Human Body, Digestion, and Absorption

Background

• Composed of cells, tissues, organs, organ systems

• All cells use ATP for reactions

• Four primary types of tissue: epithelial, connective, muscle, and nervous

• Organs have at least two or more types of tissue

• Organ systems:

Digestive System Overview Diagram

Alimentary Canal

• Also called gastrointestinal tract
• Composed of
  • Mouth Diagram
    • Two physical processes occur:
      • Mastication (chewing) - cheeks and closed lips hold the food between the teeth, the tongue mixes the food with saliva to soften it, and the teeth cut and grind solid food
      • Deglutition (swallowing) - food compacted by tongue into bolus
  • Tongue
    • Contains taste buds that detect food
    • Four tastes detected: sweet, salty, bitter, sour
  • Pharynx
    • Moves food from mouth to esophagus
  • Esophagus
    • Muscular tube leading to the stomach
  • Stomach Diagram
    • Temporary storage tank and converts bolus into chyme
    • Composed of three sections (1st third = Cardiac region, 2nd third = Body, and last third = Pyloric Region)
    • Stomach cells and what they produce:
      • Mucus cells - produces mucus
      • Parietal cells - produce hydrochloric acid and intrinsic factor
      • Chief cells - pepsinogen (inactive form of protein digestive enzyme)
      • Enteroendocrine cells - produces:
        • gastrin - cause gastric glands to increase activity
        • secretin - stimulates pancreas to produce bicarbonate to regulate pH
        • histamine - activates parietal cells to release hydrochloric acid
        • serotonin - causes contraction of stomach
        • cholecystokinin - potentiates secretin's action on liver/pancreas (increase bile and pancreas "juice")
        • somatostatin - inhibits gastric secretion of all products and inhibits gastric motility and emptying
  • Small intestine
    • Composed of three sections (duodenum, jejunum, and ileum)
    • Site of absorption of nutrients
      • types: passive absorption, facilitated absorption, active absorption, and endocytosis (phagocytosis and pinocytosis)
    • Huge surface area with modified structures called villi and microvilli
    • Villi - Diagram fingerlike projections of mucosa, contains blood capillaries and lacteals (small lymph capillaries)
    • Microvilli - tiny projections on cell membranes of absorptive (brush border) cells that contain enzymes (brush border enzymes) used to complete digestion
  • Large intestine
    • Diagram
    • Three sections: ascending, transverse, and descending colon
    • Site of water absorption and packaging waste material
    • Associated with the colon is the appendix
  • Colon
    • End of large intestine - sigmoid colon
    • Rectum and Anal canal at very end of colon

Accessory Organs

• Include
  • Teeth Diagram
  • Gallbladder
    • Stores and releases bile
  • Salivary glands
    • Functions: (1) cleans mouth, (2) moisten and dissolve food, and (3) contains enzymes
    • Composition of Saliva:
      • 97-99.5% water
      • pH 6.75-7.0
      • Sodium, potassium, chloride, phosphate, and bicarbonate
      • Mucin
      • Salivary amylase (breaks down starch)
  • Liver
    • Largest gland/organ in the body
    • Liver cells
      • process blood-borne nutrients
      • store fat-soluble vitamins
      • detoxify substances
      • produce bile (used to emulsify fats)
  • Pancreas
    • Produces hormones insulin and glucagon that help regulate glucose levels in the blood
    • Produces pancreatic juices (enzymes) used in hydrolysis of carbohydrates, lipids, and proteins

Digestive Process Diagram

1. Ingestion
2. Propulsion (swallowing and peristalsis)
3. Mechanical Digestion (chewing, churning, segmentation)
4. Chemical Digestion (acid and enzymes)
5. Absorption (lymph and blood vessels)
6. Defecation

Absorption of Nutrients

1. Passive movements (diffusion, osmosis, and facilitated diffusion)
2. Active Movements (active transport, endocytosis, and exocytosis)
3. Question: how long does it take for food to move through the body (i.e. mouth to anus)?

Regulation of the Digestive and Absorptive Process Diagram

i. Digestive activity is provoked by a range of mechanical and chemical stimuli

• Mechanoreceptors (detect movement) and chemoreceptors (detect chemicals) embedded in the lining of the GI tract
• Receptors respond to stimuli (stretching of the organ, osmolarity and pH of the contents, and the presence of substrates and end products of digestion)
• When stimulated, receptors initiate reflexes
  • activate or inhibit glands that secrete digestive juices into the lumen or hormones into the blood
  • mix lumen (organ space) contents and move them the length of the digestive tract

ii. Controls of digestive activity are both extrinsic and intrinsic

• Intrinsic - nerve plexuses and local hormone producing cells
• Extrinsic - central nervous system centers and autonomic nerves

Chemical Digestion

• Most food substance(s) are broken down by hydrolysis

Carbohydrates

• Salivary amylase in mouth - hydrolyzes starch
• Pancreatic amylase in small intestine - hydrolyzes complex carbohydrates
• Brush border enzymes in small intestine
  • Dextrinase - hydrolyzes dextrins
  • Lactase - hydrolyzes lactose
  • Maltase - hydrolyzes maltose
  • Sucrase) - hydrolyzes sucrose

Proteins

• Pepsin in stomach - cleaves peptide bonds
• Pancreatic enzymes (trypsin, chymotrypsin, elastase, carboxypeptidases) in small intesine
• Brush border enzymes (aminopeptidases, carboxypeptidases, and dipeptidases) in small intestine

Fats

• Bile salts (bile) in small intestine - hydrolyzes lipids
• pancreatic lipase in small intestine - hydrolyzes lipids

Nucleic acids

• Pancreatic ribonuclease and deoxyribonuclease in small intestine - hydrolyzes RNA and DNA
• Brush border enzymes (nucleosidases and phosphatases) in small intestine - hydrolyzes portions of nucleic acids

Clinical Related problems with the Digestive System

• Ulcers
• Indigestion (Dyspepsia)
• Difficulty swallowing (Dysphagia)
• GERD - gastroesophageal reflux disease
• Constipation
• Diverticulosis
• Appendicitis
• Colitis
• Hemorrhoids
• Irritable Bowel Syndrome
• Diarrhea