Chap17 PPT

Chapter 17: The Endocrine System

17.1 Overview of the Endocrine System

• Expected Learning Outcomes:

  • Define hormone and endocrine system.

  • Name organs of the endocrine system.

  • Contrast endocrine with exocrine glands.

  • Recognize abbreviations for many hormones.

  • Describe similarities and differences between nervous and endocrine systems.

Mechanisms of Communication

• The body uses four mechanisms for cell communication:

  • Gap junctions: Allow signaling molecules to move between cells.

  • Neurotransmitters: Released from neurons to affect adjacent cells.

  • Paracrines: Secreted to affect nearby cells in tissue fluids.

  • Hormones: Chemical messengers that travel in the bloodstream to stimulate responses in other tissues/organs.

Endocrinology

• Endocrine System: Glands, tissues, and cells that secrete hormones.

• Endocrinology: Study of the endocrine system and its disorders.

Comparison of Glands

• Endocrine Glands:

  • No ducts, release hormones directly into bloodstream.

  • Have dense capillary networks for hormone uptake.

• Exocrine Glands:

  • Have ducts, secrete substances onto epithelial surfaces.

  • Have extracellular effects (e.g., food digestion).

Nervous vs. Endocrine Systems

• Communication Speed:

  • Nervous: Quick, short-lived responses.

  • Endocrine: Slow, prolonged effects.

• Target Area:

  • Nervous: Highly specific (one organ).

  • Endocrine: General, widespread effects.”

Hormones and Receptors

• Target organs or cells have specific receptors for hormones.

• Some target cells convert hormones to more active forms.

17.2 The Hypothalamus and Pituitary Gland

• Expected Learning Outcomes:

  • Describe the anatomical relationship between the hypothalamus and pituitary gland.

  • Distinguish between anterior and posterior lobes of the pituitary.

  • List hormones produced by hypothalamus and pituitary lobes, their functions.

  • Explain pituitary control by hypothalamus and target organs.

  • Describe the effects of growth hormone.

Anatomy Overview

• Hypothalamus: Regulates functions like water balance, thermoregulation; connects to the pituitary gland.

  • Pituitary Gland (Hypophysis): Two structures - anterior and posterior pituitary.

    • Anterior pituitary linked to hypothalamus via blood vessels (hypophyseal portal system).

    • Posterior pituitary is nerve tissue with secreted hormones stored for release.

Hypothalamic Hormones

• Eight hormones produced:

  • For Anterior Pituitary: GnRH, TRH, CRH, PIH, GHRH, Somatostatin.

  • For Posterior Pituitary: Oxytocin (OT), Antidiuretic Hormone (ADH).

Anterior Pituitary Hormones

• Major Hormones: FSH, LH, TSH, ACTH, Prolactin, GH.

• Functions:

  • FSH: Stimulates sex hormone secretion, development of follicles, and sperm production.

  • LH: Stimulates ovulation and hormone secretion from ovaries and testes.

  • TSH: Stimulates the thyroid gland.

  • ACTH: Stimulates cortisol production from adrenal cortex.

  • Prolactin: Stimulates milk production post-birth.

  • GH: Stimulates growth and cellular differentiation.

Posterior Pituitary Hormones

• Hormones: ADH, OT.

  • ADH: Increases water retention by kidneys.

  • OT: Stimulates contractions during childbirth and milk ejection.

Control of Pituitary Secretion

• Rates of secretion regulated by hypothalamus, other brain areas, feedback from target organs (negative and positive feedback).

• Control of Pituitary Secretion The secretion rates of hormones from the pituitary gland are meticulously regulated by the hypothalamus, which serves as a critical link between the endocrine and nervous systems. The hypothalamus synthesizes releasing and inhibiting hormones that directly influence the anterior pituitary's secretory activity.

• Negative Feedback Mechanisms: This is the primary regulatory pathway whereby high levels of certain hormones in the bloodstream trigger the hypothalamus and anterior pituitary to reduce secretion, thereby maintaining homeostasis. For example, increased levels of cortisol will inhibit the release of Corticotropin-Releasing Hormone (CRH) from the hypothalamus and Adrenocorticotropic Hormone (ACTH) from the anterior pituitary.

• Positive Feedback Mechanisms: Contrary to negative feedback, positive feedback amplifies the process. A classic example is during childbirth; increased levels of oxytocin stimulate uterine contractions, which releases even more oxytocin until delivery occurs. In summary, the balance between negative and positive feedback mechanisms is crucial for regulating hormonal levels and ensuring the body's physiological stability. Understanding these mechanisms helps in diagnosing and treating various endocrine disorders, as disruptions in feedback loops can lead to conditions like hyperthyroidism or adrenal insufficiency.

17.3 Other Endocrine Glands

• Pineal Gland: Produces melatonin; influences circadian rhythms.

• Thymus: Releases hormones for T cell maturation; vital for immune defense.

• Thyroid Gland: Produces TH (T3, T4) impacting metabolism. Secretes calcitonin which regulates calcium levels.

• Parathyroid Glands: Produce PTH to increase blood calcium levels.

• Adrenal Glands: Two parts (medulla and cortex).

  • Medulla: produces catecholamines.

  • Cortex: produces corticosteroids for metabolism, stress response.

• Pancreatic Islets: Secrete insulin and glucagon; critical for blood glucose regulation.

• Gonads: Secrete sex hormones (estrogens, testosterone) influencing reproductive function.

• Other Tissues: Heart, kidneys, liver, and fat tissues secrete various hormones affecting multiple functions.

17.4 Hormones and Their Actions

• Chemical Classes: Steroids, monoamines, peptides.

• Synthesis and Transport: Hormones can be synthesized from cholesterol (steroids) or amino acids (monoamines, peptides).

• Modes of Action:

  • Peptides/Catecholamines: Bind to surface receptors; activate second messenger systems for quick effects.

  • Steroids/Thyroid Hormone: Enter cells, bind to intracellular receptors; influence gene expression rather slowly.

17.5 Stress and Adaptation

• Stress Response: Involves the release of epinephrine and glucocorticoids (e.g., cortisol).

• Stages: Alarm reaction, resistance, exhaustion.

17.7 Endocrine Disorders

• Hyposecretion: Inadequate hormone release due to gland issues.

• Hypersecretion: Excessive hormone release potentially leading to conditions like thyroid disorders, adrenal disorders, diabetes mellitus.

Pituitary Gland (Hypophysis): The pituitary gland is often referred to as the "master gland" of the endocrine system due to its crucial role in regulating various physiological processes. It is divided into two distinct structures:

1. Anterior Pituitary (Adenohypophysis):

   • Structure: Comprises glandular tissue and constitutes about 75% of the pituitary's total weight.

   • Connection to Hypothalamus: It is connected to the hypothalamus via the hypophyseal portal system, which allows the hypothalamus to regulate anterior pituitary hormone secretion through releasing and inhibiting hormones.

   • Hormones Secreted: The anterior pituitary produces and secretes several key hormones:

     • Follicle-Stimulating Hormone (FSH): Stimulates the growth of ovarian follicles in females and spermatogenesis in males.

     • Luteinizing Hormone (LH): Triggers ovulation in females and stimulates testosterone production in males.

     • Thyroid-Stimulating Hormone (TSH): Stimulates the thyroid gland to produce thyroid hormones, which regulate metabolism.

     • Adrenocorticotropic Hormone (ACTH): Stimulates the adrenal cortex to produce cortisol, important for stress response and metabolism.

     • Prolactin (PRL): Promotes milk production in mammary glands.

     • Growth Hormone (GH): Stimulates growth, cell reproduction, and regeneration in the body.

2. Posterior Pituitary (Neurohypophysis):

   • Structure: Composed of nerve tissue, the posterior pituitary does not produce hormones but stores and releases hormones produced in the hypothalamus.

   • Hormones Secreted: Contains two major hormones:

     • Antidiuretic Hormone (ADH): Also known as vasopressin, it regulates water balance by promoting water reabsorption in the kidneys, which helps to concentrate urine and maintain blood pressure.

     • Oxytocin (OT): Stimulates uterine contractions during childbirth and milk ejection during breastfeeding; also involved in social bonding and attachment.

• Anatomical Relations: The pituitary gland is located at the base of the brain, housed in the sella turcica of the sphenoid bone, and is connected to the hypothalamus by the infundibulum. This proximity facilitates the regulation of endocrine functions and maintains homeostasis.

• Regulatory Mechanisms: Hormonal secretion from the pituitary gland is tightly regulated by the hypothalamus and feedback from the target organs, utilizing negative and positive feedback mechanisms to maintain hormonal balance and appropriate physiological responses.

• Clinical Significance: Disorders of the pituitary gland can lead to various endocrine disorders, including growth hormone excess (acromegaly), deficiencies leading to dwarfism, and dysregulation of reproductive hormones causing infertility and menstrual irregularities. Conditions like Cushing's disease can arise from excess ACTH production, leading to adrenal hyperactivity. Whole system health can be impacted due to its regulatory role in other endocrine glands and hormones.