BIO 2640 Exam 1 Study Guide: The Endocrine System & Blood

The Endocrine System

Hormonal vs. Neural Control of Body Function

The body uses both hormonal and neural mechanisms to regulate physiological processes. These systems differ in their speed, duration, and mode of action.

• Neural Control: Utilizes electrical impulses and neurotransmitters; effects are rapid and short-lived.

• Hormonal Control: Involves hormones released into the bloodstream; effects are slower but longer-lasting.

• Comparison Table: See Table 16.1 for a detailed comparison of neural and hormonal controls.

• Example: Neural control regulates muscle contraction, while hormonal control regulates growth and metabolism.

Hormones, Paracrines, and Autocrines

Cell signaling molecules are classified based on their target and mode of action.

• Hormones: Chemical messengers secreted into the blood to act on distant target organs.

• Paracrines: Act locally on nearby cells.

• Autocrines: Affect the same cell that secretes them.

• Example: Insulin is a hormone; prostaglandins can act as paracrines.

Chemical Classification of Hormones

Hormones are classified based on their chemical structure, which determines their solubility and mechanism of action.

• Steroid Hormones: Lipid-soluble, derived from cholesterol. Examples: Cortisol, Estrogen.

• Amino Acid-Based Hormones: Water-soluble, include proteins, peptides, and amines. Examples: Insulin, Thyroxine.

Mechanisms of Hormone Action

Hormones exert their effects via two major mechanisms, depending on their solubility.

• Lipid-Soluble Hormones: Bind to intracellular receptors, directly affecting gene expression.

• Water-Soluble Hormones: Bind to membrane receptors, activating second messenger systems (e.g., cAMP, Ca2+).

• Direct vs. Second Messenger: Lipid-soluble hormones act directly; water-soluble hormones use second messengers.

• Examples: Steroid hormones use direct gene activation; peptide hormones use cAMP as a second messenger.

• Second Messengers: cAMP, IP3, Ca2+

Regulation of Hormone Release

Hormone secretion is regulated by three primary mechanisms.

• Humoral Regulation: Changes in blood levels of ions or nutrients trigger hormone release.

• Neural Regulation: Nerve fibers stimulate hormone release.

• Hormonal Regulation: Hormones stimulate other endocrine glands to release hormones.

• Example: Estrogen release from ovaries is triggered by Luteinizing Hormone (hormonal regulation).

Hypothalamus and Pituitary Gland Relationship

The hypothalamus and pituitary gland are structurally and functionally connected, forming a major regulatory axis.

• Hypothalamus: Synthesizes hormones and regulates pituitary function.

• Transport: Hormones are transported via nerve tracts (posterior pituitary) or blood vessels (anterior pituitary).

• Connection: Hypophyseal portal system (anterior), hypothalamo-hypophyseal tract (posterior).

Posterior Pituitary Structure and Hormones

The posterior pituitary (neurohypophysis) stores and releases hormones synthesized in the hypothalamus.

• Hormones: Antidiuretic hormone (ADH) and oxytocin.

• Storage vs. Synthesis: Stores, does not synthesize hormones.

• Target Organs: ADH acts on kidneys; oxytocin acts on uterus and mammary glands.

• Triggers: ADH is released in response to increased blood osmolarity; oxytocin is released during childbirth and lactation.

• Inhibitors: Alcohol inhibits ADH.

• Health Conditions: Diabetes insipidus (ADH deficiency), SIADH (ADH excess).

Anterior Pituitary Hormones

The anterior pituitary (adenohypophysis) synthesizes and releases several hormones, some of which are tropic (regulate other glands).

• Tropic Hormones: TSH, ACTH, FSH, LH.

• Non-Tropic Hormones: GH, PRL.

• Target Organs: TSH (thyroid), ACTH (adrenal cortex), FSH/LH (gonads), GH (muscle, bone), PRL (mammary glands).

• Diseases: Hypo/hypersecretion of GH leads to dwarfism/gigantism or acromegaly.

Thyroid Gland Hormones

The thyroid gland produces hormones that regulate metabolism and calcium homeostasis.

• Follicular Cells: Secrete thyroid hormone (TH) in two forms: T3 (triiodothyronine) and T4 (thyroxine).

• Active Form: T3 is more active at target tissues.

• Major Effects: Increases metabolic rate, heat production, and growth.

• Parafollicular Cells: Secrete calcitonin, which lowers blood calcium levels.

Thyroid Hormone Synthesis and Regulation

Thyroid hormone synthesis involves several steps and is regulated by negative feedback.

• Synthesis Steps: Iodine uptake, thyroglobulin production, hormone assembly, release.

• Regulation: Negative feedback via blood levels of TH.

Parathyroid Hormone (PTH) Functions

PTH is essential for calcium homeostasis.

• Response to Low Calcium: Increases blood calcium by stimulating osteoclasts, enhancing intestinal absorption, and promoting kidney reabsorption.

Adrenal Gland Hormones

The adrenal gland produces hormones that regulate stress response, metabolism, and electrolyte balance.

• Adrenal Cortex: Aldosterone (regulates sodium), cortisol (regulates metabolism and stress).

• Regulation: Aldosterone is regulated by renin-angiotensin system, ACTH, and plasma K+.

• Diseases: Cushing's syndrome (cortisol excess), Addison's disease (cortisol deficiency).

• Adrenal Medulla: Catecholamines (epinephrine, norepinephrine) mediate fight-or-flight response.

Pancreatic Hormones and Glucose Regulation

The pancreas regulates blood glucose via two major hormones.

• Insulin: Lowers blood glucose; secreted by beta cells.

• Glucagon: Raises blood glucose; secreted by alpha cells.

• Glucogenic Hormones: Glucagon, cortisol, epinephrine increase blood glucose.

Blood

Composition and Functions of Plasma

Plasma is the liquid component of blood, containing proteins, nutrients, and waste products.

• Plasma Proteins: Albumin, globulins, fibrinogen (see Table 17.1).

• Functions: Maintain osmotic pressure, transport substances, clotting.

Erythrocytes: Structure, Function, and Production

Red blood cells (RBCs) are specialized for oxygen transport.

• Hematocrit: Percentage of RBCs in blood.

• Structure: Biconcave shape, no nucleus; contains hemoglobin for gas transport and spectrin for flexibility.

• Erythropoiesis: Formation of RBCs from hematopoietic stem cells.

• Developmental Phases: Proerythroblast, erythroblast, reticulocyte (released into bloodstream).

• Lifespan: ~120 days; old RBCs are destroyed in the spleen.

• Stimulating Hormone: Erythropoietin (EPO).

• Iron: Essential for hemoglobin; stored as ferritin/hemosiderin, transported by transferrin.

Disorders of Erythrocytes

Abnormalities in RBCs can lead to various disorders.

• Anemia: Reduced oxygen-carrying capacity; classified as blood loss, not enough RBCs produced, or too many destroyed.

• Examples: Iron-deficiency anemia, sickle cell anemia.

• Polycythemia: Excess RBCs; increases blood viscosity.

• Blood Doping: Artificially increasing RBC count for athletic performance.

Leukocytes: Classes, Structure, and Function

White blood cells (WBCs) are key to immune defense and are classified by the presence of granules.

• Granulocytes: Neutrophils, eosinophils, basophils.

• Agranulocytes: Lymphocytes, monocytes.

• Identification: Based on nuclear shape and cytoplasmic granules.

• Functions: Neutrophils (phagocytosis), eosinophils (parasite defense), basophils (histamine release), lymphocytes (immunity), monocytes (macrophages).

Leukocyte Production and Disorders

Leukopoiesis is the formation of WBCs, regulated by chemical messengers.

• Leukopoiesis: Stimulated by interleukins and colony-stimulating factors (CSFs).

• Leukemia: Cancer of WBCs; abnormal proliferation.

Platelets: Structure and Function

Platelets are cell fragments essential for blood clotting.

• Origin: Derived from megakaryocytes.

• Chemicals: Contain serotonin, ADP, and thromboxane for clotting.

• Inactivation: Kept inactive by nitric oxide and prostacyclin in healthy vessels.

Hemostasis: Clot Formation and Regulation

Hemostasis is the process of stopping bleeding, involving several steps.

• Vascular Spasm: Triggered by injury; constricts blood vessels.

• Platelet Plug Formation: Platelets adhere and release chemicals (ADP, serotonin) to enhance aggregation.

• Coagulation: Involves clotting factors; two pathways (intrinsic and extrinsic) converge at Factor X activation.

• Slowest Step: Formation of prothrombin activator.

• Fibrin Formation: Fibrinogen is converted to fibrin, forming a mesh.

• Repair: Platelet-derived growth factor (PDGF) stimulates vessel repair.

Hemostatic Disorders

Disorders of clotting can lead to excessive or insufficient clot formation.

• Thrombus: Clot in an unbroken vessel.

• Embolus: Clot that travels in the bloodstream.

• Thrombocytopenia: Low platelet count.

• Hemophilia: Genetic disorder affecting clotting factors.

Additional info:

• Second messengers include cAMP ($cAMP$), IP3 ($IP_3$), and Ca2+ ($Ca^{2+}$).

• Hemoglobin structure: $Hb$ binds $O_2$ and $CO_2$.

• Coagulation pathways: Intrinsic and extrinsic pathways both activate Factor X ($X$).

Hormone	Source	Target	Major Effect
ADH	Posterior Pituitary	Kidneys	Water retention
Oxytocin	Posterior Pituitary	Uterus, Mammary glands	Contraction, milk ejection
TSH	Anterior Pituitary	Thyroid	Stimulates TH release
Insulin	Pancreas (Beta cells)	Most cells	Decreases blood glucose
Glucagon	Pancreas (Alpha cells)	Liver	Increases blood glucose
Cortisol	Adrenal Cortex	Many tissues	Stress response, metabolism