The food mass is lubricated and moisten by saliva secreted by salivary glands or the parotids produce 1.5 L/day.
The secretion contains amylase which begins to digest starch.
The starch digestion is minimal in the mouth.
Another type saliva contains is mucus, a protein that causes the food particles to stick together and lubricate the mass for swallowing. This secretion also contains lipase that is capable of digesting some lipids but it’s effect is minimal.
After that the food mass moves into the pharynx area the to esophagus by swallowing process then to the stomach.
Digestion in the stomach
The food particles are pushed forward with gastric secretions by wavelike contractions in the upper portion of the stomach.
Gastric juices are mixed with the food particles.
Around 2000-2500 ml of gastric juices are secreted daily.
Juices contains Hydrochloric acid (HCl)
Glycoprotein that facilitates B12 absorption .
Gastro-intestinal hormone gastrin
Most of the food becomes semi-liquid or chyme containing around 50% water.
Protein digestion starts by pepsin
Lipase also secreted in the stomach
This lipase is less active than pancreatic lipase which digests triglycerides.
Because of the pH of the stomach 1-4, this pH protects against microorganisms ingested with food .
GI dysfunction may increase the concentration of this microorganisms in the intestine .
The stomach continuously mixes food and release the mixture into the small intestine.
Most liquid meal empties in 1-2 hours.
Solid food empties in 2-3 hours.
Carbohydrates leaves the stomach very rapidly, then protein, fat and lastly fibrous food.
Digestion in the small intestine
The small intestine is the primary site for digestion of foods and nutrients.
The small intestine is divided into 3 sections with total length of 6-8 m.
Most of the digestion process is carried out in the upper part of the small intestine which is called duodenum.
Absorption is done in the lower part.
The entry of partially digested foods, primarily protein and fats, stimulates the release of enzymes and fluids that stimulates GI motility and satiety.
Bile is secreted form liver and gallbladder.
Bile facilitates digestion and absorption of lipids.
Pancreas secrets enzymes that capable of digesting all of the major nutrients.
Pancreas secretes lipase, colipase, phospholipase, and cholesterol esterase .
Proteolytic enzymes like trypsin and chymotrypsin, carboxpeptidase, aminopeptidase, ribonuclease .
Pancreatic amylase is secreted to hydrolyze starch into oligosaccharides and disaccharides.
Enzyme secreted by the villi further digests carbohydrates into monosaccharides before absorption.
Varying amounts of carbohydrates escapes digestion to and add fibrous materials for fermentation by colonic microbes.
Intestinal contents move at rate 1 cm per minute taking from 3-8 hours to reach colon.
The primary site for absorption is the small intestine which is characterized by expansive absorption area.
The folds are covered with fingerlike projections called Villi.
This makes an area of 200-250 m² of absorptive surface.
Each day, the small intestine absorbs:
200-300 g of monosaccharides
60-100 g of fatty acids
60-120 g of amino acids and peptides
50-100 g of ions.
Only 1 to 1.5 Liters of the 7-8 Liters of fluids secreted form the upper GI tract and 1.5-3 liters of fluids consumed daily are absorbed.
95% of bile salts are reabsorbed .
Absorptive mechanism is an extremely complex process combining a process of active transport and diffusion in which nutrients pass through the intestinal mucosal cell into bloodstream.
Digestion in the large intestine
The large intestine is the site of absorption of the remaining water and salts.
Vitamins are synthesized in the large intestine by bacterial action.
The large intestine is around 1.5 m long.
Consists of 3 sections. Cecum, colon and rectum.
Most of the water contained in the 500-1000 ml of chyme is absorbed in the colon.
Only 200-500 ml is extracted in the feces.
Colonic contents move at a rate 5 cm/hr.
Bacterial action takes place in the large intestine.
Lactobacillus flora is the dominant in infants until they start to consume solid foods.
E.coli becomes the dominant flora with other genus Bacteroides.
Colonic bacteria contribute to the formation of gases like hydrogen, carbon dioxide, nitrogen and methane.
Bacteria also is responsible for formation of organic acids like acetic, propionic, butyric and lactic acids.
Bacteria is responsible for formation of toxic substances like ammonia, all of this actions give the odor to feces.
Change in the diet can cause change in the fecal flora but response depend on the original host flora and the nature of dietary change.
Increased consumption of certain sugars and dietary fiber may lead to an increased microbial mass as well as bifidobacteria and lactobacilli that are thought to be beneficial.
Low-fiber diet, high fat and meat leads to higher ratio of clostridia and E.coli.
The feces generally consists of 75% water and 25% solids.
1/3 of the solid matter consists of dead bacteria.
Inorganic materials make up to 20%-40% .
Protein makes 2%-3%.
The remainder includes undigested dietary fiber, epithelial cells and components of digestive juices.
Expulsion of feces through the anus occurs with varying frequency.
Ranging from 3 times daily to once every 3 days.
Normal weight ranges form 100-200 g.
The total time from mouth to anus vary from 12-72 hours.
A diet contains more fruits and vegetables and whole grains results in shorter GI transit time, more defecation and larger and softer stool.