Chapter_17_THE_ENDOCRINE_SYSTEM

Overview of the Endocrine System

• The endocrine system is made up of glands, tissues, and cells that secrete hormones.

• Functions in conjunction with the nervous system to control and coordinate all other body systems.

• Key glands include: Pineal, Pituitary, Hypothalamus, Pancreas, Thymus, Adrenal glands, Thyroid, Ovaries, and Testes.

Terminology in Endocrine

• A-, An-: without (e.g., muscular atrophy)

• ADEN-: gland (e.g., adenoma)

• End-, Endo-: within (e.g., endocrine)

• Exo-: away from (e.g., exocrine)

• Glyco-, Gluco-: sugar (e.g., hyperglycaemia)

• Para-: near/beside (e.g., parathyroid)

• Poly-: much/many (e.g., polyadenitis)

• Thyro-: thyroid (e.g., thyroaplasia)

• -CRINE: to secrete (e.g., endocrine)

• -TROPHY: growth (e.g., hypertrophy)

• -PHYSIS: growth (denotes outcropping vs. trophy)

Key Components of the Endocrine System

• Endocrine System: Consists of glands that secrete hormones affecting target cells via the bloodstream.

• Hormones: Chemical messengers transported by the bloodstream; regulate physiological responses.

• Target Cells: Specific cells with receptors that bind to and interpret hormonal signals.

Function and Role of the Endocrine System

• The endocrine system provides a slower response in controlling the body’s internal environment and maintaining homeostasis.

• Key hormones are produced by endocrine glands located throughout the body, playing critical roles in regulating bodily functions.

Structures of the Endocrine System

• Main components: Pituitary, Thyroid, Parathyroid, Adrenal, and Pineal glands.

• Some glands (e.g., pancreas) have both endocrine (hormone secretion) and exocrine (digestive enzyme secretion) functions.

Types of Chemical Signaling

• Endocrine signaling: Hormones secreted into extracellular fluid, diffusing into the blood.

• Autocrine signaling: Chemicals induce a response in the same cell that secretes them.

• Paracrine signaling: Chemicals induce responses in neighboring cells.

Comparison: Endocrine vs Nervous Systems

Hormone Overview

• Hormones can be categorized into two major groups based on chemical structure:

  1. Amino acid-derived hormones (amines, peptides, proteins)

  2. Lipid-derived hormones (steroids)

• Hormones bind to specific receptors on target cells to initiate a response.

Hormonal Regulation

• Down-regulation: Cells decrease the number of receptors in response to high hormone levels, making them less sensitive.

• Up-regulation: Cells increase the number of receptors in response to low hormone levels, increasing sensitivity.

Regulation of Hormone Secretion

• Hormone levels are tightly controlled to prevent abnormal states through feedback loops:

  • Negative Feedback: High levels decrease gland secretion (e.g., thyroid hormones).

  • Positive Feedback: Hormone secretion produces more hormones (e.g., oxytocin during childbirth).

Hypothalamus-Pituitary Complex

• The hypothalamus connects to the pituitary gland via the infundibulum and regulates its function.

• Pituitary gland has two lobes with different hormone secretions influenced by the hypothalamus.

Posterior Pituitary Hormones

• Antidiuretic Hormone (ADH): Stimulates water reabsorption in kidneys.

• Oxytocin: Stimulates uterine contractions during childbirth.

Anterior Pituitary Hormones

• Secretes hormones regulating growth, metabolism, and reproduction, influenced by hypothalamic signals:

  • Growth Hormone (GH): Promotes body tissue growth.

  • Prolactin (PRL): Promotes milk production.

  • Thyroid-Stimulating Hormone (TSH): Stimulates thyroid hormone release.

  • Adrenocorticotropic Hormone (ACTH): Stimulates hormone release from adrenal glands.

  • Follicle-Stimulating Hormone (FSH): Stimulates gamete production.

  • Luteinizing Hormone (LH): Stimulates hormone production by gonads.

Thyroid Gland Functions

• Located at the base of the neck; produces Thyroxine (T4) and Triiodothyronine (T3) affecting cellular metabolism.

• Calcitonin: Works with parathyroid hormone in calcium metabolism regulation.

Regulation of Thyroid Hormones

• Classic negative feedback loop: T3 & T4 levels negatively influence TSH secretion.

• Hyperthyroidism: High T3, T4; low TSH.

• Hypothyroidism: Low T3, T4; high TSH.

Effects of Thyroid Hormones

• Increase oxygen consumption and energy use; elevates heart rate and force of contraction; stimulates red blood cell formation.

Thyroid Disorders

• Goiter: Enlargement of the thyroid gland.

• Graves Disease: Autoantibodies mimic TSH leading to hypersecretion.

Parathyroid Glands

• Located on the posterior surface of the thyroid; secrete Parathyroid Hormone (PTH) to regulate calcium levels in the blood.

• PTH increases blood calcium levels by promoting absorption and inhibiting loss.

Disorders of the Parathyroid Glands

• Hypoparathyroidism: Low calcium levels, may cause muscle twitching.

• Hyperparathyroidism: Excessive PTH leading to high blood calcium and potential health complications.

Adrenal Glands

• Located atop kidneys; involved in stress responses and metabolism regulation through hormone secretion (steroids).

Adrenal Gland Response Patterns

• General Adaptation Syndrome (GAS) for stress response:

  • Alarm Stage: Immediate fight-or-flight response.

  • Resistance Stage: Body adapts to stress.

  • Exhaustion Stage: Vital systems collapse due to prolonged stress.

Hormones of the Adrenal Cortex

• Produces corticosteroids:

  • Mineralocorticoids: Regulate electrolyte balance.

  • Glucocorticoids: Involved in glucose metabolism and stress responses.

Disorders of the Adrenal Glands

• Cushing Syndrome: Excess cortisol secretion causing various physical symptoms.

• Adrenogenital Syndrome: Excessive androgens leading to masculine traits in females and early puberty in children.

Pineal Gland Functions

• Regulates sleep cycles through the secretion of Melatonin, influenced by light exposure.

Gonadal Functions

• Ovaries and testes produce hormonal sex characteristics and maintain reproductive organs.

• Different hormones produced by ovaries (e.g., estrogen) and testes (e.g., testosterone).

Pancreas Functions

• Functions both as exocrine (digestive enzymes) and endocrine (insulin and glucagon) organ; regulates glucose metabolism.

Hormones of the Pancreas

• Insulin: Lowers blood glucose levels.

• Glucagon: Raises blood glucose levels.

• Somatostatin: Inhibits insulin and glucagon release.

• Pancreatic Polypeptide: Affects appetite regulation.

Diabetes Mellitus

• Most common metabolic disease; characterized by disrupted glucose metabolism due to insulin issues.

• Types include:

  • Type I Diabetes: Inadequate insulin production (often childhood onset).

  • Type II Diabetes: Insulin resistance often linked to obesity.

Complications of Diabetes

• Include kidney degeneration, retinal damage, early heart attacks, and peripheral nerve issues.

Endocrine Functions of Other Organs

• Heart: Secretes Atrial Natriuretic Peptide (ANP) to regulate blood pressure.

• GI Tract: Hormones aid in digestion.

• Kidneys: Produce hormones such as calcitriol and renin.

• Skin: Functions in producing vitamin D.

• Liver: Involved in several hormone productions impacting growth and blood pressure.

Aging and the Endocrine System

• Structures and functions of glands change with age leading to altered hormone production.

• Examples include menopause in females and reduced hormonal production affecting muscle mass and metabolic rate.