Digestive System Physiology (7) PDF

# **Digestive System** ## **Physiology (7)** ### **Digestive system (alimentary canal + accessory glands)** ### **Digestion** - The breaking down of large complex molecules of food into small simple units that can be absorbed by digestive enzymes & motality of GIT. - **E.g:** - **Proteins** br...

# **Digestive System** ## **Physiology (7)** ### **Digestive system (alimentary canal + accessory glands)** ### **Digestion** - The breaking down of large complex molecules of food into small simple units that can be absorbed by digestive enzymes & motality of GIT. - **E.g:** - **Proteins** breaking down into amino acids. - **Lipids** break down into fatty acids & glycerol. - **Carbohydrates** into monosaccharides. ### **Digestion achieved by many processes:** 1. **Mechanical process:** food into smaller particles. - Cutting & grinding action of teeth. - Contractions (motality) of stomach & small intestine. - No change in the chemical structure. 2. **Chemical (secretory) process:** macromolecules into micromolecules - Hydrolysis of food in digestive tract by enzymes (catabolism) - From mouth to anus. 3. **Microbial process:** - In protozoa e.g; flagellates & paramecium. - Cellulose digestion by cellulase. ### **Digestive process:** - **Ingestion:** talking of food into mouth (physical breakingdown of food). - **Mixing & movement of food:** muscular contraction (peristalsis) mix food & move it through digestive tract. - **Digestion:** breakdown of food by mechanical & chemical means. - **Absorption:** passage of digested food from digestive tract to bloodstream. - **Defecation:** elimination of wastes. ## **Buccal Digestion (in the mouth):** - The food in mouth is digested by; - Cutting & grinding by teeth & mixed with saliva. - **Saliva:** hypotonic soln. of: - 99% water & 1% (inorganic + organic). - **Salivary amylase** for begins starch (carbohydrate) digestion. - **Mucous:** lubricates food for easily swallowing (bollus). - **Lysozyme:** destroy bacteria to protect mucosa & teeth from decay. - **Electrolyte:** e.g; Na, K, Cl, P & bicarbonate ions - pH = 6.8 to 7.0 | Enzyme | Site | Action | | :------- | :----------------------------------------------------------------------- | :------------------------------------------------------------------------------ | | Amylase | constituent saliva that secreted from salivary glands & mucosa of mouth. | Digestion of carbohydrates (starch) into maltose | - **Function of saliva:** - moisten & clean oral cavity. - Moisten ingested food forming semisolid bolus to easily swallow. - Amylase breaks down carbohydrate. - Contain lysozyme inhibit bacterial growth in oral cavity. - **Note:** - there is no enzymes for lipid & protein in mouth. (incomplete digestion) - The lipid & protein only crush (grinding) (breaking down) by teeth. - Salivary glands are; sublingual, submandibular & parotid. - Saliva secretions & other digestive secretion incresed by vision or smell of food. ## **Gastric Digestion (in the stomach):** - Food in stomach digested by; Gastric juices that secreted by tubular glands. | Enzyme | Site | Action | | :----------------- | :--------------------------------------------------------------------------- | :------------------------------------------------------------------------------------------------------------------------------------- | | HCL (hydrochloric acid) | Screted by parietal (oxyntic) cells of tubular glands. | Convert pepsinogen to pepsin.<br> Convert prorennin to rennin.<br> Help in digestion.<br> Kills microrganisms (M.O.) & pathogen. | | Pepsin | Secreted from chief cell (peptic cells) of tubular glands as pepsinogen (inactive) & coveted into pepsin by HCL. | Hydrolyze proteins to peptones. | | Rennin | Secreted by GIT in young mammals as prorennin (zymogen) (inactive) & converted into rennin by HCL. | Help in digestion of milk protein (caseinogen). | | Gastric lipase | Screted by chief cells of tubular glands. | Digest lipids (slow) into glycerol & fatty acids. | | Mucin | Alkaline watery secretion. <br> Secreted by mucous cells. | Protects gastric mucosa against friction by enzymes & HCL. | - Stomach wall will not be digested by the acid or enzymes. Due to; - **Mucin:** Protects gastric mucosa against friction by enzymes & HCL - **Antipepsin** secrete with pepsin to protect stomach wall from digestion - **Mucosa:** is stratified squamous epithelium. - **Note:** - There is no absorption of food in stomach & Very slow digestion. - Saliva stop working after secretion of gastric juices. - Partially digestion. ## **Intestinal Digestion (in the small intestine):** - The food in small intestine is digested by; - Pancreatic juice, bile & intestinal juice. | Enzyme | Site | Action | | :------------ | :----------------------------------------------------------------- | :---------------------------------------------------------------------------------------------- | | Amylase | Secreted by Pancrease | Hydrolyze carbohydrates into maltose. | | Trypsin | Secreted by Pancrease as trypsinogen and activated by enterokinase. | Hydrolyze proteins (come from stomach) to amino acids. | | Lipase | Secreted by Pancrease | Hydrolyze lipids into fatty acids + glycerol. | | Sucrase | | Hydrolyze Sucrose to Glucose + fructose. | | Lactase | Secreted by the mucosa of intestine | Hydrolyze Lactose to Glucose + Galactose. | | Maltase | | Hydrolyze Maltose to Glucose + Glucose. | | Bile | Secreted from Liver cell & stored in gall bladder | Help in digestion & absorption of fats & fat -soluble vitamins.(emulsification) | - **Note:** - Protein hydrolysis; Protoses, peptones, polypeptides, dipeptides & amino acids. - Lipid hydrolysis; fatty acid + glycerol. - Carbohydrate hydrolysis; into monosaccharides. - Small intestine where digestion & absorption occur. - Absorption occur in villi & microvilli (wall of small intestine). ## **Intestinal Digestion (in the large intestine):** - Main function of colon; - Reabsorption of water (more than 1/2) & some minerals & vitamins leaving semisolid faeces for egestion. - Don't secete digestive enzymes but secrete mucin. ## **Control of digestive secretion** | Phase | Controlled by | | :--------------- | :-------------------------------------- | | Cephalic phase | Controlled by nerves | | Gastric phase | Controlled by gastrin | | Intestinal phase | Controlled by enterokinase (enterogaster) | - **Note:** - **Mucous** - Lines Gastrointestinal tract = GIT - Secreted by goblet cell. - Helps food slides down through GIT.