Physiology of the digestive tract Flashcards
common or possible nutrition diagnoses related to digestion or metabolism include:
- Altered gastrointestinal function.
- Imbalance of nutrient intake
- Altered nutrient utilization
- Altered nutrition biomarkers
- Inadequate or excessive fluid intake
- food -drug interaction
GIT:
- One of the largest organs in the body
- greatest surface area
- largest number of immune cells
- one of the most metabolically active tissues in the body
- Human GIT (9 m long)
- includes oropharyngeal structures, esophagus, stomach, liver, gallbladder, pancreas, small and large intestine.
- 45% of the energy requirement of the small in- testine and 70% of the energy requirement of cells lining the colon are supplied by nutrients passing through its lumen.
Gastrointestinal Tract designed to:
- digest the macronutrients protein, carbohydrates, and lipids from ingested foods and beverages.
- absorb fluids, micronutrients, and trace elements:
- Provide a physcial and immunologic barrier to pathogens, foreign material and potential antigens consumed with food or formed during the passage of food, through the GIT.
- provide regulatory and biochemical signaling to the nervous system, involves the intestinal microbiota, via pathway known as the gut-brain axis.
NOTE: fibrous food and undigested carbohydrates are fermented by bacteria in the human colon, can contribute to 5% too 10 % of the energy needed by humans.
Digestive and absorptive processes
1.In the mouth, chewing reduces the size of food particles, which are mixed with salivary secretions that prepare them for swallowing.
2A small amount of starch is degraded by SALIVARY AMYLASE, but digestion in the mouth is minimal.
3.The esophagus transports food and liquid from the oral cavity and pharynx to the stomach
4.In the stomach, food is mixed with acidic fluid and proteolytic and lipolytic enzymes. Small amounts of lipid digestion take place, and some proteins are changed in structure or partially digested to large peptides.
5. When food reaches the appropriate consistency and concentration, it is now called CHYME and passes from the stomach into the 6. SMALL INTESTINE, where most digestion takes place.
7. SMALL INTESTINE: first 100 cm of SI is where the digestion and absorption occurs.
8: SI: presence of food stimulates the release of hormones which stimulate the production and release of enzymes from the pancreas and bile from the GB.
9. dietary fats are reduced to microscopic droplets of triglycerides, then to free fatty acids and monoglycerides.
10. Enzymes of brush border of SI further reduce carbs to monosaccharides and proteins (peptides) to single amino acids, dipeptides, and tripeptides.
11. Along the remaining length of the small intestine, almost all the macronutrients, minerals, vitamins, trace elements, and fluid are absorbed before reaching the colon.
8. The colon and rectum absorb most of the remaining fluid delivered from the small intestine. The colon absorbs electrolytes and only a small amount of remaining nutrients
9.The movement of ingested and secreted material in the GIT is regulated primarily by hor- mones, nerves, and enteric muscles.
10. Nutrients absorbed enter the portal vein for transport to the liver where they may be stored, transformed into other substances, or released into circulation.
11. End products of fat are transported into the blood stream via the lymphatic circulation.
12. Nutrients that reach distal small intestine (dietary fiber and resistant starches) are fermented by the microbiota located within the lumen of the illeum and large intestine.
13. Fermentation produces short chain fatty acids (SCFAs) and gas.
14. SCFAs provide a preferred fuel source for cells of the intestine, stimulate intestinal cell renewal and function, enhance immune function, and regulate gene expression.
14. some carbohydrates have prebiotic properties.: induce growth and activity of beneficial microbes within the intestinal microbiota.
15. Distal colon, rectum and anus control defecation.
Enzymes of digestion
-accomplished by enzymatic hydrolysis,
-cofactors (hydrochloric acid, bile, sodium bicarbonate) facilitate the digestive and absorptive processes.
-digestive enzymes facilitate the digestive and absorptive processes.
-Digestive enzymes synthesized in specialized cells of the mouth, stomach, and pancreas are released into the GIT LUMEN,
-digestive enzymes synthesized in enterocytes of the small intestine remain embedded within the BRUSH BORDER MEMBRANE.
-FIBER AND RESISTANT CARBOHYDRATES, digestion and absoprtion of intake is completed essentially in the small intestine.
Regulation of gastrointestinal activity:
GIT movement: contractions, mixing, propulsion of luminal contents–result of movement of smooth muscle, activity of the enteric nervous system. enteroendocrine hormones, smooth muscle.
A.Muco- sal receptors sense the composition of chyme and distention of the lumen (i.e., fullness) and send impulses that coordinate the processes of digestion, secretion, absorption, and immunity.
B.Neurotransmitters and neuropeptides with small molecular weights signal nerves to contract or relax muscles, increase or decrease fluid secretions, or change blood flow.
C.signals from the central nervous system can override the enteric system and affect GIT function.
D.Hormones, neuropep- tides, and neurotransmitters in the GIT not only affect intesti- nal function but also have an effect on other nerves and tissues in many parts of the body.
E. sympathetic neurons, which are activated by fear, anger, and stress, tend to slow transit of intestinal contents by inhibiting neurons affecting muscle con- traction and inhibiting secretions. The parasympathetic nerves innervate specific areas of the alimentary tract and contribute to certain functions. For example, the sight or smell of food stimu- lates vagal activity and subsequent secretion of acid from parietal cells within the stomach. The enteric nervous system also sends signals to the central nervous system that are perceived as pain, nausea, urgency or gastric fullness, or gastric emptiness by way of the vagal and spinal nerves. In
Secretion: saliva from salivary glands in mouth
A. Enzymes: amylase
substrate: starch (linked polysaccharides)
action: hydrolysis to form dextrins and maltose
B.Enzyme:Lingual lipase
substrate: Triglyceride
action: Hydrolysis to form diglyceride and free fatty acids.
Secretion: Gastric secretion from gastric glands in stomach mucosa
A. Enzyme:Pepsin (activated from pepsinogen in the presence of hydrochloric acid)
substrate: Protein
Action: Hydrolysis of peptide bonds to form peptides and amino acids
B. Enzyme:Gastric lipase
Substrate: Trigylceride
Action: Hydrolysis to form diglyceride and free fatty acids.
secretion: exocrine secretions from pancreatic acinar cells, acting in the duodenum
A. Enzymes: Lipase
substrate: Fat (in the presence of bile salts)
Action/product: Hydrolysis to form monoglycerides and fatty acids;incorporated into micelles
Final product absorbed: Fatty acids into mucosal cells;reesterified as triglycerides
B. Cholesterol esterase
Subst: sterols (cholesterol)
Action: Hydrolysys to form esters of cholesterol and fatty acids;incorporated into micelles
Final product absorbed:
Cholesterol into mucosal cells;transferred to chylomicrons
C. Amylase
subst: starch and dextrins
Action: Hydrolysis to form dextrins and maltose
D. Trypsin (activate trypsinogen)
substrate: proteins and polypeptides
Action: Hydrolysis of interior peptide bonds to form polypeptides.
E. Chymotrypsin (activated chymotrypsinogen)
Substrate: proteins and peptides
Action; Hydrolysis of interior peptide bonds to form polypeptides.
Carboxypeptidase (activated pro-carboxypeptidase)
substrate: polypeptides
action: hydrolysis of interior peptide bonds (carboxylend) to form amino acids.
Final product: Amino Acids
F. Ribonuclease and deoxyribonuclease
Substrate: Ribonucleic acids (RNA) and deoxyribonucleic acids (DNA)
action hydrolysis to form monomucleotides
Final product: Mononucleotides
G. Elastase
Substrate: Fibrous protein (elastin)
Action: hydrolysis to form peptides and amino acids
