The Digestive System

The Digestive System

Overview

• Digestion breaks larger food particles into nutrients small enough to be absorbed into the bloodstream.
• The alimentary canal, also known as the gastrointestinal (GI) tract or digestive tract, is a long tube (approximately 30 feet) through which food passes from the mouth to the anal opening.
• Accessory organs, like the pancreas and liver, aid digestion by producing secretions that enter the GI tract.
• There are two main types of digestion:
  • Mechanical: Physical breakdown of food into smaller pieces without changing its chemical composition (e.g., tearing bread).
  • Chemical: Changing the chemical composition of food (e.g., enzymes breaking down polysaccharides into monosaccharides).

Organs of the Digestive System

• Mouth
• Tongue
• Salivary glands:
  • Parotid gland
  • Sublingual gland
  • Submandibular gland
• Pharynx
• Esophagus
• Stomach
• Small intestine:
  • Duodenum
  • Jejunum
  • Ileum
• Large intestine:
  • Cecum
  • Ascending colon
  • Transverse colon
  • Descending colon
  • Sigmoid colon
• Anus
• Accessory Organs:
  • Liver
  • Gallbladder
  • Pancreas
  • Spleen
• Appendix
• Rectum
• Anal canal

Histology of Alimentary Canal

• The alimentary canal has four tissue layers:
  • Mucosa: Innermost layer, with varying epithelium types depending on the location.
  • Submucosa: Includes blood and lymphatic vessels.
  • Muscularis: Double layer of smooth muscle (circularis and longitudinal).
  • Serosa: Outermost layer made of connective tissue, also known as adventitia or visceral peritoneum.

Peritoneum and Mesentery

• Mesentery: Holds digestive organs in place.
• Parietal peritoneum: Touches the abdominal wall.
• Visceral peritoneum: Touches the organ; also known as the serosa layer.
• A fluid-filled space exists between the parietal and visceral peritoneum.

Digestive Processes

• Chemical Digestion:
  • Starts in the mouth.
  • Breaks down:
    • Proteins into amino acids.
    • Carbohydrates into monosaccharides.
    • Lipids into fatty acids and glycerol.
    • Nucleotides into nitrogen bases.
• Absorption: Primarily occurs in the small intestine, where nutrients enter the bloodstream.
• Defecation: Removal of undigested materials.
• Ingestion: Entry of food.
• Propulsion
  • Ingestion: Entry of food into the digestive system.
  • Mastication: Chewing, which involves the use of teeth, cheeks, lips, and tongue to mechanically digest food in the mouth.

Propulsion

• Voluntary: Swallowing (deglutition) initiated when food is adequately chewed and pushed from the mouth into the pharynx.
• Involuntary: Peristalsis, wavelike contractions using circularis and longitudinal muscles, propels food through the digestive tract (similar to urine transport through ureters).
• Food is mechanically broken down in the mouth and reformed into a bolus using mucus from salivary gland secretions.
• Mucus also lubricates the bolus, facilitating its passage through the pharynx, esophagus, and large intestines.
• Longitudinal muscles in front of the bolus contract while circularis muscles relax.
• Circularis muscles behind the bolus contract while longitudinal muscles relax.

The Mouth

• Anatomical structures:
  • Oral cavity
  • Labia (lips)
  • Palate: Hard (anterior) and soft (posterior)
  • Tongue: Muscular
  • Teeth
  • Salivary glands

The Tongue

• Anatomical features:
  • Papillae: Bumps with taste buds; not all papillae are the same.
• Functions:
  • Facilitating ingestion.
  • Mechanical digestion (chewing/mastication & swallowing/deglutition).
  • Sensation: Taste.
  • Vocalization.

The Salivary Glands

• Three pairs of glands secrete saliva, each with unique secretory constituents:
  • Sublingual glands: Located just below the tongue; produce the thickest secretion with the least amount of salivary amylase.
  • Parotid glands: Located in the upper jaw; produce a watery secretion.
  • Submandibular glands: Located in the lower jaw; produce a watery secretion.
• Saliva composition:
  • 95.5% water.
  • Salivary amylase: Enzyme used to break polysaccharides into disaccharides.

Teeth

• 20 deciduous teeth (baby or milk teeth):
  • Appear at about 6 months.
  • Are eventually replaced by permanent teeth.
• 32 permanent teeth:
  • 8 incisors: Sharp, used to bite into food.
  • 4 cuspids: Pointed edge, used to tear up food.
  • 8 premolars: Flatter shape, used to mash food.
  • 12 molars: Several pointed cusps, used to crush food.

Tooth Anatomy

• Crown: Portion above the gum line.
  • Enamel: Protective covering, harder than bone.
• Neck: Transition from crown where enamel is not present.
• Root: Embedded within the maxilla and mandible.
  • Cementum & periodontal ligament hold it in place.
• Pulp cavity:
  • Loose connective tissue.
  • Contains blood vessels and nerves.
  • Surrounded by dentin.

Pharynx

• Involved in both digestion and respiration.
• Runs from posterior oral and nasal cavities to the opening of the esophagus and larynx.
• Three subdivisions:
  • Nasopharynx
  • Oropharynx
  • Laryngopharynx

Esophagus

• Muscular tube, approximately 10 inches in length.
• Located posterior to the trachea.
• Function: Passage of food.
• Sphincters:
  • Upper esophageal sphincter.
  • Lower esophageal (aka cardiac) sphincter.
• Histology:
  • Stratified squamous epithelium.
  • Mucosa: Secretes mucus to facilitate food passage.
  • Muscularis: Circularis and longitudinal muscles for peristalsis.

The Stomach

• Primary function: Storage of ingested food (limited chemical digestion).
• Four regions:
  • Cardia: Entry from the esophagus.
  • Fundus: Pouch above the cardia.
  • Body: Main area.
  • Pylorus: Exit area into the small intestines/duodenum.
• Curvatures:
  • Greater curvature: Outer.
  • Lesser curvature: Inner.

Stomach Lining and Muscularis

• The muscularis region has an extra layer of muscle (oblique) in addition to the circularis and longitudinal layers.
• These muscles facilitate propulsion and mixing of food within the stomach.
• The pyloric sphincter regulates the emptying of stomach contents into the duodenum, allowing only small amounts to pass through at a time.
• Most of the stomach contents are retropulsed back into the stomach for further mixing.
• This churning is crucial for mechanical and chemical digestion, effectively blending the food.

Stomach Histology

• Rugae: Ridges that allow for stomach expansion after large meals.
• Gastric pits: Small holes in the mucosa lining.
• Mucosa cell types:
  • Mucous (neck) cells: Line the surface and upper portion (neck) of the pits; secrete mucus to protect the stomach wall.
• Chyme: Collective term for food and enzymes in the stomach.
• The mucosal barrier protects the stomach from self-digestion/ulcers.
  • Alkaline-rich mucus (neutralizes acid).
  • Tight junctions between epithelial cells.
  • Stem cells quickly replace damaged cells.
  • Parietal cells: Secrete HCl and intrinsic factor (HCl "activates" pepsinogen, and intrinsic factor is necessary for the absorption of vitamin B12).
  • Chief cells: Secrete pepsinogen (enzyme that breaks down proteins when activated).
  • Enteroendocrine cells: Secrete hormones that control digestion.

Small Intestine

• Three regions:
  • Duodenum: First 10 inches; connects to the stomach; contains Brunner’s glands that secrete bicarbonate to neutralize the acid from the stomach.
  • Jejunum: 3 feet.
  • Ileum: 6 feet.
• The mucosal epithelial cells vary among the three regions.
• The small intestines produce enzymes for chemical digestion.
• Secretions from the liver (bile) and pancreas are emptied into the duodenum.

Pancreas

• Functions as both an endocrine (hormone) and exocrine (digestive) organ.
• Produces enzymes for breaking down proteins, carbohydrates, lipids, and nucleotides.
• The pancreatic duct merges with the common bile duct (from the gallbladder) to empty into the duodenum.

Digestion within the Small Intestine

• Circular folds, villi, and microvilli increase the surface area for nutrient absorption.
• With the help of secretions from the pancreas and gallbladder, the small intestines complete the chemical digestion of ingested food and absorb the resulting nutrients into the bloodstream.
• Mechanical digestion:
  • Segmentation: Divides food into smaller pieces.
• Chemical digestion: Breaks down proteins, carbohydrates, lipids (fats), and nucleic acids (DNA/RNA).

Large Intestines

• Six regions:
  • Cecum: First part of the large intestine; pouch below the ileum (appendix is located here); ileocecal sphincter.
  • Colon: Ascending, transverse, descending, sigmoid (S-shaped).
  • Rectum: Muscular portion.
  • Anal canal: Not part of the large intestine; contains sphincters (internal and external) to control the elimination of feces (defecation).
• Majority of the large intestine is simple columnar epithelium.
• Functions:
  • Absorption
  • Goblet cells
  • Teniae coli: Bands of smooth muscle; longitudinal layer of muscularis.
• Haustra: Pouches that result from tonic contractions of teniae coli, giving the large intestine its “pouchy” appearance.
• The arrangement and movement (contractions) within the large intestines facilitate water absorption.

Accessory Organs

• Liver: Produces bile (from the breakdown of hemoglobin); bile is stored in the gallbladder until needed.
• Bile is released when consuming fatty meals and emulsifies fats (NOT an enzyme).
• The liver has numerous vital functions.
• Has two primary lobes (four total); the right lobe is larger.

Liver Histology

• Hepatocytes: Liver cells.
• Bile canaliculus: Ducts that collect bile.
• Hepatic sinusoid: Space within the liver.

Gallbladder

• Right and left hepatic ducts from the respective lobes join to form the common hepatic duct.
• The common hepatic duct joins the cystic duct (from the gallbladder) to form the common bile duct.
• The common bile duct joins with the pancreatic duct.