Overview
This lecture explains the structure and function of the digestive system, detailing its main organs, processes, and the journey of food through the gastrointestinal tract.
Digestive System Overview
- The digestive system includes the alimentary canal (GI tract) and accessory digestive organs.
- The GI tract is a continuous tube from mouth to anus, where food is broken down and absorbed.
- Accessory organs (teeth, tongue, gallbladder, salivary glands, liver, pancreas) assist with digestion but are not part of the GI tract.
Digestive Processes
- Six digestive actions: ingestion, propulsion, mechanical breakdown, chemical digestion, absorption, and defecation.
- Propulsion involves swallowing and peristalsis, which moves food through the GI tract.
- Segmentation is the local constriction that mixes food.
- Mechanical breakdown includes chewing and churning in the stomach.
- Chemical digestion uses enzymes to break polymers into absorbable monomers.
- Absorption is the passage of nutrients into blood and lymph.
- Defecation removes indigestible substances as feces.
Structure of the GI Tract
- Main layers: mucosa (secretes, absorbs, protects), submucosa (connective tissue with vessels and nerves), muscularis externa (movement), serosa (outermost protective layer).
- Two nerve plexuses: submucosal (controls glands and smooth muscle) and myenteric (controls GI tract motility).
- Peritoneum surrounds abdominal organs; serous fluid allows mobility.
Mouth, Pharynx, and Esophagus
- The mouth has a thick, protective lining and features like the palate, uvula, and tonsils.
- The tongue aids movement; the lingual frenulum secures it.
- Salivary glands produce saliva with enzymes; teeth carry out mastication (chewing).
- The esophagus is a muscular tube; its lining shifts at the stomach entry for secretion.
Stomach Structure and Function
- The stomach has regions: cardia, fundus, body, pylorus.
- Gastric pits and glands produce acidic gastric juice needed for digestion.
- Mucosal barrier protects stomach lining from acid.
- Food becomes chyme and moves to the small intestine via the pyloric sphincter.
Small Intestine and Accessory Organs
- Three sections: duodenum, jejunum, ileum.
- The duodenum receives bile (from liver/gallbladder) and pancreatic juice (from pancreas).
- Absorption is maximized by circular folds, villi, and microvilli.
- Peyer's patches provide immune protection.
- Liver produces bile for fat breakdown; gallbladder stores bile.
- Pancreas produces enzymes; acinar cells secrete digestive juices.
Large Intestine
- Main functions: absorb water, form and expel feces.
- Features: tenia coli (muscle bands), haustra (pouches), epiploic appendages (fat sacs).
- Regions: cecum, appendix, colon (ascending, transverse, descending, sigmoid), rectum, anal canal.
Key Terms & Definitions
- Alimentary canal (GI tract) — continuous digestive tube from mouth to anus.
- Peristalsis — involuntary muscle contractions moving food through the GI tract.
- Mechanical breakdown — physical digestion of food (chewing, churning).
- Chemical digestion — enzyme-driven breakdown of food molecules.
- Absorption — transfer of nutrients from the GI tract to blood/lymph.
- Mucosa — innermost GI tract layer; secretes and absorbs.
- Serosa — outermost GI tract layer providing protection.
- Chyme — semi-fluid paste of partially digested food.
- Bile — liver-produced fluid that emulsifies fats.
- Peyer’s patches — lymphoid tissue in small intestine for immune defense.
Action Items / Next Steps
- Review details of each digestive organ and their roles.
- Study specific enzyme pathways for macromolecule digestion in upcoming nutrition lessons.