Endocrine System Quiz

Endocrine System Overview

• The endocrine system comprises glands, tissues, and cells that secrete hormones.
• Endocrinology is the study of the endocrine system and its associated disorders.
• Endocrine glands are organs that produce hormones.
• Hormones are chemical messengers transported in the bloodstream, stimulating responses in other tissues or organs, often at a distance.

Cell Communication Mechanisms

• Internal communication is crucial for coordinating cell activities.
• Four mechanisms facilitate communication between cells:
  • Gap junctions: pores in cell membranes allowing signaling between cells.
  • Neurotransmitters: released from neurons, affecting nearby cells.
  • Paracrines: (local secretions) secreted into tissue fluids to impact nearby cells.
  • Hormones: chemical messengers traveling in the bloodstream to influence target cells.

Comparison of Endocrine and Exocrine Glands

• Exocrine glands have ducts, releasing secretions onto epithelial surfaces or the digestive tract mucosa for extracellular effects (e.g., digestion).
• Endocrine glands lack ducts, releasing hormones into the bloodstream for intracellular effects (e.g., altering cellular metabolism).

Comparison of Nervous and Endocrine Systems (Differences)

• Both systems are involved in internal communication.
• The nervous system employs both electrical and chemical signals. The endocrine system depends only on chemical signals.
• Nervous system responses are rapid and short-lived. Endocrine system effects are slower but longer-lasting.
• Nervous system responses adapt quickly. Endocrine system responses adapt slowly.
• Nervous system effects are targeted and specific to one organ. Endocrine system effects are widespread.

Hormone Chemistry

• Two main categories of hormones exist:
  • Steroids: derived from cholesterol (e.g., estrogens, progesterone, testosterone, cortisol).
  • Amino-acid based: chains of amino acids (e.g., epinephrine, norepinephrine, melatonin, thyroid hormone).

Paracrine Secretions

• Paracrines are chemical messengers that diffuse short distances to affect nearby cells.
• They are distinct from neurotransmitters (not produced by neurons) and hormones (not transported in the blood).
• A single chemical can act as a hormone, paracrine, or neurotransmitter depending on the location and context. Illustrative examples include histamine (vascular relaxation) and nitric oxide (vasodilation).

Eicosanoids: A Family of Paracrine Secretions

• Eicosanoids are a category of paracrines derived from arachidonic acid.
• Leukotrienes mediate allergic and inflammatory reactions. Thromboxanes stimulate vasoconstriction and clotting.
• Prostaglandins (a diverse group) exhibit varying effects, including relaxing smooth muscle in the bladder, intestines, and bronchioles but contracting blood vessels.

Control of Hormone Secretion

• Endocrine glands are usually controlled by negative feedback loops.
• When hormone levels reach a threshold, the process is halted.
• Humoral stimuli: gland activity is regulated by factors in body fluids (e.g., glucose levels and insulin secretion).
• Neural stimuli: glands release hormones in response to nerve impulses (e.g., activation of the adrenal medulla).

Hormone Interactions

• Synergistic effects: multiple hormones act together for a greater effect (e.g., FSH and testosterone in sperm production).
• Permissive effects: one hormone enhances the target organ's response to a subsequent hormone (e.g., estrogen priming the uterus for progesterone action).
• Antagonistic effects: one hormone opposes the action of another hormone (e.g., insulin lowering blood glucose and glucagon increasing it).

Enzyme Amplification

• Hormones can exert profound effects.
• One hormone molecule can trigger the synthesis of numerous enzyme molecules.
• Very small stimuli can elicit a substantial response.

Hormone Receptors

• Hormones only influence cells with specific receptors.
• Receptor molecules are proteins or glycoproteins located on plasma membranes, in the cytoplasm, or in the nucleus.
• Hormone-receptor interactions display specificity and saturation. Specific receptors bind to specific hormones. Saturation occurs when all receptor sites are bound.

Hormone Mode of Action

• Hydrophobic hormones penetrate membranes and directly influence gene expression.
• Hydrophilic hormones remain outside cells and engage secondary messengers for intracellular signaling.

Thyroid Hormone (Hydrophobic)

• Thyroid hormone (T₃ and T₄) is essential for metabolism.
• Diffuses into target cells, converting to a more potent form (T₃).
• Binds to receptors on chromosomes, triggering gene activation.
• Increases metabolic rate and strengthens the heartbeat.

Action of Hydrophilic Hormones

• Hydrophilic hormones cannot cross cell membranes, requiring a multi-step intracellular signaling cascade.
• The hormone binds to a receptor, activating a G-protein.
• The G-protein triggers cAMP production, a secondary messenger.
• cAMP activates protein kinases that modify specific cellular proteins, initiating the appropriate cellular response.

Animations and Additional Resources

• Web links provide animations illustrating the action of hydrophilic and hydrophobic hormones.

Modulation of Target Cell Sensitivity

• Target cell sensitivity to hormones shifts based on receptor density adjustments due to up-regulation or down-regulation.
• Increased receptor density (up-regulation) enhances response.
• Decreased receptor density (down-regulation) reduces response to prolonged high hormone concentrations.

Anatomy of Hypothalamus

• The hypothalamus acts as a bridge between the nervous and endocrine systems.
• It manages primitive bodily functions, like water balance, temperature regulation, and sex drive.

Pituitary Gland (Hypophysis)

• A pea-sized gland, attached to the hypothalamus via the infundibulum.
• It's divided into two portions: the anterior and posterior pituitary, each with distinct origins and functionalities.

Embryonic Development

• The pituitary gland develops from two different embryonic tissues.

Histology of Pituitary Gland

• The anterior pituitary gland's structure comprises different cell types.
• The posterior pituitary is primarily composed of nerve fibers.

Anterior and Posterior Pituitary

• The anterior pituitary connects to the hypothalamus through a portal system.
• The posterior pituitary is composed of neural tissue, directly extending from hypothalamic neurons.

Hypophyseal Portal System

• A set of capillaries linked between the hypothalamus and the anterior pituitary allows hormones to travel from the hypothalamus to the anterior pituitary

Hypothalamic Hormones

• The hypothalamus synthesizes and delivers hormones regulating the anterior pituitary glands.
• Some hormones are stored and released by the posterior pituitary.

Anterior Pituitary Hormones

• Six primary hormones are produced, including gonadotropins (FSH and LH) for gonadal regulation and others influencing the thyroid (TSH), adrenal cortex (ACTH), mammary glands (PRL) and general growth (GH).

Hypothalamic-Pituitary-Target Organ Relationships

• Diagram illustrates the relationships between the hypothalamic hormone, pituitary hormone and target organ.

Posterior Pituitary Hormones

• Produced in the hypothalamus, transported to the posterior pituitary for storage and release.
• ADH (antidiuretic hormone) increases water retention.
• OT (oxytocin) is crucial for childbirth, milk lactation and bonding.

Control of Pituitary Secretion

• Secretion rates are controlled by complex processes and factors including the hypothalamus, other parts of the brain, and target organ feedback.
• Anterior lobe regulation is influenced by hypothalamic hormones that are regulated by externals factors such as body temperature.
• Posterior lobe control is triggered by neuroendocrine reflexes triggered by external signals.

Control of Pituitary: Feedback from Target Organs

• Negative feedback loops help maintain homeostasis in hormone levels.
• Positive feedback loops amplify hormone release in specific processes.

Growth Hormone (GH)

• Has diverse effects on body tissues, crucial for cartilage, bone, muscle, and fat growth.
• It stimulates the liver to produce insulin-like growth factors (IGFs) for cellular effects.
• Influences protein synthesis. It impacts lipid metabolism and carbohydrate use.

Growth Hormone (continued)

• Regulates bone growth, development and remodelling throughout adolescence and sleep.
• Responses to vigorous exercise.
• Reduced secretion levels may contribute to aging.

Pineal Gland

• Located in the brain, the pineal gland syncs bodily functions with the day-night cycle.
• It produces melatonin, associated with circadian rhythms and puberty timing, possibly involved with seasonal affective disorder symptoms.

Thymus

• The thymus is involved in endocrine, lymphatic, and immune systems.
• It's a bilobed organ in the upper chest's mediastinum.
• Its produces hormones important in immune defense.

Thyroid Gland Anatomy

• A butterfly-shaped gland in the neck, secreting thyroxine (T4, containing four iodine atoms) and triiodothyronine (T3) for metabolic rate, growth, and development.
• Has parafollicular cells that secrete calcitonin to lower blood calcium levels, important during childhood.

Thyroid Gland (continued)

• The only gland that stores its secretory hormones.
• Synthesis steps include iodine trapping by follicular cells followed by amino acid iodination of thyroglobulin to produce T3 and T4, exocytosis of the thyroid hormones.
• Thyroxine binding globulin in the blood transports the thyroid hormones throughout the body.

Parathyroid Glands

• Typically four small glands situated behind the thyroid gland regulating blood calcium levels using parathyroid hormone (PTH).
• PTH increases calcium levels, activates vitamin D, lowers calcium excretion via urine, and promotes bone resorption.

Adrenal Gland

• A small gland on top of each kidney, formed by the merger of two fetal glands with distinct functions.
• Adrenal cortex surrounds the medulla and produces steroid hormones like mineralocorticoids (aldosterone), glucocorticoids, and sex steroids.

Adrenal Medulla

• The adrenal medulla is the inner core of the adrenal gland and is part of the sympathetic nervous system.
• Its role is to release catecholamines (epinephrine and norepinephrine), and dopamine, as hormones to the blood and their function is similar to the sympathetic nervous system functions.

Adrenal Cortex (Continued)

• Creates more than 25 steroid hormones called corticosteroids and corticoids.

Categories of Corticosteroids

• Mineralocorticoids (e.g., aldosterone): regulate mineral balance, chiefly sodium (Na+) retention to maintain blood volume and blood pressure.
• Glucocorticoids (e.g., cortisol): regulate metabolism, especially glucose and other fuels; crucial for stress response, tissue repair, and anti-inflammatory processes.
• Sex steroids (e.g., androgens and estradiol): influence sexual development and function.

Pancreas

• The pancreas is both an exocrine (digestive enzymes) and endocrine (hormones) organ.
• Endocrine clusters (islets of Langerhans) produce vital hormones.

Pancreatic Hormones (Insulin)

• Insulin is secreted by beta cells in response to elevated blood glucose and amino acid levels.
• It lowers blood glucose by stimulating cells to absorb glucose, convert it to glycogen and promote synthesis of fat and protein.

Pancreatic Hormones (Glucagon)

• Glucagon, released from alpha cells, is secreted when blood glucose levels are low.
• It increases blood glucose by stimulating liver glucose release and fat catabolism in adipose tissue.

Pancreatic Hormones (Somatostatin)

• Somatostatin released from delta cells inhibits secretion of insulin and glucagon thus modulating their overall effect.

Hormones and the Pancreas

• Categorizes hormones based on their effect on blood glucose levels.

The Gonads (Ovaries and Testes)

• Both ovaries and testes are both exocrine and endocrine.
  • Exocrine product (e.g., eggs, sperm).
  • Endocrine product (e.g., gonadal hormones, chiefly steroids).
• Ovaries produce estradiol (an estrogen), progesterone, and inhibin.
• Testes secrete testosterone and lesser amounts of estrogens and inhibin.

Histology of Gonads

• Ovary histology: Granulosa cells produce estrogens, egg encircled by follicle cells and theca.
• Testis histology: Interstitial cells produce testosterone; seminiferous tubules lined with Sertoli cells and contain germ cells.

Ovary

• Produces estradiol and related estrogens, which affect female reproductive development, physique, menstrual cycle, pregnancy support and lactation.
• Progesterone is secreted following ovulation to maintain pregnancy.
• Inhibin, secreted by the ovary, inhibits FSH, thus controlling sperm and egg production.

Testes

• Produce testosterone and other steroids, essential for male reproductive development, physique, and sexual drive.
• Supports sperm production.
• Inhibin from Sertoli cells modulates FSH secretion, influencing sperm production.

Endocrine Functions of Other Organs

• Skin: Keratinocytes synthesize cholecalciferol (vitamin D precursor) using UV light.
• Liver: Creates several hormones including calcidiol (a vitamin D precursor), angiotensinogen, and erythropoietin that influences red blood cells production. Also produces IGF-I regulating cell growth.
• Kidneys: Convert vitamin D precursors to calcitriol (active form), produce renin for blood pressure regulation and erythropoietin promoting red blood cell production.
• Heart: Generates atrial natriuretic peptide to lower blood pressure.
• Stomach and Small Intestines: Produce many hormones coordinating digestion.
• Adipose Tissue: Secretes leptin influencing appetite.
• Osseous Tissue (Bone): Produces osteocalcin, impacting insulin sensitivity.
• Placenta: Secretes hormones regulating pregnancy and fetal development.

Stress Response

• The body responds to stress with distinct phases.
  • Fight-or-flight response (immediate): Focuses resources on vital organs.
  • Resistance reaction (days to weeks): Mobilizes energy resources.
  • Exhaustion phase (weeks to months): Body's resources are depleted.

Anti-inflammatory Drugs

• Steroidal anti-inflammatory drugs (SAIDs) inhibit the synthesis of eicosanoids.
• Nonsteroidal anti-inflammatory drugs (NSAIDs) block cyclooxygenase (COX) enzymes, inhibiting prostaglandin and thromboxane synthesis.

Endocrine Disorders

• Variations in hormone levels or target cell responses impact the body.
• Hyposecretion: inadequate hormone release, potentially due to gland damage or impaired signal reception (e.g., diabetes insipidus).
• Hypersecretion: excessive hormone release, stemming from tumors or autoimmune disorders (e.g., Cushing syndrome, toxic goiter).

Pituitary Disorders

• Acromegaly: Excessive growth hormone secretion in adults, characterized by bone and soft tissue thickening.
• Gigantism: Excessive growth hormone secretion in children.
• Pituitary dwarfism: Inadequate growth hormone secretion.

Thyroid Gland Disorders

• Congenital hypothyroidism: Inadequate thyroid hormone (TH) production at birth.
• Myxedema: Adult hypothyroidism, marked by reduced metabolism.
• Goiter: Thyroid enlargement, potentially due to iodine deficiency or autoimmune stimulation

Parathyroid Disorders

• Hypoparathyroidism: Insufficient parathyroid hormone (PTH), causing dangerously low blood calcium levels.
• Hyperparathyroidism: Excessive PTH causing elevated blood calcium, leading to bone fragility.

Adrenal Disorders

• Cushing syndrome: Excess cortisol secretion.
• Adrenogenital syndrome (AGS): Excess androgen secretion, impacting sexual development.

Diabetes Mellitus

• Diabetes mellitus is the most common metabolic disease, involving impaired insulin function.
• The disease leads to high blood glucose levels, excess glucose in urine, and frequent urination.

Types of Diabetes Mellitus

• Type 1 Diabetes: Failure of insulin production (autoimmune destruction of beta cells).
• Type 2 Diabetes: Insulin resistance or insufficient insulin production.

Pathology of Diabetes

• Chronic high blood glucose in diabetes leads to damage to blood vessels and nerves, including those servicing the kidneys, eyes, and heart.
• Diabetic neuropathy, characterized by nerve damage, poor wound healing and erectile dysfunction.
• Ketoacidosis, a dangerous complication, results from elevated ketone levels and acidification of the blood.