endocrine

13.1: General Characteristics of the Endocrine System

  • Endocrine Glands:

    • Composed of cells, tissues, and organs that constitute the endocrine system.

    • Ductless Glands: They release hormones directly into the bloodstream.

    • Definition: "Endocrine" refers to "internal secretion."

    • Hormonal Targeting: Hormones only affect target cells that have specific receptors; non-target cells do not respond.

  • Exocrine Glands:

    • Secrete substances through ducts or tubes leading to a body surface.

    • Deliver products externally to specific sites.

  • Other Types of Secretions:

    • Paracrine Secretion: Affects nearby cells.

    • Autocrine Secretion: Affects the cells that secrete the substances themselves.

Endocrine and Exocrine Glands

  • Endocrine Glands: Secrete hormones into body fluids.

  • Exocrine Glands: Secrete substances into the outside environment.

Comparison Between Nervous & Endocrine Systems

  • Communication:

    • Both the nervous and endocrine systems function in communication, utilizing chemicals to bind to receptor molecules.

    • Nervous System: Releases neurotransmitters into synapses.

    • Endocrine System: Secretes hormones into the bloodstream.

  • Precision of Endocrine System:

    • Only target cells with specific receptors respond to a hormone, indicating high specificity in communication.

13.2: Hormone Action

  • Release: Hormones are secreted into extracellular spaces around endocrine cells and diffuse into the bloodstream for systemic distribution.

  • Endocrinology: The scientific study of hormones and their actions.

Hormone Names and Abbreviations

  • Hypothalamic Hormones:

    • Corticotropin-releasing hormone (CRH)

    • Gonadotropin-releasing hormone (GnRH)

    • Luteinizing hormone-releasing hormone (LHRH)

    • Somatostatin (SS)

    • Growth hormone-releasing hormone (GHRH)

    • Prolactin-inhibiting hormone (PIH)

    • Prolactin-releasing factor (PRF)

    • Thyrotropin-releasing hormone (TRH)

  • Anterior Pituitary Hormones:

    • Adrenocorticotropic hormone (ACTH)

    • Follicle-stimulating hormone (FSH)

    • Growth hormone (GH)

    • Luteinizing hormone (LH)

    • Prolactin (PRL)

    • Thyroid-stimulating hormone (TSH)

  • Posterior Pituitary Hormones:

    • Antidiuretic hormone (ADH, vasopressin)

    • Oxytocin (OT)

  • Thyroid Hormones:

    • Thyroxine (T4)

    • Triiodothyronine (T3)

  • Parathyroid Hormone:

    • Parathyroid hormone (PTH, also parathormone)

  • Adrenal Hormones:

    • The adrenal medulla secretes Epinephrine (EPI, adrenaline) and Norepinephrine (NE, noradrenaline).

    • The adrenal cortex secretes hormones like Aldosterone and Cortisol.

  • Pancreatic Hormones:

    • Insulin (Beta Cells)

    • Glucagon (Alpha Cells)

    • Somatostatin (Delta Cells)

Chemistry of Hormones

  • Types of Hormones:

    • Steroid or Steroid-like Hormones:

    • Steroids: Lipids with complex carbon-hydrogen rings, derived from cholesterol. Examples include sex hormones (testosterone, estrogens) and adrenal cortex hormones (cortisol, aldosterone).

    • Non-steroid Hormones:

    • Amines: Derived from tyrosine (e.g., epinephrine, norepinephrine).

    • Proteins: Long chains of amino acids (e.g., growth hormone).

    • Peptides: Short chains of amino acids (e.g., oxytocin).

    • Glycoproteins: Proteins with carbohydrate moieties (e.g., TSH).

Actions of Hormones

  • Mechanism of Action:

    • Hormones alter metabolic processes, including enzyme activity and membrane transport rates.

    • Each hormone binds to specific receptors either on the surface or inside target cells, leading to varied physiological responses.

  • Receptor Interaction:

    • The number of receptors affects the response strength, with the potential for modification via:

    • Upregulation: Increased receptor numbers in response to low hormone levels.

    • Downregulation: Decreased receptor numbers when hormone levels are elevated.

  • Steroid and Thyroid Hormones:

    • Both have limited water solubility but can diffuse through cell membranes.

    • Hormones bind to internal receptors, usually located in the nucleus, and form hormone-receptor complexes that activate gene transcription leading to protein synthesis.

  • Non-steroid Hormones:

    • Cannot penetrate the lipid bilayer and must bind to surface receptors, activating second messengers such as cyclic adenosine monophosphate (cAMP) to evoke changes within the target cell.

13.3: Control of Hormonal Secretions

  • Regulation: Hormone secretion predominantly controlled by negative feedback mechanisms, which maintain hormonal balance.

  • Hormone Lifetime: Hormones may act for minutes or extend to days.

  • Hormonal Breakdown: Hormones are metabolized by enzymes, especially in the liver, or expelled through urine to cease their effects.

  • Positive Feedback: Though rare, involves stimulation of hormone release, primarily noted in reproductive functions.

  • Mechanisms of Hormonal Control:

    • Tropic Hormones: Influence other glands' hormone secretion.

    • Nervous System Control: Direct nervous stimulation of glands to produce hormones.

    • Internal Environment Changes: Internal substance level changes (e.g., ions, glucose) trigger secretion.

13.4: Pituitary Gland

  • Location and Structure: Located at the base of the brain, nestled within the sella turcica of the sphenoid bone and connected to the hypothalamus via the pituitary stalk (infundibulum).

    • Anatomy: Divided into two parts: anterior lobe (adenohypophysis) and posterior lobe (neurohypophysis).

  • Regulation:

    • Anterior Lobe: Hormones release stimulated by hypothalamic releasing hormones carried through the hypophyseal portal system.

    • Posterior Lobe: Nerve impulses from the hypothalamus directly trigger hormone release.

Hypothalamic Control of Peripheral Endocrine Glands

  • Function: The hypothalamus governs hormone secretion in peripheral glands through specific pathways, using releasing hormones to stimulate anterior pituitary cells, which in turn affect target glands.

    • Negative Feedback: Critical in regulating the entire system, ensuring hormonal balance and homeostasis.

Anterior Pituitary Hormones

  • Hormone Types:

    • Growth Hormone (GH): Stimulates cell growth and division.

    • Prolactin (PRL): Promotes milk production in females.

    • Thyroid-stimulating Hormone (TSH): Influences thyroid hormone release.

    • Adrenocorticotropic Hormone (ACTH): Stimulates cortisol production from the adrenal cortex.

    • Follicle-stimulating Hormone (FSH): Participates in gonadal growth and gamete production.

    • Luteinizing Hormone (LH): Triggers ovulation in females and hormone production in both genders.

Control of TSH Secretion

  • TSH secretion regulation:

    • Influenced by levels of thyrotropin-releasing hormone (TRH) and thyroid hormone negative feedback.

Disorders Related to Growth Hormone

  • Hypopituitary Dwarfism: Short stature due to GH deficiency during childhood.

  • Gigantism: Height increase due to GH excess during childhood, associated with pituitary tumors.

  • Acromegaly: Bone thickening caused by GH excess in adulthood, leading to enlargement in various body features.

Posterior Pituitary Hormones

  • Composition: Primarily consists of nerve fibers, unlike the glandular tissue of the anterior lobe.

  • Hormones Produced:

    • Antidiuretic Hormone (ADH): Reduces urine output and can constrict blood vessels.

    • Oxytocin: Induces uterine contractions during childbirth and milk ejection in lactation.

13.5: Thyroid Gland

  • Structure: Comprises two lateral lobes and an isthmus, situated below the larynx beside the trachea, with iodine absorption capabilities.

  • Hormones Produced: Thyroid hormones include:

    • T4 (Thyroxine)

    • T3 (Triiodothyronine)

    • Calcitonin

    • Follicular Cells: Produce T3 and T4.

    • Parafollicular Cells: Produce calcitonin.

Actions of Thyroid Hormones

  • Effects of Hormones:

    • T4: Enhances energy release, accelerates growth and protein synthesis, and stimulates nervous activity.

    • T3: More potent than T4 with similar actions.

    • Calcitonin: Lowers blood calcium levels by acting on bones and kidneys, promoting calcium deposition and excretion regulation.

Thyroid Disorders

  • Hyperthyroidism: Symptoms include weight loss and increased metabolic activity, caused by autoimmune disorders (Grave's Disease).

  • Hypothyroidism: Symptoms range from sluggishness to growth retardation, arising from autoimmune thyroid damage or iodine deficiency.

13.6: Parathyroid Glands

  • Location: Situated on the posterior surface of the thyroid gland, usually four in number.

  • Function: Secretes parathyroid hormone (PTH), regulating calcium and phosphate ions in blood.

PTH Actions

  • Increased Blood Calcium Levels: Elevated through bone resorption, kidney conservation, and intestinal absorption by influencing vitamin D metabolism.

13.7: Adrenal Glands

  • Anatomy: Also known as suprarenal glands, located atop each kidney, consisting of the adrenal cortex and adrenal medulla.

  • Adrenal Medulla Hormones:

    • Epinephrine (80%) and Norepinephrine (20%) for stress response.

  • Adrenal Cortex Hormones:

    • Aldosterone, cortisol, and sex hormones produced in three distinct zones:

    1. Zona Glomerulosa: Mineralocorticoids (e.g., Aldosterone)

    2. Zona Fasciculata: Glucocorticoids (e.g., Cortisol)

    3. Zona Reticularis: Androgens (male sex hormones).

  • Renin-Angiotensin System: Helps regulate blood pressure via hormone signaling that affects sodium and water retention.

Disorders of the Adrenal Cortex

  • Addison's Disease: Lack of steroid hormone production leading to electrolyte imbalances and low blood pressure.

  • Cushing's Syndrome: Excess cortisol, resulting in muscle and bone loss.

13.8: Pancreas

  • Dual Function: Acts as both an endocrine (hormone secretion) and exocrine (digestive juice secretion) gland.

  • Islets of Langerhans produce hormones:

    • Alpha Cells: Secrete glucagon.

    • Beta Cells: Secrete insulin.

    • Delta Cells: Secrete somatostatin.

Regulation of Blood Glucose

  • Insulin Effects: Lowers blood glucose by promoting its uptake by cells.

  • Glucagon Effects: Raises blood glucose levels when low.

  • Homeostasis: Hormones work antagonistically with negative feedback mechanisms to maintain glucose levels.

Diabetes Mellitus

  • Type 1: Autoimmune destruction of insulin-producing cells.

  • Type 2: Body's cells unresponsive to insulin; more common.

    • Symptoms include hyperglycemia, tissue wasting, and dehydration from glycosuria.

13.9: Other Endocrine Glands

  • Pineal Gland: Secretes melatonin, regulating sleep-wake cycles.

  • Thymus Gland: Produces thymosins for T-lymphocyte development and immune function.

  • Reproductive Organs:

    • Ovaries produce estrogens and progesterone.

    • Testes produce testosterone.

    • Placenta produces estrogen, progesterone, and gonadotropin.

Stress Responses

  • Short-term Response: Activating fight or flight mechanisms via the adrenal medulla, increasing glucose and oxygen delivery.

  • Long-term Response: Managed by the adrenal cortex with cortisol affecting metabolism and immune function.

  • Hormonal and Neural Signals: Initiated upon stressful stimuli, resulting in a cascade of hormonal changes to prepare the body for immediate action or adapt to ongoing stress.