What is Digestion?
o Physical and chemical breakdown of food with enzymes for the absorption of substances into blood.
o Breakdown of macronutrients to form molecules is necessary so that it can cross the epithelium of the intestine to the bloodstream of the body.
o Physical / mechanical digestion is breaking the food substances into small pieces for the chemical digestion to take place easily. Whereas chemical digestion further digests the molecules by degrading with the help of enzymes.
o The nutrients of food reach the body through digestive system.
o The macronutrients like carbohydrates, proteins lipids are digested into small units for the small intestine to digest it completely.
o The functions and the signaling of the action are between endocrine system and nervous system from cellular level to throughout the body.
Process of Digestion
Process of Digestion: Step 1
o Digestion of food begins in the buccal cavity by both the process of physical (grinding the food into small pieces by teeth) and chemical process takes place by salivary amylase and lipase present in the saliva.
o Salivary amylase responsible for the conversion of starch into simple sugars and lipase helps in hydrolysis of triglycerides to form diacylglycerols and monoacylglycerols.
o After the chemical digestion in the buccal cavity the food is swallowed and passed into the esophagus.
Digestive System Diagram
Process of Digestion: Step 2
o Then digestion continues in the stomach where again physical and chemical digestion takes place.
o Physical digestion is done by the peristalsis of the smooth muscles.
o The food is churned that reduces the food particles to size 2mm so the passage of food to the duodenum is easy.
o The digested food is particles move forward to duodenum and rest of the bolus is sent back to the stomach for the digestion.
o This whole process is repeated until all of the bolus is of the size to enter small intestine.
o Chemical digestion of stomach takes place by two type of gland – pyloric and oxyntic glands.
o Oxyntic glands have parietal and chief cells where the parietal cells secrete the hydrochloric acid that has functions to denature the proteins and helps in activity of pepsinogen and release intrinsic factor for the absorption of vitamin B12.
o Chief cells secrete zymogen pepsinogen which precursor to the pepsin and is activated to pepsin by acidic pH less than 3.5. stomach has mucus cells and G cells.
o G cells secrete hormone called gastrin that helps in stimulation of hydrochloric acids by parietal cells.
o Mucus cells secrete bicarbonate rich mucous that the protects the lining of stomach walls from acid produces and other enzymes.
Process of Digestion: Step 3
o Small intestine helps in digestion with mostly chemical digestion. There is presence of minor physical digestion that takes place in small intestine.
o As the churned bolus from stomach enters, it mixes with secretion of pancreas and duodenum.
o Pancreatic secretion includes several enzymes like pancreatic amylase (digest starch into maltose), lipase (hydrolyze triglycerides to diacylglycerols and monoacylglycerols), trypsinogen, chymo-trypsinogen, procarboxypeptidase, proelastase (all are precursor that helps in activating peptidases).
o The trypsinogen, chymotrypsinogen, procarboxypeptidase and proelastase enzymes are converted to trypsin, chymotrypsin, carboxypeptidase and elastase that help in the hydrolysis of peptide bonds of proteins.
o Bile from the liver is released from the bile duct to the duodenum is a mixture of bile salts, bilirubin, fatty acids and cholesterols.
o Bile helps in emulsification of hydrophobic lipids that is necessary for the action of lipase which is hydrophilic in nature.
o All the secretion enter duodenum there is activation of cascade that helps to activate trypsinogen to trypsin that will activate pancreatic peptidases.
o Enzymes of duodenum contains disaccharides (includes maltase, lactase and sucrase) and dipeptidases.
o Disaccharides helps in conversion of maltose to glucose, lactose to glucose and fructose and dipeptidase helps in cleaving the peptide bond in dipeptides.
o All the major nutrients and macromolecules are digested to form absorbable for the bloodstream needed for the body.
Process of Digestion: Step 4
o The large intestine plays significant role, as it is responsible for the conversion of food to feces by absorbing important vitamins.
o The chyme from the small intestine passed onto the large intestine and moves slowly through it by the peristalsis.
o The bacteria present in the large intestine digest the remaining food and release gas like methane and carbon dioxide as the byproduct of the process.
o The absorption of remaining water by the large intestines helps in solidifying the feces.
o The fecal matter is then stored in the rectum and sigmoid colon until signal of elimination.
o Then the fecal matter is eliminated through the process of defecation.
You may like to read;
Human Digestive System: Diagram, Parts and Function
Alimentary Canal: Diagram, Parts and Function
Digestive Glands: Diagram, Parts and Function
Krebs Cycle: Definition, Diagram, Steps, and Mechanism
Digestion of Food Citations
- Gastrointestinal digestion of food allergens: effect on their allergenicity. Biomed Pharmacother . 2007 Jan;61(1):50-60.
- Human digestion–a processing perspective. J Sci Food Agric . 2016 May;96(7):2275-83.