Ch7 Digestive System

GastroIntestinal tract

from mouth to anus, its external to body tissue (not through any membrane)

Oral cavity (mouth), pharynx, esophagus, stomach, small/large intestine, anus

Accessory Organs

Salivary glands, liver, gallbladder, pancreas

Functions:

Motility

Movement via the tract

Secretion of digestive enzymes

Into the lumen of the GI tract

Digestion of food

Enzymes break down large molecules into smaller ones

Absorption

small molecules from lumen to the circulatory system

The movements does not requires the activities of the motor neurons (smooth muscle can response to regulation but does not requires motor neurons)

Mouth, Jaw, top 1/3 esophagus, skeletal muscle, some voluntary

Rest of esophagus, stomach, intestines, smooth involuntary muscle

Remaining, skeletal voluntary

Swallowing

Oral phase, voluntary, bolus of food forced into pharynx, soft palate lodges against pharynx

Pharyngeal phase, involuntary, mechanoreceptors initiate reflex in brainstem, epiglottis closes and upper esophageal sphincter relaxes

Esophageal phase: involuntary phase, esophageal sphincter contracts, peristaltic contractions move bolus to stomach

Peristalsis

Rhythmic wave of smooth muscle contraction that results in movement of bolus through the GI tract

Occurs in the esophagus, stomach, small intestine, large intestine

Segmentation

Mixing movement

Pinching of the intestine into compartments and subsequent mixing of undigested material

No net movement

Occurs in the small intestine, large intestine

Seliva

With enzymes, mucus, water

Moisten food

amylase, lipase begin digestion of carbohydrates and triglycerides

Lysozyme, antimicrobial

Stomach

Holds ingested food (chyme-food)

passes chyme on small increments to the small intestine (via peristaltic wave)

kills most microbes

degrades food physically

begin digestion of proteins with pepsin

Secret intrinsic factor for absorption vitamin B12

Hydrochloric acid

Does not digest chyme

kills most microbe (acidity)

denatures proteins and inactivate enzymes in food (acidity)

break down plant cell walls and connective tissues in meat (acidity)

Activates pepsinogen to pepsin

Secretion of HCl

Hydrogen-K ATPase (proton to lumen, K+ to cell) (apical side)

K, Cl channel apical side (flow towards lumen)

CO2 and H2O convert to H2CO3 with Carbonic anhydrase in the parietal cell and H+ flows to lumen and HCO3- flows to capillary

HCO3-, Cl- (AE2) cotransporter in basolateral side that HCO3- out of cell and Cl into the cell

In summary

Cl- enters from basolateral side to lumen of stomach

HCO3- to the capillary

H+ to the lumen of stomach

Intestine

From liver and pancreas will secreted to small intestine

Most absorption occurs

Lining with villi and microvilli

The surface of the internal side of small intestine is called brush border

Digestion of Carbohydrates

Salivary amylase, salivary glans in mouth, from carbohydrates into double sugar

Pancreatic amylase, pancreas, from carbohydrates into double sugar

Other, wall of intestine, from double sugar to single sugar

Carbohydrates

Apical side with Brush border enzymes

From polysaccharides, break with pancreatic amylase to disaccharides, and by brush border enzyme to monosaccharides

Fructose enters via GLUT5

Glucose and galactose enters with Na

Basolateral side

With NaK pump

Monosaccharides enters via GLUT2

with K channel that K flows out

Protein

Pepsin, released from pepsinogen from the stomach walls that needs to be activated by HCl, breaks down proteins into small chain amino acids

Trypsin, released from Trypsinogen from pancreas which needs to be activated by enteropeptidase (secreted by enteroendocrine cells on the lining of small intestin), breaks down proteins into small chain amino acids and activates other digestive enzymes

Amino acid small chain was absorbed via transporter

Fats (triglycerides)

Salivary lipase, released by salivary glands and breaks triglycerides to monoglycerides and fatty acids (absorb by small intestine)

Pancreatic lipase, released by pancreas and breaks triglycerides to monoglycerides and fatty acids (absorb by small intestine)

Bile salts, released by liver and emulsifies fats from large globules to small globules, with -OH regions that forbid the small globules into large ones

Large intestine

Store, concentrate, excrete undigested material

Some absorption (water and ion)

Shorter than small intestine, not covered with villi and microvilli

Contains most microbes in GI tract

GI tract was regulated with hormone and neurons

Enteroendocrine cells lines GI tract and release hormone according to environment

Stomach:

Stretching of stomach and sensed by mechanoreceptive neurons lining

Neuron stimulate cell linings to secrete more HCl

Small intestine

Sensed by enteroendocrine cells, which releases CholeCytosKinin into blood

Increase pancreatic enzyme secretion, facilitate liver and pancreas secretions, decrease stomach contractility

CCK causes relaxation of sphincter of Oddi, which is a structure that restrict secretions of liver and pancreas

Muscles:

Circular muscle

Thinning tract on contraction

responsible for segmental contractions and peristalsis

Longitudinal muscle

Shorten tract on contraction

responsible for peristalsis

Sphincter

Smooth muscle that is involuntary

Muscularis Mucosa

Inner layer of GI tract in charge of local movements

Muscularis Externa

Outer layer of Gi tract in charge of large scale movements