Endocrine System

Hormone Action & Target Cells

The endocrine system regulates body functions through hormones, which act on specific target cells. Hormones can be classified based on their chemical structure and mechanism of action.

• Steroid Hormones: Lipid-soluble hormones derived from cholesterol. They can cross cell membranes and bind to intracellular receptors, influencing gene expression directly.

• Amino Acid-Based Hormones: Water-soluble hormones (e.g., peptides, proteins) that bind to membrane receptors, often activating second messenger systems (e.g., cAMP, Ca2+).

• Second Messengers: Substances like cAMP, Ca2+, and IP3 that relay signals from membrane receptors to intracellular targets.

• Target Cell Specificity: A cell must possess the correct receptor to respond to a hormone.

Example: Steroid hormones (e.g., estrogen) enter cells and regulate transcription, while peptide hormones (e.g., insulin) act via membrane-bound receptors and second messengers.

Pituitary & Hypothalamus

The pituitary gland is divided into anterior and posterior regions, each with distinct functions and hormone sources.

• Anterior Pituitary: Produces hormones that stimulate other endocrine glands (e.g., TSH, ACTH) and hormones affecting non-endocrine tissues (e.g., GH, prolactin).

• Posterior Pituitary: Stores and releases hormones (ADH, oxytocin) produced in the hypothalamus; not a true endocrine gland.

Example: ADH is produced in the hypothalamus and released by the posterior pituitary.

Major Hormones

Key hormones, their sources, and effects:

• ADH (Antidiuretic Hormone): Source: Posterior pituitary; Effect: Water retention, vasoconstriction.

• Aldosterone: Source: Adrenal cortex; Effect: Increases Na+ reabsorption.

• Prolactin: Source: Anterior pituitary; Effect: Promotes milk production.

• Thyroid Hormone (Thyroxine): Source: Thyroid gland; Effect: Increases basal metabolic rate and heat production.

• PTH (Parathyroid Hormone): Source: Parathyroid glands; Effect: Increases blood Ca2+, affects bone tissue.

• Erythropoietin (EPO): Source: Kidneys; Effect: Stimulates RBC production in response to low O2.

• Estrogen & Progesterone: Source: Ovaries; Effect: Female reproductive development.

• hCG (Human Chorionic Gonadotropin): Source: Placenta; Effect: Maintains pregnancy.

• Glucocorticoids (Cortisol): Source: Adrenal cortex; Effect: Stress response, metabolism regulation.

• Catecholamines: Source: Adrenal medulla; Effect: Fight-or-flight response.

Hormonal Disorders

Common endocrine disorders and their causes:

• Cushing's Syndrome: Hypersecretion of cortisol (adrenal cortex).

• Addison's Disease: Hyposecretion of adrenal cortex hormones.

• Graves' Disease: Hypersecretion of thyroid hormone.

• Myxedema: Hyposecretion of thyroid hormone.

• Chronic PTH Elevation: Leads to bone resorption and decreased bone density.

Digestive System

Digestion vs. Metabolism

Digestion is the breakdown of food in the GI tract, while metabolism encompasses all biochemical reactions in the body.

• Catabolism: Breakdown of molecules for energy.

• Anabolism: Building larger molecules from smaller ones.

• Metabolism: Sum of all biochemical reactions.

Example: Digestion breaks down carbohydrates; metabolism includes glycolysis, gluconeogenesis, etc.

Functions of Digestive Organs

Major functions of digestive organs:

• Oral Cavity: Mechanical digestion, some chemical digestion.

• Esophagus: Propulsion of food.

• Stomach: Mechanical and chemical digestion, protein breakdown.

• Small Intestine: Chemical digestion, absorption.

• Large Intestine: Water absorption, bacterial fermentation.

Layers of the Alimentary Canal

The GI tract has four basic layers, from lumen outward:

• Mucosa: Secretion, absorption, protection.

• Submucosa: Blood vessels, nerves, support.

• Muscularis Externa: Motility (peristalsis, segmentation).

• Serosa: Protective outer layer.

GI Motility

Movements in the GI tract:

• Peristalsis: Propels food forward.

• Segmentation: Mixes food, enhances absorption.

• Bolus: Food mass; becomes chyme in the stomach.

Stomach Physiology

Gastric Acid

Functions of stomach acid (low pH):

• Protein digestion (denatures proteins).

• Activates enzymes (e.g., pepsinogen to pepsin).

• Protects against microorganisms.

Stomach Cells & Secretions

Major stomach cell types and their products:

• Chief Cells: Secrete pepsinogen (inactive).

• Parietal Cells: Secrete HCl and intrinsic factor.

• G Cells: Secrete gastrin (stimulated by peptides, stomach stretch).

Pepsinogen is activated to pepsin by HCl.

Protection of the Stomach

The stomach protects itself from acid and enzymes via:

• Bicarbonate-rich mucus.

• Tight junctions between epithelial cells.

• Rapid replacement of epithelial cells.

• Inactive proteases (e.g., pepsinogen).

Regulation of Digestion

Enteric Nervous System

Digestive regulation involves short and long reflexes:

• Short Reflexes: Local responses within the GI tract.

• Long Reflexes: Involve CNS; allow integration of external stimuli.

GI Hormones

Key GI hormones, their sources, and actions:

• Gastrin: Source: Stomach; Action: Stimulates gastric acid secretion.

• Secretin: Source: Duodenum; Action: Stimulates bicarbonate secretion from pancreas, inhibits gastric activity.

• Cholecystokinin (CCK): Source: Duodenum; Action: Stimulates bile release, pancreatic enzyme secretion.

• GIP (Gastric Inhibitory Peptide): Source: Small intestine; Action: Inhibits gastric activity, stimulates insulin release.

Secretin & CCK

• Stimulate accessory organs (pancreas, liver, gallbladder).

• Regulate digestive secretions and bile flow.

• Inhibit stomach emptying.

Gastric Phase

Stimuli promoting gastric secretion:

• Stomach stretch.

• Presence of peptides.

• Changes in gastric pH.

Digestive Secretions & Enzymes

Saliva

Major components of saliva:

• Water, electrolytes, mucus, amylase, lysozyme.

• Amylase digests carbohydrates.

Digestive Enzymes

Enzymes and their substrates:

• Carbohydrates: Amylase, maltase, sucrase, lactase.

• Proteins: Pepsin, trypsin, chymotrypsin.

• Lipids: Lipase.

Example: A meal high in carbohydrates requires amylase for digestion.

Vitamin Absorption

Vitamin B12 requires intrinsic factor for absorption. Without intrinsic factor, B12 absorption is impaired, leading to anemia.

Large Intestine

Bacterial Functions

Bacteria in the large intestine:

• Produce vitamins (e.g., vitamin K, some B vitamins).

• Ferment undigested carbohydrates.

Nutrient Absorption

Lipid Absorption

Lipid absorption involves several steps:

• Fatty acids and monoglycerides form micelles.

• Micelles deliver lipids to enterocytes (intestinal cells).

• Inside enterocytes, lipids are reassembled into triglycerides and packaged as chylomicrons.

• Chylomicrons enter lymphatic vessels before reaching the bloodstream.

Nutrition & Metabolism

Carbohydrates

Fates of absorbed carbohydrates:

• Glycolysis: Glucose breakdown for energy.

• Glycogenesis: Storage of glucose as glycogen.

• Glycogenolysis: Release of glucose from glycogen.

• Lipogenesis: Conversion of excess glucose to fat.

• Gluconeogenesis: Production of glucose from non-carbohydrate sources.

Proteins

• Complete Protein: Contains all essential amino acids.

• Amino acids cannot be stored; excess is deaminated.

• Deaminated amino acids can be used for energy; nitrogen is excreted as urea.

Deamination & Transamination

• Deamination: Removal of amino group, producing ammonia (converted to urea).

• Transamination: Transfer of amino group to another molecule.

• Major waste product: Urea, excreted in urine.

Lipids

• Lipolysis: Breakdown of triglycerides into glycerol and fatty acids.

• Lipogenesis: Formation of triglycerides from glycerol and fatty acids.

• Excess fat metabolism produces ketone bodies.

Catabolism & Chemical Reactions

• Hydrolysis: Breaking molecules apart using water (common in digestion).

• Dehydration Synthesis: Building larger molecules by removing water.

Cellular Respiration

Stages of Cellular Respiration

Cellular respiration is the process of oxidizing glucose to produce ATP. The stages are:

1. Glycolysis: Occurs in cytoplasm; glucose is converted to pyruvate, producing ATP and NADH.

2. Citric Acid/Krebs Cycle: Occurs in mitochondria; acetyl-CoA enters, producing CO2, ATP, NADH, FADH2.

3. Electron Transport Chain: Occurs in mitochondria; NADH and FADH2 donate electrons, creating a proton gradient.

4. Oxidative Phosphorylation: ATP is produced as protons flow through ATP synthase.

Equation:

Glycolysis

• Input: Glucose.

• Output: Pyruvate, ATP, NADH.

• Does not require oxygen directly.

• ATP produced by substrate-level phosphorylation.

Citric Acid/Krebs Cycle

• Input: Acetyl-CoA.

• Output: CO2, NADH, FADH2, ATP.

• Carbon atoms are released as CO2.

• Electron carriers (NADH, FADH2) are produced.

Electron Carriers

• NADH: Carries high-energy electrons to the electron transport chain.

• FADH2: Also carries electrons; contributes less ATP than NADH.

ATP yield: FADH2 yields approximately 1.5 ATP per molecule.

Oxidation-Reduction Reactions

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

ATP Production

• Substrate-Level Phosphorylation: Direct transfer of phosphate to ADP.

• Oxidative Phosphorylation: ATP produced via electron transport chain; majority of ATP is produced here.

Electron Transport Chain & Oxidative Phosphorylation

• NADH and FADH2 donate electrons, driving ATP synthesis.

• Oxidative phosphorylation produces much more ATP than glycolysis or Krebs cycle.

Lipoproteins & Cholesterol

LDL vs. HDL

Example: LDL is associated with cholesterol delivery; HDL with cholesterol removal.

Summary: Key Exam Concepts

• Compare steroid and amino acid-based hormone actions.

• Explain hormone specificity and receptor importance.

• Identify sources and functions of major hormones.

• Match endocrine disorders with hormone/gland and secretion type.

• Distinguish digestion, metabolism, catabolism, anabolism.

• Order alimentary canal layers.

• Explain stomach acid functions and protection mechanisms.

• Match stomach cells with secretions.

• Explain inactive protease secretion.

• Compare GI reflexes.

• Describe actions of gastrin, secretin, CCK.

• Match enzymes with nutrients.

• Trace lipid absorption and transport.

• Define complete protein and amino acid metabolism.

• Distinguish metabolic pathways (glycogenesis, etc.).

• Compare deamination and transamination.

• Explain ketone body production.

• Sequence cellular respiration stages.

• Identify inputs/outputs of glycolysis and Krebs cycle.

• Explain NADH and FADH2 roles.

• Distinguish oxidation/reduction.

• Compare ATP production mechanisms.

• Identify main ATP-producing process.

• Compare LDL and HDL functions.

Additional info: Academic context and explanations were expanded for clarity and completeness.