Digestive System Notes

Digestive System

Introduction

Humans obtain basic organic molecules from food for ATP production, tissue building, and cofactor/coenzyme functions.

  • Digestion: Polymers (carbohydrates, fats, and proteins) are broken down into monomers via hydrolysis reactions.
  • Absorption: Monomers are taken into the bloodstream for cell use.

Digestion Through Hydrolysis Reactions

  • Carbohydrates:
    • Disaccharides (e.g., Maltose) are broken down into monosaccharides (e.g., Glucose).
    • Disaccharide+H2O→Monosaccharides\text{Disaccharide} + H_2O \rightarrow \text{Monosaccharides}
  • Proteins:
    • Peptides are broken down into amino acids.
    • Peptide+H2O→Amino acids\text{Peptide} + H_2O \rightarrow \text{Amino acids}
  • Lipids:
    • Triglycerides are broken down into fatty acids and glycerol.
    • Triglyceride+3H2O→Fatty acids+Glycerol\text{Triglyceride} + 3H_2O \rightarrow \text{Fatty acids} + \text{Glycerol}

The Digestive Tract

The digestive tract is open at both ends and continuous with the external environment.

  • Materials that cannot be digested (e.g., cellulose) never truly "enter" the body.
  • One-way transport allows for functional specialization.

Anatomy of the Digestive System

  • Digestive Tract (all):
    • Oral cavity
    • Pharynx
    • Esophagus
  • Gastrointestinal Tract:
    • Stomach
    • Small intestine
    • Large intestine

Accessory Organs

  • Teeth
  • Salivary glands
  • Pancreas
  • Liver
  • Gallbladder

Digestive Processes

  • Motility: Movement of food through the digestive system.
  • Secretion: Release of substances into the lumen to enhance food breakdown.
  • Digestion: Physical and chemical breakdown of food.
  • Absorption: Transfer of materials from the lumen to the internal environment.
  • Excretion: Temporary storage of undigested material followed by expulsion from the body.

Four Basic Digestive Processes

  1. Motility: Food moves through the digestive tract.
  2. Secretion: Release of enzymes and other substances to aid digestion.
  3. Digestion: Breakdown of food particles into smaller nutrient molecules.
  4. Absorption: Movement of nutrient molecules into the cardiovascular system.

Organs of Digestion

[Diagram of the digestive system]

Upper Digestive Tract

  • Nasal Cavity
  • Oral Cavity
  • Pharynx (Nasal, Oral)
  • Larynx/Trachea
  • Esophagus

Gastrointestinal Tract Structure

  • Mucosa (lumen side): Epithelial tissue
  • Submucosa: Elastic connective tissue containing lymph and blood vessels
  • Muscularis externa: Smooth muscle layers
  • Adventitia or Serosa: Outer layer of connective tissue that secretes serous fluid

Esophagus

  • Transports food and water to the stomach; secretes mucus.
  • Movement of food bolus via peristalsis
  • Empties into the stomach through the lower esophageal sphincter.

Stomach Location

  • The stomach is located near the liver, transverse colon, ascending colon and small intestine.

Stomach Functions

  • Stores food
  • Churns food to mix with gastric secretions
  • Begins protein digestion
  • Kills bacteria in food (due to acidity)
  • Moves food into the small intestine in the form of chyme

Stomach Details

  • Muscular, sac-like organ.
  • Chemical and physical digestion occur, forming chyme.
  • Initiates protein digestion.
  • Stores food and releases small amounts to the small intestine (empties in 2-6 hours).
  • The inner surface is lined with gastric rugae.

Stomach Mucosal Cells

  • Gastric glands (small folds in mucosa) contain specialized secretory cells:
    • Parietal cells:
    • Hydrochloric acid (HCl)
    • Intrinsic factor
    • Non-goblet mucous cells: Secrete mucus (Gastric Mucosal Barrier protects stomach epithelium)
    • Chief cells: Pepsinogen (digests protein)
    • Endocrine cells (into blood):
    • ECL cells: Histamine
    • G-cells: Gastrin
    • Ghrelin-secreting cells: Promotes hunger

Stomach Acid Secretion

  • Parietal cells secrete H+H^+ into the gastric lumen using active transport (H+/K+H^+/K^+ pump).
  • Cl−Cl^- follows H+H^+ from cells into the lumen.
  • HCl is used to:
    • Denature ingested proteins
    • Activate pepsinogen
    • Maintain catalytic activity of gastric enzymes
  • HCl secretion is stimulated by:
    • Histamine (from ECL cells)
    • Gastrin (from G cells; stimulates both parietal and ECL cells)

Hydrochloric Acid Production by Parietal Cells

[Diagram illustrating HCl production]

Protein Digestion in the Stomach

  • Pepsinogen (precursor enzyme) is secreted by chief cells into the lumen.
  • Low pH of gastric juice activates pepsinogen's protein-digestive activity.
  • Pepsinogen autocatalyzes itself into pepsin (fully activated, more effective at digesting protein).
  • Pepsin converts additional pepsinogen molecules into pepsin.
  • Pepsin cleaves proteins into shorter polypeptides.

Regulation of Gastric Function

Three basic phases:

  1. Cephalic phase:
    • Regulation of the stomach by the brain via the vagus nerve.
    • Stimulates G and ECL cells in response to stimuli associated with food.
  2. Gastric phase:
    • Arrival of food in the stomach.
    • Amino acids and short polypeptides stimulate pepsinogen and gastrin secretion.
  3. Intestinal phase:
    • Arrival of chyme in the small intestine stimulates a neural reflex that inhibits gastric motility and secretion.
    • Fats in chyme stimulate secretion of enterogastrones from the intestine, which inhibit stomach function.

Three Phases of Gastric Secretion

[Diagram illustrating the three phases]

Pyloric Sphincter

Regulates the passage of chyme between the stomach and small intestine.

Small Intestine

  • Duodenum
  • Jejunum
  • Ileum

Most digestion and absorption occur in the small intestine.

Small Intestine Structure and Function

  • Functions:
    • Completes digestion of carbohydrates, proteins, and fats.
    • Absorption of nutrients.
    • Sugars, lipids, amino acids, calcium, and iron are absorbed in the duodenum and jejunum.
    • Bile salts, vitamin B12, water, and electrolytes are absorbed in the ileum.
  • Very rapid due to villi and microvilli.

Small Intestine Details

  • Approximately 3 meters long in living adults.
    • Duodenum: First 20-30 cm
    • Jejunum: Next 2/5ths
    • Ileum: Remainder
  • Large surface area due to:
    • Plicae circularis: Circular folds in the mucosa
    • Villi: Fingerlike projections containing capillaries and a central lacteal (for absorption)

Small Intestine - Microvilli

  • Microvilli ("brush border") on the surface of mucosal cells.
  • Hold brush-border enzymes (bound to epithelial cell membranes).
    • These enzymes hydrolyze disaccharides, small peptides, etc.
    • Activate pancreatic enzymes (e.g., enterokinase activates trypsin).

Small Intestine Motility

  • Forms of movement:
    • Peristalsis: Propels chyme through the small intestine.
    • Segmentation: Major type of contraction; contractions occur at different segments to mix chyme with digestive secretions.

Small Intestine Motility Regulation

  • Stimulated through pacemaker potentials in smooth muscles.
  • Parasympathetic innervation stimulates motility.
  • Enteric nervous system:
    • Neurons within the walls of the digestive tract.
    • ~100 million neurons, complete with association neurons, sensory neurons, and autonomic motor neurons.
    • Controls digestive function independent of the CNS.

Liver - Circulatory Route

Absorbed material is transported via the hepatic portal vein to the liver and then to the vena cava.

Large Intestine Structure and Function

  • Functions:
    • Absorption of water, electrolytes, vitamin K, and some B vitamins.
    • Production of vitamin K and B vitamins via microbial organisms.
    • Storage of feces.

Large Intestine Details

  • Cecum, appendix, colon, rectum (5' long).
  • Salt, water, B vitamins, and vitamin K are absorbed here.
  • Main function is to store undigested material (feces).
  • Enormous number of symbiotic bacteria in the colon:
    • Synthesize vitamin K and folic acid.
    • Synthesize short-chain fatty acids (fuel, aid in mineral absorption).
    • Inhibit pathogenic bacteria.

Intestinal Microbiota

Benefits from Microbes:

  • Microbes make vitamin K and some B vitamins.
  • They also make fatty acids from cellulose; some are used for energy by large intestine epithelial cells. They help absorb electrolytes such as sodium, calcium, bicarbonate, magnesium, and iron.
  • They outcompete harmful species of bacteria.
  • Disruption of normal microflora can lead to inflammatory bowel disease.

Reflexes in the Colon

[Diagram illustrating reflexes in the colon]

Accessory Digestive Organs: Liver

  • Largest internal organ.
  • Consists of thin hepatic plates of hepatocytes separated by capillary spaces (sinusoids).
  • Organized into liver lobules.
  • Blood is delivered into the liver from:
    • Hepatic artery
    • Hepatic portal vein (transports blood from the GI tract)
  • Blood is delivered into the periphery of lobules and drains out via a central vein that empties into the hepatic vein.
  • Bile is secreted into bile canaliculi, which drain into bile ducts.

Liver Functions

  • Production and secretion of bile:
    • Stored in the gallbladder, secreted into the duodenum.
    • Bile salts are used for lipid emulsification.
    • Bilirubin secretion.
    • Bilirubin is derived from heme from the hemoglobin of destroyed red blood cells.
    • Bacteria in the intestine convert conjugated bilirubin into urobilinogen.

More Liver Functions

  • Detoxification of the blood.
    • Degradation of waste, hormones, drugs, etc.
  • Regulation of blood glucose.
    • Removes excess glucose from the blood and converts it to glycogen.
    • Can secrete glucose into the blood when needed.
  • Metabolism of lipids.
    • Converts cholesterol into bile salts and excretes it from the body.
    • Produces ketone bodies.
  • Synthesizes plasma proteins.
    • Albumins, clotting factors, and carrier proteins for nonpolar materials.

Accessory Digestive Organs: Pancreas

  • Produces Pancreatic Juice:
    • Bicarbonate: Neutralizes stomach acidity.
    • Enzymes:
    • Pancreatic amylase: Breaks down starch.
    • Trypsin and other proteases: Break down polypeptides.
    • Pancreatic lipase: Digests triglycerides.
    • Others (phospholipases, nucleases, etc.).
  • Pancreatic juice enters the duodenum through the duodenal papilla.

Pancreas - Exocrine Tissue

  • Pancreatic juice is secreted by exocrine tissue in the pancreatic acini.
  • Cells secrete enzymes principally in the form of zymogens (inactive enzymes).
  • Zymogens are activated in the small intestine.
    • E.g., enterokinase (brush border enzyme) converts trypsinogen into trypsin.
    • Trypsin converts other zymogens into active enzymes.

Regulation of Pancreatic Juice and Bile Secretion

  • Enterogastrone is released when chyme enters the duodenum through the pyloric sphincter (slows motility).
  • Secretin:
    • Stimulated by a drop in pH below 4.5 (from the influx of chyme from the stomach).
    • Stimulates the secretion of bicarbonate into the pancreatic juice.
    • Stimulates bile secretion from the liver.
  • Cholecystokinin (CCK):
    • Stimulated by fat and protein in the chyme.
    • Stimulates digestive enzyme secretion into pancreatic juice.
    • Stimulates bile secretion from the liver and contraction of the gallbladder (more bile is released into the duodenum).

Nutrient Digestion and Absorption: Carbohydrates

  • Amylase (secreted in saliva and pancreatic juice) breaks down starch (polysaccharide) into smaller units.
  • Small carbohydrates are digested into monosaccharides by brush border enzymes.
  • Absorbed into the mucosa.
  • Transported into the blood entering the hepatic portal system.

Enzyme Action and the Hydrolysis of Sucrose

[Diagram illustrating enzyme action]

Chemical Digestion Summary

  • Carbohydrate digestion:
    • Oral cavity: Polysaccharides are broken down into disaccharides by salivary amylase; smaller polysaccharides and maltose are also formed.
    • Small intestine: Pancreatic amylases break down disaccharides into monosaccharides.
  • Protein digestion:
    • Stomach: Proteins are broken down into small polypeptides by pepsin.
    • Small intestine: Pancreatic trypsin and chymotrypsin break down small polypeptides, pancreatic carboxypeptidase acts on them, and dipeptidases/carboxypeptidase/aminopeptidase break them down into amino acids.
  • Nucleic acid digestion:
    • Small intestine: Pancreatic nucleases break down DNA and RNA into nucleotides, and nucleotidases/nucleosidases/phosphatases break them down into nitrogenous bases, sugars, and phosphates.
  • Fat digestion:
    • Small intestine: Fat (triglycerides) is broken down by pancreatic lipase into glycerol, fatty acids, and monoglycerides.

Nutrient Digestion and Absorption: Protein

  • Pepsin in the stomach cleaves proteins into smaller polypeptides.
  • Pancreatic enzymes (e.g., trypsin) and brush border enzymes digest polypeptides into amino acids/small peptides (di- and tripeptides).
  • Transported into the mucosa.
  • Small peptides are broken into amino acids inside mucosal cells.
  • Transferred into the blood entering the hepatic portal system.

Lipids - Special Digestion/Absorption Problem

  • Lipids are not water-soluble.
  • They do not mix with stomach or intestinal contents.
  • They form fat droplets.

Lipid Digestion

  • Enzymes of digestion = lipases.
    • Secreted from the pancreas.
    • Lipases can only act on molecules near the edge of a fat droplet.
  • Bile salts increase the surface area of droplets by breaking large droplets into several small droplets = emulsification.

Bile Salts

  • Synthesized in the liver from cholesterol.
  • Secreted in bile to the duodenum.
  • Part polar, part non-polar molecule.
  • Emulsify fat.

Emulsification of a Fat Globule

  • Before: Little surface exposed to lipases.
  • After: Much more surface exposed to lipases.

Triglycerides Digestion

  • Pancreatic lipases break emulsified fat into free fatty acids and monoglycerides.
  • Absorbed by epithelium.

Nutrient Digestion and Absorption: Lipids

  • Reform triglycerides in the ER of epithelial cells.
  • Combined with protein in the Golgi Apparatus to form chylomicrons.
  • Chylomicrons are released via exocytosis into the submucosa.
  • Enter lacteals of villi.
  • Transported through the lymphatic system.
  • Enters the blood at the thoracic duct at the left subclavian vein.

Hypothalamus Function in Digestion/Nutrition

  • Caloric expenditure of the body depends on:
    • Basal metabolic rate
    • Adaptive thermogenesis (energy expended to maintain body temperature and to digest/absorb food)
    • Physical activity
  • Body mass homeostasis is achieved when caloric intake balances caloric expenditure.
  • The arcuate nucleus of the hypothalamus regulates hunger and feeding behavior.
    • Some neurons inhibit hunger.
    • Others stimulate hunger.

Hypothalamus Regulation

  • Arcuate nucleus is influenced by:
    • Other brain areas
    • GI tract hormones:
    • Ghrelin (secreted when the stomach is empty; stimulates hunger)
    • CCK (reduces appetite)
    • Polypeptide Y-Y (PYY): Intermediate-term hunger suppression
    • Adipose tissue:
    • Satiety factors (e.g., leptin) suppress hunger and elevate metabolic rate; released in response to increasing storage of fat in adipocytes.
    • Pancreas:
    • Insulin acts as a satiety factor