Digestive System Lecture 18

Study Notes

Learning Goals: The lecture aims to cover the functions, anatomy, accessory organs, chemical reactor theory, and mechanical vs. chemical digestion processes, including absorption, for proteins and carbohydrates.

The digestive tract has four core functions:
- Digestion: Breakdown of food.
- Motility: Movement of food through the tract.
- Secretion: Release of enzymes and fluids into the tract.
- Absorption: Uptake of nutrients into the bloodstream.

The digestive system includes organs like the oral cavity, salivary glands, esophagus, stomach (with rugae), gallbladder, pancreas, small intestine (duodenum, jejunum, ileum), large intestine, and rectum.
- Rugae: folds in the stomach lining to expand its surface area.
- Various accessory organs such as the liver aid in digestion.

Duodenum: First part of the SI, receiving chyme (partially digested food) from the stomach.
- Brunner's Glands: Secrete mucus, protect the duodenum.
- Digestive enzymes for starch, fat, protein digestion are present.
Jejunum: Middle part of the SI, primarily responsible for nutrient absorption.
- Membrane-bound enzymes break down nutrients.
Ileum: Final part of the SI, primarily for absorbing nutrients.
- Rich in Peyer's Patches (lymph nodes).
- Vitamins, bile salts absorbed here.

Antigens (bad microbes) are sampled by M-cells.
M-cells then pass the antigens to immune cells in the underlying gut-associated lymphoid tissue (GALT). This plays a role in combating infection and maintaining immune response.

The large intestine includes the ascending, transverse, descending, sigmoid, and rectum portions.
Primary functions:
- Electrolyte, water absorption.
- Site of microbial communities (for fermentation).
- Absorption of some vitamins (e.g., Vitamin K) and amino acids from microbes.

Exocrine function:
- The pancreas secretes digestive enzymes.
- Bicarbonate buffer (NaHCO3) is present to neutralize acidic chyme.
Endocrine function:
- Islet cells secrete hormones (e.g., insulin and glucagon).

Functions:
- Nutrient and fat metabolism.
- Protein and hormone synthesis.
- Urea production.
- Detoxification.
- Storage.
- Substances absorbed in the gastrointestinal tract are processed by the liver.
- The liver produces and secretes substances into the duodenum like bile, bilirubin.
- Nutrients from the small intestine are carried to the liver via the hepatic portal vein.

Continuous Stirred-Tank Reactor (CSTR):
- Continuous mixing and consistent reaction rates like in the stomach.
Plug-Flow Reactor (PFR):
- Sequential processing and changing reaction rates as food moves through the intestines.

Mechanical Digestion:
- Mastication (chewing) physically breaks down food.
- Chemical Digestion:
  - Salivary enzymes (amylase, lipase, lysozyme).
  - Acid and enzymes used in the digestive process.

Acid (e.g., hydrochloric acid in the stomach) unfolds proteins and makes peptide bonds vulnerable to attack.
Pepsin: Enzyme digesting proteins in the stomach. -Disrupts protein structure.
Other enzymes (trypsin, carboxypeptidases, aminopeptidases): Important for digesting proteins in the small intestine.

Amylase is an enzyme vital to the digestion of carbohydrates.
Carbohydrates are broken down to monosaccharides (like glucose) by amylase and other enzymes.

Glucose, fructose, and galactose are absorbed through specific transporters.
- Active transport and diffusion mechanisms essential for uptake of nutrients.

Lecture involves showing data comparing starch digestion and levels across different human populations, reflecting potential evolutionary and dietary adaptation.