After the teeth chew the food, swallowing occurs voluntarily with the combined action of the tongue and pharyngeal muscles. The food is pushed along in addition to being mixed thanks to the synchronized combination of specially design muscles and nerves.
After swallowing, it takes about 1-3 hours for solid food to leave the stomach. Typically , it takes about 1-2 days for food to completely pass through the digestive system.
1.- Digestion of Carbohydrates: Starches are polymers of glucose ( C6H12O6).
Monosaccharides: glucose, galactose and fructose
Disaccharides : Maltose = glucose + glucose
Lactose = glucose + galactose
Sucrose = glucose + fructose
Absorption of carbohydrates requires that they first be broken down to monosaccharides.
This process begins in the mouth when the food is mixed with the saliva that is a mix of mucus that is a lubricant plus the Salivary Amylase that is an enzyme that partially breaks down starch. The saliva has antibodies and proteolytic enzymes that have an antibiotic effect.
After this mix is swallowed, , it goes to the stomach where the Chloride Acid ( HCL) hydrolyze the starches mildly. When this mix goes to the Duodenum, the enzyme Pancreatic Amylase changes starch to oligosaccharides, which are then absorbed into the small intestine and enter the portal blood.
2.- Fat Digestion: Fat leaving the stomach is first broken into small droplets. Pancreatic and intestinal lipase break down triglycerides to monoglycerides and free fatty acids. Pancreatic and intestinal lipase's break down cholesterol esters to free cholesterol and free fatty acids. Pancreatic phospholipases remove fatty acids from phospholipids (lysophospholipids).
The bile salt molecules then mix with the free fatty acids , monoglycerides, cholesterol and lysophospholipids to form micelles tiny carrier vehicles that transport the digested lipid products to the intestinal cells for absorption.
Inside the epithelial intestinal cell, the free fatty acids recombine with the monoglycerides, free cholesterol and lisophospholipids to form triglycerides, cholesterol esters and phospholipids. These together with apoprotein ( special proteins that become active when combined with something else) form chylomicrons, that leave the cells to enter the lymphatics.
Other lipoproteins vehicles ( VLDL, LDL, IDL, HDL) form after further processing in the liver. Cholesterol is found in all lipoprotein vehicles but is relatively concentrated in LDL and HDL. Triglycerides are relative concentrated in chylomicrons and VLDL.
The chylomicrons droop off their triglycerides largely in fat, skeletal and heart muscles cells.
3.- Digestion of proteins: Proteins need to be broken down to amino acids , but small peptides ( more tha one aminoacid) can also be absorbed ( one of the causes of allergies to certain proteins ).
Protein digestion begins in the stomach with HCl and pepsin and ends in the small intestine with trypsin, chymotrypsin and carboxypolypeptidase from pancreas and intestinal peptidases.
4.- Digestive Hormones
4.1. Gastrin: Produced in stomach. Stimulate parietal cells to secrete HCl. Increases intestinal motility, stimulates pyloric contraction and relaxation of pyloric sphincter, stimulates pancreatic enzyme secretion; relaxes ileocecal valve.
4.2. Produced in the small intestine
Secretin: Decreases gastric secretion and motility, stimulate bile sale secretion in liver and pancreatic bicarbonate secretion that will protect mucosa against acid. Inhibits gastrointestinal motility.
Gastric inhibitory Peptide (GIP): Inhibits gastric secretion and motility; decreases the speed of gastric emptying.
Cholecystokinin: Decreases gastric motility, stimulates pancreatic enzyme and bicarbonate secretion; stimulates gall bladder contraction and relaxation of sphincter of Oddi; causes vasodilation in intestinal mucosa.
4.3. Produced in the Pancreas ( endocrine)
Insulin: Increases intestinal motility.
Glucagon: Decreases intestinal motility.
Somatostatin: Decreases digestion and absorption. Inhibits insulin and glucagon secretion
4.4. Other Chemical produced during Digestion:
4.4.1. Stomach
Pepsinogen: Changes to pepsin in presence of H+ to break down proteins to peptides
Intrinsic Factor: Binds to B12 and facilitates its absorption in small intestine.
HCl: Facilitates pepsinogen change to pepsin, mild hydrolysis of starches, bactericidal; PH facilitates some mineral absorption in intestine ( e.g. Iron)
Bicarbonate: Produced in the stomach, pancreas and colon. Protects mucosa against acid trough the digestive system.
Mucus: Produced in the mouth, esophagus, stomach. It function is as a lubricant and protects the mucosa from acid and pepsin.
4.4.2. Duodenum/ Small Intestine
Enzymes:
- Maltose, lactose. Sucrose
- Peptidases
- Intestinal lipases
- Enterokinase ( Activates changes of trypsinogen to trypsin and other hormones break down proteins to peptides and amino acids).
4.4.3. Liver: Bile salts . Bile salt micelles emulsify fats into smaller particles that can be attacked by pancreatic lipase. Micelles carry fats ti villi for absorption. Bile salt excretion decreases body cholesterol. The gall bladder storages bile.
4.4.4. Colon: Produces bicarbonate that helps neutralize bacterial products and mucus that lubricates , protects and solidifies the feces.
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