Digestive System Overview

The Digestive System

• Fuel provider for the body's cells
• Composed of two groups of organs
  • Alimentary canal (GI)
    • Digestion and absorption of food
    • Mouth, pharynx, esophagus, stomach, small intestine, large intestine
  • Accessory digestive organs
    • Teeth, tongue, gallbladder
    • Digestive glands
      • Salivary glands
      • Liver
      • Pancreas

Six Functions of the Digestive System

• Ingestion: Intake of food and drink through the mouth
• Mechanical processing: Crushing of food for easier movement through the digestive tract
  • Increases surface area for enzyme action
  • Begins in the oral cavity with teeth and tongue
• Chemical processing: Chemical breakdown of food into absorbable sizes
• Secretion: Release of water, acids, enzymes, and buffers into the lumen of the digestive tract
• Absorption: Movement of small organic molecules, electrolytes, and water across the digestive epithelium and into the interstitial fluid of the digestive tract
• Excretion: Removal of waste products from the body via feces

The Mouth

• Parotid gland
• Sublingual gland
• Submandibular gland
• Tongue

The Teeth

• Perform chewing (mastication)
• Tooth Parts
  • Neck: Boundary between root and crown
  • Enamel: Covers the crown
    • Hardest biologically manufactured substance
    • Requires Ca2+, phosphate, and vitamin D3 for formation
  • Dentin: Majority of the tooth
  • Pulp cavity: Contains blood vessels and nerves entering through root canals
  • Periodontal ligament: Connects to bone in tooth socket (alveolus) with cementum

Types of Teeth

• Incisors: Blade-shaped teeth at the front of the mouth, used for clipping or cutting
• Cuspids (canines): Sharp ridgeline and pointed tip, used for tearing or slashing
• Bicuspids (premolars) and molars: Flattened crowns with ridges, used for crushing, mashing, and grinding

Dental Formulas

• Shorthand representation of the number and position of teeth
  • Ratio of upper to lower teeth for one-half of the mouth
  • Primary: 2I, 1C, 2M
  • Permanent: 2I, 1C, 2PM, 3M

Dental Succession

• Two sets of teeth form during development
  • Deciduous teeth (primary, baby, or milk teeth): First to appear, generally 20 in number
    • Periodontal ligaments and roots erode during adult tooth eruption
  • Secondary dentition (permanent dentition): Usually 32 in number
    • Third molars (wisdom teeth) are the last to erupt

The Pharynx

• Also known as the throat
• Common passageway for food, liquid, and air
• Food passes through the oropharynx and laryngopharynx to the esophagus
• Mucosa is stratified squamous epithelium
• Lamina propria contains mucous glands and tonsils
• Pharyngeal muscles work with oral cavity and esophageal muscles for swallowing

The Esophagus

• Muscular tube (25 cm long and 2 cm wide) that connects the pharynx to the stomach
• Located posterior to the trachea
• Enters the abdominal cavity through the esophageal hiatus in the diaphragm
  • Hiatal hernia involves movement of abdominal organs upward through the esophageal hiatus
• Lined with stratified squamous epithelium
• Circular muscles at either end form the upper and lower esophageal sphincters

Lips and Cheeks

• Contain orbicularis oris and buccinator muscles
• Vestibule: Recess internal to lips and cheeks, external to teeth and gums
• Oral cavity: Located within the teeth and gums
• Labial frenulum: Median attachment of each lip to the gum

The Stomach

• Four primary functions
  • Storage of ingested food
  • Mechanical breakdown of food
  • Chemical digestion of food, particularly proteins
  • Production of intrinsic factor, required for vitamin B12 absorption

Gastric Secretions

• Gastric juice is a mixture of water, electrolytes, enzymes, and hydrochloric acid (HCl)
• Parietal cells produce HCl
• Chief cells produce pepsinogen (inactive form of pepsin)
• Mucus cells secrete alkaline mucus for protection
• G cells produce gastrin, a hormone that stimulates gastric acid secretion

Digestion in the Stomach

• Pepsin initiates protein digestion into small peptides
• Salivary amylase digests carbohydrates until pH falls below 4.5
• No nutrients are absorbed in the stomach

The Small Intestine

• Three segments: Duodenum, jejunum, and ileum
• Most digestion and absorption occur here
• Surface area is increased by:
  • Circular folds
  • Villi
  • Microvilli

Intestinal Secretions

• Mucus, enzymes, hormones:
  • Brunner's glands in the duodenum secrete alkaline mucus
  • Intestinal enzymes (e.g., peptidases, lactase, sucrase, maltase) digest various nutrients
  • Hormones (e.g., secretin, cholecystokinin (CCK), gastric inhibitory peptide (GIP)) regulate digestive processes

The Liver

• Largest gland in the body
• Performs numerous vital functions:
  • Bile production: Breaks down fat
  • Protein synthesis
  • Storage of glucose, vitamins, and iron
  • Detoxification of the blood
  • Production of blood clotting factors

The Gallbladder

• Small sac located beneath the liver
• Stores bile until it is released into the duodenum

The Pancreas

• Endocrine gland: Produces hormones like insulin and glucagon
• Exocrine gland: Produces pancreatic juice containing enzymes for digestion:
  • Pancreatic amylase (carbs)
  • Pancreatic lipase (fats)
  • Proteases (proteins)

Absorption in the Small Intestine

• Most ingested nutrients are absorbed here
• Mechanisms include:
  • Simple diffusion
  • Facilitated diffusion
  • Active transport
  • Osmosis

Monosaccharide Absorption

• Glucose, fructose, and galactose are absorbed from the small intestine into the bloodstream
• Glucose and galactose are actively transported via the sodium-glucose cotransporter (SGLT1)
• Fructose is absorbed by facilitated diffusion

Amino Acid Absorption

• Dipeptides and tripeptides are transported into intestinal cells by specific carriers
• Within cells, these peptides are broken down into individual amino acids
• Amino acids are then transported into the bloodstream

Lipid Absorption

• Lipids are broken down by pancreatic lipase into fatty acids and monoglycerides
• These products are absorbed into the intestinal cells and re-assembled into triglycerides
• Triglycerides are packaged into chylomicrons and transported into the lymphatic system

Vitamin Absorption

• Fat soluble vitamins (A, D, E, K) are absorbed with dietary fats
• Water soluble vitamins (B complex and C) are absorbed by diffusion or active transport

Water Absorption

• Most water is absorbed by osmosis in the small intestine
• The absorption is coupled with solute uptake, creating an osmotic gradient

Minerals

• Inorganic ions released by dissociation of electrolytes
• Important for:
  • Determining osmotic concentration of body fluids
  • Physiological processes (membrane potential, muscle contraction, action potentials, buffers)
  • Cofactors in enzymatic reactions

Potassium (K+)

• Absorbed by simple diffusion
• Water absorption increases luminal K+ concentration
• Diarrhoea can lead to K+ loss

Calcium (Ca+)

• Absorption is strongly controlled by reflexes due to changes in plasma calcium
• Regulation is mediated by Vitamin D

Iron (Fe)

• Ferrous (Fe2+) and ferric (Fe3+) are actively transported into the intestinal epithelium
• Incorporated into Ferritin and transported out of enterocyte by ferroportin (Hepcidin mediated)
• Hepcidin is a key regulator of iron entry into circulation in mammals
• Released into the bloodstream and bound to transferrin

Water

• Daily requirement averages 2500 mL/day
  • Can vary based on activity and body temperature
• Most comes from consumed food and water
• Small amount produced in mitochondria by electron transport system (metabolic water, about 300mL/day)

Water Absorption

• 95% absorbed in the small intestine by osmosis
• Net osmosis occurs because of concentration gradients established by active transport of solutes
• Water uptake is linked to solute uptake

The Large Intestine

• Four segments: Cecum, ascending colon, transverse colon, descending colon, sigmoid colon, and rectum
• Primarily involved in:
  • Water and electrolyte absorption
  • Formation and storage of feces
  • Bacterial fermentation of undigested material

Defecation

• Elimination of feces from the rectum
• Triggers:
  • Stretch receptors in rectal walls stimulate local peristalsis
  • Increased distention in the rectum
• Reflexes:
  • Long reflex: Mediated by parasympathetic innervation causing internal anal sphincter relaxation
  • Somatic reflex: Stimulates external anal sphincter contraction (skeletal muscle)

Control of Digestive Functions

• Local Factors
  • Prostaglandins, histamine, and other chemicals affect adjacent cells within the digestive tract
  • Coordinate responses to changing conditions (pH, chemical or physical stimuli)
  • Affect only a portion of the tract

Neural Mechanisms

• Control movement of material and secretory functions
• Motor neurons: Control smooth muscle contraction and glandular secretion (myenteric plexus)
• Short reflexes: Local reflexes controlling small segments of the digestive tract
  • Sensory neurons
  • Motor neurons
  • Interneurons
• Long reflexes: Higher level control of digestive and glandular activities
  • Involve interneurons and motor neurons in the CNS
  • May involve parasympathetic motor fibers that synapse in the myenteric plexus
    • Glossopharyngeal, vagus, or pelvic nerves

Hormonal Mechanisms

• At least 18 peptide hormones affect:
  • Most digestive functions
  • Activities of other systems
• Produced by enteroendocrine cells in the digestive tract
• Distributed via bloodstream to target organs

Short Bowel Syndrome

• A malabsorption disorder caused by a significant reduction in the length of the small intestine
• Results in:
  • Decreased nutrient absorption
  • Increased nutrient losses in feces
  • Potential dehydration
  • Electrolyte imbalances

Effects of SBS on GIT function

• Impaired digestion and absorption of nutrients
• Reduced surface area for nutrient uptake
• Rapid transit of food through the gastrointestinal tract
• Increased risk of malnutrition, dehydration, and electrolyte imbalances
• Potential for functional damage to the remaining small intestine

Effects of SBS on Health

• Malnutrition: Weight loss, anemia, vitamin deficiencies
• Dehydration: Diarrhea, electrolyte imbalances, fatigue
• Osteoporosis: Calcium malabsorption
• Impaired growth and development (in children)
• Increased risk of intestinal infections
• Potential for long-term intestinal failure requiring specialized treatments such as parenteral nutrition (PN)