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Intestinal Hormones Inhibit:

•Gastric Secretion and Motility

Intestinal Hormones Promote:

•Intestinal Digestion and Glucose Assimilation

QUIZ/TEST REVIEW NOTES

SECTION 3 INTESTINAL PHASE OF DIGESTION

DIGESTIVE SYSTEM CHAPTER 21

I. INTESTINAL PHASE AND FEEDBACK SIGNALS a. Intro

Net result of gastric phase is o Digestion of proteins in the stomach by pepsin

o Formation of chime by action of pepsin, acid and

peristaltic contractions

o Controlled entry of chyme into the small intestine

b. Slowing of feedforward and positive feedback escalation of gastric function 1. Positive Feedback of gastric phase to regulate delivery rate of chyme from the

stomach 2. Feedforward to promote digestion, motility and utilization of nutrients

c. Intestinal Hormones

Secretin Hormone

Chyme/Stimulus For Release: Acid

Secreted By: Duodenum

(-) Negative Effect (Inhibitory): Gastric secretion and motility

(+) Positive Effect (Excitatory): Pancreatic Bicarbonate

Targets: Pancreas and Stomach

Inhibitor: Somatostatin

GIP Hormone (Gastric Inhibitory

Peptide)

Chyme/Stimulus For Release: Carbohydrates/glucose

Secreted By: Duodenum

(-) Negative Effect (Inhibitory): Gastric Secretion

(+) Positive Effect (Excitatory): Pancreatic Insulin

Targets: β-cells of pancreas

CCK Hormone (Cholecystokinin)

Chyme/Stimulus For Release: Fats and Proteins

Secreted By: Duodenum (also brain)

(-) Negative Effect (Inhibitory): Gastric Secretion and Motility

(+) Positive Effect (Excitatory): Pancreatic Enzymes

Targets: Gall bladder, pancreas, gastric smooth muscle

Learning Objectives:

Describe processes of digestion,

absoprotion, motility and

secretion in intestine/colon

Neuroendocrine controls of

intestinal phase of digestion

Digestive processes of

carbohydrates, fats, and proteins

Role of bicarbonate throughout

digestion

Summarize function of pancreatic enzymes and specific functions of

the brush boarder disaccaridases

Explain significance of water

absorption and motility in colon

Insulin release is a

feedforward

mechanism/response in

anticipation of high levels of

sugar

CCK enhances Satiety, the

feeling that hunger has been

satisfied

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d. Intestinal Osmoreceptors Mixture of acid, enzymes, and digested chyme forms a hyperosmotic solution

Osmoreceptors in intestine walls of lumen are sensitive to osmolarity of entering chyme

When stimulated by high osmolarity the receptors inhibit gastric emptying in a reflex mediated

by unknown blood-borne substance

II. SMALL INTESTINAL DIGESTION

a. Pancreatic duct cells release bicarbonate to neutralize the acidic chyme o Bicarbonate secretion into the small intestine neutralizes highly acidic chyme

entering the stomach

o Most bicarbonate comes from the pancreas and is released in response to neural

stimuli and secretin

o Signals for pancreatic enzyme release:

Food in the intestine

Neural signals

Hormone CCK

b. Bicarbonate Secretion o Small amount of bicarbonate comes from duodenal cells and the rest comes from the

pancreas which secretes NaHCO3

o Exocrine Function

Duct Cells (Secrete Bicarbonate; NaHCO3)

Acinar Cells (Secrete Digestive Enzymes)

Enzyme Optimal pH: - Gastric Enzymes: Acidophiles - Intestinal Enzymes: Alkalophiles

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Cystic Fibrosis: Inherited defect causes

the CFTR channel to be defective or

absent. Result: Secretion of chloride and

fluid ceases, but goblet cells continue to

secrete mucus, resulting in thickened

mucus

- In the G.I. the mucus clogs small

pancreatic ducts and prevents digestive enzyme secretion into

intestine

- In Respiratory system, the CFTR

channel failure to secrete fluid

clogs the mucociliary escalator,

leading to lung infections

o Bicarbonate production requires high levels of carbonic anhydrase

o Bicarbonate produced by CO2 and H2O is secreted by Cl--HCO3

- anti-port proteins

o Chloride (Cl-) influx by NKCC cotransporter and efflux by CFTR channel

o Hydrogen ions produced along with bicarbonate leave cell by Na+-H+ anti-port

c. NaCl Secretion o In addition to ion secretion by

pancreas, crypt cells in small

intestine/colon secrete NaCl solution

o Active step is Cl- secretion

which is followed by Na+ and

H2O secretion

o Secreted saline solution with

mucus secreted by goblet cells

helps to lubricate contents of

gut

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d. Pancreatic Acinar-cell Digestive Enzymes that have neutral pH Optimum

PANCREATIC AMYLASE Breakdown Glycogen and Starch to Maltose;

(All intestinal carbohydrate transport is restricted to

monosaccharide’s)

PANCREATIC LIPASE AND COLIPASE Breakdown Fats to Monoglycerides and Free Fatty

Acids

TRYPSIN AND CHYMOTRYPSIN Breakdown proteins (Endopeptidases: Attack peptide

bonds and activated by GI tract lumen; when first

released from Pancreas they inactive proenzymes)

PHOSPHOLIPASE Breakdown Phospholipids

e. Brush Boarder Enzymes o Created by epithelia cells on apical surface of a Intestinal Villus; consists of

Microvillus

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Bilirubin

•A bile pigment that is the waste product of hemoglobin degradation

Bile salts

•Act as detergents to solubilize fats during digestion

•Made from steroid bile acids combinged with amino acids

Cholesterol

•Excreted in the feces

f. Addition of bile from the liver and gall bladder

a. Bile composition

o Is a nonenzymatic solution secreted from hepatocytes/liver cells and is

stored in the gallbladder of the liver

o Key components

BILE SALTS CONTINUED…

BILE SALTS FACILITATE FAT DIGESTION Bile salts interact with lipids to break down coarse emulsion into smaller, more stable

particles

Bile salts are amphipathic (polar/non-polar) containing hydrophobic and hydrophilic regions

o Hydrophobic: associates with surface of lipid droplets

o Hydrophilic: associates with water, creating stable emulsion of small water-soluble

fat droplets

Bile salt coating of emulsion fat prevents pancreatic lipase penetration, so colipase is needed

o Colipase

Secreted from pancreas

Displaces some bile salts allowing some lipase to access fat inside bile

coating Creation of micelles where final stages of fat catabolism occur

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III. ABSORPTION

a. Stomach o Lipid-soluble substances

o Alcohol and Drugs

b. Small Intestine o 90% of all nutrients

Peptides

Amino Acids

Glucose

Fructose

Fats

Water

Ions

Minerals

Vitamins

c. Colon o Water Absorption (primary)

ALSO

Ions

Minerals

Vitamins

Small organic molecules produced by bacteria