is regulated by negative feedback from target gland hormones (e.g., thyroid hormones inhibit TSH and TRH release).

Module 16.3 The Thyroid and Parathyroid Glands

Anatomy of the Thyroid and Parathyroid Glands

• Thyroid gland: Butterfly-shaped, located anterior to the trachea; composed of follicles and parafollicular cells.

• Parathyroid glands: Four small glands on the posterior thyroid surface.

Cell Types and Hormones

• Follicular cells: Produce thyroid hormones (T3 and T4).

• Parafollicular cells: Produce calcitonin.

Thyroid Hormone

• Stimulus for release: TSH from anterior pituitary.

• Target tissues: Most body cells.

• Effects: Increases metabolic rate, promotes growth and development.

• Regulation: Negative feedback by circulating thyroid hormones on hypothalamus and pituitary.

Parathyroid Hormone (PTH)

• Stimulus for release: Low blood calcium levels.

• Target tissues: Bones, kidneys, intestines.

• Effects: Increases blood calcium by stimulating bone resorption, increasing calcium reabsorption in kidneys, and activating vitamin D.

Module 16.4 The Adrenal Glands

Anatomy of the Adrenal Glands

• Adrenal cortex: Outer region; produces steroid hormones.

• Adrenal medulla: Inner region; produces catecholamines.

Mineralocorticoids (e.g., Aldosterone)

• Stimulus for release: Low blood sodium, high potassium, or activation of the renin-angiotensin system.

• Target tissue: Kidneys.

• Effects: Increases sodium reabsorption and potassium excretion, regulating blood pressure and volume.

Glucocorticoids (e.g., Cortisol)

• Stimulus for release: ACTH from anterior pituitary.

• Target tissues: Most body cells.

• Effects: Increases blood glucose, suppresses immune response, aids in stress adaptation.

Adrenal Medulla and the Sympathetic Nervous System

• The adrenal medulla is functionally part of the sympathetic nervous system, releasing epinephrine and norepinephrine during stress (fight-or-flight response).

Catecholamines

• Stimulus for release: Sympathetic nervous system activation.

• Target tissues: Heart, blood vessels, lungs, liver, etc.

• Effects: Increases heart rate, blood pressure, blood glucose, and prepares body for rapid action.

Module 16.5 The Endocrine Pancreas

Structure and Hormone-Secreting Cells

• Islets of Langerhans: Clusters of endocrine cells in the pancreas.

• Alpha cells: Secrete glucagon.

• Beta cells: Secrete insulin.

Glucagon

• Stimulus for release: Low blood glucose.

• Target tissue: Liver.

• Effect: Increases blood glucose by promoting glycogen breakdown and gluconeogenesis.

Insulin

• Stimulus for release: High blood glucose.

• Target tissues: Most body cells, especially muscle and adipose tissue.

• Effect: Lowers blood glucose by promoting cellular uptake and storage of glucose.

Regulation of Blood Glucose

• Insulin and glucagon act antagonistically to maintain blood glucose homeostasis.

Diabetes Mellitus

• Type 1: Autoimmune destruction of beta cells; requires insulin therapy.

• Type 2: Insulin resistance; managed with lifestyle changes, oral medications, and sometimes insulin.

• Symptoms: Polyuria, polydipsia, polyphagia, hyperglycemia.

Module 16.6 Other Endocrine Glands and Hormone-Secreting Tissues

Pineal and Thymus Glands

• Pineal gland: Secretes melatonin; regulates circadian rhythms.

• Thymus: Secretes thymosins; important for T cell maturation in the immune system.

Other Hormone-Secreting Tissues

• Gonads: Ovaries (estrogen, progesterone), testes (testosterone); regulate reproduction.

• Adipose tissue: Secretes leptin; regulates appetite and metabolism.

• Heart: Secretes atrial natriuretic peptide (ANP); lowers blood pressure.

• Kidneys: Secrete erythropoietin (stimulates red blood cell production) and renin (regulates blood pressure).

Module 16.7 Examples of Endocrine Control of Physiological Variables

Hormonal Maintenance of Homeostasis

• Example: Regulation of blood glucose by insulin and glucagon.

• Example: Regulation of blood calcium by PTH and calcitonin.

• Example: Regulation of blood pressure by aldosterone and ANP.

Hormones and the Stress Response

• Hormones such as cortisol and catecholamines work with the nervous system to mediate the body's response to stress, ensuring energy availability and physiological adaptation.