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Ch 14: Digestive Physiology

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The GI tract is a hollow tube

• ~30 ft, mouth to anus

• Transports food/liquid/chime

• Accessory organs (liver, pancreas) & glands outside

tubular system secrete substances that aid digestion.

• Allows for absorption of nutrients across intestinal

membrane to blood.

• Packages un-used substances for excretion as solid waste.

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GI tract:Mouth, teeth, salivary

glandsPharynxEsophagusStomachSmall intestine

- duodenum- Jejunum- ileum

Accessory to small Intestine:

- Liver- Pancreas

Large intestineRectumAnus

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4 basic GI processes• Motility- muscles mix &

move food through successive compartments

• Digestion- ingested food broken into small molecules

• Absorption- molecules move from GI lumen into blood stream

• Secretion- movement of substances into the lumen, e.g. enzymes, HCO3

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GI Tract LayersFour layers (a.k.a. tunics):

1. Mucosa:

2. Submucosa:

3. Muscularis:

4. Serosa:

1. Mucosa: - innermost layer with:

- mucus-secreting (goblet) cells or other exocrine cells

- endocrine cells

- absorptive cells

- lymph nodes (immune function)

- may be folded (“plicae”) to surface area

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2. Submucosa: - layer with:

- blood supply & lymph vessels

- nerve supply= Miessner’s plexi

– sensory neurons &

– motor neurons transmitting vagus nerve commands

to muscularis,

parasympathetic = ____________________

sympathetic = __________________________

3. Muscularis: - layer with:

- 2 muscle layers (circular layer for diameter, & longtitudinal for length)

- nerve supply= Auerbach’s plexis

4. Serosa: outermost layer of connective tissue, continuous with

peritoneum and mesentery.

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Upper GI tract begins mouth

• mastication, saliva, amylase

• Pharynx or throat- common passage for air & food

• Epiglottis covers opening to trachea (glottis) when swallowing.

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Saliva is a cocktail produced by several glands

• HCO3-

• mucus

• amylase

• lysozyme: lyses bacteria

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Esophagus is a muscular tube

• Beginning is skeletal muscle, lower portions all smooth muscle.

• Each end controlled by circular sphincter muscle

• Passes thru hole in diaphragm: esophageal hiatus

• Failure of lower esophageal sphincter allows gastric reflux- backflow of acidic stomach contents = ______________________________

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Barrett’s esophagus is pre-cancerous condition

• Chronic acid damage

• Misplaced glandular cells replace normal squamous epithelium

• Higher risk of adenocarcinoma (cancer of glandular tissue)

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Barrett’s endoscopy

Pink- normal esophageal

Squamous epithelium

Red- Barrett’s columnar

epithelium

Esophageal cancer:

• On the rise- top 10 most common cancers worldwide

• Risk factors: alcohol, smoking, acid reflux

• Very poor 5 yr survival rate for advanced stages (<40%)

• 90% of cases diagnosed in advanced stages

• Therapy- surgery, radiation, chemo

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• Neoplasm- any new growth, designated by the suffix “-oma”

• Carcinoma- cancerous cells arising from epithelial tissue

• Adenocarcinoma- cancerous cells with glandular (secretory) properties that begin growing in the cells that line some internal organs

Cancer terminology

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Esophageal cancer

• On the rise- top 10 most common cancers worldwide

• Risk factors: alcohol, smoking, reflux

• Very poor 5 yr survival rate for advanced stages (<40%)

• 90% of cases diagnosed in advanced stages

• Therapy- surgery, radiation, chemo

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Stomach is J-shaped muscular sac

• Folded lining (rugae) contains gastric glands

• Contractile activity mixes gastric juice w/ food chyme

• pyloric sphincter controls emptying

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Gastric pits, in fundus & body, have gastric cells:

• Neck cells: make mucus

• Parietal cells: secret HCl, & intrinsic factor for B12 absorption

• Chief cells: pepsinogen

• G-cells: hormone gastrin,stimulates parietal cells

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Peptic ulcer disease results from erosion of stomach lining

• Stomach mucus protects against pH~2!• Open lower layers to acid & pepsin attack• Affects ~10% of US population• Many are caused by H. pylori infection

Perforated ulcer

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Helicobacter pylori: an acidophile• Cause immune response that destroys tissue

• Treated w/: 1) antibiotics2) histamine receptor (H2) antagonists

3) proton pump blockersEx.__________________

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Sm Intestine - site of digestion for most foods

• 3 regions: duodenum, jejunum, ileum (Most nutrient absorption completed in first 20%, before reaching ileum)

• Duodenum receives chyme from stomach, pancreatic enzymes & bicarbonate, & bile from liver.

• Sm intestine endocrine secretions: (REVIEW!)- secretin - stimulate water and bicarbonate secretion from pancreas- cholecystokinin – stimulates gallbladder contraction & stimulates pancreatic

enzyme secretion- gastric inhibitory peptide - slows gastric motility (slow down) & stimulates

pancreatic enzyme secretion

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Hepatic portal system

• Substances absorbed into blood from intestine go to liver

• “1st pass metabolism”- orally administered drugs are taken directly to liver

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Liver produces bile• Contains HCO3

-, phospholipids, cholesterol-like bile salts

• Excess bile stored in gall bladder- bile secreted thru common bile duct to duodenum.

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Gallstones or “cholelethiasis”

Bile salts precipitate out of bile and form stones.

10% of americans

TX:- Medication to dissolve stones

- Surgical removal of gallbladder (laproscopically)

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Liver: a most versatile organ• Glucose/glycogen metabolism

• Removal of old RBCs

• Clears hormones, medications, toxins from body

• Cori cycle

• Blood glucose regulation

• Amino acid metabolism

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Pancreas has endocrine & exocrine functions

• Acinar cells – exocrine secretions (digestive enzymes)Amylase for starch & glycogen

Lipases for fats

Proteases for proteins

Nucleases for nucleic acids

• Islet cells – endocrine secretionBeta cells - insulinalpha cells - glucagon

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Large intestine removes H2O from chyme• Lg intestine:

- Cecum- colon (ascending, transverse, descending)- rectum

• Transforms chyme into solid feces(water reabsorbed from chyme)

• Solid feces stored in rectum- has stretch receptors that signal for defecation

• Anus has involuntary and voluntary sphincters

• Blockage/inflammation of vermiform appendix =appendicitis

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Defecation involves voluntary & involuntary anal sphincters

• Colon contraction pushes feces into rectum

• As rectum fills, stretch receptors stimulate urge to defecate.

• To defecate or not defecate depends on state of the anal sphincters:

• Internal anal sphincter = smooth muscle (involuntary)

parasympathetic - internal sphincter relaxes, allowing feces to pass through anus.

sympathetic – internal sphincter contract, keep feces in (moves back into colon, more water resorbed)

• External anal sphincter = skeletal muscle (voluntary)

External sphincter

Internal sphincter

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Defecation Reflex

Stimulus:

Autonomic Response:

Delay defecation (hold it!): Defecation:

Rectum stretches

- Rectum & sigmoid colon contracts (push feces into anus)- Internal anal sphincter relaxes (allow anus to fill)- External anal sphincter contracts (keep it in temporarily)

- External anal sphincter contracts(keep feces in)

- Puborectalis muscle contracts(keep feces in)

- External anal sphincter relaxes (allow feces out)

- Puborectalis muscle relaxes (allow feces out)

- Straining (abdominal)

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Carbohydrate digestion

A. Simple carbohydrates

> Monosaccharides = single sugar unit. Ex.: Glucose, galactose, fructose

> Disaccharides = 2 sugar units. Ex.: Sucrose, lactose, maltose

B. Complex carbohydrates(polysaccharides)

- Made of more than 2 sugar units

Ex.:> Starch = storage form of sugar in plants (ex. Potatoes)

> Cellulose = indigestible polysaccharide in plant cell walls.

> Glycogen = storage form of glucose in muscles & liver.

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Different types of enzymes can break down different nutrients:

• Amylase enzymes break down starch (polysaccharides) into sugar.(secreted in saliva & by pancreas)

• Protease enzymes break down proteins into amino acids.(secreted by stomach and pancreas)

• Lipase enzymes break down fats into fatty acids and glycerol.(secreted by pancreas)

BUT lipid digestion 1st relies on bile to break down to size enzymes can be effective

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Proteases stored as zymogens

• Inactive, stored in vesicle: protect cells

from proteolytic digestion

• undergo proteolytic activation in lumen

of gut

• Enterokinase: sm intestine brush

boarder enzyme that activates

zymogens