Biochem 510 - Digestion

What is digestion?

in the GI tract, the series of mechanical, chemical, and enzymatic processes that break food down for absorption

Primary organs of the GI tract

oral cavity, esophagus, stomach, small intestine (duodenum, jejunum, ileum), large intestine

Function of oral cavity (Primary)

mastice food, mix with saliva, which contains enzymes (amylase to digest carbs) and mucus (lubricant), mostly mechanical digestion, very little enzymatic or chemical digestion

Function of the esophagus (Primary)

tube to the stomach, lined with muscles for swallowing, peristalsis

Peristalsis (Primary)

the coordinated (wave-like) series of muscle contractions that move food through the GI tract beginning in the esophagus

Function of the stomach (Primary)

holding tank (50ml-1.5L), controls delivery to small intestine (pyloric sphincter), site of some mechanical (~3 contractions per min), some enzymatic (salivary amylase, pepsin, gastric lipase), but mostly chemical (HCl secretion, protein denaturation, mineral release, kills most bacteria)

Function of the small intestine (Primary)

major site of digestion (and absorption), some mechanical digestion (8-12 contractions per minute), but mostly enzymatic digestion

Function of Colon (Primary)

lots of digestion performed by intestinal bacteria, also an important site of absorption, water, electrolytes (K, Cl, Na)

Accessory organs of the GI tract

salivary glands, pancreas, liver, gallbladder, the microbiota, and appendix

Function of salivary glands

excretion of saliva, which contains enzymes and mucus-lubricant

Function of the pancreas

endocrine function (1%) - production of insulin and glucagon, released into blood stream, and control glucose metabolism

exocrine function (99%) - produces secretions for digestion that are released into the duodenum, contains enzymes (lipases, proteases, amylases, etc.) and bicarbonate (to buffer stomach acid, critical for enzyme activity, protect small intestine from acidity)

Acinar cells and ductal cells

part of the exocrine function of the pancrea, duct cells secrete aqueous bicarbonate solution, and acinar cells secrete digestive enzymes

Why don’t pancreatic enzymes pancreas?

zymogens - inactive enzymes

Zymogens

enzymes are first made in inactive forms, and are then activated by proteolysis after their release into the duodenum

Functions of the liver and gallbladder

liver makes bile acid and bile salts (made from cholesterol), stored and concentrated in the gallbladder and emptied into the intestine when needed

Bile acids and salts

act as detergents to break up fat “blobs” vie emulsification, needed for lipid absorption and absorption of fat-soluble vitamins

The microbiota

lots of microbes live in GI tract, which can train our immune systems, make vitamins so we can use them, digest unused energy sources to convert into usable energy, but composition can affect risk of obesity

Appendix

proposed vestigial nature, but new theory is that potentially a reservoir of good intestinal bacteria that can repopulate your intestine after diarrhea or antibiotic treatment

Absorption

the process where nutrients are taken up into the body, occurs in the stomach, small intestine, and large intestine, though different nutrients are absorbed in different organs

Why is the small intestine the major site of nutrient absorption?

Primary site of digestion, site of lipid emulsification (micelles), enterocyte protein expression, and has large surface area

How can the small intestine have such a large surface area?

Villi → epithelial cells → microvilli

Villi/villus

finger-like projections of cells in the small intestine

Enterocytes

the epithelial cells that line the lumenal surface of the small intestine

Microvilli/microvillus

finger-like projections of membranes of enterocytes into the gut lumen

Brush border

the lumenal membrane of enterocytes

Tight junctions

structures between enterocytes that block movement of gut material into the body

Blood capillaries

Inside the villi, lead to portal vein and to the liver — the pathway of water soluble nutrients including sugars, amino acids, water-soluble vitamins, and minerals

Lacteal

inside the villi, leads to the lymphatic system — the pathway of fat-soluble nutrients including lipids, cholesterol, and fat-soluble vitamins, which drains into circulation at the left subclavian vein

Neuronal regulation of digestion

nerves make contacts (innervate) throughout the GI tract, either stimulatory or inhibitory

Hormonal regulation of digestion

over 100 regulatory peptides control digestion

Gastrin

produced in stomach and small intestine — stimulates GI motility and acid release

Cholecystokinin

produced in the small intestine — stimulates pancreatic secretion and gallbladder contraction

Motilin

produced in the small intestine — stimulates gastric and GI motility

Ghrelin

produced in the stomach and pancrease — increases appetite

Leptin

produced in white adipose tissue — suppresses appetite

Gastroesophageal Reflux Disease (GERD)

stomach acid entering the esophagus, sensation of heartburn, damages the esophagus and poses cancer risk

Gastroesophageal Reflux Disease (GERD) - Causes

increased pressed on lower esophageal sphincter, diet (high fat foods, chocolate, nicotine, and peppermint) that weakens the LES, and exacerbated by products that increase stomach acid (coffee, tea, acidic and spicy foods)

Chronic Pancreatitis

decreased function of the pancreas due to blocking release of pancreatic enzymes, zymogen activation, and release of HCO3 which causes hyperacidity of SI and inhibits brush border enzyme activity

Chronic Pancreatitis - Causes

chronic alcohol consumption, gallstones blocking pancreatic duct

Inflammatory Bowel Disease (IBD)

the inflammation of intestinal mucosa, disrupts absorption which leads to loss of brush border enzyme activity, direct damage to enterocytes, and increased rate of food movement

Inflammatory Bowel Disease (IBD) - Causes

unknown, genetic link strong

Infectious Diarrhea

irritation of the intestinal mucosa, increases fecal transit rate, decreased reabsorption of water and electrolytes, leading to imbalances

Infectious Diarrhea - Causes

“food poisoning” - ingestion of toxin producing bacteria or viruses