Unit I: MetabolismDigestive Processes

Chapter 21 and 22

Mastication or Chewing

surface area exposed to digestive enzymes

• Contact of food with sensory receptors triggers chewing reflex– tongue, buccinator and orbicularis oris manipulate food– masseter and temporalis elevate the teeth to crush food– medial and lateral pterygoids swing teeth in side-to-side

grinding action of molars

Salivation

• Total of 1 to 1.5 L of saliva per day

• Cells filter water from blood and add other substances

• Food stimulates receptors that signal salivatory nuclei in medulla and pons

– parasympathetic stimulation salivary glands produce thin saliva, rich in enzymes

– sympathetic stimulation produce less abundant, thicker saliva, with more mucus

Swallowing / Deglutition

Tongue

Esophagus

1. Buccal Phase

2. Pharyngeal Phase

Bolus

• Tactile receptors on the palatal arches and uvula are stimulated. • Swallowing center in the medulla oblongata then coordinates:

• Elevation of the larynx • Folding of the epiglottis• Elevate the uvula and soft palate

• Involuntary

• Compression of the bolus

• Tongue then forces the bolus

into the oropharynx

• Voluntary

Soft palate

Bolus

Swallowing / Deglutition

3. Esophageal Phase

Peristalsis

Stomach

Thoraciccavity

Lower esophageal sphincter (LES)

• Myenteric nerve plexis signals

peristaltic waves

• LES opens

• Bolus continues to the stomach • Typical bolus = 9 seconds• Liquids = 2 seconds• A dry or poorly lubricated bolus may need secondary peristaltic waves

Peristalsis allows us to move a bolus regardless of our body position.

Gastric Motility

• Swallowing center signals stomach to relax

• Receptive-relaxation response

• Peristalsis

– contraction every 20 seconds

– stronger contractions after 30 minutes; ejects 3 ml

– typical meal emptied from stomach in 4 hours

Vomiting

• Induced by

– excessive stretching of stomach, psychological stimuli or chemical irritants (bacterial toxins)

• Emetic center in medulla causes

– retching

• Lower esophageal sphincter to relax

• stomach and duodenum to contract spasmodically

– vomiting

• when abdominal contraction forces upper esophageal sphincter to open

1. Cephalic Phase 2. Gastric Phase• See, smell, taste, or think of food. • Directed by the CNS• Prepares the stomach to receive food• Production of gastric juice accelerates

• reaching rates of about 500 mL/h• lasts only minutes.

• Arrival of food in the stomach (1) distension of the stomach (2) an increase in the pH (3) the presence of undigested proteins • Lasts 3-4 hours • Gastrin stimulates contractions in themuscularis externa of the stomach.

Mucous cells

Stimulation

Mucus

Pepsinogen

HClGastrin

KEY

Submucosalplexuses

Vagus nerve (N X)Medulla Oblongata

Chief cells

Parietal cells

G cells

Regulation of Gastric Secretion

Chemoreceptors

Stretchreceptors

Elevated pH

Distension

Submucosaland myentericplexuses

viabloodstream

Gastrin

Mucus

Pepsinogen

HCl

Partlydigestedpeptides

Mixingwaves

Chief cells

Parietal cells

G cells

Mucous cells

Peristalsis

Enterogastricreflex

Myentericplexus

via bloodstream Chiefcells

Parietalcells

Duodenalstretch andchemoreceptors

CCK

GIP

Secretin

Presence oflipids andcarbohydrates

Decreased pHInhibition

KEY

3. Intestinal Phase• Chyme first enters the small intestine

• Function: control the rate of gastric emptying

• Enterogastric reflex:Stimuli:↓ stimulation of stretch receptors in stomach and

• ↑stimulation of stretch receptors in intestinePurpose: Inhibit gastric activity• Stimulates contraction of pyloric sphincter• Mucus production

Regulation of Gastric Secretion

Intestinal Hormones

• Cholecystokinin (CCK)

1. Contraction of gallbladder

2. Secretions of pancreatic enzymes

3. Relaxation of hepatopancreatic sphincter

• Secretin

• Glucose dependent insulin peptide/ Gastric inhibitory peptide

Overall effect: reduce gastrin secretion and passage of chyme

Small Intestine Motility

• Segmentation -− stationary ring-like constrictions− not as in peristalsis

• Purpose:– mix – churn

• Pacemaker cells in Muscularis externa

• Provides the most contact digestion

http://youtu.be/PfnKvErPwY4

Carbohydrate Digestion

• Salivary amylase oligosaccharide– 50% of dietary starch digested before it reaches small intestine

• Pancreatic amylase oligosaccharide and maltose• Brush border enzymes monosaccharides

Carbohydrate Absorption

• 80% of glucose taken up by Na-glucose transport proteins →

• Facilitated diffusion → blood capillaries of villus →

• Delivered to liver

Disaccharides

Simple sugars

Epithelial cell

Enzymes

Simple sugars(such as glucose

or fructose)

Proteinsingested

½ of amino acids digested come from dietary proteins

Protease pepsin breaks down complex proteins into smaller peptide and polypeptide chains

Pancreatic enzymes: Trypsin, chymotrypsin, and carboxypeptidase break down proteins into a mixture of dipeptides, tripeptides, and amino acids.

The epithelial surfaces of the small intestine contain several brush border enzymes that release individual amino acids.

amino acids

Facilitated diffusionand cotransport

Dipeptidase

Amino acids

Epithelial cellFacilitated diffusionand cotransport

Amino acids

Protein Digestion

Protein Absorption

Behaves like monosaccharides:

•Taken up by Na-dependent transport proteins → epithelial cells →

•Facilitated diffusion → blood capillaries of villus → bloodstream

Lipid Digestion

1. Lipase – Lingual lipase, gastric lipase

2.

3.

Bile

Lipid Absorption

2.

1.

Nucleic Acids, Vitamins, and Minerals

• Nucleases

– brush border split them completely apart

• Vitamins are absorbed unchanged

– A, D3, E and K with other lipids

– B complex and C by simple diffusion

– B12 if bound to intrinsic factor

• Minerals are absorbed all along small intestine

– Na+ cotransported with sugars and amino acids

– Cl- exchanged for bicarbonate reversing stomach

– Iron and calcium absorbed as needed

Water Balance

• Digestive tract receives about 9 L of water/day

−0.7 L in food, 1.6 L in drink, 6.7 L in secretions

−8 L is absorbed by small intestine and 0.8 L by large intestine

• Diarrhea

–feces pass through too quickly if irritated

–feces contains high concentrations of a solute (lactose)

Absorption and Motility of Large Intestine

• Transit time is 12 to 24 hours

– reabsorbs water and electrolytes

• Haustral contractions occur every 30 minutes

– distension of a haustrum stimulates it to contract

• Mass movements occur 1 to 3 times a day

– filling of the stomach and duodenum

Vitamin AbsorptionVitamin K Vitamin B5 Biotin

1500mL/day enters Organic waste products, such asurobilinogen, and various toxins.

Over 1 L of water isreabsorbed

through osmosis.

Only 200 mL of feces is ejected. 75% water, 25% solids

Intrinsic Defecation Reflex

ParasympatheticDefecation Reflex

When external sphincteris relaxed:

DEFECATION OCCURS

Involuntary contractionof external anal

sphincter

Relaxation of internalanal sphincter

DISTENSIONOF RECTUM

Increased localperistalsis

Stimulation ofstretch receptors

Start

Stimulation ofmyenteric plexus

Increasedperistalsisthroughout largeintestine

Stimulation ofmotor neuronsin sacral spinalcord

Stimulation ofsomatic motorneurons

stimulates

inhibits

Defecation

External sphinctercan be voluntarily relaxed