GI 8

Key Hormones Involved

• Gastrin: Stimulates gastric acid and HCl secretion; regulated by pH levels in the stomach and presence of food.

• Histamine: Makes the parietal cell more susceptible to activation (permissive hypothesis)

Functions of the Small Intestine

• Contains brush border enzymes for carbohydrate and protein digestion.

• Pancreatic secretions aid in fat emulsification via bile salts and lipases.

  • Common entry point - sphincter of Oddi

• Pancreas also releases trypsin inhibitor to prevent damage from the active form

Digestive Process in the Stomach and Small Intestine

• Pre-Intestinal Changes:

  • Pancreatic lipases

    • Pancreatic pro-colipase -trypsin→ colipase

    • Triglycerides -pancreatic lipases→ fatty acids, di and mono-glycerides

• Meal Processing: Chyme becomes semi-liquid; limited digestion occurs in the stomach.

• Small Intestine: Processes chyme with brush border enzymes and pancreatic juices, leading to further digestion of fats and proteins.

Bile and its Functions

• Liver: Largest gland; functions in storage, synthesis, detoxification, and metabolism. It secretes bile to emulsify fats.

• Bile Composition:

  • Isotonic

  • 7.8-8.2 pH

  • 3% solids:

    • Bile acids (bile salts)

      • Synthesized in the liver from cholesterol

      • Facilitate digestion, transport, and absorption of fat by forming water-soluble complexes with the fats

      • Facilitate transport and absorption of fat-soluble vitamins (A,D,E,K)

      • Reduce surface tension and stabilize emulsion

      • Amphipathic

      • Bile salts surround lipid globules, creating a micelle, and break it down into smaller micelles, making the lipids more accessible to lipases

      • Bile salt pool - 3.5g

      • Daily synthesis - 0.5g

      • Daily release into intestine - 15-20g

        • Bile salts are reabsorbed into the portal blood and returned to the liver via enterohepatic circulation

      • Bile salts regulate bile flow

        • The more bile salt returned in the portal blood, the larger the volume of bile secreted

        • Positive feedback loop

        • Without the ileum (the ability to recycle bile), bile synthesis will be decreased

      • Bile salts also regulate the synthesis of new bile

        • The more bile salt returned in portal blood, the smaller the amount of new bile synthesized

        • Negative feedback

        • Without the ileum

      • Bile salts prevent cholesterol precipitation and gallstone formation

      • In the colon, bile salts inhibit the transport and absorption of water and sodium

        • Excess bile salts → diarrhea as water absorption is limited

    • Bile pigments

    • Cholesterol

    • Phospholipids

• Bile Secretion:

  • Continuous process (0.5-1L/day)

  • Stored in the gall bladder when the sphincter of Oddi is closed

    • 10-20% solid

    • 7.0 - 7.5 pH

    • Gallstones are cholesterol or pigment precipitates

      • Cholecystectomy - gallbladder removal

      • Can still process fatty meals because there are bile salts coming from the liver, but it’s not as concentrated as that coming form the gallbladder

  • Entrance of bile into SI is intermittent (500-700 mL.day)

Bile flow regulation:

• Choleretics - agents which cause the liver to secrete a larger volume of bile

  • Ex. secretin

• Cholagogues - agents which cause an increase in gall bladder emptying

  • Ex. CCK

Digestive Enzymes in the Small Intestine

• Villi:

  • Contain brush border enzymes attached to the microvilli of SI epithelial cells

    • Go to lumen:

      • Enterokinase

      • Amylase

      • Lipase

      • Aminopeptidases

      • Dipeptidases

    • Stay in brush border

      • Sucrase

      • Maltase

      • Isomaltase

      • Lactase

    • Constant cell turnover (3-5 days) to replace damaged cells from acidic environment

  • Digestion absorption is completed at the enterocytes of the SI villi

  • 
    

• Crypts:

  • The crypt cells lack digestive enzymes, but secrete 3 liters/day of succus entericus, an alkaline fluid

  • Crypt cells mature and become villi cells

• The only GI organ essential for life is the SI

  • Postprandial blood flow to intestine is very large (1-2 L/min)

  • Lymph flow (1-2 mL/min)

Nutrient Absorption Mechanisms

• Carbohydrates: Absorbed as monosaccharides via capillaries; effective absorption due to large surface area and brush border enzymes.

  • Via capillaries

  • Absorption efficiency - 99%

• Proteins: Absorbed as amino acids and small peptides; digestion facilitated by pepsin and pancreatic proteases.

  • Via capillaries

  • Absorption efficiency - 92%

• Fats: Absorbed via micelles into intestinal cells, resynthesized into triglycerides and packaged as chylomicrons for lymphatic transport.

  • Absorbed via lacteals

  • Absorption efficiency - 95%

• Iron and Ca2+ are only absorbed at the duodenum

• Vitamin B12 is only absorbed at the ileum

• Bile salts are mainly absorbed at the ileum

Absorption of Water and Ions in the Colon

• Secretions:

  • Alkaline

  • Mucin

  • Small volume

  • No digestive enzymes and absorption

  • Lots of bacterial activity

    • Must stay in the colon

• Absorption and Reabsorption:

  • Intake 2000 mL, 500g solid

  • Secrete 100 mL, 50g solid

    • 30% bacteria

    • 30% undigested fiber

    • 10-20% lipids

    • 10-20% inorganic matter

  • 2 L ingested + 7 liters secreted unb the GIT, 9 liters must be reabsorbed

    • Also need to reabsorb 50g of enzymes and 30g of cells (80g of protein) made each day

    • 7L of water is reabsorbed in the SI via osmosis

      • Max capacity - 15 L/day

    • 2L of water is reabsorbed in the colon via osmosis

      • Max capacity - 4-5 L/day