Digestive Physiology: mouth to stomach

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principles of digestion

the alimentary canal, or gastrointestinal (gi) tract, digests and absorbs food while the accessory organs assist the alimentary canal in this process

alimentary canal

mouth, pharynx, esophagus, stomach, small intestine, and large intestine

accessory organs

teeth, tongue, gallbladder, salivary glands, liver, and pancreas

Functions of the digestive tract

1. ingestion

2. propulsion (movement)

3. mechanical digestion

4. chemical digestion

5. absorption

6. defecation

ingestion

the process of taking food, drink, or other substances into the body by swallowing.

propulsion (movement)

swallowing (oropharynx)

peristalsis (esophagus, stomach, small intestine, large intestine)

Mechanical digestion

chewing, churning (stomach), segmentation (small intestine)

physical breakdown of feedstuffs (teeth, stomach, intestines)

chemical digestion

macromolecules to monomers

enzymes from saliva, stomach, pancreas, intestines

results:

• polysaccharides to monosaccharides

• proteins to amino acids

• fats to glycerol and fatty acids

“-ase”

micro-organisms

absorption

to take in vitamins, minerals, water, etc from digestion

motility

lots of movement

muscle contractions that break up food, mix it with enzymes, and keep it moving

secretion

digestive enzymes and hormones

membrane transport

nutrients absorb through ________

GI tract- nervous control

enteric nervous system

intrinsic controls

extrinsic control

control of the GI tract

nervous control

hormones

paracrine secretions

intrinsic controls

short reflexes mediated by local nerve plexuses (gut brain)

extrinsic control

long reflexes involving CNS centers

structure of GI tract- Peritoneum

serous membrane of the abdominal cavity

• visceral

• parietal

structure of GI tract- Peritoneal cavity

lubricates digestive cavity

slide across one another

mesentery

fold of peritoneum that wraps around intestines

oral cavity functions

ingestion

mechanical digestion (except birds)

propulsion initiated by swallowing

salivary amylase (chemical breakdown of starch)

pharynx + esophagus pass food to the stomach

oral cavity- tongue

functions:

• acquisition of food/water

• reposition and mix food

• formation of bolus (working food into a ball)

• initiation of swallowing, speech, taste

papillae and taste buds on dorsal surface

lingual glands secrete saliva, tonsils in root

skeletal muscle in tongue

oral cavity- saliva

saliva is a hypotonic solution (97-99.5% water +solutes)

• pH of 6.8-7.0

• contains electrolytes Na+, K+, Cl-, phosphate + bicarbonate

mucus

lysozyme

immunoglobulin A

Salivary amylase

mucus

aids in swallowing

lysozyme

enzyme that kills bacteria

immunoglobulin A

inhibits bacteria growth

salivary amylase

starch digestion

Layers of GI tract- Mucosa**

innermost layers of cells

Layers of GI tract- Submucosa

vessels and nerves

flexible movement during peristalsis

Layers of GI tract- muscularis externa

(peristalsis/segmentation)

contraction and relaxation

physically break down food and propel it forward

muscle layer

Layers of GI tract- serosa

epithelial layer and connective tissue

connection

esophagus

straight, muscular tube

• nonkeratinized stratified squamous epithelium

• esophageal glands in submucosa

• skeletal (striated) muscle in upper part, smooth at bottom (varies by species)

pharynx —> stomach

Deglutition (swallowing)

involves tongue, soft palate, pharynx, esophagus, and 22 muscle groups

buccal phase —> oropharynx (conscious control)

pharyngeal-esophageal phase (subconscious control)
all routes except into digestive tract sealed

peristalsis moves food through pharynx to esophagus

peristalsis

moves food from mouth to anus

• circular smooth muscle contracts and relaxes in waves to propel food forward

segmentation

subdivide, break up, and mix contents

• localized contractions of circular muscle of the GI tract

• contractions isolate small sections of the intestine, moving contents back and forth

stomach

mechanically breaks up and liquefies food (food becomes chyme)

begins chemical digestion of fats and proteins

does not absorb significant nutrients'

• aspirin and some lipid-soluble drugs & alcohol

several regions:

• cardiac region, fundus, body

surface of stomach differs from rest of tract

cardiac region of the stomach

surrounds cardiac valve

fundus region of the stomach

dome-shaped region (anterior position)

body region of the stomach

midportion

fundus and body of the stomach

house gastric pits which contain gastric glands

• specialized cells

produce most of the gastric juice (H2O, HCL, pepsin)

• 2-3 liters of gastric juice per day

surface mucus cells and mucus neck cells

secrete mucus

glycoprotein/liquid gel that protects the lining of the stomach

parietal cells

secrete HCL (hydrochloric acid)

• pH 1.5-3.5; denatures proteins, activates pepsin, kills many bacteria

intrinsic factor

• glycoprotein required for absorbing vitamin B12 in small intestine

chief cells

secrete inactive enzyme pepsinogen

activated to pepsin by HCL and by pepsin itself (pos feedback)

enteroendocrine cell

secretes chemical messengers into lamina propria (area underlying mucosa) or lumen

• paracrine- serotonin, histamine

• hormones- somatostatin, gastrin

G cell

specialized enteroendocrine cell that secretes gastrin

regulation of gastric secretion

neural and hormonal mechanisms

3 phases:

• cephalic (reflex) phase

• gastric phase

• intestinal phase

cephalic (reflex) phase

prior to food entry

sight, smell, taste, thought of food

vagus nerve stimulates

gastric phase

food enters stomach

stretch or increase in pH

secretion stimulated by several factors

intestinal phase

partially digested food enters duodenum

first gastric activity increases- if the duodenum is stretched; amino acids in chyme cause gastrin release

enterogastric reflex- duodenum inhibits stomach

hydrochloric acid secretion

parietal cells contain carbonic anhydrase (CA)

• CO2 +H2O —> H2CO3- + H+

• H+ pumped into stomach lumen by hydrogen-potassium -ATPase

  • antiporter uses ATP to pump H+ out and K+ in

• HCO3- exchanged for Cl- (chloride shift)

  • Cl- pumped out to join H+ forming HCl

  • increased HCO3- in blood causes alkaline tide (blood pH increases)

functions of HCl

activates pepsin and lingual lipase

breaks up connective tissue and plant cell walls

• liquifies food —> chyme

converts ingested ferric ions (Fe3+) to ferrous ions (Fe2+)

• absorbed and used to make hemoglobin

destroys ingested bacteria/pathogens

ruminant stomach

rumen- bacterial fermentation, absorption

reticulum- particle separation

omasum- water, electrolyte, mineral absorption

abomasum- glandular stomach (similar to monogastric)

rumen physiology

microbial fermentation and absorption of volatile fatty acids (VFAs) in rumen (about 75%), reticulum, & omasum

• bacteria, protozoa, fungi

no digestive secretions

symbiosis

protein fermentation

dietary proteins —> amino acids in rumen

• fermented by bacteria

• bacterial protein made with nitrogen and carbon

• non-protein nitrogen (urea) can be used by bacteria to build bacterial proteins

while bacteria die, flushed to small intestine to be used by animals

• bacteria are about 60% protein

some proteins bypass ruminal fermentation

fat metabolism in rumen

rumen highly reducing environment (gain electrons)

end products of fermentation

cellulose - glucose- VFAs (acetic, propionic, butyric)

other sugars- glucose- VFAs

proteins- amino acids

• carbon skeleton- VFAs

• amino group- ammonia, urea, recycling

triglycerides- glycerol + fatty acids

• glycerol- VFAs

• fatty acids- saturation and chain elongation

B vitamin synthesis

microbial protein

heat and gas- losses of energy

saliva adaptations for the ruminant

high in Na, K, P, HCo3: buffers the rumen

high in mucin: anti-foaming to prevent bloat

contains urea: urea recycling; NPN used by microbes

liquid for the fermentation vat

avian stomach

esophagus- crop (food storage)- proventriculus- gizzard- small intestine- large intestine- cloaca

proventriculus secretes HCl and pepsinogen

gizzard- muscular organ

• mechanical digestion

• feed may move back and forth between gizzard and proventriculus